View source for Galileo project

{{short description|American space program to study Jupiter (1989–2003)}}
{{About|the Jupiter probe project|the spacecraft itself|Galileo (spacecraft){{!}}''Galileo'' (spacecraft)|the navigation satellites|Galileo (satellite navigation)|the project to study UFOs|The Galileo Project|astronomy education resource|Project Galileo}}
{{Featured article}}
{{Italic title|string=Galileo}}
{{Infobox spaceflight
| auto = all
| name = ''Galileo''
| names_list = Jupiter Orbiter Probe
| image = Artwork Galileo-Io-Jupiter.JPG
| image_caption = Artist's concept of ''Galileo'' at Io with Jupiter in the background. The high-gain antenna is fully deployed in this illustration but in reality the antenna failed to extend.
| image_alt = refer to caption
| image_size =
| mission_type = [[Jupiter]] orbiter
| operator = [[NASA]]
| COSPAR_ID = 1989-084B
| SATCAT = 20298
| website = {{URL|solarsystem.nasa.gov/galileo/}}
| mission_duration = {{plainlist|
* Planned: {{time interval|18 October 1989|7 December 1997|show=ymd|sep=,}}
* Jupiter orbit: {{time interval|8 December 1995 01:16|21 September 2003 18:57|show=ymd|sep=,}}
* Final: {{time interval|18 October 1989|21 September 2003 18:57|show=ymd|sep=,}}
}}
| distance_travelled = {{convert|4631778000|km|e9mi|sigfig=3|abbr=unit}}<ref name="spref20030919">{{cite web |url=https://spaceref.com/press-release/the-final-day-on-galileo-sunday-september-21-2003/ |title=The Final Day on Galileo – Sunday, September 21, 2003 |publisher=Spaceref.com |date=September 19, 2003 |access-date=August 11, 2023 }}</ref>
| manufacturer = {{plainlist|
* [[Jet Propulsion Laboratory]]<ref name="galileo-arrival">{{cite web |url=http://www.jpl.nasa.gov/news/press_kits/gllarpk.pdf |title=Galileo Jupiter Arrival |type=Press Kit |publisher=NASA/Jet Propulsion Laboratory |date=December 1995 |access-date=December 22, 2016 |archive-date=November 16, 2001 |archive-url=https://web.archive.org/web/20011116223923/http://www.jpl.nasa.gov/news/press_kits/gllarpk.pdf |url-status=live }}</ref> 
* [[Messerschmitt-Bölkow-Blohm]]<ref name="galileo-arrival" />
* [[General Electric]]<ref name="galileo-arrival" />
* [[Hughes Aircraft Company]]<ref name="galileo-arrival" />
}}
| launch_mass = {{plainlist|
* Total: {{convert|2560|kg|lb|abbr=on}}<ref name="galileo-arrival" />
* Orbiter: {{convert|2220|kg|lb|abbr=on}}<ref name="galileo-arrival" />
* Probe: {{convert|340|kg|lb|abbr=on}}<ref name="galileo-arrival" />
}}
| dry_mass = {{plainlist|
* Orbiter: {{convert|1880|kg|lb|abbr=on}}<ref name="galileo-arrival" />
* Probe: {{convert|340|kg|lb|abbr=on}}<ref name="galileo-arrival" />
}}
| payload_mass = {{plainlist|
* Orbiter: {{convert|118|kg|lb|abbr=on}}<ref name="galileo-arrival" />
* Probe: {{convert|30|kg|lb|0|abbr=on}}<ref name="galileo-arrival" />
}}
| power = {{plainlist|
* Orbiter: 570 watts<ref name="galileo-arrival" />
* Probe: 730 watt-hours<ref name="galileo-arrival" />
}}
| launch_date = {{start date text|October 18, 1989, 16:53:40}}&nbsp;[[UTC]]<ref name="Beyer1992">{{cite report |url=http://ipnpr.jpl.nasa.gov/progress_report/42-109/109T.PDF |title=Galileo Early Cruise, Including Venus, First Earth, and Gaspra Encounters |series=The Telecommunications and Data Acquisition Report |publisher=NASA/Jet Propulsion Laboratory |first1=P. E. |last1=Beyer |first2=R. C. |last2=O'Connor |first3=D. J. |last3=Mudgway |pages=265–281 |date=May 15, 1992 |id=TDA Progress Report 42-109 |access-date=December 22, 2016 |archive-date=January 25, 2021 |archive-url=https://web.archive.org/web/20210125143531/https://ipnpr.jpl.nasa.gov/progress_report/42-109/109T.PDF |url-status=live }}</ref>
| launch_rocket = {{OV|104}} <br /> [[STS-34]]/[[Inertial Upper Stage|IUS]]
| launch_site = [[Kennedy Space Center|Kennedy]] [[Kennedy Space Center Launch Complex 39|LC-39B]]
| launch_contractor =
| entered_service = December 8, 1995, 01:16&nbsp;UTC&nbsp;[[Spacecraft Event Time|SCET]]
| disposal_type = Controlled entry into Jupiter
| decay_date = {{end date text|September 21, 2003, 18:57:18}} UTC
| interplanetary         = 
 {{Infobox spaceflight/IP
   |type                = flyby
   |note                = gravity assist
   |object              = [[Venus]]
   |distance            = {{convert|16000|km|mi|sp=us}}
   |arrival_date        = February 10, 1990<ref name="PDS Atmospheres Node 1989">{{cite web | title=Welcome to the Galileo Orbiter Archive Page | website=PDS Atmospheres Node | date=1989-10-18 | url=https://pds-atmospheres.nmsu.edu/data_and_services/atmospheres_data/Galileo/galileo_orbiter.html | access-date=2023-04-11 | archive-date=April 11, 2023 | archive-url=https://web.archive.org/web/20230411212904/https://pds-atmospheres.nmsu.edu/data_and_services/atmospheres_data/Galileo/galileo_orbiter.html | url-status=live }}</ref>
 }}
 {{Infobox spaceflight/IP
   |type                = flyby
   |note                = gravity assist
   |object              = [[Earth]]
   |distance            = {{convert|960|km|mi|sp=us}} and {{convert|303|km|mi|sp=us}}
   |arrival_date        = December 8, 1990 and December 8, 1992
 }}
 {{Infobox spaceflight/IP
   |type                = flyby
   |note                = 
   |object              = [[951 Gaspra]]
   |distance            = {{convert|1601|km|mi|sp=us}}
   |arrival_date        = October 29, 1991
 }}
 {{Infobox spaceflight/IP
   |type                = flyby
   |note                = 
   |object              = [[243 Ida]]
   |distance            = {{convert|2400|km|mi|sp=us}}
   |arrival_date        = August 28, 1993
 }}
 {{Infobox spaceflight/IP
  |type                 = orbiter
  |object               = [[Jupiter]]
  |component            = [[Galileo (spacecraft)|Orbiter]]
  |arrival_date         = December 8, 1995, 01:16&nbsp;UTC&nbsp;SCET
 }}
 {{Infobox spaceflight/IP
  |type                 = atmospheric
  |object               = [[Jupiter]]
  |component            = [[Galileo Probe|Probe]]
  |arrival_date         = December 7, 1995, 22:04&nbsp;UTC&nbsp;SCET<ref name = "Meltzer">Michael Meltzer, [https://history.nasa.gov/sp4231.pdf ''Mission to Jupiter: a History of the ''Galileo'' Project''] {{Webarchive|url=https://web.archive.org/web/20170214204726/https://history.nasa.gov/sp4231.pdf |date=February 14, 2017 }}, NASA SP 2007–4231, p. 188</ref>
  |location             = {{Coord|06|05|N|04|04|W|globe:jupiter|name=Galileo Probe}} <br /> <small>at entry interface</small>
 }}
| instruments_list = {{Infobox spaceflight/Instruments
  | acronym1 = SSI  | name1 = Solid-State Imager
  | acronym2 = NIMS | name2 = Near-Infrared Mapping Spectrometer
  | acronym3 = UVS  | name3 = Ultraviolet Spectrometer
  | acronym4 = PPR  | name4 = Photopolarimeter-Radiometer
  | acronym5 = DDS  | name5 = Dust Detector Subsystem
  | acronym6 = EPD  | name6 = Energetic Particles Detector
  | acronym7 = HIC  | name7 = Heavy Ion Counter
  | acronym8 = MAG  | name8 = Magnetometer
  | acronym9 = PLS  | name9 = Plasma Subsystem
  | acronym10 = PWS | name10 = Plasma Wave Subsystem
 }}
| programme        = '''[[Large Strategic Science Missions]]'''<br><small>''Planetary Science Division''</small>
| previous_mission = ''[[Voyager 1]]''
| next_mission     = ''[[Cassini–Huygens]]''
| insignia         = Galileo mission patch.png
| insignia_size    = 180px
}}

'''''Galileo''''' was an American robotic space program that studied the planet [[Jupiter]] and [[Moons of Jupiter|its moons]], as well as several other [[Solar System]] bodies. Named after the Italian astronomer [[Galileo Galilei]], the [[Galileo (spacecraft)|''Galileo'' spacecraft]] consisted of an orbiter and an [[atmospheric entry]] probe. It was delivered into Earth orbit on October 18, 1989, by {{OV|Atlantis}} on the [[STS-34]] mission, and arrived at Jupiter on December 7, 1995, after [[gravity assist]] [[planetary flyby|flybys]] of [[Venus]] and [[Earth]], and became the first spacecraft to orbit Jupiter. The spacecraft then launched the first probe to directly measure its [[atmosphere]]. Despite suffering major antenna problems, ''Galileo'' achieved the first [[asteroid]] flyby, of [[951 Gaspra]], and discovered the first [[asteroid moon]], [[Dactyl (moon)|Dactyl]], around [[243 Ida]]. In 1994, ''Galileo'' observed [[Comet Shoemaker–Levy 9]]'s collision with Jupiter.

Jupiter's atmospheric composition and [[ammonia]] clouds were recorded, as were the [[volcanism]] and [[Plasma (physics)|plasma]] interactions on [[Io (moon)|Io]] with Jupiter's atmosphere. The data ''Galileo'' collected supported [[Europa (moon)#Subsurface ocean|the theory of a liquid ocean]] under the icy surface of [[Europa (moon)|Europa]], and there were indications of similar liquid-[[saline water|saltwater]] layers under the surfaces of [[Ganymede (moon)|Ganymede]] and [[Callisto (moon)|Callisto]]. Ganymede was shown to possess a [[magnetic field]] and the spacecraft found new evidence for [[exosphere]]s around Europa, Ganymede, and Callisto. ''Galileo'' also discovered that Jupiter's faint [[ring system]] consists of dust from [[impact event]]s on the four small inner moons. The extent and structure of Jupiter's [[magnetosphere]] was also mapped.

The primary mission concluded on December 7, 1997, but the ''Galileo'' orbiter commenced an extended mission known as the ''Galileo'' Europa Mission (GEM), which ran until December 31, 1999. By the time GEM ended, most of the spacecraft was operating well beyond its original design specifications, having absorbed three times the radiation exposure that it had been built to withstand. Many of the instruments were no longer operating at peak performance, but were still functional, so a second extension, the ''Galileo'' Millennium Mission (GMM) was authorized. On September 20, 2003, after 14 years in space and 8 years in the Jovian system, ''Galileo''{{'s}} mission was terminated by sending the orbiter into Jupiter's atmosphere at a speed of over {{convert|48|km/s|mi/s|0|sp=us}} to eliminate the possibility of [[Interplanetary contamination|contaminating]] the moons with bacteria.

==Background==
[[Jupiter]] is the largest planet in the [[Solar System]], with more than twice the mass of all the other planets combined.<ref>{{cite web |title=In Depth {{pipe}} Jupiter |date=October 5, 2017 |publisher=NASA Solar System Exploration |url=https://solarsystem.nasa.gov/planets/jupiter/in-depth/ |access-date=27 October 2020 |archive-date=March 24, 2018 |archive-url=https://web.archive.org/web/20180324111152/https://solarsystem.nasa.gov/planets/jupiter/in-depth/ |url-status=live }}</ref> Consideration of sending a probe to Jupiter began as early as 1959, when the [[National Aeronautics and Space Administration]] (NASA) [[Jet Propulsion Laboratory]] (JPL) developed four mission concepts:
* Deep space flights would fly through interplanetary space;
* [[Planetary flyby]] missions would fly past planets close enough to collect scientific data and could visit multiple planets on a single mission;
* Orbiter missions would place a spacecraft in orbit around a planet for prolonged and detailed study;
* [[Atmospheric entry]] and [[lander (spacecraft)|lander]] missions would explore a planet's atmosphere and surface.{{sfn|Meltzer|2007|pp=9–10}}

Two missions to Jupiter, ''[[Pioneer 10]]'' and ''[[Pioneer 11]]'', were approved in 1969, with NASA's [[Ames Research Center]] given responsibility for planning the missions.{{sfn|Meltzer|2007|pp=21–22}} ''Pioneer 10'' was launched in March 1972 and passed within {{convert|200,000|km|sp=us}} of Jupiter in December 1973. It was followed by ''Pioneer 11'', which was launched in April 1973, and passed within {{convert|34,000|km|sp=us}} of Jupiter in December 1974, before heading on to an [[:wikt:encounter|encounter]] with [[Saturn]].{{sfn|Meltzer|2007|pp=28–29}} They were followed by the more advanced ''[[Voyager 1]]'' and ''[[Voyager 2]]'' spacecraft, which were launched on 5 September and 20 August 1977 respectively, and reached Jupiter in March and July 1979.<ref>{{cite web |title=NSSDCA: Voyager Project Information |publisher=NASA |url=https://nssdc.gsfc.nasa.gov/planetary/voyager.html |access-date=27 October 2020 |archive-date=October 28, 2020 |archive-url=https://web.archive.org/web/20201028144943/https://nssdc.gsfc.nasa.gov/planetary/voyager.html |url-status=live }}</ref>{{efn|Although Voyager 2 was launched before Voyager 1, the latter reached Jupiter and Saturn first.<ref>{{cite web |title=Voyager 2 Launched Before Voyager 1 - NASA |publisher=NASA |url=https://www.nasa.gov/image-article/voyager-2-launched-before-voyager-1/ |access-date=April 7, 2024}}</ref>}}

==Planning==
{| class="wikitable floatright" style="font-size: 91%" 
|+ ''Galileo'' Project managers{{sfn|Meltzer|2007|p=265}}
! Manager || Date
|-
| [[John R. Casani]]|| October 1977 – February 1988
|-
| Dick Spehalski    || February 1988 – March 1990
|-
| Bill O'Neil       || March 1990 – December 1997
|-
| Bob Mitchell      || December 1997 – June 1998
|-
| Jim Erickson      || June 1998 – January 2001
|-
| Eilene Theilig    || January 2001 – August 2003
|-
| Claudia Alexander || August 2003 – September 2003
|}

===Initiation===
Following the approval of the ''[[Voyager program|Voyager]]'' missions, NASA's Scientific Advisory Group for Outer Solar System Missions considered the requirements for Jupiter orbiters and atmospheric probes. It noted that the technology to build a [[heat shield]] for an atmospheric probe did not yet exist, and indeed facilities to test one under the conditions found on Jupiter would not be available until 1980. There was also concern about the effects of radiation on spacecraft components, which would be better understood after ''Pioneer 10'' and ''Pioneer 11'' had conducted their flybys. ''Pioneer 10''{{'}}s flyby in December 1973 indicated that the effects were not as severe as had been feared.{{sfn|Meltzer|2007|pp=29–30}} NASA management designated JPL as the lead center for the Jupiter Orbiter Probe (JOP) Project.{{sfn|Meltzer|2007|pp=32–33}} [[John R. Casani]], who had headed the ''[[Mariner program|Mariner]]'' and ''Voyager'' projects, became the first project manager.<ref>{{cite web |publisher=NASA |title=NASA's 50 Year Men and Women |url=https://www.nasa.gov/50th/50th_magazine/50yearsEmployees.html |access-date=October 28, 2020 |archive-date=March 19, 2010 |archive-url=https://web.archive.org/web/20100319002842/https://www.nasa.gov/50th/50th_magazine/50yearsEmployees.html |url-status=dead }}</ref> The JOP would be the fifth spacecraft to visit Jupiter, but the first to orbit it, and the probe the first to enter its atmosphere.{{sfn|Dawson|Bowles|2004|pp=190–191}}

Ames and JPL decided to use a ''Mariner'' spacecraft for the Jupiter orbiter like the ones used for ''Voyager'' rather than a ''Pioneer'' spacecraft. ''Pioneer'' was stabilized by spinning the spacecraft at 60 [[rpm]], which gave a 360-degree view of the surroundings, and did not require an [[attitude control system]]. By contrast, ''Mariner'' had an attitude control system with three [[gyroscopes]] and two sets of six [[nitrogen]] jet thrusters. Attitude was determined with reference to the Sun and [[Canopus]], which were monitored with two primary and four secondary [[star tracker]] sensors. There was also an [[inertial reference unit]] and an [[accelerometer]]. The attitude control system allowed the spacecraft to take high-resolution images, but the functionality came at the cost of increased weight: a ''Mariner'' weighed {{convert|722|kg}} compared to just {{convert|146|kg}} for a ''Pioneer''.{{sfn|Meltzer|2007|pp=30–32}}

The increase in weight had implications. The Voyager spacecraft had been launched by [[Titan IIIE]] rockets with a ''[[Centaur (rocket stage)|Centaur]]'' upper stage, but Titan was retired afterwards. In the late 1970s, NASA was focused on the development of the reusable [[Space Shuttle]], which was expected to make expendable rockets obsolete.<ref>{{cite news |newspaper=[[The New York Times]] |title=Test Rocket for Planetary Exploration Rolled Out |first=John Noble |last=Wilford |author-link=John Noble Wilford |date=3 October 1973 |url=https://www.nytimes.com/1973/10/03/archives/test-rocket-for-planetary-exploration-rolled-out-forthcoming.html |access-date=8 October 2020 |archive-date=August 10, 2020 |archive-url=https://web.archive.org/web/20200810122304/https://www.nytimes.com/1973/10/03/archives/test-rocket-for-planetary-exploration-rolled-out-forthcoming.html |url-status=live }}</ref> In late 1975, NASA decreed that all future planetary missions would be launched by the Space Shuttle. The JOP would be the first to do so.{{sfn|Mudgway|2001|p=294}} The Space Shuttle was supposed to have the services of a [[space tug]] to launch payloads requiring something more than a [[low Earth orbit]], but this was never approved. The [[United States Air Force]] (USAF) instead developed the [[solid-fueled]] Interim Upper Stage (IUS), later renamed the [[Inertial Upper Stage]] (with the same acronym), for the purpose.{{sfn|Meltzer|2007|pp=32–33}}

The IUS was constructed in a modular fashion, with two stages, a large one with {{convert|21400|lb|order=flip}} of propellant, and a smaller one with {{convert|6000|lb|order=flip}}. This was sufficient for most satellites. It could also be configured with two large stages to launch multiple satellites.{{sfn|Heppenheimer|2002|pp=330–335}} A configuration with three stages, two large and one small, would be enough for a planetary mission, so NASA contracted with [[Boeing]] for the development of a three-stage IUS.{{sfn|Heppenheimer|2002|pp=368–370}} A two-stage IUS was not powerful enough to launch a payload to Jupiter without resorting to using a series of [[gravity-assist]] maneuvers around planets to garner additional speed. Most engineers regarded this solution as inelegant and planetary scientists at JPL disliked it because it meant that the mission would take months or even years longer to reach Jupiter.{{sfn|Bowles|2002|p=420}}{{sfn|Heppenheimer|2002|pp=368–370}} Longer travel times meant that the spacecraft's components would age and possibly fail, and the onboard power supply and propellant would be depleted. Some of the gravity assist options also involved flying closer to the Sun, which would induce thermal stresses that also might cause failures.{{sfn|Meltzer|2007|p=82}}

It was estimated that the JOP would cost $634 million (equivalent to ${{Inflation|US-GDP|0.634|1979|r=3}} billion in {{Inflation/year|US-GDP}}), and it had to compete for [[fiscal year]] 1978 funding with the Space Shuttle and the [[Hubble Space Telescope]]. A successful lobbying campaign secured funding for both JOP and Hubble over the objections of [[United States Senate|Senator]] [[William Proxmire]], the chairman of the Independent Agencies Appropriations Subcommittee.{{sfn|Meltzer|2007|p=38}} The [[United States Congress]] approved funding for the Jupiter Orbiter Probe on July 19, 1977,<ref>{{cite web |title=Congressional Record |publisher=United States Congress |url=https://www.congress.gov/95/crecb/1977/07/19/GPO-CRECB-1977-pt19-3-1.pdf |access-date=11 July 2024}}</ref> and JOP officially commenced on October 1, 1977, the start of the fiscal year.{{sfn|Meltzer|2007|pp=33–36}} Project manager Casani solicited suggestions for a more inspirational name for the project from people associated with it. The most votes went to "Galileo", after [[Galileo Galilei]], the first person to view Jupiter through a telescope, and the discoverer of what are now known as the [[Galilean moons]] in 1610. It was noted at the time that the name was also that of a [[Galileo (Star Trek)|spacecraft]] in the ''[[Star Trek: The Original Series|Star Trek]]'' television show. In February 1978, Casani officially announced the choice of the name "Galileo".{{sfn|Meltzer|2007|p=38}}

===Preparation===
To enhance reliability and reduce costs, the project engineers decided to switch from a pressurized [[Galileo Probe|atmospheric probe]] to a vented one, so the pressure inside the probe would be the same as that outside, thus extending its lifetime in Jupiter's atmosphere, but this added {{convert|100|kg}} to its weight. Another {{convert|165|kg}} was added in structural changes to improve reliability. This required additional fuel in the IUS, but the three-stage IUS was itself overweight with respect to its design specifications, by about {{convert|7000|lb|order=flip}}.<ref name="Hurdles">{{cite news |title=More Hurdles Rise In Galileo Project To probe Jupiter |first=Thomas |last=O'Toole |author-link=Thomas O'Toole |newspaper=[[The Washington Post]] |date=11 August 1979 |url=https://www.washingtonpost.com/archive/politics/1979/08/15/more-hurdles-rise-in-galileo-project-to-probe-jupiter/a30ddbe5-d805-418f-8a93-5d8deaa69f93/ |access-date=11 October 2020 |archive-date=May 23, 2021 |archive-url=https://web.archive.org/web/20210523182517/https://www.washingtonpost.com/archive/politics/1979/08/15/more-hurdles-rise-in-galileo-project-to-probe-jupiter/a30ddbe5-d805-418f-8a93-5d8deaa69f93/ |url-status=live }}</ref>{{sfn|Finley|1988|p=23}}{{sfn|Meltzer|2007|pp=41–43}} Lifting ''Galileo'' and the three-stage IUS required a special lightweight version of the [[Space Shuttle external tank]], the [[Space Shuttle orbiter]] stripped of all non-essential equipment, and the [[Space Shuttle main engine]]s (SSME) running at full power level—109 percent of their rated power level.{{sfn|Heppenheimer|2002|pp=368–370}}{{efn|The rated power level (RPL) is the power at which an engine can be normally operated. In the case of the Space Shuttle, the specification called for 27,000 seconds operation at 100 percent of the RPL, or 14,000 seconds at 109 percent of the RPL, which was designated full power level (FPL).{{sfn|Jenkins|2016|p=II-158}} }} Running at this power level necessitated the development of a more elaborate engine cooling system. Concerns were raised over whether the engines could be run at 109 percent by the launch date, so a gravity-assist maneuver using Mars was substituted for a direct flight.{{sfn|Meltzer|2007|pp=41–43}}

[[File:Galileo Preparations - GPN-2000-000672.jpg|thumb|left|In the Vertical Processing Facility (VPF), ''Galileo'' is prepared for mating with the [[Inertial Upper Stage]] booster.|alt=High gain antenna is folded. ]]
Plans called for the {{OV|Columbia}} to launch ''Galileo'' on the [[Canceled Space Shuttle missions#STS-23 (Columbia)|STS-23 mission]], tentatively scheduled for sometime between January 2 and 12, 1982,<ref>{{cite magazine |last=Portree |first=David S. F. |title=What Shuttle Should Have Been: The October 1977 Flight Manifest |date=March 24, 2012 |magazine=[[Wired (magazine)|Wired]] |issn=1059-1028 |url=https://www.wired.com/2012/03/what-shuttle-should-have-been-the-october-1977-flight-manifest/ |access-date=October 30, 2020 |archive-date=March 17, 2014 |archive-url=https://web.archive.org/web/20140317100957/http://www.wired.com/wiredscience/2012/03/what-shuttle-should-have-been-the-october-1977-flight-manifest |url-status=live }}</ref> this being the launch window when Earth, Mars and Jupiter were aligned to permit Mars to be used for the gravity-assist maneuver.{{sfn|Finley|1988|p=23}} By 1980, delays in the Space Shuttle program pushed the launch date for ''Galileo'' back to 1984.<ref>{{cite web |title=STS Flight Assignment Baseline |publisher=John H. Evans Library Digital Collections |url=https://digcollections.lib.fit.edu/items/show/45381 |access-date=October 31, 2020 |archive-date=November 30, 2021 |archive-url=https://web.archive.org/web/20211130095340/https://digcollections.lib.fit.edu/items/show/45381 |url-status=live }}</ref> While a Mars slingshot was still possible in 1984, it would no longer be sufficient.{{sfn|Meltzer|2007|pp=46–47}}

NASA decided to launch ''Galileo'' on two separate missions, launching the orbiter in February 1984 with the probe following a month later. The orbiter would be in orbit around Jupiter when the probe arrived, allowing the orbiter to perform its role as a relay. This configuration required a second Space Shuttle mission and a second carrier spacecraft to be built for the probe to take it to Jupiter, and was estimated to cost an additional $50 million (equivalent to ${{Inflation|US-GDP|50|1979}} million in {{Inflation/year|US-GDP}}), but NASA hoped to be able to recoup some of this through competitive bidding. The problem was that while the atmospheric probe was light enough to launch with the two-stage IUS, the Jupiter orbiter was too heavy to do so, even with a gravity assist from Mars, so the three-stage IUS was still required.<ref name="Deferring">{{cite web |title=NASA Weighs Deferring 1982 Mission to Jupiter |first=Thomas |last=O'Toole |author-link=Thomas O'Toole |newspaper=The Washington Post |date=September 19, 1979 |url=https://www.washingtonpost.com/archive/politics/1979/09/04/nasa-weighs-deferring-1982-mission-to-jupiter/bfe8bb4a-20fe-41f5-af14-d6b1c003b470/ |access-date=11 October 2020 |archive-date=August 27, 2017 |archive-url=https://web.archive.org/web/20170827161312/https://www.washingtonpost.com/archive/politics/1979/09/04/nasa-weighs-deferring-1982-mission-to-jupiter/bfe8bb4a-20fe-41f5-af14-d6b1c003b470/ |url-status=live }}</ref>{{sfn|Meltzer|2007|pp=46–47}}

By late 1980, the price tag for the IUS had risen to $506 million (equivalent to ${{Inflation|US-GDP|0.506|1979|r=3}} billion in {{Inflation/year|US-GDP}}).{{sfn|Heppenheimer|2002|pp=330–335}} The USAF could absorb this cost overrun on the development of the two-stage IUS (and indeed anticipated that it might cost far more), but NASA was faced with a quote of $179 million (equivalent to ${{Inflation|US-GDP|179|1979}} million in {{Inflation/year|US-GDP}}) for the development of the three-stage version,{{sfn|Heppenheimer|2002|pp=368–370}} which was $100 million (equivalent to ${{Inflation|US-GDP|100|1979}} million in {{Inflation/year|US-GDP}}) more than it had budgeted for.{{sfn|Meltzer|2007|p=43}} At a press conference on January 15, 1981, [[Robert A. Frosch]], the [[NASA Administrator]], announced that NASA was withdrawing support for the three-stage IUS, and going with a [[Centaur G Prime]] upper stage because "no other alternative upper stage is available on a reasonable schedule or with comparable costs."{{sfn|Janson|Ritchie|1990|p=250}}

[[File:Model of Centaur G with Galileo probe (upright).jpg|thumb|right|Model of ''Galileo'' atop a [[Centaur G Prime]] upper stage in the [[San Diego Air and Space Museum]] |alt=refer to caption]]
Centaur provided many advantages over the IUS. The main one was that it was far more powerful. The probe and orbiter could be recombined, and the probe could be delivered directly to Jupiter in two years' flight time.{{sfn|Heppenheimer|2002|pp=368–370}}{{sfn|Bowles|2002|p=420}} The second was that, despite this, it was gentler than the IUS, because it had lower thrust. This reduced the chance of damage to the payload. Thirdly, unlike solid-fuel rockets which burned to completion once ignited, a Centaur could be switched off and on again. This gave it flexibility, which increased the chances of a successful mission, and permitted options like asteroid flybys. Centaur was proven and reliable, whereas the IUS had not yet flown. The only concern was about safety; solid-fuel rockets were considered safer than liquid-fuel ones, especially ones containing [[liquid hydrogen]].{{sfn|Heppenheimer|2002|pp=368–370}}{{sfn|Bowles|2002|p=420}} NASA engineers estimated that additional safety features might take up to five years to develop and cost up to $100 million (equivalent to ${{Inflation|US-GDP|100|1979}} million in {{Inflation/year|US-GDP}}).{{sfn|Meltzer|2007|p=43}}<ref name="Deferring" />

In February 1981, JPL learned that the [[Office of Management and Budget]] (OMB) was planning major cuts to NASA's budget, and was considering cancelling ''Galileo''. The USAF intervened to save ''Galileo'' from cancellation. JPL had considerable experience with autonomous spacecraft that could make their own decisions.{{sfn|Meltzer|2007|pp=49–50}} This was a necessity for deep space probes, since a signal from Earth takes from 35 to 52 minutes to reach Jupiter, depending on the relative position of the planets in their orbits.<ref>{{cite web |title=Seeing in the Dark. Astronomy Topics. Light as a Cosmic Time Machine |publisher=PBS |url=https://www.pbs.org/seeinginthedark/astronomy-topics/light-as-a-cosmic-time-machine.html |access-date=12 October 2020 |archive-date=October 26, 2020 |archive-url=https://web.archive.org/web/20201026023044/https://www.pbs.org/seeinginthedark/astronomy-topics/light-as-a-cosmic-time-machine.html |url-status=live }}</ref> The USAF was interested in providing this capability for its satellites, so that they would be able to determine their [[orientation (geometry)|attitude]] using onboard systems rather than relying on [[ground station]]s, which were not "hardened" against [[nuclear weapons]], and could take independent evasive action against [[anti-satellite weapon]]s. It was also interested in the manner in which JPL was designing ''Galileo'' to withstand the intense radiation of the [[magnetosphere of Jupiter]], as this could be used to harden satellites against the [[electromagnetic pulse]] of nuclear explosions. On February 6, 1981 [[Strom Thurmond]], the [[President pro tempore of the United States Senate|President pro tem of the Senate]], wrote directly to [[David Stockman]], the director of the OMB, arguing that ''Galileo'' was vital to the nation's defense.{{sfn|Waldrop|1982|p=1013}}{{sfn|Meltzer|2007|pp=50–51}}{{efn|[[Sandia National Laboratories]] produced 12,000 microprocessor and integrated circuit components for ''Galileo'' that were hardened against radiation.{{sfn|Olmstead|Johnson|2024|p=246}} }}

[[File:Astronauts John Fabian and Dave Walker pose in front of a model of the Shuttle-Centaur.jpg|thumb|left|Astronauts [[John M. Fabian]] and [[David M. Walker (astronaut)|David M. Walker]] pose in front of a model of the [[Shuttle-Centaur]] with ''Galileo'' in mid-1985 |alt=refer to caption]]
In December 1984, Casani proposed adding a flyby of asteroid [[29 Amphitrite]] to the ''Galileo'' mission. In plotting a course to Jupiter, the engineers wanted to avoid asteroids. Little was known about them at the time, and it was suspected that they could be surrounded by dust particles. Flying through a dust cloud could damage the spacecraft's optics and possibly other parts of the spacecraft as well. To be safe, JPL wanted to avoid asteroids by at least {{convert|10000|km|sp=us}}. Most of the asteroids in the vicinity of the flight path like [[1219 Britta]] and [[1972 Yi Xing]] were only a few kilometers in diameter and promised little scientific value when observed from a safe distance, but 29 Amphitrite was one of the largest, and a flyby at even {{convert|10000|km|sp=us}} could have great value. The flyby would delay the spacecraft's arrival in Jupiter orbit from August 29 to December 10, 1988, and the expenditure of propellant would reduce the number of orbits of Jupiter from eleven to ten. This was expected to add $20 to $25 million (equivalent to ${{Inflation|US-GDP|20|1984}} to ${{Inflation|US-GDP|25|1984}} million in {{Inflation/year|US-GDP}}) to the cost of the ''Galileo'' project. The 29 Amphitrite flyby was approved by NASA Administrator [[James M. Beggs]] on December 6, 1984.<ref>{{cite press release |id=1062 |date=January 17, 1985 |title=Asteroid 29 Flyby Approved |publisher=NASA/Jet Propulsion Laboratory |url=https://www.jpl.nasa.gov/releases/80s/release_1985_1062.html |archive-url=https://web.archive.org/web/20081006064236/https://www.jpl.nasa.gov/releases/80s/release_1985_1062.html |archive-date=2008-10-06 |url-status=dead |df=mdy-all}}</ref>{{sfn|Meltzer|2007|pp=66–67}}

During testing, contamination was discovered in the system of metal [[slip ring]]s and brushes used to transmit electrical signals around the spacecraft, and they were returned to be refabricated. The problem was traced back to a [[chlorofluorocarbon]] used to clean parts after soldering. It had been absorbed, and was then released in a vacuum environment. It mixed with debris generated as the brushes wore down, and caused intermittent problems with electrical signal transmission. Problems were also detected in the performance of memory devices in an electromagnetic radiation environment. The components were replaced, but then a [[read disturb]] problem arose, in which reads from one memory location disturbed the contents of adjacent locations. This was found to have been caused by the changes made to make the components less sensitive to electromagnetic radiation. Each component had to be removed, retested, and replaced. All of the spacecraft components and spare parts received a minimum of 2,000 hours of testing. The spacecraft was expected to last for at least five years—long enough to reach Jupiter and perform its mission. On December 19, 1985, it departed JPL in [[Pasadena, California]], on the first leg of its journey, a road trip to the [[Kennedy Space Center]] in [[Florida]].{{sfn|Meltzer|2007|pp=68–69}} The ''Galileo'' mission was scheduled for [[STS-61-G]] on May 20, 1986, using {{OV|Atlantis|full=nolink}}.{{sfn|Hitt|Smith|2014|pp=282–285}}<ref>{{cite press release |title=NASA Names Flight Crews for ''Ulysses'', ''Galileo'' Missions |id=85-022 |date=31 May 1985 |first=Steve |last=Nesbitt |publisher=NASA |url=https://www.nasa.gov/centers/johnson/pdf/83137main_1985.pdf |access-date=17 October 2020 |archive-url=https://web.archive.org/web/20221101030918/https://www.nasa.gov/centers/johnson/pdf/83137main_1985.pdf |archive-date=1 November 2022 }}</ref>

===Spacecraft===
{{main|Galileo (spacecraft)|l1=''Galileo'' (spacecraft)}}
JPL built the Galileo spacecraft and managed the Galileo program for NASA, but West Germany's [[Messerschmitt-Bölkow-Blohm]] supplied the propulsion module, and Ames managed the atmospheric probe, which was built by the [[Hughes Aircraft Company]]. At launch, the orbiter and probe together had a mass of {{convert|2562|kg|lb|abbr=on}} and stood {{convert|6.15|m|ft|abbr=on}} tall. There were twelve experiments on the orbiter and seven on the atmospheric probe. The orbiter was powered by a pair of [[GPHS-RTG|general-purpose heat source radioisotope thermoelectric generator]]s (GPHS-RTGs) fueled by [[plutonium-238]] that generated 570&nbsp;watts at launch. The atmospheric probe had a [[lithium–sulfur battery]] rated at 730&nbsp;watt-hours.{{efn|A 12-Volt car battery has about 600 Watt-Hours.<ref>{{cite web |title=How Many Watt Hours in a Car Battery |publisher=Large Power |url=https://www.large.net/news/90u43pm.html |access-date=13 April 2024 |archive-date=April 9, 2024 |archive-url=https://web.archive.org/web/20240409192148/https://www.large.net/news/90u43pm.html |url-status=live }}</ref>}}<ref name="Press Kit" />

Probe instruments included sensors for measuring atmospheric temperature and pressure. There was a [[mass spectrometer]] and a [[helium]]-abundance detector to study atmospheric composition, and a [[Whistler (radio)|whistler]] detector for measurements of lightning activity and Jupiter's radiation belt. There were magnetometer sensors, a plasma-wave detector, a [[high-energy particle]] detector, a [[cosmic dust|cosmic]] and Jovian dust detector, and a [[heavy ion]] counter. There was a [[near-infrared spectroscopy|near-infrared mapping spectrometer]] for [[multispectral images]] for atmospheric and moon surface chemical analysis, and an [[ultraviolet–visible spectroscopy|ultraviolet spectrometer]] to study gases.<ref name="Press Kit">{{cite web |url=http://www.jpl.nasa.gov/news/press_kits/gllarpk.pdf |title=Galileo Jupiter Arrival |type=Press Kit |publisher=NASA{{\}}Jet Propulsion Laboratory |date=December 1995 |access-date=December 22, 2016 |archive-date=November 16, 2001 |archive-url=https://web.archive.org/web/20011116223923/http://www.jpl.nasa.gov/news/press_kits/gllarpk.pdf |url-status=live }}</ref>

===Reconsideration===
On January 28, 1986, {{OV|Challenger|full=nolink}} lifted off on the [[STS-51-L]] mission. A failure of the solid rocket booster 73 seconds into flight tore the spacecraft apart, resulting in the deaths of all seven crew members.{{sfn|Meltzer|2007|pp=72–77}} The [[Space Shuttle Challenger disaster|Space Shuttle ''Challenger'' disaster]] was America's worst space disaster up to that time.{{sfn|Dawson|Bowles|2004|pp=206–207}} The immediate impact on the ''Galileo'' project was that the May launch date could not be met because the Space Shuttles were grounded while the cause of the disaster was investigated. When they did fly again, ''Galileo'' would have to compete with high-priority [[United States Department of Defense|Department of Defense]] launches, the [[tracking and data relay satellite]] system, and the Hubble Space Telescope. By April 1986, it was expected that the Space Shuttles would not fly again before July 1987 at the earliest, and ''Galileo'' could not be launched before December 1987.{{sfn|Meltzer|2007|p=78}}

[[File:Animation of Galileo trajectory.gif|thumb|right|Animation of ''Galileo''{{'s}} trajectory from October 19, 1989, to September 30, 2003 <br/>{{legend2|magenta|''Galileo''}}{{·}}{{legend2|lime|
[[Jupiter]]}}{{·}}{{legend2|royalblue|[[Earth]]}}{{·}}{{legend2|PaleGreen|[[Venus]]}}{{·}}{{legend2|Gold|[[951 Gaspra]]}}{{·}}{{legend2|Cyan|[[243 Ida]]}} |alt=Depicts slingshot maneuvers around Venus and Earth.]]
The [[Rogers Commission]] into the ''Challenger'' disaster handed down its report on June 6, 1986.{{sfn|Meltzer|2007|p=78}} It was critical of NASA's safety protocols and risk management.{{sfn|Rogers|1986|pp=160–162}} In particular, it noted the hazards of a Centaur-G stage.{{sfn|Meltzer|2007|pp=176–177}} On June 19, 1986, NASA Administrator [[James C. Fletcher]] canceled the Shuttle-Centaur project.{{sfn|Meltzer|2007|p=79}} This was only partly due to the NASA management's increased aversion to risk in the wake of the ''Challenger'' disaster; NASA management also considered the money and manpower required to get the Space Shuttle flying again, and decided that there were insufficient resources to resolve lingering issues with Shuttle-Centaur as well.{{sfn|Dawson|Bowles|2004|pp=216–218}} The changes to the Space Shuttle proved more extensive than anticipated, and in April 1987, JPL was informed that ''Galileo'' could not be launched before October 1989.{{sfn|Meltzer|2007|p=93}} The ''Galileo'' spacecraft was shipped back to JPL.{{sfn|Meltzer|2007|p=177}}

Without Centaur, it looked like there was no means of getting ''Galileo'' to Jupiter. For a time, ''[[Los Angeles Times]]'' science reporter [[Usha Lee McFarling]] noted, "it looked like ''Galileo''{{'}}s only trip would be to the [[Smithsonian Institution]]."<ref name="McFarling">{{cite news |first=Usha Lee |last=McFarling |author-link=Usha Lee McFarling |date=September 22, 2003 |title=Stalwart Galileo Is Vaporized Near Jupiter |newspaper=[[Los Angeles Times]] |url=https://www.latimes.com/archives/la-xpm-2003-sep-22-me-galileo22-story.html |access-date=May 19, 2024 |archive-date=May 19, 2024 |archive-url=https://web.archive.org/web/20240519195233/https://www.latimes.com/archives/la-xpm-2003-sep-22-me-galileo22-story.html |url-status=live }}</ref> The cost of keeping it ready to fly in space was reckoned at $40 to $50 million per year (equivalent to ${{Inflation|US-GDP|40|1986}} to ${{Inflation|US-GDP|50|1986}} million in {{Inflation/year|US-GDP}}), and the estimated cost of the whole project had blown out to $1.4 billion (equivalent to ${{Inflation|US-GDP|1.4|1986}} billion in {{Inflation/year|US-GDP}}).<ref>{{cite web |title=NASA's Galileo mission clears hurdles for Jupiter voyage. In flying past Venus, probe could learn much about 'greenhouse effect' |newspaper=Christian Science Monitor |date=December 3, 1987 |first=Peter N. |last=Spotts |url=https://www.csmonitor.com/1987/1203/agal.html |access-date=November 7, 2020 |archive-date=December 7, 2021 |archive-url=https://web.archive.org/web/20211207041458/https://www.csmonitor.com/1987/1203/agal.html |url-status=live }}</ref>

At JPL, the ''Galileo'' Mission Design Manager and Navigation Team Chief, Robert Mitchell, assembled a team that consisted of Dennis Byrnes, Louis D'Amario, Roger Diehl and himself, to see if they could find a trajectory that would get ''Galileo'' to Jupiter using only a two-stage IUS. Roger Diehl came up with the idea of using a series of gravity assists to provide the additional velocity required to reach Jupiter. This would require ''Galileo'' to fly past Venus, and then past Earth twice. This was referred to as the Venus-Earth-Earth Gravity Assist (VEEGA) trajectory.{{sfn|Meltzer|2007|pp=293–294}}

[[File:Galileo probe deployed (large).jpg|thumb|left|''Galileo'' is prepared for release from {{OV|Atlantis}}. The [[Inertial Upper Stage]] (white) is attached.|alt=refer to caption]]
The reason no one had considered the VEEGA trajectory before was that the second encounter with Earth would not give the spacecraft any extra energy. Diehl realised that this was not necessary; the second encounter would merely change its direction to put it on a course for Jupiter.{{sfn|Meltzer|2007|pp=293–294}} In addition to increasing the flight time, the VEEGA trajectory had another drawback from the point of view of [[NASA Deep Space Network]] (DSN): ''Galileo'' would arrive at Jupiter when it was at the maximum range from Earth, and maximum range meant minimum signal strength. It would have a [[declination]] of 23 degrees south instead of 18 degrees north, so the tracking station would be the [[Canberra Deep Space Communication Complex]] in Australia, with its two 34-meter and one 70-meter antennae. A northerly declination could have been supported by two sites, at [[Goldstone Deep Space Communications Complex|Goldstone]] and [[Madrid Deep Space Communications Complex|Madrid]]. The Canberra antennae were supplemented by the 64-meter antenna at the [[Parkes Observatory]].{{sfn|Mudgway|2001|p=301}}{{sfn|Taylor|Cheung|Seo|2002|p=23}}

Initially it was thought that the VEEGA trajectory demanded a November launch, but D'Amario and Byrnes calculated that a mid-course correction between Venus and Earth would permit an October launch as well.{{sfn|Meltzer|2007|p=157}} Taking such a roundabout route meant that ''Galileo'' would require sixty months to reach Jupiter instead of just thirty, but it would get there.<ref name="McFarling" /> Consideration was given to using the USAF's [[Titan IV]] launch system with its Centaur G Prime upper stage.{{sfn|Dawson|Bowles|2004|p=215}} This was retained as a backup for a time, but in November 1988 the USAF informed NASA that it could not provide a Titan IV in time for the May 1991 launch opportunity, owing to the backlog of high priority Department of Defense missions.{{sfn|Office of Space Science and Applications|1989|p=2{{hyphen}}19}} However, the USAF supplied IUS-19, which had originally been earmarked for a Department of Defense mission, for use by the ''Galileo'' mission.{{sfn|Bangsund|Knutson|1988|p=10{{hyphen}}12}}

===Nuclear concerns===
As the launch date of ''Galileo'' neared, [[Anti-nuclear movement in the United States|anti-nuclear groups]], concerned over what they perceived as an unacceptable risk to the public's safety from the [[plutonium]] in ''Galileo''{{'}}s GPHS-RTG modules, sought a court injunction prohibiting ''Galileo''{{'s}} launch.<ref name="Groups Protest Use of Plutonium">{{cite news |newspaper=[[The New York Times]] |title=Groups Protest Use of Plutonium on Galileo |first=William J. |last=Broad |author-link=William Broad |date=October 10, 1989 |url=https://www.nytimes.com/1989/10/10/science/groups-protest-use-of-plutonium-on-galileo.html |access-date=November 4, 2020 |archive-date=February 12, 2021 |archive-url=https://web.archive.org/web/20210212025607/http://www.nytimes.com/1989/10/10/science/groups-protest-use-of-plutonium-on-galileo.html |url-status=live }}</ref> RTGs were necessary for deep space probes because they had to fly distances from the Sun that made the use of solar energy impractical.<ref name="Sagan" /> They had been used for years in planetary exploration without mishap: the Department of Defense's [[Lincoln Experimental Satellite]]s 8/9 had 7 percent more plutonium on board than ''Galileo'', and the two [[Voyager program|''Voyager'' spacecraft]] each carried 80 percent of ''Galileo''{{'s}} load of plutonium.<ref name="RTG">{{cite web |title=What's in an RTG? |publisher=NASA/Jet Propulsion Laboratory |url=http://www2.jpl.nasa.gov/galileo/messenger/oldmess/RTG.html |access-date=May 15, 2011 |url-status=dead |archive-url=https://web.archive.org/web/20100411024748/http://www2.jpl.nasa.gov/galileo/messenger/oldmess/RTG.html |archive-date=April 11, 2010 }}</ref> By 1989, plutonium had been used in 22 spacecraft.<ref>{{cite magazine |title=Plutonium gets a lift from Galileo |magazine=[[New Scientist]] |issn=0262-4079 |url=https://www.newscientist.com/article/mg12216683-100-plutonium-gets-a-lift-from-galileo/ |date=June 10, 1989 |access-date=November 4, 2020 |archive-date=October 28, 2020 |archive-url=https://web.archive.org/web/20201028151529/https://www.newscientist.com/article/mg12216683-100-plutonium-gets-a-lift-from-galileo/ |url-status=live }}</ref>

Activists remembered the crash of the [[Soviet Union]]'s nuclear-powered [[Kosmos 954]] satellite in Canada in 1978, and the ''Challenger'' disaster, while it did not involve nuclear fuel, raised public awareness about spacecraft failures. No RTGs had ever done a non-orbital swing past the Earth at close range and high speed, as ''Galileo''{{'s}} VEEGA trajectory required it to do. This created the possibility of a mission failure in which Galileo struck Earth's atmosphere and dispersed plutonium. [[Planetary scientist]] [[Carl Sagan]], a strong supporter of the ''Galileo'' mission, wrote that "there is nothing absurd about either side of this argument."<ref name="Sagan">{{cite web |last=Sagan |first=Carl |author-link=Carl Sagan |title=Galileo: To Launch or not to Launch? |date=October 9, 1989 |url=http://www.dartmouth.edu/~chance/course/Syllabi/97Dartmouth/day-6/sagan.html |access-date=November 4, 2020 |archive-date=January 26, 2021 |archive-url=https://web.archive.org/web/20210126152333/http://www.dartmouth.edu/~chance/course/Syllabi/97Dartmouth/day-6/sagan.html |url-status=dead }}</ref>

Before the ''Challenger'' disaster, JPL had conducted shock tests on the RTGs that indicated that they could withstand a pressure of {{convert|2,000|psi|order=flip}} without a failure, which would have been sufficient to withstand an explosion on the launch pad. The possibility of adding additional shielding was considered but rejected, mainly because it would add an unacceptable amount of extra weight.{{sfn|Meltzer|2007|p=77}} After the ''Challenger'' disaster, NASA commissioned a study on the possible effects if such an event occurred with ''Galileo'' on board. Angus McRonald, a JPL engineer, concluded that what would happen would depend on the altitude at which the Space Shuttle broke up. If the ''Galileo''/IUS combination fell free from the orbiter at {{convert|90000|ft|order=flip|sp=us}}, the RTGs would fall to Earth without melting, and drop into the Atlantic Ocean about {{convert|150|mi|order=flip|sp=us}} from the Florida coast. On the other hand, if the orbiter broke up at an altitude of {{convert|323,800|feet|order=flip|sp=us}} it would be traveling at {{convert|7957|ft/s|order=flip|sp=us}} and the RTG cases and GPHS modules would melt before falling into the Atlantic {{convert|400|mi|order=flip|sp=us}} off the Florida coast.<ref>{{cite magazine |last=Portree |first=David S. F. |title=If Galileo Had Fallen to Earth (1988) |date=December 18, 2012 |magazine=[[Wired (magazine)|Wired]] |issn=1059-1028 |url=https://www.wired.com/2012/12/galileo-and-an-uncontrolled-shuttle-orbiter-reentry-1988/ |access-date=November 4, 2020 |archive-date=October 27, 2020 |archive-url=https://web.archive.org/web/20201027072205/https://www.wired.com/2012/12/galileo-and-an-uncontrolled-shuttle-orbiter-reentry-1988/ |url-status=live }}</ref><ref>{{cite report |title=Galileo: Uncontrolled STS Orbiter Reentry |id=JPL D-4896 |first=Angus D. |last=McRonald |publisher=NASA |date=April 15, 1988 |url=https://www.nasa.gov/pdf/24889main_JPL_Report.pdf |access-date=November 4, 2020 |archive-url=https://web.archive.org/web/20221111183533/https://www.nasa.gov/pdf/24889main_JPL_Report.pdf |archive-date=11 November 2022 }}</ref>

NASA concluded that the chance of a disaster was 1 in 2,500, although anti-nuclear groups thought it might be as high as 1 in 430.<ref name="Groups Protest Use of Plutonium" />{{sfn|Office of Space Science and Applications|1989|p=2-23}} NASA assessed the risk to an individual at 1 in 100 million, about two orders of magnitude less than the danger of being killed by lightning.{{sfn|Office of Space Science and Applications|1989|p=2{{hyphen}}24}} The prospect of an inadvertent re-entry into the atmosphere during the VEEGA maneuvers was reckoned at less than 1 in 2 million,<ref name="RTG"/> but an accident might have released a maximum of {{convert|11,568|Ci|lk=on}}. This could result in up to 9 fatalities from cancer per 10 million exposed people.{{sfn|Office of Space Science and Applications|1989|pp=2{{hyphen}}21, 4{{hyphen}}18}}

==Launch==
{{main|STS-34}}
[[File:STS-34 Launch 1.jpg|thumb|right|Launch of [[STS-34]] with ''Galileo'' on board|alt=refer to caption]]
[[STS-34]] was the mission designated to launch ''Galileo'', scheduled for October 12, 1989, in the Space Shuttle ''Atlantis''.<ref>{{cite press release |first=Jeffrey |last=Carr |id=88-049 |date=November 10, 1988 |title=Four New Shuttle Crews Named (STS-32, STS-33, STS-34, STS-35) |publisher=NASA |url=https://www.nasa.gov/centers/johnson/pdf/83140main_1988.pdf |access-date=November 5, 2020 |archive-url=https://web.archive.org/web/20220802220724/https://www.nasa.gov/centers/johnson/pdf/83140main_1988.pdf |archive-date=2 August 2022 }}</ref> The spacecraft was delivered to the Kennedy Space Center by a high-speed truck convoy that departed JPL in the middle of the night. There were fears that the trucks might be hijacked by anti-nuclear activists or terrorists after the plutonium, so the route was kept secret from the drivers beforehand, and they drove through the night and the following day and only stopped for food and fuel.{{sfn|Meltzer|2007|p=69}}

Last-minute efforts by three environmental groups (the [[Christic Institute]], the Florida Coalition for Peace and Justice and the [[Foundation on Economic Trends]]) to halt the launch were rejected by the [[United States Court of Appeals for the District of Columbia Circuit|District of Columbia Circuit]] on technical grounds rather than the merits of the case, but in a concurring opinion, Chief Justice [[Patricia Wald]] wrote that while the legal challenge was not [[frivolous litigation|frivolous]], there was no evidence of the plaintiffs' claim that NASA had acted improperly in compiling the mission's environmental assessment. On October 16, eight protesters were arrested for trespassing at the Kennedy Space Center; three were jailed and the remaining five released.<ref name="Galileo Launch Nears">{{cite news |title=Galileo Launch Nears |first=Kathy |last=Sawyer |date=October 17, 1989 |newspaper=[[The Washington Post]] |url=https://www.washingtonpost.com/archive/politics/1989/10/17/galileo-launch-nears/d61c4140-7588-4637-b1c1-6b3ce69ac473/ |access-date=November 5, 2020 |archive-date=August 27, 2017 |archive-url=https://web.archive.org/web/20170827224401/https://www.washingtonpost.com/archive/politics/1989/10/17/galileo-launch-nears/d61c4140-7588-4637-b1c1-6b3ce69ac473/ |url-status=live }}</ref><ref>{{cite magazine |first=Helen |last=Gavaghan |date=March 3, 1990 |title=Protest groups move to halt space mission... |magazine=[[New Scientist]] |issn=0262-4079 |url=https://www.newscientist.com/article/mg12517060-100-protest-groups-move-to-halt-space-mission/ |access-date=May 14, 2024 |archive-date=May 14, 2024 |archive-url=https://web.archive.org/web/20240514210946/https://www.newscientist.com/article/mg12517060-100-protest-groups-move-to-halt-space-mission/ |url-status=live }}</ref> Federal judge [[Oliver Gasch]] ruled on October 21 that the launch was in the public interest, as canceling it would cost the public $164 million and increased knowledge of the Solar system.<ref>{{cite magazine |title=Green Light for Galileo |date=October 21, 1989 |magazine=[[New Scientist]] |issn=0262-4079 |url=https://www.newscientist.com/article/mg12416871-200-green-light-for-galileo/ |access-date=May 14, 2024 |archive-date=May 14, 2024 |archive-url=https://web.archive.org/web/20240514210947/https://www.newscientist.com/article/mg12416871-200-green-light-for-galileo/ |url-status=live }}</ref>

The launch was twice delayed; first by a faulty main engine controller that forced a postponement to October 17, and then by inclement weather, which necessitated a postponement to the following day,<ref name="STS-34" /> but this was not a concern since the launch window extended until November 21.<ref name="Galileo Launch Nears" /> ''Atlantis'' finally lifted off at 16:53:40 [[UTC]] on October 18, and went into a {{convert|213|mi|order=flip|sp=us|adj=on}} orbit.<ref name="STS-34">{{cite web |url=https://www.nasa.gov/mission_pages/shuttle/shuttlemissions/archives/sts-34.html |title=Mission Archives: STS-34 |publisher=NASA |date=February 18, 2010 |access-date=January 7, 2017 |archive-date=October 11, 2006 |archive-url=https://web.archive.org/web/20061011184151/https://www.nasa.gov/mission_pages/shuttle/shuttlemissions/archives/sts-34.html |url-status=live }}</ref> ''Galileo'' was successfully deployed at 00:15 UTC on October 19.{{sfn|Meltzer|2007|p=78}} Following the IUS burn, the ''Galileo'' spacecraft adopted its configuration for solo flight, and separated from the IUS at 01:06:53 UTC on October 19.<ref name="PDS" /> The launch was perfect, and ''Galileo'' was soon headed towards Venus at over {{convert|9000|mph|order=flip|abbr=on}}.<ref>{{cite news |title=Galileo Travels 292,500 Miles Toward Venus |newspaper=[[The Washington Post]] |url=https://www.washingtonpost.com/archive/politics/1989/10/20/galileo-travels-292500-miles-toward-venus/0ca00a5a-a443-4cc2-85ac-817b9514941b/ |access-date=November 5, 2020 |archive-date=August 28, 2017 |archive-url=https://web.archive.org/web/20170828014426/https://www.washingtonpost.com/archive/politics/1989/10/20/galileo-travels-292500-miles-toward-venus/0ca00a5a-a443-4cc2-85ac-817b9514941b/ |url-status=live }}</ref> ''Atlantis'' returned to Earth safely on October 23.<ref name="STS-34"/>

==Venus encounter==
The encounter with [[Venus]] on February 9 was in view of the DSN's Canberra and [[Madrid Deep Space Communications Complex]]es.{{sfn|Mudgway|2001|p=306}} ''Galileo''{{'}}s closest approach to Venus came at 05:58:48 UTC on February 10, 1990, at a range of {{convert|16106|km|mi|abbr=on|sp=us}}.<ref name="PDS">{{cite web |title=PDS: Mission Information |publisher=NASA |url=https://pds.nasa.gov/ds-view/pds/viewMissionProfile.jsp?MISSION_NAME=GALILEO |access-date=November 9, 2020 |archive-date=July 17, 2017 |archive-url=https://web.archive.org/web/20170717152844/https://pds.nasa.gov/ds-view/pds/viewMissionProfile.jsp?MISSION_NAME=GALILEO |url-status=live }}</ref> Due to the [[Doppler effect]], the spacecraft's velocity relative to Earth could be computed by measuring the change in carrier frequency of the spacecraft's transmission compared to the nominal frequency.<ref>{{cite news |first=Morgan |last=Bettex |date=August 3, 2010 |title=Explained: The Doppler Effect |publisher=[[MIT News]] |url=https://news.mit.edu/2010/explained-doppler-0803 |access-date=April 15, 2024 |archive-date=September 28, 2023 |archive-url=https://web.archive.org/web/20230928014545/https://news.mit.edu/2010/explained-doppler-0803 |url-status=live }}</ref> Doppler data collected by the DSN allowed JPL to verify that the gravity-assist maneuver had been successful, and the spacecraft had obtained the expected {{convert|2.2|km/s|abbr=on|sp=us}} increase in speed. Unfortunately, three hours into the flyby, the tracking station at Goldstone had to be shut down due to high winds, and Doppler data was lost.{{sfn|Mudgway|2001|p=306}}

[[File:Galileo Venus global view.jpg|thumb|left|upright|Violet light image of Venus taken in February 1990 by ''Galileo''{{'s}} solid state imaging (SSI) system|alt=refer to caption]]
Because Venus was much closer to the Sun than the spacecraft had been designed to operate, great care was taken to avoid thermal damage. In particular, the [[X-band]] [[high gain antenna]] (HGA) was not deployed, but was kept folded up like an umbrella and pointed away from the Sun to keep it shaded and cool. This meant that the two small [[S-band]] low-gain antennae (LGAs) had to be used instead.{{sfn|Meltzer|2007|p=152}} They had a maximum bandwidth of 1,200 [[bits per second]] (bit/s) compared to the 134,000 bit/s expected from the HGA. As the spacecraft moved further from Earth, reception necessitated the use of the DSN's 70-meter dishes, to the detriment of other users, who had lower priority than ''Galileo''. Even so, the [[downlink]] [[telemetry]] rate fell to 40 bit/s within a few days of the Venus flyby, and by March it was down to just 10 bit/s.{{sfn|Mudgway|2001|p=306}}{{sfn|Johnson|Yeates|Young|Dunne|1991|p=1516}}

Venus had been the focus of many automated flybys, probes, balloons and landers, most recently the 1989 ''[[Magellan (spacecraft)|Magellan]]'' spacecraft, and ''Galileo'' had not been designed with Venus in mind. Nonetheless, there were useful observations that it could make, as it carried some instruments that had never flown on spacecraft to Venus, such as the near-infrared mapping spectrometer (NIMS).{{sfn|Johnson|Yeates|Young|Dunne|1991|p=1516}} Telescopic observations of Venus had revealed that there were certain parts of the infrared spectrum that the [[greenhouse gas]]es in the Venusian atmosphere did not block, making them transparent on these wavelengths. This permitted the NIMS to both view the clouds and obtain maps of the equatorial- and mid-latitudes of the night side of Venus with three to six times the resolution of Earth-based telescopes.{{sfn|Johnson|Yeates|Young|Dunne|1991|p=1517}} The ultraviolet spectrometer (UVS) was also deployed to observe the Venusian clouds and their motions.{{sfn|Johnson|Yeates|Young|Dunne|1991|p=1517}}{{sfn|Carlson et al.|1991|pp=1541–1544}}{{sfn|Belton et al.|1991|pp=1531–1536}}

Another set of observations was conducted using Galileo's energetic-particles detector (EPD) when ''Galileo'' moved through the [[bow shock]] caused by Venus's interaction with the [[solar wind]]. [[Earth's magnetic field]] causes the bow shock to occur at around {{convert|65000|km|mi|sp=us}} from its center, but Venus's weak magnetic field causes it to occur nearly on the surface, so the solar wind interacts with the atmosphere.{{sfn|Williams et al|1991|pp=1525–1528}}{{sfn|Meltzer|2007|pp=154–157}} A search for [[lightning]] on Venus was conducted using the [[plasma wave instrument|plasma-wave detector]], which noted nine bursts likely to have been caused by lightning, but efforts to capture an image of lightning with the solid-state imaging system (SSI) were unsuccessful.{{sfn|Belton et al.|1991|pp=1531–1536}}
{{Clear}}

==Earth encounters==
===Flybys===
''Galileo'' made two course corrections on April 9 to 12 and May 11 to 12, 1990, to alter its velocity by {{convert|35|m/s|sp=us}}.{{sfn|Meltzer|2007|p=157}} The spacecraft flew by Earth twice; the first time at a range of {{convert|960|km|mi|abbr=on}} at 20:34:34&nbsp;UTC on December 8, 1990.<ref name="PDS"/> This was {{convert|5|mi|order=flip|0|abbr=on}} higher than predicted, and the time of the closest approach was within a second of the prediction. It was the first time that a deep space probe had returned to Earth from interplanetary space.{{sfn|Meltzer|2007|p=157}} A second flyby of Earth was at {{convert|304|km|mi|abbr=on}} at 15:09:25&nbsp;UTC on December 8, 1992.<ref name="PDS"/> This time the spacecraft passed within a kilometer of its aiming point over the South Atlantic. This was so accurate that a scheduled course correction was cancelled, thereby saving {{convert|5|kg|sp=us}} of propellant.{{sfn|Meltzer|2007|p=164}}

===Earth's bow shock and the solar wind===
[[File:Galileo Earth - PIA00114.jpg|thumb|right|''Galileo'' image of Earth, taken in December 1990|alt=refer to caption]]
The Earth encounters provided an opportunity for a series of experiments. A study of Earth's bow shock was conducted as ''Galileo'' passed by Earth's day side. The solar wind travels at {{convert|200|to|800|km/s|sp=us}} and is deflected by [[Earth's magnetic field]], creating a [[magnetic tail]] on Earth's dark side over a thousand times the radius of the planet. Observations were made by ''Galileo'' when it passed through the magnetic tail on Earth's dark side at a distance of {{convert|56000|km|sp=us}} from the planet. The magnetosphere was quite active at the time, and ''Galileo'' detected magnetic storms and [[whistler (radio)|whistlers]] caused by lightning strikes.<ref>{{cite web |title=Collaborative Study of Earth's Bow Shock |publisher=NASA |url=https://spdf.gsfc.nasa.gov/bowshock/ |access-date=14 November 2020 |archive-date=November 16, 2020 |archive-url=https://web.archive.org/web/20201116112941/https://spdf.gsfc.nasa.gov/bowshock/ |url-status=live }}</ref>{{sfn|Meltzer|2007|pp=158–159}}

The NIMS was employed to look for [[mesospheric clouds]], which were thought to be caused by [[methane]] released by industrial processes. The water vapor in the clouds breaks down the [[ozone]] in the upper atmosphere. Normally the clouds are only seen in September or October, but ''Galileo'' was able to detect them in December, an indication of possible damage to Earth's [[ozone layer]].{{sfn|Meltzer|2007|pp=158–159}}

===Remote detection of life on Earth===
Carl Sagan, pondering the question of whether [[detecting Earth from distant star-based systems|life on Earth could be easily detected from space]], devised a set of experiments in the late 1980s using ''Galileo''{{'s}} remote sensing instruments during the mission's first Earth flyby in December 1990. After data acquisition and processing, Sagan published a paper in ''[[Nature (journal)|Nature]]'' in 1993 detailing the results of the experiment. ''Galileo'' had indeed found what are now referred to as the "Sagan criteria for life". These included strong absorption of light at the red end of the visible spectrum (especially over [[continents]]) by [[chlorophyll]] in photosynthesizing plants; absorption bands of molecular oxygen as a result of plant activity; infrared bands caused by the approximately 1 micromole per [[mole (unit)|mole]] of methane (a gas which must be replenished by volcanic or biological activity) in the atmosphere; and modulated narrowband radio wave transmissions uncharacteristic of any known natural source. ''Galileo''{{'s}} experiments were thus the first [[scientific control]]s in the newborn science of [[astrobiological]] remote sensing.{{sfn|Sagan et al.|1993|pp=715–721}}

===Lunar observations===
<gallery class="center" widths="220px" heights="220px">
File:Moon-galileo-color.jpg|[[Mare Orientale]] on the [[Moon]]|alt=The maria are large areas with less cratering
File:The Moon from Galileo - GPN-2000-000473.jpg|''Galileo'' shot of the [[lunar north pole]]|alt=The far side is cratered; maria on the near side
File:Moon Crescent - False Color Mosaic.jpg|[[False-color]] mosaic by ''Galileo'' showing [[Geology of the Moon|compositional variations]] of the Moon's surface |alt=refer to caption
</gallery>

En route to ''Galileo''{{'s}} second gravity-assist flyby of Earth, the spacecraft flew over the [[lunar north pole]] on December 8, 1992, at an altitude of {{convert|110,000|km|sp=us}}. The north pole had been photographed before, by ''[[Mariner 10]]'' in 1973, but ''Galileo''{{'s}} cameras, with their {{convert|1.1|km|sp=us}} per [[pixel]] imagery, provided new information about a region that still held some scientific mysteries. The infrared spectromer surveyed the surface minerals and revealed that the region was more minerallogically diverse than expected. There was evidence that the [[Moon]] had been volcanically active earlier than originally thought, and the spectrometer clearly distinguished different lava flows on the [[Mare Serenitatis]]. Areas where titanium-rich material had been blasted from vents, like the one sampled by [[Apollo 17]], showed up clearly.{{sfn|Harland|2000|pp=65–67}}

===''Galileo'' Optical Experiment===
During the second Earth flyby, another experiment was performed. Optical communications in space were assessed by detecting light pulses from powerful lasers with ''Galileo''{{'s}} CCD. The experiment, dubbed ''Galileo'' Optical Experiment or GOPEX,<ref name="GOPEX">{{cite web |url=http://lasers.jpl.nasa.gov/PAPERS/GOPEX/gopex_s2.pdf |title=GOPEX SPIE 1993 (Edited) |publisher=NASA/Jet Propulsion Laboratory |access-date=May 15, 2011 |url-status=dead |archive-url=https://web.archive.org/web/20110721050447/http://lasers.jpl.nasa.gov/PAPERS/GOPEX/gopex_s2.pdf |archive-date=July 21, 2011}}</ref> used two separate sites to beam laser pulses to the spacecraft, one at [[Table Mountain Observatory]] in California and the other at the [[Starfire Optical Range]] in [[New Mexico]]. The Table Mountain site used a [[Nd:YAG laser]] operating at a [[nonlinear optics#Frequency doubling|frequency-doubled]] wavelength of 532&nbsp;nm, with a repetition rate of 15 to 30&nbsp;Hertz and a pulse power [[full width at half maximum]] (FWHM) in the tens of megawatts range, which was coupled to a {{convert|0.6|m|ft|abbr=on}} [[Cassegrain reflector]] telescope for transmission to ''Galileo''. The Starfire range site used a similar setup with a larger {{convert|1.5|m|ft|abbr=on}} transmitting telescope. Long-exposure (~0.1 to 0.8&nbsp;s) images using ''Galileo''{{'s}} 560&nbsp;nm centered green filter produced images of Earth clearly showing the laser pulses even at distances of up to {{convert|6|e6km|e6mi|abbr=unit}}.<ref name="GOPEX" />

Adverse weather conditions, restrictions placed on laser transmissions by the U.S. [[Space Defense Operations Center]] ([[Cheyenne Mountain Complex|SPADOC]]) and a pointing error caused by the scan platform on the spacecraft not being able to change direction and speed as quickly as expected (which prevented laser detection on all frames with less than 400&nbsp;ms exposure times) contributed to a reduction in the number of successful detections of the laser transmission to 48 of the total 159 frames taken.<ref name="GOPEX" /> Nonetheless, the experiment was considered a resounding success and the data acquired were used to design laser downlinks to send large volumes of data very quickly from spacecraft to Earth. The scheme was studied in 2004 for a data link to a future Mars-orbiting spacecraft.<ref name="lasers">{{cite news |url=https://www.space.com/534-nasa-test-laser-communications-mars-spacecraft.html |title=NASA To Test Laser Communications With Mars Spacecraft |publisher=[[Space.com]] |date=November 15, 2004 |access-date=January 19, 2021 |archive-date=November 8, 2021 |archive-url=https://web.archive.org/web/20211108200952/https://www.space.com/534-nasa-test-laser-communications-mars-spacecraft.html |url-status=live }}</ref> On December 5, 2023, NASA's [[Deep Space Optical Communications]] experiment on the ''[[Psyche (spacecraft)|Psyche]]'' spacecraft used infrared lasers for two-way communication between Earth and the spacecraft.<ref>{{cite press release |title=NASA's Deep Space Optical Comm Demo Sends, Receives First Data |date= |publisher=NASA |url=https://www.jpl.nasa.gov/news/nasas-deep-space-optical-comm-demo-sends-receives-first-data |access-date=April 2, 2024 |archive-date=April 2, 2024 |archive-url=https://web.archive.org/web/20240402195233/https://www.jpl.nasa.gov/news/nasas-deep-space-optical-comm-demo-sends-receives-first-data |url-status=live }}</ref><ref>{{cite news |title=Pew! Pew! Pew! NASA's 1st successful two-way laser experiment is a giant leap for moon and Mars communications |first=Andrew |last=Jones |date=December 20, 2023 |publisher=[[Space.com]] |url=https://www.space.com/nasa-laser-communication-1st-two-way-link-iss |access-date=2 April 2024 |archive-date=April 2, 2024 |archive-url=https://web.archive.org/web/20240402195233/https://www.space.com/nasa-laser-communication-1st-two-way-link-iss |url-status=live }}</ref>

==High-gain antenna problem==
[[File:Galileo orbiter arrival at Jupiter (cropped).jpg|thumb|right|Illustration of ''Galileo'' with antenna not fully deployed|alt=refer to caption]]
After the Venus flyby and ''Galileo'' passing beyond Earth’s orbit, it was no longer risky to employ the {{abbr|HGA|high-gain antenna}}, so on April 11, 1991, ''Galileo'' was ordered to unfurl it. This was done using two small dual drive actuator (DDA) motors to drive a [[worm gear]], and was expected to take 165 seconds, or 330 seconds if one actuator failed. The antenna had 18 [[carbon-fiber reinforced polymer|graphite-epoxy]] ribs; when the driver motor started and put pressure on the ribs, they were supposed to pop out of the cup their tips were held in, and the antenna would unfold like an umbrella. When it reached the fully deployed configuration, redundant [[microswitch]]es would shut down the motors. Otherwise they would run for eight minutes before being automatically shut down to prevent them from overheating.{{sfn|Johnson|1994|pp=362–366}}{{sfn|Meltzer|2007|pp=171–172}}

Through telemetry from ''Galileo'', investigators determined that the electric motors had stalled at 56 seconds. The spacecraft's spin rate had decreased due to an increase in its [[moment of inertia]] and its wobble increased, indicative of an asymmetric unfolding. Only 15 ribs had popped out, leaving the antenna looking like a lop-sided, half-open umbrella. It was not possible to re-fold the antenna and try the opening sequence again; although the motors were capable of running in reverse, the antenna was not designed for this, and human assistance was required when it was done on Earth to ensure that the wire mesh did not snag.{{sfn|Johnson|1994|p=372}}{{sfn|Meltzer|2007|pp=173–174}}

The first thing the ''Galileo'' team tried was to rotate the spacecraft away from the Sun and back again on the assumption that the problem was with friction holding the pins in their sockets. If so, then heating and cooling the ribs might cause them to pop out of their sockets. This was done seven times, but with no result. They then tried swinging LGA-2 (which faced in the opposite direction to the HGA and LGA-1) 145 degrees to a hard stop, thereby shaking the spacecraft. This was done six times with no effect. Finally, they tried shaking the antenna by pulsing the DDA motors at 1.25 and 1.875 Hertz. This increased the torque by up to 40 percent. The motors were pulsed 13,000 times over a three-week period in December 1992 and January 1993, but only managed to move the ballscrew by one and a half revolutions beyond the stall point.{{sfn|Johnson|1994|p=372}}{{sfn|Meltzer|2007|p=181}}

[[File:Galileo in 1983.jpg|thumb|left|''Galileo'' with its high gain antenna open in the VPF on Earth |alt=refer to caption]]
Investigators concluded that during the 4.5 years that ''Galileo'' spent in storage after the ''Challenger'' disaster, the [[lubricant]]s between the tips of the ribs and the cup were eroded. They were then worn down by [[vibration]] during the three cross-country journeys by truck between California and Florida for the spacecraft. The failed ribs were those closest to the flat-bed trailers carrying ''Galileo'' on these trips.{{sfn|Meltzer|2007|pp=177–178}} The use of land transport was partly to save costs—air transport would have cost an additional $65,000 ({{Inflation|US-GDP|65000|r=-3|1989|fmt=eq}}) or so per trip—but also to reduce the amount of handling required in loading and unloading the aircraft, which was considered a major risk of damage.{{sfn|Meltzer|2007|p=183}} The spacecraft was also subjected to severe vibration in a vacuum environment by the IUS. Experiments on Earth with the test HGA showed that having a set of stuck ribs all on one side reduced the DDA torque produced by up to 40 percent.{{sfn|Meltzer|2007|pp=177–178}}

The antenna lubricants were applied only once, nearly a decade before launch. Furthermore, the HGA was not subjected to the usual rigorous testing, because there was no backup unit that could be installed in ''Galileo'' in case of damage. The flight-ready HGA was never given a thermal evaluation test, and was unfurled only a half dozen or so times before the mission. Testing might not have revealed the problem in any case; the [[Lewis Research Center]] was never able to replicate the problem on Earth, and it was assumed to be the combination of loss of lubricant during transportation, vibration during launch by the IUS, and a prolonged period of time in the vacuum of space where bare metal touching could undergo [[cold welding]]. Whatever the cause, the HGA was rendered useless.{{sfn|Meltzer|2007|pp=182–183}}

The two LGAs were capable of transmitting information back to Earth, but since it transmitted its signal over a cone with a 120-degree [[half-angle]], allowing it to communicate even when not pointed at Earth, its [[bandwidth (computing)|bandwidth]] was significantly less than that of the HGA would have been, as the HGA transmitted over a half-angle of one-sixth of a degree. The HGA was to have transmitted at 134&nbsp;[[kilobit]]s per second, whereas LGA-1 was only intended to transmit at about 8 to 16&nbsp;bits per second. LGA-1 transmitted with a power of about 15 to 20&nbsp;watts, which by the time it reached Earth and had been collected by one of the large aperture 70-meter DSN antennas, had a total power of about 10<sup>{{nbh}}20</sup>&nbsp;watts.<ref>{{cite web |url=http://www2.jpl.nasa.gov/galileo/faqhga.html |title=''Galileo'' FAQ – ''Galileo''{{'s}} Antennas |publisher=NASA/Jet Propulsion Laboratory |access-date=May 15, 2011 |archive-url=https://web.archive.org/web/20100528014039/http://www2.jpl.nasa.gov/galileo/faqhga.html |archive-date=May 28, 2010}}</ref> The change to mission plan required a series of software changes to be uploaded.{{sfn|Marr|1994|pp=150–157}}

Image data collected was buffered and collected in ''Galileo''{{'}}s Command and Data Subsystem (CDS) memory. This represented 192 kilobytes of the 384 kilobyte CDS storage, and had been added late, out of concern that the 6504 Complementary metal–oxide–semiconductor ([[CMOS]]) memory devices might not be reliable during a {{abbr|VEEGA|Venus-Earth-Earth Gravity Assist}} mission. As it happened, they gave no trouble, but the CDS memory could store up to 31 minutes of data from the Radio Relay Hardware (RRH) channels.{{sfn|Marr|1994|pp=150–157}} To conserve bandwidth, [[data compression|data-compression]] software was implemented. Image compression used an integer approximation of the [[discrete cosine transform]], while other data were compressed with variant of the [[Lempel–Ziv–Welch]] algorithm.{{sfn|Cheung|Tong|1993|p=99}} Using compression, the arraying of several Deep Space Network antennas, and sensitivity upgrades to the receivers used to listen to ''Galileo''{{'s}} signal, data throughput was increased to a maximum of 160&nbsp;bits per second.<ref name="parkes.tracks">{{cite web |url=https://www.scss.tcd.ie/Stephen.Farrell/ipn/background/five-antennae-for-galileo.html |title=The Parkes Galileo Tracks |publisher=Trinity College Dublin |first=John M. |last=Sarkissian |date=November 1997 |access-date=January 19, 2021 |archive-date=June 18, 2021 |archive-url=https://web.archive.org/web/20210618152442/https://www.scss.tcd.ie/Stephen.Farrell/ipn/background/five-antennae-for-galileo.html |url-status=live }}</ref><ref name="DeepSpaceNasaDataRate">{{cite web |url=http://deepspace.jpl.nasa.gov/technology/95_20/gll_case_study.html |title=Advanced Systems Program and the Galileo Mission to Jupiter |publisher=NASA/Jet Propulsion Laboratory |url-status=dead |archive-url=https://web.archive.org/web/20110614133445/http://deepspace.jpl.nasa.gov/technology/95_20/gll_case_study.html |archive-date=June 14, 2011}}</ref> By further using data compression, the effective bandwidth could be raised to 1,000&nbsp;bits per second.<ref name=DeepSpaceNasaDataRate /><ref>{{cite web |url=http://nssdc.gsfc.nasa.gov/nmc/spacecraftTelemetry.do?id=1989-084B |title=NASA – NSSDCA – Spacecraft – PDMP Details |publisher=NASA/Goddard Space Flight Center |url-status=dead |archive-url=https://web.archive.org/web/20090404070908/http://nssdc.gsfc.nasa.gov/nmc/spacecraftTelemetry.do?id=1989-084B |archive-date=April 4, 2009}}</ref>

The data collected on Jupiter and its moons were stored in the spacecraft's onboard tape recorder, and transmitted back to Earth during the long [[apsis|apoapsis]] portion of the probe's orbit using the low-gain antenna. At the same time, measurements were made of Jupiter's magnetosphere and transmitted back to Earth. The reduction in available bandwidth reduced the total amount of data transmitted throughout the mission,<ref name="parkes.tracks" /> but William J. O'Neil, ''Galileo''{{'s}} project manager from 1992 to 1997,{{sfn|Meltzer|2007|p=201}} expressed confidence that 70 percent of ''Galileo''{{'s}} science goals could still be met.{{sfn|Mudgway|2001|p=312}}<ref>{{cite web |url=http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/22009/1/97-0449.pdf |title=Galileo's Telecom Using The Low-Gain Spacecraft Antenna |archive-url=https://web.archive.org/web/20111124054217/http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/22009/1/97-0449.pdf |archive-date=2011-11-24 |date=November 24, 2011 |publisher=NASA/Jet Propulsion Laboratory |orig-year=1996 |url-status=dead |access-date=January 29, 2012}}</ref> The decision to use magnetic tape for storage was a conservative one, taken in the late 1970s when the use of tape was common. Conservatism was not restricted to engineers; a 1980 suggestion that the results of ''Galileo'' could be distributed electronically instead of on paper was regarded as ridiculous by geologists, on the grounds that storage would be prohibitively expensive; some of them thought that taking measurements on a computer involved putting a wooden ruler up to the screen.{{sfn|Greenberg|2005|pp=40–41}}

==Asteroid encounters==
===951 Gaspra===
[[File:951 Gaspra.jpg|[[951 Gaspra]] (enhanced colorization)|thumb|right|alt=Potato-shaped asteroid]]
Two months after entering the [[asteroid belt]], ''Galileo'' performed the first asteroid encounter by a spacecraft.{{sfn|Meltzer|2007|pp=161–164}} ''Galileo'' passed [[951 Gaspra]], an [[S-type asteroid]], at a distance of {{convert|1604|km|mi|abbr=on}} at 22:37&nbsp;UTC on October 29, 1991, at a relative speed of about {{convert|8|km/s|mi/s|sp=us}}.<ref name="PDS"/> Fifty-seven images of Gaspra were taken with the SSI, covering about 80 percent of the asteroid.{{sfn|Veverka|Belton|Klaasen|Chapman|1994|p=2}} Without the HGA, the bit rate was only about 40 bit/s, so an image took up to 60 hours to transmit back to Earth. The ''Galileo'' project was able to secure 80 hours of Canberra's 70-meter dish time between 7 and 14 November 1991,{{sfn|Veverka|Belton|Klaasen|Chapman|1994|p=7}} but most of images taken, including low-resolution images of more of the surface, were not transmitted to Earth until November 1992.{{sfn|Meltzer|2007|pp=161–164}}

The imagery revealed a cratered and irregular body, measuring about {{convert|19|by|12|by|11|km|mi|sp=us}}.{{sfn|Veverka|Belton|Klaasen|Chapman|1994|p=2}} Its shape was not remarkable for an asteroid of its size.{{sfn|Veverka|Belton|Klaasen|Chapman|1994|p=8}} Measurements were taken using the NIMS to indicate the asteroid's composition and physical properties.{{sfn|Granahan|2011|pp=265–272}} While Gaspra has plenty of small craters—over 600 of them ranging in size from {{convert|100|to|500|m|sp=us}}—it lacks large ones, hinting at a relatively recent origin,{{sfn|Meltzer|2007|pp=161–164}} although it is possible that some of the depressions were eroded craters. Several relatively flat planar areas were found, suggesting that Gaspra was formed from another body by a collision.{{sfn|Veverka|Belton|Klaasen|Chapman|1994|p=8}} Measurements of the solar wind in the vicinity of the asteroid showed it changing direction a few hundred kilometers from Gaspra, which hinted that Gaspra might have a magnetic field, but this was not certain.{{sfn|Meltzer|2007|pp=161–164}}

===243 Ida and Dactyl===
{{Main|243 Ida#Galileo flyby|l1=243 Ida}}
Following the second Earth encounter, ''Galileo'' performed close observations of another asteroid, [[243 Ida]]. A slight trajectory correction was made to enable this on August 26, 1993. With four hours to go before the encounter with Ida, ''Galileo'' spontaneously abandoned the observation configuration and resumed its cruise configuration. Engineers were able to correct the problem and have the instruments ready by 16:52:04&nbsp;UTC on August 28, 1993, when ''Galileo'' flew past Ida at a range of {{convert|2410|km|mi|abbr=on}}. High-resolution images were taken to create a color mosaic of one side of the asteroid, with the highest resolution image taken at a range of {{convert|10,500|mi|abbr=on}}.{{sfn|Harland|2000|pp=72–77}} Measurements were taken using SSI and NIMS.{{sfn|Belton et al|1996|pp=2–3}}{{sfn|Chapman|Veverka|Thomas|Klaasen|1995|pp=783–785}}

[[File:243 ida.jpg|[[243 Ida]], with its moon Dactyl to the right|thumb|left|alt=Another potato-shaped asteroid; Dactyl is just a small dot]]
Transmission was still limited to the 40 bit/s data rate available during the Gaspra flyby. At that rate, it took thirty hours to send each of the five frames. In September, the line of sight between ''Galileo'' and Earth was close to the Sun, so there was only time to send one mosaic before it was blocked by the Sun on September 29, 1993; the rest of the mosaics were transmitted in February and March, after Earth had come around the Sun. ''Galileo''{{'s}} tape recorder was used to store the images, but tape space was also required for the primary Jupiter mission. A technique was developed whereby only image fragments of two or three lines out of every 330 were initially sent. A determination could then be made as to whether the image was of 243 Ida or of empty space. Ultimately, only about 16 percent of the SSI data recorded could be sent back to Earth.{{sfn|Harland|2000|pp=72–77}}{{sfn|Belton et al|1996|p=7}}

When astronomer Ann Harch examined the images on February 17, 1994, she found that Ida had a small moon measuring around {{convert|1.6|km|mi|sp=us|sigfig=1}} in diameter, which appeared in 47 images.{{sfn|Harland|2000|pp=72–77}} A competition was held among ''Galileo'' project members to select a name for the moon, which was ultimately dubbed [[Dactyl (moon)|Dactyl]] after the legendary [[Dactyls (mythology)|Dactyls]], mythical beings which lived on [[Mount Ida (Crete)|Mount Ida]], the geographical feature on [[Crete]] the asteroid was named for. Craters on Dactyl were named after individual dactyloi. Regions on 243 Ida were named after cities where [[Johann Palisa]], the discover of 243 Ida, made his observations, while ridges on 243 Ida were named in honor of deceased ''Galileo'' team members.{{sfn|Belton et al|1996|p=10}}<ref>{{cite web |title=243 Ida |date=November 17, 2017 |publisher=NASA |url=https://science.nasa.gov/solar-system/asteroids/243-ida/ |access-date=April 3, 2024 |archive-date=April 3, 2024 |archive-url=https://web.archive.org/web/20240403200221/https://science.nasa.gov/solar-system/asteroids/243-ida/ |url-status=live }}</ref>

Dactyl was the first [[asteroid moon]] to be discovered. Moons of asteroids had been assumed to be rare, but the discovery of Dactyl hinted that they might in fact be quite common. From subsequent analysis of this data, Dactyl appeared to be an [[S-type asteroid]], and spectrally different from 243 Ida, although Ida is also an S-type asteroid. It was hypothesized that both may have been produced by the breakup of a [[Koronis family|Koronis]] parent body.{{sfn|Belton et al|1996|pp=2–3}}{{sfn|Chapman|Veverka|Thomas|Klaasen|1995|pp=783–785}}

==Voyage to Jupiter==
=== Comet Shoemaker–Levy 9 ===
{{main|Comet Shoemaker–Levy 9}}
[[File:SL9ImpactGalileo.jpg|thumb|right|Four images of Jupiter and [[Comet Shoemaker–Levy 9]] in visible light taken by ''Galileo'' at {{frac|2|1|3}}-second intervals from a distance of {{convert|238|e6km|e6mi|sp=us|abbr=off}} |alt=refer to caption]]
''Galileo''{{'s}} prime mission was a two-year study of the Jovian system, but on March 26, 1993, while it was en route, astronomers [[Carolyn S. Shoemaker]], [[Eugene Merle Shoemaker|Eugene M. Shoemaker]] and [[David H. Levy]] discovered fragments of a comet orbiting Jupiter, the remains of a comet that had passed within Jupiter's [[Roche limit]] and had been torn apart by [[tidal force]]s. It was named [[Comet Shoemaker–Levy 9]]. Calculations indicated that it would crash into the planet sometime between July 16 and 24, 1994. Although ''Galileo'' was still {{convert|238|e6km|sp=us|abbr=off}} away, Jupiter was 66 pixels wide in its camera, and it was perfectly positioned to observe this event. Terrestrial telescopes had to wait to see the [[impact event]] sites as they rotated into view because it would occur on Jupiter's night side.{{sfn|Meltzer|2007|pp=188–189}}{{sfn|Harland|2000|p=80}}

Instead of burning up in Jupiter's atmosphere as expected, the first of the 21 comet fragments struck the planet at around {{convert|320000|km/h|sp=us}} and exploded with a fireball {{convert|3000|km|sp=us}} high, easily discernible to Earth-based telescopes even though it was on the night side of the planet. The impact left a series of dark scars on the planet, some two or three times as large as the Earth, that persisted for weeks. When ''Galileo'' observed an impact in ultraviolet light, the fireballs lasted for about ten seconds, but in the infrared they persisted for 90 seconds or more. When a fragment hit the planet, it increased Jupiter's overall brightness by about 20 percent. The NIMS observed one fragment create a fireball {{convert|7|km|sp=us}} in diameter that burned with a temperature of {{convert|8000|K|C F|sigfig=2}}, which was hotter than the surface of the Sun.{{sfn|Meltzer|2007|pp=190–191}}{{sfn|Harland|2000|pp=82–83}}

===Probe deployment===
The ''Galileo'' probe separated from the orbiter at 03:07&nbsp;UTC on July 13, 1995,<ref name="galileo-arrival" /> five months before its rendezvous with the planet on December 7.{{sfn|D'Amario|Bright|Wolf|1992|p=24}} At this point, the spacecraft was {{convert|83|e6km|e6mi|sp=us|abbr=off}} from Jupiter, but {{convert|664|e6km|e6mi|sp=us}} from Earth, and telemetry from the spacecraft, transmitted at the [[speed of light]], took 37 minutes to reach JPL. A tiny frequency change in the radio signal indicated that the separation had been accomplished. The ''Galileo'' orbiter was still on a collision course with Jupiter. Previously, course corrections had been made using the twelve {{convert|10|N|adj=on}} thrusters, but with the probe on its way, the ''Galileo'' orbiter could now fire its {{convert|400|N|adj=on}} [[Messerschmitt-Bölkow-Blohm]] main engine which had been covered by the probe until then. At 07:38 UTC on July 27, it was fired for the first time to place the ''Galileo'' orbiter on course to enter orbit around Jupiter, whence it would act as a communications relay for the ''Galileo'' probe. The ''Galileo'' probe's project manager, Marcie Smith at the [[Ames Research Center]], was confident that the LGAs could be used as relays. The burn lasted for five minutes and eight seconds, and changed the velocity of the ''Galileo'' orbiter by {{convert|61.9|m/s|sp=us}}.<ref>{{cite press release |title=Critical Engine Firing Successful for Jupiter-Bound Galileo |publisher=NASA/Jet Propulsion Laboratory |url=https://www.jpl.nasa.gov/news/critical-engine-firing-successful-for-jupiter-bound-galileo/ |access-date=19 January 2021 |archive-date=February 1, 2021 |archive-url=https://web.archive.org/web/20210201000728/https://www.jpl.nasa.gov/news/critical-engine-firing-successful-for-jupiter-bound-galileo/ |url-status=live }}</ref>{{sfn|Meltzer|2007|pp=194–195}}

===Dust storms===
In August 1995, the ''Galileo'' orbiter encountered a severe dust storm {{convert|63|e6km|e6mi|sp=us}} from Jupiter that took several months to traverse. Normally the spacecraft's dust detector picked up a dust particle every three days; now it detected up to 20,000 particles a day. Interplanetary dust storms had previously been encountered by the ''[[Ulysses (spacecraft)|Ulysses]]'' probe, which had passed by Jupiter three years before on its mission to study the Sun's polar regions, but those encountered by ''Galileo'' were more intense. The dust particles were 5 to 10&nbsp;nm in size, about the same as those in cigarette smoke, and had speeds ranging from {{convert|90,000|to|450,000|mi/h|km/h|order=flip|sp=us}} depending on their size. The existence of the dust storms had come as a complete surprise to scientists when ''Ulysses'' encountered them. While data from both ''Ulysses'' and ''Galileo'' hinted that they originated somewhere in the Jovian system, it was a mystery how they had been created and how they had escaped from Jupiter's strong [[gravitational field|gravitational]] and [[electromagnetic field]]s.<ref>{{cite press release |title=Galileo Flying Through Intense Dust Storm |publisher=NASA/Jet Propulsion Laboratory |first1=Douglas |last1=Isbel |first2=James H. |last2=Wilson |id=95-147 |url=https://www.jpl.nasa.gov/news/galileo-flying-through-intense-dust-storm-on-way-to-jupiter |access-date=15 April 2024}}</ref>{{sfn|Meltzer|2007|pp=195–196}}{{sfn|Graps|Grün|Svedhem|Krüger|2000|p=49}}

===Tape recorder anomaly===
The failure of ''Galileo''{{'s}} high-gain antenna meant that data storage to the tape recorder for later compression and playback was crucial in order to obtain any substantial information from the flybys of Jupiter and its moons. The four-track, 114-[[megabyte]] digital tape recorder was manufactured by [[Iteris|Odetics Corporation]].<ref>{{cite web |url=http://www2.jpl.nasa.gov/galileo/faqtape.html |archive-url=https://web.archive.org/web/20090403160253/http://www2.jpl.nasa.gov/galileo/faqtape.html |archive-date=April 3, 2009 |title=''Galileo'' FAQ – Tape Recorder |publisher=NASA/Jet Propulsion Laboratory |url-status=dead |access-date=May 15, 2011}}</ref> On October 11, it was stuck in rewind mode for 15 hours before engineers learned what had happened and were able to send commands to shut it off. Although the recorder itself was still in working order, the malfunction had possibly damaged a length of tape at the end of the reel. This section of tape was declared "off limits" to any future data recording, and was covered with 25 more turns of tape to secure the section and reduce any further stresses, which could tear it. Because it happened only weeks before ''Galileo'' entered orbit around Jupiter, the anomaly prompted engineers to sacrifice data acquisition of almost all of the [[Io (moon)|Io]] and [[Europa (moon)|Europa]] observations during the orbit insertion phase in order to focus on recording data sent from the atmospheric probe during its descent.<ref>{{cite press release |url=https://www.jpl.nasa.gov/news/galileo-on-track-after-tape-recorder-recovery/ |title=Galileo on Track After Tape Recorder Recovery |publisher=NASA/Jet Propulsion Laboratory |date=October 26, 1995 |access-date=January 19, 2021 |archive-date=November 30, 2021 |archive-url=https://web.archive.org/web/20211130055714/https://www.jpl.nasa.gov/news/galileo-on-track-after-tape-recorder-recovery |url-status=live }}</ref>

==Jupiter==
{{see also|Timeline of Galileo (spacecraft)|l1=Timeline of ''Galileo'' (spacecraft)}}
{{multiple image |align=center |direction=horizontal |total_width=900
 |image1=E4 True and False Color Jupiter Hot Spot Mosaic (PIA00602).jpg
 |width1=1300 |height1=1000 |caption1=True and false color images of Jupiter's cloud layers |alt1=refer to caption
 |image2=Great Red Spot at Four Different Wavelengths (PIA00721).jpg
 |width2=2800 |height2=1600 |caption2=Jupiter's [[Great Red Spot]] at 757&nbsp;nm, 415&nbsp;nm, 732&nbsp;nm, and 886&nbsp;nm |alt2=refer to caption
 |image3=Jupiter lightnings.jpg
 |width3=1700 |height3=900 |caption3=Jovian lightning amidst clouds lit by Io's moonlight |alt3=lightning is just small dots
}}

===Arrival===
The ''Galileo'' orbiter's magnetometers reported that the spacecraft had encountered the bow shock of Jupiter's magnetosphere on November 16, 1995, when it was {{convert|15|e6km|e6mi|abbr=off|sp=us}} from Jupiter. The bow shock moved to and fro in response to solar wind gusts, and was therefore crossed multiple times between 16 and 26 November, by which time ''Galileo'' was {{convert|9|e6km|e6mi|abbr=off|sp=us}} from Jupiter.{{sfn|Meltzer|2007|pp=201–202}}

On December 7, 1995, the orbiter arrived in the Jovian system. That day it made a {{convert|32500|km|sp=us|adj=on}} flyby of Europa at 11:09 UTC, and then an {{convert|890|km|sp=us|adj=on}} flyby of Io at 15:46 UTC, using Io's gravity to reduce its speed, and thereby conserve propellant for use later in the mission. At 19:54 it made its closest approach to Jupiter. The orbiter's electronics had been heavily shielded against radiation, but the radiation surpassed expectations, and nearly exceeded the spacecraft's design limits. One of the navigational systems failed, but the backup took over. Most robotic spacecraft respond to failures by entering [[Safe mode (spacecraft)|safe mode]] and awaiting further instructions from Earth, but this was not possible for ''Galileo'' during the arrival sequence due to the great distance and consequent long turnaround time.{{sfn|Meltzer|2007|pp=201–202}}

===Atmospheric probe===
[[File:Descent Module.jpeg|thumb|right|Inner descent module of the ''Galileo'' entry probe|alt=Spherical spaccreaft with some portuding instruments]]
The descent probe awoke in response to an alarm at 16:00 UTC and began powering up its instruments. It passed through the [[rings of Jupiter]] and encountered a previously undiscovered [[radiation belt]] ten times as strong as Earth's [[Van Allen radiation belt]] {{convert|31,000|mi|order=flip|sp=us}} above Jupiter's cloud tops.{{sfn|Ragent|Colburn|Avrin|Rages|1996|pp=854–856 }}{{sfn|Meltzer|2007|pp=202–204}} It had been predicted that the probe would pass through three layers of clouds; an upper one consisting of [[ammonia]]-ice particles at a pressure of {{convert|0.5|to|0.6|bar|psi}}; a middle one of [[ammonium hydrosulfide]] ice particles at a pressure of {{convert|1.5|to|2|bar|psi}}; and one of water vapor at {{convert|4|to|5|bar|psi}}.{{sfn|Meltzer|2007|p=212}} The atmosphere through which the probe descended was much denser and hotter than expected. Jupiter was also found to have only half the amount of helium expected and the data did not support the three-layered cloud structure theory: only one significant cloud layer was measured by the probe, at a pressure of around {{convert|1.55|bar|psi}} but with many indications of smaller areas of increased particle densities along the whole length of its trajectory.{{sfn|Ragent|Colburn|Avrin|Rages|1996|pp=854–856 }}

The descent probe entered [[atmosphere of Jupiter|Jupiter's atmosphere]], defined for the purpose as being {{convert|450|km|sp=us}} above the {{convert|1|bar|psi|adj=on}} pressure level,{{sfn|Young|1998|p=22,776}} without any braking at 22:04 UTC on December 7, 1995. At this point it was moving at {{convert|170700|km/h|sp=us}} relative to Jupiter.<ref name="Probe Events">{{cite web |url=http://spaceprojects.arc.nasa.gov/Space_Projects/galileo_probe/htmls/probe_events.html |archive-url=https://web.archive.org/web/20070102143553/http://spaceprojects.arc.nasa.gov/Space_Projects/galileo_probe/htmls/probe_events.html |archive-date=January 2, 2007 |title=Galileo Probe Mission Events |publisher=NASA |url-status=dead |date=June 14, 1996 }}</ref> This was by far the most difficult [[atmospheric entry]] yet attempted by any spacecraft; the probe had to withstand a peak [[deceleration]] of {{convert|228|g0|lk=in|abbr=on}}.{{sfn|Heppenheimer|2009|p=257}}<ref>{{cite news |url=http://www.space.com/searchforlife/070719_seti_probing.html |title=Probing Planets: Can You Get There From Here? |first=Lisa |last=Chu-Thielbar |date=July 19, 2007 |publisher=[[Space.com]] |access-date=2007-07-27 |archive-date=February 12, 2009 |archive-url=https://web.archive.org/web/20090212005251/http://www.space.com/searchforlife/070719_seti_probing.html |url-status=live }}</ref> The rapid flight through the atmosphere produced a plasma with a temperature of about {{convert|14,000|C}}, and the probe's [[carbon phenolic]] heat shield lost more than half of its mass, {{convert|80|kg|sp=us}}, during the descent.{{sfn|Meltzer|2007|p=118}}<ref>{{cite web|url=http://spaceprojects.arc.nasa.gov/Space_Projects/galileo_probe/htmls/Heat_Shield.html |title=''Galileo'' Probe Heat Shield Ablation |first=Julio |last=Magalhães |publisher=NASA Ames Research Center |date=1997-09-17 |access-date=2006-12-12 |url-status=dead |archive-url=https://web.archive.org/web/20060929185050/http://spaceprojects.arc.nasa.gov/Space_Projects/galileo_probe/htmls/Heat_Shield.html |archive-date=2006-09-29 }}</ref><ref>{{cite web |url=http://spaceprojects.arc.nasa.gov/Space_Projects/galileo_probe/htmls/probe_spacecraft.html |title=The ''Galileo'' Probe Spacecraft |first=Julio |last=Magalhães |publisher=NASA Ames Research Center |date=1996-12-06 |access-date=2006-12-12 |url-status=dead |archive-url=https://web.archive.org/web/20070101114453/http://spaceprojects.arc.nasa.gov/Space_Projects/galileo_probe/htmls/probe_spacecraft.html |archive-date=2007-01-01 }}</ref> As the probe passed through Jupiter's cloud tops, it started transmitting data to the orbiter, {{convert|215000|km|sp=us}} above.{{sfn|Harland|2000|p=105}} The data was not immediately relayed to Earth, but a single bit was transmitted from the orbiter as a notification that the signal from the probe was being received and recorded, which would then take days to be transmitted using the LGA.{{sfn|Meltzer|2007|pp=202–204}}

The atmospheric probe deployed its {{convert|2.5|m|ft|abbr=off|adj=on|sp=us}} [[parachute]] fifty-three seconds later than anticipated, resulting in a small loss of upper-atmospheric readings. This was attributed to wiring problems with an accelerometer that determined when to begin the parachute deployment sequence. The probe then dropped its heat shield, which fell into Jupiter's interior.{{sfn|Harland|2000|p=105}}<ref name="gpr">{{cite web |url=http://www2.jpl.nasa.gov/sl9/gll38.html |title=Galileo Probe Science Results |date=January 22, 1996 |first1=Douglas |last1=Isbell |first2=David |last2=Morse |publisher=NASA/Jet Propulsion Laboratory |access-date=March 4, 2016 |archive-url=https://web.archive.org/web/20220424134149/http://www2.jpl.nasa.gov/sl9/gll38.html |archive-date=24 April 2022 }}</ref><ref>{{cite web |title=The Galileo Probe Mission Events Timeline |publisher=NASA |url=http://ccf.arc.nasa.gov/galileo_probe/htmls/probe_mission_events.html |archive-url=https://web.archive.org/web/19990424091538/http://ccf.arc.nasa.gov/galileo_probe/htmls/probe_mission_events.html |archive-date=1999-04-24 |url-status=dead |df=mdy-all}}</ref><ref>{{cite web |title=In Depth |publisher=Galileo – NASA Solar System Exploration |url=https://solarsystem.nasa.gov/missions/galileo/in-depth/ |access-date=6 March 2021 |archive-date=February 19, 2018 |archive-url=https://web.archive.org/web/20180219070520/https://solarsystem.nasa.gov/missions/galileo/in-depth/ |url-status=live }}</ref> The parachute reduced the probe's speed to {{convert|430|km/h|sp=us}}. The signal from the probe was no longer detected by the orbiter after 61.4 minutes, at an elevation of {{convert|112|miles|order=flip|sp=us}} below the cloud tops and a pressure of {{convert|22.7|atm|bar psi atm|order=out}}.<ref>{{cite web |title=In Depth Galileo Probe |url=https://solarsystem.nasa.gov/missions/galileo-probe/in-depth/ |website=NASA Solar System Exploration |date=December 2017 |access-date=3 July 2023 |archive-date=January 27, 2023 |archive-url=https://web.archive.org/web/20230127141502/https://solarsystem.nasa.gov/missions/galileo-probe/in-depth/ |url-status=live }}</ref> It was believed that the probe continued to fall at [[terminal velocity]], as the temperature increased to {{convert|1700|C}} and the pressure to {{convert|5000|atm|bar psi atm|order=out}}, destroying it.{{sfn|Meltzer|2007|pp=204–205}}

<gallery class="center" mode="packed" heights="240px">
File:Galileo Probe - AC81-0174.jpg|Artist's impression of the probe's entry into [[Atmosphere of Jupiter|Jupiter's atmosphere]] |alt=refer to caption
Image:Galileo atmospheric probe.jpg|Timeline of the probe's atmospheric entry |alt=Probe enters, deploys parachute, transmission ends 61.4 minutes after entry where the pressure is ~<!--(The Probe transmitted data to the Orbiter continuously for 57.6 minutes reaching a depth of 23 bars but the relay link to the Orbiter began at four minutes after entry, so transmission ended 61.4 minutes after entry.) -->
File:Jupiter's clouds.jpg|Jupiter's clouds – expected and actual results of ''Galileo''{{'}}s atmospheric probe mission |alt=The clouds of [[ammonia]] and [[ammonium sulfide]] were much thinner than expected, and clouds of water vapor were not detected.
</gallery>

The probe detected less lightning, less water, but stronger winds than expected. Scientists had expected to find wind speeds of up to {{convert|220|mph|order=flip|sp=us}}, but winds of up to {{convert|330|mph|order=flip|sp=us}} were detected. The implication was that the winds are not produced by heat generated by sunlight (as Jupiter gets less sunlight than Earth) or the condensation of water vapor (the main causes on Earth), but are due to an internal heat source. It was already well known that the atmosphere of Jupiter was mainly composed of hydrogen, but the clouds of [[ammonia]] and [[ammonium sulfide]] were much thinner than expected, and clouds of water vapor were not detected. This was the first observation of ammonia clouds in another planet's atmosphere. The atmosphere creates ammonia-ice particles from material coming up from lower depths.<ref name="endkit">{{cite web |url=https://solarsystem.nasa.gov/system/downloadable_items/1028_galileo-end_presskit.pdf |title=Galileo End of Mission Press Kit |access-date=October 29, 2011 |archive-date=October 23, 2020 |archive-url=https://web.archive.org/web/20201023104131/https://solarsystem.nasa.gov/system/downloadable_items/1028_galileo-end_presskit.pdf |url-status=live }}</ref>

The atmosphere was more turbulent than expected. Wind speeds in the outermost layers were {{convert|290|to|360|km/h|sp=us}}, in agreement with previous measurements from afar, but those wind speeds increased dramatically at pressure levels of {{convert|1|to|4|bar|psi}}, then remaining consistently high at around {{convert|170|m/s|km/h|disp=flip|sp=us}}.{{sfn|Atkinson|Ingersoll|Seiff|1997|pages=649–650}} The abundance of [[nitrogen]], [[carbon]] and [[sulfur]] was three times that of the Sun, raising the possibility that they had been acquired from other bodies in the Solar system,<ref>{{cite web |title=Galileo Probe Mission Science Summary |publisher=NASA |url=http://spaceprojects.arc.nasa.gov/Space_Projects/galileo_probe/htmls/Science_summary.html |archive-url=https://web.archive.org/web/20060221225640/http://spaceprojects.arc.nasa.gov/Space_Projects/galileo_probe/htmls/Science_summary.html |archive-date=2006-02-21 |url-status=dead |df=mdy-all}}</ref><ref name=gpr/> but the low abundance of water cast doubt on theories that Earth's water had been acquired from comets.<ref>{{cite news |title=Jupiter Retains Atmosphere of Mystery; Surprise Galileo Data Could Force New Theories of Planetary Formation|last=Sawyer |first=Kathy |newspaper=[[The Washington Post]] |date=January 23, 1996 |page=A.03 |url=https://www.washingtonpost.com/archive/politics/1996/01/23/jupiter-retains-atmosphere-of-mystery/07c4f386-749d-43d5-8adf-c9fa3fb1cc28/ |access-date=23 May 2024}}</ref>

There was far less lightning activity than expected, only about a tenth of the level of activity on Earth, but this was consistent with the lack of water vapor. More surprising was the high abundance of [[noble gas]]es ([[argon]], [[krypton]] and [[xenon]]), with abundances up to three times that found in the Sun. For Jupiter to trap these gases, it would have had to be much colder than today, around {{convert|-240|C|F|0}}, which suggested that either Jupiter had once been much further from the Sun, or that the interstellar debris that the Solar system had formed from was much colder than had been thought.<ref>{{cite press release |publisher=NASA/Jet Propulsion Laboratory |title=Galileo Probe Results Suggest Jupiter Had an Ancient, Chilly Past |url=https://www.jpl.nasa.gov/news/galileo-probe-results-suggest-jupiter-had-an-ancient-chilly-past/ |date=November 17, 1999 |access-date=January 19, 2021 |archive-date=November 30, 2021 |archive-url=https://web.archive.org/web/20211130054857/https://www.jpl.nasa.gov/news/galileo-probe-results-suggest-jupiter-had-an-ancient-chilly-past |url-status=live }}</ref>

===Orbiter===
[[File:Animation of Galileo trajectory around Jupiter.gif|thumb|right|Animation of ''Galileo''{{'s}} trajectory around Jupiter from August 1, 1995, to September 30, 2003<br/>{{legend2|magenta|''Galileo''}}{{·}}{{legend2|Lime|[[Jupiter]]}}{{·}}{{legend2|OrangeRed|Io}}{{·}}{{legend2|RoyalBlue|Europa}}{{·}}{{legend2|Gold|[[Ganymede (moon)|Ganymede]]}}{{·}}{{legend2|Cyan|[[Callisto (moon)|Callisto]]}} |alt=refer to caption]]
With the probe data collected, the ''Galileo'' orbiter's next task was to slow down in order to avoid heading off into the outer solar system. A burn sequence commencing at 00:27 UTC on December 8 and lasting 49 minutes reduced the spacecraft's speed by {{convert|600|m/s|sp=us}} and it entered a [[parking orbit]] with an [[orbital period]] of 198 days. The ''Galileo'' orbiter thus became the first artificial satellite of Jupiter.{{sfn|Meltzer|2007|pp=208–209}}<ref>{{cite web |url=http://solarsystem.nasa.gov/missions/profile.cfm?MCode=Galileo&Display=ReadMore |title=Solar System Exploration – Galileo |archive-url=https://web.archive.org/web/20121006010150/http://solarsystem.nasa.gov/missions/profile.cfm?MCode=Galileo&Display=ReadMore |archive-date=October 6, 2012 |publisher=NASA |url-status=dead |access-date=April 24, 2012}}</ref> Most of its initial orbit was occupied transmitting the data from the probe back to Earth. When the orbiter reached its [[apojove]] on March 26, 1996, the main engine was fired again to increase the orbit from four times the radius of Jupiter to ten times. By this time the orbiter had received half the radiation allowed for in the mission plan, and the higher orbit was to conserve the instruments for as long as possible by limiting the radiation exposure.{{sfn|Meltzer|2007|pp=208–209}}

The spacecraft traveled around Jupiter in elongated [[ellipse]]s, each orbit lasting about two months. The differing distances from Jupiter afforded by these orbits allowed ''Galileo'' to sample different parts of the planet's extensive [[magnetosphere]]. The orbits were designed for close-up flybys of Jupiter's largest moons. A naming scheme was devised for the orbits: a code with the first letter of the moon being encountered on that orbit (or "J" if none was encountered) plus the orbit number.{{sfn|Meltzer|2007|pp=232–233}}

===Mission extension===

After the primary mission concluded on December 7, 1997, most of the mission staff departed, including O'Neil, but about a fifth of them remained. The ''Galileo'' orbiter commenced an extended mission known as the ''Galileo'' Europa Mission (GEM), which ran until December 31, 1999. This was a low-cost mission, with a budget of $30 million (equivalent to ${{Inflation|US-GDP|30|1997}} million in {{Inflation/year|US-GDP}}).{{sfn|Meltzer|2007|pp=234–237}} The reason for calling it as the "Europa" mission rather than the "Extended" mission was political; although it was wasteful to scrap a spacecraft that was still functional and capable of performing a continuing mission, Congress took a dim view of requests for more money for projects that had already been fully funded. This was avoided through rebranding.{{sfn|Greenberg|2005|pp=337–338}}

The smaller GEM team did not have the resources to deal with problems, but when they arose it was able to temporarily recall former team members for intensive efforts to solve them. The spacecraft performed several flybys of [[Europa (moon)|Europa]], [[Callisto (moon)|Callisto]] and [[Io (moon)|Io]]. On each one the spacecraft collected only two days' worth of data instead of the seven it had collected during the prime mission. The [[radiation]] environment near Io, which ''Galileo'' approached to within {{convert|201|km|sp=us}} on November 26, 1999, on orbit I25, was very unhealthy for ''Galileo''{{'s}} systems, and so these flybys were saved for the extended mission when loss of the spacecraft would be more acceptable.{{sfn|Meltzer|2007|pp=234–237}}

By the time GEM ended, most of the spacecraft was operating well beyond its original design specifications, having absorbed more than 600 [[kilorad]]s in between 1995 and 2002,{{sfn|Fieseler|Ardalan|Frederickson|2002}} three times the radiation exposure that it had been built to withstand. Many of the instruments were no longer operating at peak performance, but were still functional, so a second extension, the ''Galileo'' Millennium Mission (GMM) was authorized. This was intended to run until March 2001, but it was subsequently extended until January 2003. GMM included return visits to Europa, Io, Ganymede and Callisto, and for the first time to [[Amalthea (moon)|Amalthea]].{{sfn|Meltzer|2007|pp=237–238}} The total cost of the original ''Galileo'' mission was about {{US$|1.39 billion}} (equivalent to ${{Inflation|US-GDP|1.39|2003}} billion in {{Inflation/year|US-GDP}}). Of this, {{US$|892 million}} (equivalent to ${{Inflation|US-GDP|892|2003}} million in {{Inflation/year|US-GDP}}) was spent on spacecraft development.<ref name="galileo-arrival" /> Another $110 million (equivalent to ${{Inflation|US-GDP|110|2003}} million in {{Inflation/year|US-GDP}}) was contributed by international agencies.<ref>{{cite web |url=https://solarsystem.nasa.gov/missions/galileo/overview/#otp_quick_facts |title=Galileo: Quick Facts |publisher=NASA |access-date=January 19, 2021 |archive-date=July 19, 2009 |archive-url=https://web.archive.org/web/20090719111109/http://www2.jpl.nasa.gov/galileo/messenger/oldmess/2Probe.html#otp_quick_facts |url-status=live }}</ref>

===Io===
The innermost of the four Galilean moons, Io is roughly the same size as Earth's moon, with a [[mean radius]] of {{convert|1821.3|km|sp=us}}. It is in [[orbital resonance]] with Ganymede and Europa, and [[tidal locking|tidally locked]] with Jupiter, so just as the Earth's Moon always has the same side facing Earth, Io always has the same side facing Jupiter. It has a faster orbit though, with a rotation period of 1.769 days. As a result, the rotational and tidal forces on Io are 220 times as great as those on Earth's moon.{{sfn|Anderson|Sjogren|Schubert|1996|p=709}} These frictional forces are sufficient to melt rock, creating volcanoes and lava flows. Although only a third of the size of Earth, Io generates twice as much heat. While geological events occur on Earth over periods of thousands or even millions of years, cataclysmic events are common on Io. Visible changes occurred between orbits of ''Galileo''. The colorful surface is a mixture of red, white and yellow sulfur compounds.{{sfn|Meltzer|2007|pp=242–244}}

[[File:Io - Tvashtar Catena.jpg|thumb|right|upright=2.0|[[Tvashtar Paterae|Tvashtar]] [[Crater chain|Catena]] on Io, showing changes in hot spots between 1999 and 2000. Infrared imaging shows a hot lava flow more than {{convert|60|km|mi}} long. |alt=Different lava flows]]
''Galileo'' flew past Io, but in the interest of protecting the tape recorder, O'Neil decided to forego collecting images. To use the SSI camera meant operating the tape recorder at high speed, with sudden stops and starts, whereas the fields and particles instruments only required the tape recorder to run continuously at slow speeds, and it was believed that it could handle this. This was a crushing blow to scientists, some of whom had waited years for the opportunity.{{sfn|Meltzer|2007|pp=245–246}} No other Io encounters were scheduled during the prime mission because it was feared that the high radiation levels close to Jupiter would damage the spacecraft.{{sfn|Meltzer|2007|p=231}} However, valuable information was still obtained; Doppler data used to measure Io's gravitational field revealed that Io had a core of molten [[iron]] and [[iron sulfide]].{{sfn|Anderson|Sjogren|Schubert|1996|p=709}}<ref>{{cite press release |date=May 3, 1996 |publisher=NASA/Jet Propulsion Laboratory |title=NASA's Galileo Finds Giant Iron Core in Jupiter's Moon Io |url=https://www.jpl.nasa.gov/news/nasas-galileo-finds-giant-iron-core-in-jupiters-moon-io/ |access-date=January 19, 2021 |archive-date=February 1, 2021 |archive-url=https://web.archive.org/web/20210201080131/https://www.jpl.nasa.gov/news/nasas-galileo-finds-giant-iron-core-in-jupiters-moon-io/ |url-status=live }}</ref>

Another opportunity to observe Io arose during the ''Galileo'' Europa Mission (GEM), when ''Galileo'' flew past Io on orbits I24 and I25, and it would revisit Io during the ''Galileo'' Millennium Mission (GMM) on orbits I27, I31, I32 and I33.{{sfn|Meltzer|2007|pp=240–241}} As ''Galileo'' approached Io on I24 at 11:09 UTC on October 11, 1999, it entered safe mode. Apparently, high-energy electrons had altered a bit on a memory chip. When it entered safe mode, the spacecraft turned off all non-essential functions. Normally it took seven to ten days to diagnose and recover from a safe mode incident; this time the ''Galileo'' Project team at JPL had nineteen hours before the encounter with Io. After a frantic effort, they managed to diagnose a problem that had never been seen before, and restore the spacecraft systems with just two hours to spare. Not all of the planned activities could be carried out, but ''Galileo'' obtained a series of high-resolution color images of the [[Pillan Patera]], and [[Zamama (volcano)|Zamama]], [[Prometheus (volcano)|Prometheus]], and [[Pele (volcano)|Pele]] volcanic eruption centers.{{sfn|Meltzer|2007|pp=246–248}}

When ''Galileo'' next approached Io on I25 at 03:40 UTC on November 26, 1999, JPL were eating their [[Thanksgiving dinner]] at the ''Galileo'' Mission Control Center when, with the encounter with Io just four hours away, the spacecraft again entered safe mode. This time the problem was traced to a software patch implemented to bring ''Galileo'' out of safe mode during I24. Fortunately, the spacecraft had not shut down as much as it had on I24, and the team at JPL were able to bring it back online. During I24 they had done so with two hours to spare; this time, they had just three minutes. Nonetheless, the flyby was successful, with ''Galileo''{{'s}} NIMS and SSI camera capturing an erupting volcano that generated a {{convert|20|mi|order=flip|sp=us|adj=on}} long plume of lava that was sufficiently large and hot to have also been detected by the [[NASA Infrared Telescope Facility]] atop [[Mauna Kea]] in [[Hawaii]]. While such events were more common and spectacular on Io than on Earth, it was extremely fortuitous to have captured it; [[planetary scientist]] [[Alfred McEwen]] estimated the odds at 1 in 500.<ref>{{cite press release |date=December 17, 1999 |publisher=NASA/Jet Propulsion Laboratory |title=Galileo Sees Dazzling Lava Fountain on Io |url=https://www.jpl.nasa.gov/news/galileo-sees-dazzling-lava-fountain-on-io/ |access-date=January 19, 2021 |archive-date=November 30, 2021 |archive-url=https://web.archive.org/web/20211130071105/https://www.jpl.nasa.gov/news/galileo-sees-dazzling-lava-fountain-on-io |url-status=live }}</ref>

[[File:Io rotating 2.ogv|thumb|left|Io in sped-up motion; a rotation actually takes 1.769 days |alt=refer to caption]]
The safe-mode incidents on I24 and I25 left some gaps in the data, which I27 targeted. This time ''Galileo'' passed {{convert|198|km|sp=us}} over the surface of Io. At this time, the spacecraft was nearly at the maximum distance from Earth, and there was a [[solar conjunction]], a period when the Sun blocked the line of sight between Earth and Jupiter. As a consequence, three quarters of the observations had to be taken over a period of three hours. NIMS images revealed fourteen active volcanoes in a region thought to contain just four. Images of [[Loki Patera]] showed that in the four and half months between I24 and I27, some {{convert|10000|km2|sp=us}} had been covered in fresh lava. A series of observations of [[extreme ultraviolet]] (EUV) had to be cancelled due to yet another safe-mode event. Radiation exposure caused a transient [[Bus (computing)|bus]] reset, a computer hardware error resulting in a safe mode event. A software patch implemented after the Europa encounter on orbit E19 guarded against this when the spacecraft was within 15 Jupiter radii of the planet, but this time it occurred at 29 Jupiter radii. The safe mode event also caused a loss of tape playback time, but the project managers decided to carry over some Io data into orbit G28, and play it back then. This limited the amount of tape space available for that Ganymede encounter, but the Io data was considered to be more valuable.{{sfn|Meltzer|2007|pp=249–250}}

The discovery of Io's iron core raised the possibility that it had a magnetic field. The I24, I25 and I27 encounters had involved passes over Io's equator, which made it difficult to determine whether Io had its own magnetic field or one induced by Jupiter. Accordingly, on orbit I31, ''Galileo'' passed within {{convert|200|km|sp=us}} of the surface of the north pole of Io, and on orbit I32 it flew {{convert|181|km|sp=us}} over the south pole.{{sfn|Meltzer|2007|pp=251–253}}<ref>{{cite press release |first=Guy |last=Webster |id=2001-161 |publisher=NASA/Jet Propulsion Laboratory |title=Spacecraft to Fly Over Source of Recent Polar Eruption on Io |date=August 1, 2001 |url=https://www.jpl.nasa.gov/news/spacecraft-to-fly-over-source-of-recent-polar-eruption-on-io |access-date=April 16, 2024 |archive-date=April 28, 2024 |archive-url=https://web.archive.org/web/20240428014501/https://www.jpl.nasa.gov/news/spacecraft-to-fly-over-source-of-recent-polar-eruption-on-io |url-status=live }}</ref> After examining the magnetometer results, planetary scientist [[Margaret G. Kivelson]], announced that Io had no intrinsic magnetic field, which meant that its molten iron core did not have the same [[convection (heat transfer)|convective]] properties as that of Earth.<ref>{{cite press release |title=Jupiter's Io Generates Power and Noise, But No Magnetic Field |date=December 10, 2001 |first=Guy |last=Webster |publisher=NASA/Jet Propulsion Laboratory |url=https://www.jpl.nasa.gov/news/jupiters-io-generates-power-and-noise-but-no-magnetic-field/ |access-date=November 29, 2020 |archive-date=February 1, 2021 |archive-url=https://web.archive.org/web/20210201071553/https://www.jpl.nasa.gov/news/jupiters-io-generates-power-and-noise-but-no-magnetic-field/ |url-status=live }}</ref>

On I31 ''Galileo'' sped through an area that had been in the plume of the [[Tvashtar Paterae]] volcano, and it was hoped that the plume could be sampled. This time, Tvashtar was [[effusive eruption|quiet]], but the spacecraft flew through the plume of another, previously unknown, volcano {{convert|600|km|sp=us}} away. What had been assumed to be hot ash from the volcanic eruption turned out to be sulfur dioxide snowflakes, each consisting of 15 to 20 molecules clustered together.{{sfn|Meltzer|2007|pp=251–253}}<ref>{{cite web |title=Dashing through the Snows of Io |publisher=NASA |url=https://science.nasa.gov/science-news/science-at-nasa/2001/ast16oct_1 |access-date=November 29, 2020 |archive-url=https://web.archive.org/web/20221207005035/https://science.nasa.gov/science-news/science-at-nasa/2001/ast16oct_1 |archive-date=7 December 2022}}</ref><ref>{{cite web |title=Eruption at Tvashtar Catena, Io |publisher=The Planetary Society |url=https://www.planetary.org/space-images/pia02550 |access-date=May 30, 2024}}</ref> ''Galileo''{{'s}} final return to Io on orbit I33 was marred by another safe mode incident, and much of the hoped-for data was lost.<ref>{{cite press release |title=Farewell, Io; Galileo Paying Last Visit to a Restless Moon |date=January 15, 2002 |first=Guy |last=Webster |publisher=NASA/Jet Propulsion Laboratory |url=https://www.jpl.nasa.gov/news/farewell-io-galileo-paying-last-visit-to-a-restless-moon/ |access-date=January 19, 2021}}</ref>

===Europa===
[[File:Europa-moon-with-margins.jpg|thumb|right|Europa photographed by ''Galileo''|alt=Europe is criss-crossed by lines]]
Although the smallest of the four Galilean moons, with a radius of {{convert|1565|km|sp=us}}, Europa is the sixth-largest moon in the solar system.<ref name="Europa - Jupiter's Icy Moon">{{cite web |title=Europa – Jupiter's Icy Moon |publisher= Teachlink @ Utah State University |url=http://teacherlink.ed.usu.edu/tlnasa/OtherPRINT/poster/Europa_Poster.pdf |access-date=December 3, 2020 |archive-date=February 4, 2021 |archive-url=https://web.archive.org/web/20210204170152/http://teacherlink.ed.usu.edu/tlnasa/OtherPRINT/poster/Europa_Poster.pdf |url-status=dead }}</ref> Observations from Earth indicated that it was covered in ice.{{sfn|Greenberg|2005|p=9}} Like Io, Europa is tidally locked with Jupiter. It is in orbital resonance with Io and Ganymede, with its 85-hour orbit being twice that of Io, but half that of Ganymede. Conjunctions with Io always occur on the opposite side of Jupiter to those with Ganymede.{{sfn|Greenberg|2005|pp=51–52}} Europa is therefore subject to tidal effects.{{sfn|Greenberg|2005|pp=49–51}} There is no evidence of volcanism like on Io, but ''Galileo'' revealed that the surface ice was covered in cracks.{{sfn|Greenberg|2005|pp=12–14}}

Some observations of Europa were made during orbits G1 and G2. On C3, ''Galileo'' conducted a {{convert|34,800|km|sp=us|adj=on}} "nontargeted" encounter of Europa—meaning a secondary flyby at a distance of up to {{convert|100,000|km|sp=us}}—on November 6, 1996. During E4 from December 15 to 22, 1996, ''Galileo'' flew within {{convert|692|km|sp=us}} of Europa, but data transmission was hindered by a Solar [[occultation]] that blocked transmission for ten days.{{sfn|Meltzer|2007|pp=254–256}}

''Galileo'' returned to Europa on E6 in January 1997, this time at a height of {{convert|586|km|sp=us}}, to analyze oval-shaped features in the infrared and ultraviolet spectra. Occultations by Europa, Io and Jupiter provided data on the atmospheric profiles of them, and measurements were made of Europa's gravitational field. On E11 from November 2 to 9, 1997, data was collected on the magnetosphere.{{sfn|Meltzer|2007|pp=254–256}} Due to the problems with the HGA, only about two percent of the anticipated number of images of Europa were obtained by the primary mission.{{sfn|Greenberg|2005|p=160}} On the GEM, the first eight orbits (E12 through E19) were all dedicated to Europa, and ''Galileo'' paid it a final visit on E26 during the GMM.{{sfn|Meltzer|2007|pp=256–259}}

[[File:PIA03002 Blocks in the Europan Crust Provide More Evidence of Subterranean Ocean.jpg|thumb|left|This false color image on the left shows a region of Europa's crust made up of blocks which are thought to have broken apart and "rafted" into new positions.|alt=refer to caption]]
Images of Europa also showed few impact craters. It seemed unlikely that it had escaped the meteor and comet impacts that scarred Ganymede and Callisto, so this indicated Europa has an active geology that renews the surface and obliterates craters.{{sfn|Greenberg|2005|pp=12–14}}<ref name="Europa - Jupiter's Icy Moon" /> Astronomer [[Clark Chapman]] argued that, assuming a {{convert|20|km|adj=on|sp=us}} crater occurs in Europa once every million years, and given only about twenty have been spotted on Europa, the implication is that the surface must only be about 10 million years old.{{sfn|Meltzer|2007|pp=260–216}} With more data on hand, in 2003 a team led by Kevin Zahle at NASA's [[Ames Research Center]] arrived at a figure of 30 to 70 million years.{{sfn|Zahnle|Schenk|Levison|Dones|2003|p=277}} [[Tidal flexing]] of up to {{convert|100|m|sp=us}} per day was the most likely culprit.<ref name="Off kilter">{{cite press release |title=Long-stressed Europa Likely Off-kilter at One Time |date=September 18, 2013 |publisher=NASA/Jet Propulsion Laboratory |first1=Jia-Rui |last1=Cook |first2=Elizabeth |last2=Zubritsky |first3=Nancy |last3=Neal-Jones |url=https://www.jpl.nasa.gov/news/long-stressed-europa-likely-off-kilter-at-one-time/ |access-date=December 4, 2020 |archive-date=February 19, 2021 |archive-url=https://web.archive.org/web/20210219074046/https://www.jpl.nasa.gov/news/long-stressed-europa-likely-off-kilter-at-one-time |url-status=live }}</ref> But not all scientists were convinced; Michael Carr, a planetologist from the [[US Geological Survey]], argued that, on the contrary, Europa's surface age was closer to a billion years. He compared the craters on Ganymede with those on Earth's moon, and concluded that the satellites of Jupiter were not subject to the same amount of cratering.{{sfn|Meltzer|2007|pp=260–261}}<ref name="New Images Hint">{{cite press release |id=97-66 |date=April 9, 1997 |first1=Donald |last1=Savage |first2=Jane |last2=Platt |title=New Images Hint at Wet and Wild History For Europa |publisher=NASA/Jet Propulsion Laboratory |url=https://www.jpl.nasa.gov/news/new-images-hint-at-wet-and-wild-history-for-europa |access-date=April 2, 2024 |archive-date=April 2, 2024 |archive-url=https://web.archive.org/web/20240402205836/https://www.jpl.nasa.gov/news/new-images-hint-at-wet-and-wild-history-for-europa |url-status=live }}</ref>

Evidence of surface renewal hinted at the possibility of a viscous layer below the surface of warm ice or liquid water. NIMS observations by ''Galileo'' indicated that the surface of Europa appeared to contain magnesium- and sodium-based salts. A likely source was [[brine]] below the ice crust. Further evidence was provided by the magnetometer, which reported that the magnetic field was induced by Jupiter. This could be explained by the existence of a spherical shell of conductive material like salt water. Since the surface temperature on Europa was {{convert|-162|C}}, any water breaching the surface ice would instantly freeze over. Heat required to keep water in a liquid state could not come from the Sun, which at that distance had only 4 percent of the intensity it had on Earth, but ice is a good insulator, and the heat could be provided by the tidal flexing.<ref name="New Images Hint" />{{sfn|Meltzer|2007|pp=261–263}} ''Galileo'' also yielded evidence that the crust of Europa had slipped over time, moving south on the hemisphere facing Jupiter, and north on the far side.<ref name="Off kilter" />{{sfn|Greenberg|2005|pp=173–178}}{{sfn|Sarid et al.|2002|p=24}}

[[File:Plate Tectonics on Europa.jpg|thumb|right|Plate tectonics on Europa|alt=Illustration of model of Europa with a liquid ocaen surrounded by warmer ice and then an outer layer of a cold ice shell, with outbreaks of cryolarva.]]
There was acrimonious debate among scientists over the thickness of the ice crust, and those who presented results indicating that it might be thinner than the {{convert|20|to|30|km|sp=us}} proposed by the accredited scientists on the ''Galileo'' Imaging Team faced intimidation, scorn, and reduced career opportunities.{{sfn|Greenberg|2005|pp=313–321}} The ''Galileo'' Imaging Team was led by [[Michael J. Belton]] from the [[Kitt Peak National Observatory]]. Scientists who planned imaging sequences had the exclusive right to the initial interpretation of the ''Galileo'' data, most which was performed by their research students.{{sfn|Greenberg|2005|pp=31–32}} The scientific community did not want a repetition of the 1979 Morabito incident, when [[Linda A. Morabito]], an engineer at JPL working on ''Voyager 1'', discovered the first active extraterrestrial volcano on Io.{{sfn|Chaisson|1994|p=102}} The Imaging Team controlled the manner in which discoveries were presented to the scientific community and the public through press conferences, conference papers and publications.{{sfn|Greenberg|2005|pp=31–32}}

Observations by the Hubble Space Telescope in 1995 reported that Europa had a thin oxygen atmosphere. This was confirmed by ''Galileo'' in six experiments on orbits E4 and E6 during occultations when Europa was between ''Galileo'' and the Earth. This allowed Canberra and Goldstone to investigate the [[ionosphere]] of Europa by measuring the degree to which the radio beam was diffracted by charged particles. This indicated the presence of water ions, which were most likely water molecules that had been dislodged from the surface ice and then ionized by the Sun or the Jovian magnetosphere. The presence of an ionosphere was sufficient to deduce the existence of a thin atmosphere on Europa.<ref>{{cite press release |title=Galileo finds Europa has an Atmosphere |publisher=NASA/Jet Propulsion Laboratory |first=Jane |last=Platt |date=July 18, 1997 |url=https://www.jpl.nasa.gov/news/galileo-spacecraft-finds-europa-has-atmosphere/ |access-date=January 19, 2021 |archive-date=November 30, 2021 |archive-url=https://web.archive.org/web/20211130052248/https://www.jpl.nasa.gov/news/galileo-spacecraft-finds-europa-has-atmosphere |url-status=live }}</ref>

On December 11, 2013, NASA reported, based on results from the ''Galileo'' mission, the detection of "[[Clay mineral|clay-like minerals]]" (specifically, [[phyllosilicates]]), often associated with [[organic materials]], on the icy crust of [[Europa (moon)|Europa]]. The presence of the minerals may have been the result of a collision with an [[asteroid]] or [[comet]].<ref name="NASA-20131211">{{cite web |last=Cook |first=Jia-Rui c. |title=Clay-Like Minerals Found on Icy Crust of Europa |url=https://www.jpl.nasa.gov/news/clay-like-minerals-found-on-icy-crust-of-europa/ |date=December 11, 2013 |publisher=NASA/Jet Propulsion Laboratory |access-date=January 19, 2021 |archive-date=May 28, 2019 |archive-url=https://web.archive.org/web/20190528024022/https://www.jpl.nasa.gov/news/news.php?release=2013-362 |url-status=live }}</ref>

{{Clear}}

===Ganymede===
[[File:Ganymede - June 26 1996 (26781123830).jpg|thumb|right|Ganymede, photographed on June 26, 1996|alt=Ganymede looks like the Moon, with craters and darker and lighter grey regions]]
The largest of the Galilean moons with a radius of {{convert|2620|km|sp=us}}, Ganymede is larger than Earth's moon, the [[dwarf planet]] [[Pluto]] or the planet [[Mercury (planet)|Mercury]].{{sfn|Meltzer|2007|pp=267–268}} It is the largest of the moons in the Solar system that are characterized by large amounts of water ice, which also includes Saturn's moon [[Titan (moon)|Titan]], and Neptune's moon [[Triton (moon)|Triton]]. Ganymede has three times as much water for its mass as Earth has.{{Sfn|Stevenson|1996|pp=511–512}}

When ''Galileo'' entered Jovian orbit, it did so at an [[orbital inclination]] to the Jovian equator, and therefore in the orbital plane of the four Galilean moons. To transfer orbit while conserving propellant, two slingshot maneuvers were performed. On G1, the gravity of Ganymede was used to slow the spacecraft's orbital period from 210 to 72 days to allow for more encounters and to take ''Galileo'' out of the more intense regions of radiation. On G2, the gravity assist was employed to put it into a coplanar orbit to permit subsequent encounters with Io, Europa and Callisto.{{sfn|Meltzer|2007|pp=267–268}}

Although the primary purpose of G1 and G2 was navigational, the opportunity to make some observations was not missed. The plasma-wave experiment and the magnetometer detected a magnetic field with a strength of about {{convert|750|nT|nT|lk=on|disp=out|abbr=off}}, more than strong enough to create a separate magnetosphere within that of Jupiter.{{efn|Earth's magnetic field varies from 22,000 to 67,000 nanoteslas.<ref>{{cite web |title=An Overview of the Earth's Magnetic Field |publisher=British Geological Survey |url=http://www.geomag.bgs.ac.uk/education/earthmag.html#_Toc2075549 |access-date=16 April 2024 |archive-date=September 7, 2023 |archive-url=https://web.archive.org/web/20230907044147/http://www.geomag.bgs.ac.uk/education/earthmag.html#_Toc2075549 |url-status=live }}</ref> }} This was the first time that a magnetic field had ever been detected on a moon contained within the magnetosphere of its host planet.{{sfn|Kivelson|Khurana|Russell|Walker|1996|pp=537–541}}<ref>{{cite press release |title=Galileo Makes Discoveries at Ganymede |publisher=NASA/Jet Propulsion Laboratory |date=October 7, 1996 |url=https://www2.jpl.nasa.gov/releases/96/gany1res.html |access-date=December 5, 2020 |archive-url=https://web.archive.org/web/20221202092949/https://www2.jpl.nasa.gov/releases/96/gany1res.html |archive-date=2 December 2022 }}</ref><ref>{{cite press release|title= New Discoveries From Galileo – Big Icy Moon of Jupiter Found to Have a 'Voice' After All; Europa Flyby Next for Galileo |date=December 12, 1996 |first1=Douglas |last1=Isbell |first2=Mary Beth |last2=Murrill |id=96-255|publisher=NASA/Jet Propulsion Laboratory |url=http://www2.jpl.nasa.gov/galileo/status961212.html |archive-url=https://web.archive.org/web/20100602211623/http://www2.jpl.nasa.gov/galileo/status961212.html |archive-date=2010-06-02 |url-status=dead |df=mdy-all}}</ref> This discovery led naturally to questions about its origin. The evidence pointed to an iron or iron sulfide core and [[mantle (geology)|mantle]] {{convert|400|to|1,300|km|sp=us}} below the surface, encased in ice. Margaret Kivelson, the scientist in charge of the magnetometer experiment, contended that the induced magnetic field required an iron core, and speculated that an electrically conductive layer was required, possibly a brine ocean {{convert|200|km|sp=us}} below the surface.{{sfn|Meltzer|2007|pp=270–272}}<ref name="Hidden ocean">{{cite press release |title=Solar System's Moon Likely Has a Hidden Ocean |publisher=NASA |first=Guy |last=Webster |date=December 16, 2000 |url=https://solarsystem.nasa.gov/news/169/solar-systems-moon-likely-has-a-hidden-ocean/ |access-date=December 5, 2020 |archive-date=October 18, 2020 |archive-url=https://web.archive.org/web/20201018122955/https://solarsystem.nasa.gov/news/169/solar-systems-moon-likely-has-a-hidden-ocean/ |url-status=live }}</ref>

[[File:Ganymede diagram.svg|thumb|left|The internal structure of Ganymede]]
''Galileo'' returned to Ganymede on orbits G7 and G9 in April and May 1997, and on G28 and G29 in May and December 2000 on the GMM.{{sfn|Meltzer|2007|pp=268–270}} Images of the surface revealed two types of terrain: highly cratered dark regions and grooved terrain [[Sulcus (geology)|sulcus]]. Images of the Arbela Sulcus taken on G28 made Ganymede look more like Europa, but tidal flexing could not provide sufficient heat to keep water in liquid form on Ganymede, although it may have made a contribution. One possibility was radioactivity, which might provide sufficient heat for liquid water to exist {{convert|50|to|200|km|sp=us}} below the surface.<ref name="Hidden ocean" />{{sfn|Meltzer|2007|pp=271–273}} Another possibility was volcanism. Slushy water or ice reaching the surface would quickly freeze over, creating areas of a relatively smooth surface.<ref>{{cite magazine |last=Cowen |first=Ron |title=Images Suggest Icy Eruptions on Ganymede |magazine=[[Science News]] |issn=0036-8423 |date=March 3, 2001 |volume=159 |issue=9 |page=133 |doi=10.2307/3981750 |jstor=3981750 }}</ref>
{{Clear}}

===Callisto===
[[File:Callisto, moon of Jupiter, NASA.jpg|thumb|right|Callisto, photographed by ''Galileo'']]
Callisto is the outermost of the Galilean moons, and the most pockmarked, indeed the most of any body in the Solar system. So many craters must have taken billions of years to accumulate, which gave scientists the idea that its surface was as much as four billion years old, and provided a record of meteor activity in the Solar system. ''Galileo'' visited Callisto on orbits C3, C9 and C100 during the prime mission, and then on C20, C21, C22 and C23 during the GEM. When the cameras observed Callisto close up, there was a puzzling absence of small craters. The surface features appeared to have been eroded, indicating that they had been subject to active geological processes.{{sfn|Meltzer|2007|pp=273–277}}<ref name="The Galileo Mission to Jupiter and Its Moons">{{cite magazine |last=Johnson |first=Torrence V. |title=The Galileo Mission to Jupiter and Its Moons |magazine=Scientific American |issn=0036-8733 |volume=282 |issue=2 |date=February 2000 |pages=40–49 |doi=10.1038/scientificamerican0200-40 |jstor=26058599 |pmid=10710785 |bibcode=2000SciAm.282b..40J }}</ref>

''Galileo''{{'s}} flyby of Callisto on C3 marked the first time that the Deep Space Network operated a link between its antennae in Canberra and Goldstone that allowed them to [[aperture synthesis|operate as a gigantic array]], thereby enabling a higher bit rate. With the assistance of the antenna at Parkes, this raised the effective bandwidth to as much as 1,000 bits per second.<ref>{{cite press release |title=Galileo makes close pass by Callisto |date=November 4, 1996 |publisher=NASA/Jet Propulsion Laboratory |url=https://www.jpl.nasa.gov/news/galileo-makes-close-pass-by-callisto/ |access-date=January 19, 2021 |archive-date=November 30, 2021 |archive-url=https://web.archive.org/web/20211130052441/https://www.jpl.nasa.gov/news/galileo-makes-close-pass-by-callisto |url-status=live }}</ref>

Data accumulated on C3 indicated that Callisto had a homogeneous composition, with heavy and light elements intermixed. This was estimated to be composed of 60 percent [[silicate]], iron and iron sulfide rock and 40 percent water ice.{{sfn|Harland|2000|p=172}}<ref>{{cite press release |title=Galileo Returns New Insights into Callisto and Europa |date=May 23, 1997 |publisher=NASA/Jet Propulsion Laboratory |url=https://www.jpl.nasa.gov/news/galileo-returns-new-insights-into-callisto-and-europa/ |access-date=December 6, 2020 |archive-date=January 25, 2021 |archive-url=https://web.archive.org/web/20210125212928/https://www.jpl.nasa.gov/news/galileo-returns-new-insights-into-callisto-and-europa/ |url-status=live }}</ref> This was overturned by further radio Doppler observations on C9 and C10, which indicated that rock had settled towards the core, and therefore that Callisto indeed has a differentiated internal structure, although not as much so as the other Galilean moons.{{sfn|Meltzer|2007|pp=273–277}}<ref>{{cite press release |first=Jane |last=Pratt |title=Galileo Mission Finds Strange Interior of Jovian Moon |publisher=NASA |url=https://solarsystem.nasa.gov/news/141/galileo-mission-finds-strange-interior-of-jovian-moon/ |access-date=December 6, 2020 |archive-date=October 17, 2020 |archive-url=https://web.archive.org/web/20201017184949/https://solarsystem.nasa.gov/news/141/galileo-mission-finds-strange-interior-of-jovian-moon/ |url-status=live }}</ref>

[[File:Callisto diagram.svg|thumb|left|The internal structure of Callisto|alt=Cut away diagram of Ganymede, with a solid iron core successively surrounded by liquid iron and iron sulfide, rocky mantle, tetragonal ice, salt water and hexagonal ice.]]
Observations made with ''Galileo''{{'s}} magnetometer indicated that Callisto had no magnetic field of its own, and therefore lacked an iron core like Ganymede's, but that it did have an induced field from Jupiter's magnetosphere. Because ice is too poor a conductor to generate this effect, it pointed to the possibility that Callisto, like Europa and Ganymede, might have a subsurface ocean of brine.{{sfn|Meltzer|2007|pp=273–277}}<ref>{{cite press release |first=Jane |last=Pratt |title=Jupiter's Moon Callisto May Hide Salty Ocean |publisher=NASA |url=https://solarsystem.nasa.gov/news/145/jupiters-moon-callisto-may-hide-salty-ocean/ |access-date=December 6, 2020 |archive-date=December 6, 2020 |archive-url=https://web.archive.org/web/20201206005510/https://solarsystem.nasa.gov/news/145/jupiters-moon-callisto-may-hide-salty-ocean/ |url-status=live }}</ref> ''Galileo'' made its closest encounter with Callisto on C30, when it made a {{convert|138|km|adj=on|sp=us}} pass over the surface, during which it photographed the [[Asgard (crater)|Asgard]], [[Valhalla (crater)|Valhalla]] and Bran craters.{{sfn|Meltzer|2007|pp=273–277}} This was used for slingshot maneuvers to set up the final encounters with Io on I31 and I32.<ref>{{cite press release |first=Guy |last=Webster |title=Galileo Succeeds in its Closest Flyby of a Jovian Moon |publisher=NASA |url=https://solarsystem.nasa.gov/news/196/galileo-succeeds-in-its-closest-flyby-of-a-jovian-moon/ |access-date=December 6, 2020 |archive-date=December 26, 2020 |archive-url=https://web.archive.org/web/20201226145659/https://solarsystem.nasa.gov/news/196/galileo-succeeds-in-its-closest-flyby-of-a-jovian-moon/ |url-status=live }}</ref>{{Clear}}

===Amalthea===
[[File:Galileo Amalthea artwork.jpg|thumb|right|Artist's concept of Galileo passing near Jupiter's small inner moon Amalthea|alt=Amalthea looks like a large rock.]]
Although ''Galileo''{{'s}} main mission was to explore the Galilean moons, it also captured images of four of the inner moons, [[Thebe (moon)|Thebe]], [[Adrastea (moon)|Adrastea]], [[Amalthea (moon)|Amalthea]], and [[Metis (moon)|Metis]]. Such images were only possible from a spacecraft; to Earth-based telescopes they were [[Diffraction-limited system|merely specks of light]].<ref name="The Galileo Mission to Jupiter and Its Moons" /> Two years of Jupiter's intense radiation took its toll on the spacecraft's systems, and its fuel supply was running low in the early 2000s. ''Galileo''{{'s}} cameras were deactivated on January 17, 2002, after they had sustained irreparable radiation damage.<ref>{{cite web |title= 30 Years Ago: Galileo off to Orbit Jupiter |date= October 17, 2019 |publisher= NASA |url= https://www.nasa.gov/feature/30-years-ago-galileo-off-to-orbit-jupiter |access-date= December 6, 2020 |archive-date= August 31, 2020 |archive-url= https://web.archive.org/web/20200831053435/https://www.nasa.gov/feature/30-years-ago-galileo-off-to-orbit-jupiter/ |url-status= live }}</ref>

NASA engineers were able to recover the damaged tape-recorder electronics, and ''Galileo'' continued to return scientific data until it was deorbited in 2003, performing one last scientific experiment: a measurement of Amalthea's mass as the spacecraft swung by it. This was tricky to arrange; to be useful, ''Galileo'' had to fly within {{convert|300|km|sp=us}} of Amalthea, but not so close as to crash into it. This was complicated by its irregular {{convert|146|by|262|km|sp=us|adj=on}} potato-like shape. It was tidally locked, pointing its long axis towards Jupiter. A successful flyby meant knowing which direction the asteroid was pointed in relation to ''Galileo'' at all times.<ref name="The Long Goodbye">{{cite magazine |last=Carroll |first=M. |year=2003 |title=The Long Goodbye |magazine=[[Astronomy (magazine)|Astronomy]] |issn=0091-6358 |volume=31 |issue=10 |pages=36–41 |url-access=subscription |url=https://www.proquest.com/docview/215919054 |access-date=23 May 2024|id={{ProQuest|215919054}} }}</ref>

''Galileo'' flew by Amalthea on November 5, 2002, during its 34th orbit<!-- 21 September 2002 11:58:55.818 UTC to 28 January 2003 00:58:55.815 UTC -->, allowing a measurement of the moon's mass as it passed within {{convert|160|km|mi|abbr=on}} of its surface<!-- S030916A.LBL uses "Altitude"; presumably the uncertainty comes from Amalthea's irregular shape -->.{{sfn|Meltzer|2007|p=280}} The results startled the scientific team; they revealed that Amalthea had a mass of {{convert|2.08e18|kg}}, and with a volume of {{convert|2.43e6|km3|sp=us}}, it therefore had a density of 857 ± 99 kilograms per cubic meter, less than that of water.<ref name="The Long Goodbye" />{{sfn|Anderson et al.|2005|pp=1291–1293}}

A final discovery occurred during the last two orbits of the mission. When the spacecraft passed the orbit of Amalthea, the star scanner detected unexpected flashes of light that were reflections from seven to nine moonlets. None of the individual moonlets was reliably sighted twice, so no orbits were determined. It is believed that they were most likely debris ejected from Amalthea that formed a tenuous, and perhaps temporary, ring around Jupiter.{{sfn|Fieseler et al.|2004|pp=399–400}}

{{multiple image |align=center |direction=horizontal |total_width=900
 |image1=Jupiter's Main Ring and Halo (PIA01622).jpg
 |width1=1019 |height1=577 |caption1=Jupiter's rings. Enhanced top image shows the halo of ring particles suspended by Jupiter's powerful electromagnetic field. |alt1=refer to caption
 |image2=Jupiter's moon Amalthea photographed by Galileo.jpg
 |width2=798 |height2=573 |caption2=Inner moon Amalthea |alt2=refer to caption
 |image3=Thebe.jpg
 |width3=229 |height3=229 |caption3=Inner moon Thebe |alt3=refer to caption
}}

===Star scanner===
''Galileo''{{'s}} star scanner was a small optical telescope that provided an absolute attitude reference, but it made several scientific discoveries serendipitously. In the prime mission, it was found that the star scanner was able to detect high-energy particles as a noise signal. This data was eventually calibrated to show the particles were predominantly >{{convert|2|MeV|abbr=on|lk=on}} electrons that were trapped in the Jovian magnetic belts, and released to the Planetary Data System.<ref>{{cite web |url=http://www.mindspring.com/~feez/ |title=Science with The Galileo Star Scanner |archive-url=https://web.archive.org/web/20080719195042/http://www.mindspring.com/~feez/ |archive-date=2008-07-19 |date=July 19, 2008 |website=Mindspring.com |access-date=December 8, 2012}}</ref>

A second discovery occurred in 2000. The star scanner was observing a set of stars that included the second [[Magnitude (astronomy)|magnitude]] star [[Delta Velorum]]. At one point, this star dimmed for 8 hours below the star scanner's detection threshold. Subsequent analysis of ''Galileo'' data and work by amateur and professional astronomers showed that Delta Velorum is the brightest known [[eclipsing binary]], brighter at maximum than [[Algol]]. It has a primary period of 45 days and the dimming is just visible with the naked eye.<ref>{{cite web |title=Galileo Mystery Solved: The Star, Not The Instrument, Was On The Blink |publisher=ScienceDaily |url=https://www.sciencedaily.com/releases/2001/03/010326072946.htm |access-date=April 7, 2024 |archive-date=April 7, 2024 |archive-url=https://web.archive.org/web/20240407031918/https://www.sciencedaily.com/releases/2001/03/010326072946.htm |url-status=live }}</ref>

===Radiation-related anomalies===
[[File:Jupiter's Magnetosphere animation.png|thumb|left|Jupiter's inner magnetosphere and radiation belts|alt=lines of magnetism come from the poles and loop around.]]
Jupiter's uniquely harsh radiation environment caused over 20 anomalies over the course of ''Galileo''{{'s}} mission, in addition to the incidents expanded upon below. Despite having exceeded its radiation design limit by at least a factor of three, the spacecraft survived all these anomalies. Work-arounds were found eventually for all of these problems, and ''Galileo'' was never rendered entirely non-functional by Jupiter's radiation. The radiation limits for ''Galileo''{{'s}} computers were based on data returned from ''[[Pioneer 10]]'' and ''[[Pioneer 11]]'', since much of the design work was underway before the two ''Voyagers'' arrived at Jupiter in 1979.{{sfn|Tomayko|1988|p=200}}

A typical effect of the radiation was that several of the science instruments suffered increased [[signal-to-noise ratio|noise]] while within about {{convert|700000|km|mi|abbr=on}} of Jupiter. The SSI camera began producing totally white images when the spacecraft was hit by the exceptional [[Bastille Day event|Bastille Day coronal mass ejection]] in 2000, and did so again on subsequent close approaches to Jupiter.{{sfn|Fieseler|Ardalan|Frederickson|2002|pp=2748–2751}} The quartz crystal used as the frequency reference for the radio suffered permanent frequency shifts with each Jupiter approach.{{sfn|Fieseler|Ardalan|Frederickson|2002|pp=2743–2744}} A spin detector failed, and the spacecraft gyro output was biased by the radiation environment.{{sfn|Fieseler|Ardalan|Frederickson|2002|pp=2744–2746}}

The most severe effects of the radiation were current leakages somewhere in the spacecraft's power bus, most likely across [[Brush (electric)|brushes]] at a [[Bearing (mechanical)|spin bearing]] connecting rotor and stator sections of the orbiter. These current leakages triggered a reset of the onboard computer and caused it to go into safe mode. The resets occurred when the spacecraft was either close to Jupiter or in the region of space magnetically downstream of Jupiter. A change to the software was made in April 1999 that allowed the onboard computer to detect these resets and autonomously recover, so as to avoid safe mode.<ref>{{cite web |url=http://starbase.jpl.nasa.gov/go-a-nims-3-tube-v1.0/go_1117/catalog/insthost.cat~ |title=Instrument Host Overview |publisher=NASA/Jet Propulsion Laboratory |date=1999 |access-date=November 29, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20160315192151/http://starbase.jpl.nasa.gov/go-a-nims-3-tube-v1.0/go_1117/catalog/insthost.cat~ |archive-date=March 15, 2016}}</ref>

===Tape recorder problems===
Routine maintenance of the tape recorder involved winding the tape halfway down its length and back again to prevent it sticking.{{sfn|Meltzer|2007|p=226}} In November 2002, after the completion of the mission's only encounter with Jupiter's moon Amalthea, problems with playback of the tape recorder again plagued ''Galileo''. About 10 minutes after the closest approach of the Amalthea flyby, ''Galileo'' stopped collecting data, shut down all of its instruments, and went into safe mode, apparently as a result of exposure to Jupiter's intense radiation environment. Though most of the Amalthea data was already written to tape, it was found that the recorder refused to respond to commands telling it to play back data.<ref>{{cite press release |first=Guy |last=Webster |id=2002-213 |url=https://www.jpl.nasa.gov/news/galileo-millennium-mission-status-3 |title=Galileo Millennium Mission Status |publisher=NASA/Jet Propulsion Laboratory |date=November 25, 2002 |access-date=April 27, 2024 |archive-date=April 28, 2024 |archive-url=https://web.archive.org/web/20240428014837/https://www.jpl.nasa.gov/news/galileo-millennium-mission-status-3 |url-status=live }}</ref>

After weeks of troubleshooting of an identical flight spare of the recorder on the ground, it was determined that the cause of the malfunction was a reduction of light output in three infrared Optek OP133 [[light-emitting diode]]s (LEDs) located in the drive electronics of the recorder's motor [[rotary encoder|encoder]] wheel. The [[gallium arsenide]] LEDs had been particularly sensitive to [[proton]]-irradiation-induced [[crystal|atomic lattice]] displacement defects, which greatly decreased their effective light output and caused the drive motor's electronics to falsely believe the motor encoder wheel was incorrectly positioned.{{sfn|Swift|Levanas|Ratliff|Johnston|2003|pp=1991–1993}}

''Galileo''{{'s}} flight team then began a series of "[[Annealing (metallurgy)|annealing]]" sessions, where current was passed through the LEDs for hours at a time to heat them to a point where some of the crystalline lattice defects would be shifted back into place, thus increasing the LED's light output. After about 100 hours of annealing and playback cycles, the recorder was able to operate for up to an hour at a time. After many subsequent playback and cooling cycles, the complete transmission back to Earth of all recorded Amalthea flyby data was successful.{{sfn|Swift|Levanas|Ratliff|Johnston|2003|pp=1993–1997}}

===End of mission and deorbit===
[[File:Galileo End.jpg|thumb|right|Illustration of ''Galileo'' entering Jupiter's atmosphere|alt=refer to caption]]
When the exploration of Mars was being considered in the early 1960s, Carl Sagan and [[Sidney Coleman]] produced a paper concerning contamination of the red planet. In order that scientists could determine whether native life forms existed before the planet became contaminated by micro-organisms from Earth, they proposed that space missions should aim at a 99.9 percent chance that contamination should not occur. This figure was adopted by the [[Committee on Space Research]] (COSPAR) of the [[International Council of Scientific Unions]] in 1964, and was subsequently applied to all planetary probes.<ref name="Macroscope">{{cite magazine |last1=Greenberg |first1=Richard |last2=Tufts |first2=B. Randall |title=Macroscope: Infecting Other World |magazine=[[American Scientist]] |issn=0003-0996 |date=July–August 2001 |volume=89 |issue=4 |pages=296–299 |doi=10.1511/2001.28.3356 |jstor=27857494 }}</ref>

The danger was highlighted in 1969 when the [[Apollo 12]] astronauts returned components of the [[Surveyor 3]] spacecraft that had landed on the Moon three years before, and it was found that microbes were still viable even after three years in that harsh climate. An alternative was the [[Prime Directive]], a philosophy of non-interference with alien life forms enunciated by the [[Star Trek: The Original Series|original ''Star Trek'' television series]] that prioritized the interests of the life forms over those of scientists. Given the (admittedly slim) prospect of life on Europa, scientists Richard Greenberg and Randall Tufts proposed that a new standard be set of no greater chance of contamination than that which might occur naturally by meteorites.<ref name="Macroscope" />

''Galileo'' had not been [[sterilization (microbiology)|sterilized]] prior to launch and could conceivably have carried [[bacteria]] from Earth. Therefore, a plan was formulated to send the probe directly into Jupiter, in an intentional crash to eliminate the possibility of an impact with Jupiter's moons, particularly Europa, and prevent a [[forward contamination]]. On April 14, 2003, the ''Galileo'' orbiter reached its greatest orbital distance from Jupiter for the entire mission since orbital insertion, {{convert|26|e6km|e6mi|abbr=unit}}, before plunging back towards the gas giant for its final impact.<ref>{{cite web |url=http://solarsystem.nasa.gov/galileo/ |title=Galileo Legacy Site |publisher=NASA |year=2010 |access-date=April 24, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120819145316/http://solarsystem.nasa.gov/galileo/ |archive-date=August 19, 2012}}</ref> At the completion of J35, its final orbit<!-- 13 September 2003 13:58:55.818 UTC to 30 September 2003 11:58:55.818 UTC -- note that the last 9 days did not occur --> around the Jovian system, ''Galileo'' struck Jupiter in darkness just south of the equator on September 21, 2003, at 18:57&nbsp;UTC. Its impact speed was approximately {{convert|30|mi/s|km/s|abbr=on|order=flip}}.<ref name="spref20030919"/><ref>{{cite news |first=Peter |last=Bond |title=Galileo spacecraft crashes into Jupiter |url=http://spaceflightnow.com/galileo/030921galileogone.html |publisher=Spaceflight Now |date=September 21, 2003 |access-date=December 5, 2020 |archive-date=December 5, 2020 |archive-url=https://web.archive.org/web/20201205122235/https://spaceflightnow.com/galileo/030921galileogone.html |url-status=live }}</ref>

===Major findings===
# The composition of Jupiter differs from that of the Sun, indicating that Jupiter has evolved since the formation of the Solar System.<ref name="endkit" /><ref name="Key Science Discoveries">{{cite web |publisher=NASA |title=Galileo – 10 Key Science Discoveries |url=https://solarsystem.nasa.gov/missions/galileo/overview/ |access-date=November 29, 2020 |archive-date=July 19, 2009 |archive-url=https://web.archive.org/web/20090719111109/http://www2.jpl.nasa.gov/galileo/messenger/oldmess/2Probe.html |url-status=live }}</ref>
# ''Galileo'' made the first observation of ammonia clouds in another planet's atmosphere. The atmosphere creates ammonia ice particles from material coming up from lower depths.<ref name="endkit" />
# Io was confirmed to have extensive volcanic activity that is 100 times greater than that found on Earth. The heat and frequency of eruptions are reminiscent of early Earth.<ref name="endkit" /><ref name="Key Science Discoveries" />
# Complex plasma interactions in Io's atmosphere create immense electrical currents which couple to Jupiter's atmosphere.<ref name="endkit" /><ref name="Key Science Discoveries" />
# Several lines of evidence from ''Galileo'' support the theory that liquid oceans exist under Europa's icy surface.<ref name="endkit" /><ref name="Key Science Discoveries" />
# Ganymede possesses its own, substantial magnetic field – the first satellite known to have one.<ref name="endkit" /><ref name="Key Science Discoveries" />
# ''Galileo'' magnetic data provided evidence that Europa, Ganymede and Callisto have a liquid salt water layer under the visible surface.<ref name="endkit" />
# Evidence exists that Europa, Ganymede, and Callisto all have a thin atmospheric layer known as a "surface-bound [[exosphere]]".<ref name="endkit" /><ref name="Key Science Discoveries" />
# Jupiter's [[ring system]] is formed by dust kicked up as interplanetary [[meteoroid]]s smash into the planet's [[Inner satellites of Jupiter|four small inner moons]]. The outermost ring is actually two rings, one embedded with the other. There is probably a separate ring along [[Amalthea (moon)|Amalthea]]'s orbit as well.<ref name="endkit" /><ref name="Key Science Discoveries" />
# The ''Galileo'' spacecraft identified the global structure and dynamics of a giant planet's [[magnetosphere]].<ref name="endkit" />

==Follow-on missions==
There was a spare ''Galileo'' spacecraft that was considered by the NASA–ESA Outer Planets Study Team in 1983 for a mission to Saturn, but it was passed over in favor of a newer design, which became ''[[Cassini–Huygens]]''.{{sfn|National Research Council|European Space Science Committee|1998|p=61}} While ''Galileo'' was operating, ''[[Ulysses (spacecraft)|Ulysses]]'' passed by Jupiter in 1992 on its mission to study the Sun's polar regions, and ''[[Cassini–Huygens]]'' coasted by the planet in 2000 and 2001 en route to Saturn.{{sfn|Meltzer|2007|p=38}} ''[[New Horizons]]'' passed close by Jupiter in 2007 for a gravity assist en route to Pluto, and it too collected data on the planet.<ref>{{cite web |title=New Horizons: The Path to Pluto and Beyond |publisher=Johns Hopkins University Applied Physics Laboratory |url=http://pluto.jhuapl.edu/Mission/The-Path-to-Pluto-and-Beyond.php |access-date=December 6, 2020 |archive-date=December 24, 2020 |archive-url=https://web.archive.org/web/20201224232442/http://pluto.jhuapl.edu/Mission/The-Path-to-Pluto-and-Beyond.php |url-status=live }}</ref>

===''Juno''===
The next mission to orbit Jupiter was NASA's ''[[Juno (spacecraft)|Juno]]'' spacecraft, which was launched on August 5, 2011, and entered Jovian orbit on July 4, 2016. Although intended for a two-year mission, it is still active in 2024 and expected to continue until September 2025.<ref>{{cite web |title=Missions {{pipe}} Juno |publisher=NASA/Jet Propulsion Laboratory |url=https://www.jpl.nasa.gov/missions/juno/ |access-date=December 6, 2020 |archive-date=March 1, 2018 |archive-url=https://web.archive.org/web/20180301015624/https://www.jpl.nasa.gov/missions/juno/ |url-status=live }}</ref><ref>{{cite web |url=http://www.nasa.gov/missions/highlights/schedule.html |title=NASA's Shuttle and Rocket Launch Schedule |publisher=NASA |access-date=February 17, 2011 |archive-url=https://web.archive.org/web/20110218005402/http://www.nasa.gov/missions/highlights/schedule.html |archive-date=February 18, 2011}}</ref><ref>{{cite web |title=Juno |date=June 15, 2023 |publisher=NASA |url=https://science.nasa.gov/mission/juno/ |access-date=April 3, 2024 |archive-date=April 3, 2024 |archive-url=https://web.archive.org/web/20240403092500/https://science.nasa.gov/mission/juno/ |url-status=live }}</ref> ''Juno'' provided the first views of Jupiter's north pole and new insights into Jupiter's aurorae, magnetic field, and atmosphere.<ref>{{Cite web|title=Overview {{!}} Juno|url=https://solarsystem.nasa.gov/missions/juno/overview/|url-status=live|access-date=May 19, 2021|website=[[NASA]]|archive-date=May 19, 2021|archive-url=https://web.archive.org/web/20210519143102/https://solarsystem.nasa.gov/missions/juno/overview/}}</ref> Information gathered about Jovian lightning prompted revision of earlier theories,{{sfn|Connerney|Gurnett|Hospodarsky|Kurth|2018|pp=87–90}} and analysis of the frequency of interplanetary dust impacts (primarily on the backs of the solar panels), as ''Juno'' passed between Earth and the asteroid belt, indicated that this dust comes from [[Mars]], rather than from comets or asteroids, as was previously thought.<ref>{{Cite web |last=Shekhtman |first=Lonnie |date=March 9, 2021 |title=Serendipitous Juno Detections Shatter Ideas About Origin of Zodiacal Light |url=https://www.jpl.nasa.gov/news/serendipitous-juno-detections-shatter-ideas-about-origin-of-zodiacal-light |url-status=live |archive-url=https://web.archive.org/web/20210318004153/https://www.jpl.nasa.gov/news/serendipitous-juno-detections-shatter-ideas-about-origin-of-zodiacal-light |archive-date=March 18, 2021 |access-date=March 19, 2021 |website=[[Jet Propulsion Laboratory]] |publisher=[[NASA]]}}</ref>

===Jupiter Icy Moons Explorer===
The European Space Agency is planning to return to the Jovian system with the [[Jupiter Icy Moons Explorer]] (JUICE).<ref>{{cite web |url=https://sci.esa.int/web/juice/-/59905-juice-s-primary-target-ganymede |title=JUICE's primary target: Ganymede |publisher=European Space Agency |date=September 1, 2019 |access-date=August 28, 2021 |archive-date=October 2, 2023 |archive-url=https://web.archive.org/web/20231002005055/https://sci.esa.int/web/juice/-/59905-juice-s-primary-target-ganymede |url-status=live }}</ref> This was launched from Europe's Spaceport in French Guiana on April 14, 2023, and is expected to reach Jupiter in July 2031.<ref>{{cite web |title=JUICE |date=December 5, 2017 |publisher=NASA |url=https://science.nasa.gov/mission/juice/ |access-date=April 3, 2024 |archive-date=April 3, 2024 |archive-url=https://web.archive.org/web/20240403191543/https://science.nasa.gov/mission/juice/ |url-status=live }}</ref><ref>{{cite web |title=Juice |publisher=ESA |url=https://www.esa.int/Science_Exploration/Space_Science/Juice |access-date=April 3, 2024 |archive-date=October 12, 2022 |archive-url=https://web.archive.org/web/20221012133556/https://www.esa.int/Science_Exploration/Space_Science/Juice |url-status=live }}</ref>

===''Europa Clipper''===
Even before ''Galileo'' concluded, NASA considered the [[Europa Orbiter]],<ref>{{cite conference |title=The Europa Orbiter Mission Design |conference=49th International Astronomical Congress. September 28 – October 2, 1998. Melbourne, Australia. |first1=Jan M. |last1=Ludwinski |first2=Mark D. |last2=Guman |first3=Jennie R. |last3=Johannesen |first4=Robert T. |last4=Mitchell |first5=Robert L. |last5=Staehle |date=1998 |hdl = 2014/20516|id=IAF 98-Q.2.02}}</ref> but it was canceled in 2002.<ref>{{cite news |url=http://www.space.com/news/nasa_budget_020204.html |title=NASA Kills Europa Orbiter; Revamps Planetary Exploration |publisher=[[Space.com]] |first=Brian |last=Berger |date=February 4, 2002 |archive-url=https://web.archive.org/web/20090524234825/http://www.space.com/news/nasa_budget_020204.html |archive-date=May 24, 2009}}</ref> A lower-cost version was then studied, which led to ''[[Europa Clipper]]'' being approved in 2015.<ref>{{cite press release |title=Missions {{pipe}} Europa Clipper |publisher=NASA/Jet Propulsion Laboratory |url=https://www.jpl.nasa.gov/missions/europa-clipper/ |access-date=December 5, 2020 |archive-date=March 23, 2021 |archive-url=https://web.archive.org/web/20210323162742/https://www.jpl.nasa.gov/missions/europa-clipper |url-status=live }}</ref> This mission launched from Kennedy Space Center on October 14, 2024 and is expected to reach Jupiter in April 2030.<ref>{{cite web |title=Mission Updates &#124; NASA's Europa Clipper |date=December 2017 |publisher=NASA |url=https://europa.nasa.gov/news/mission-updates/ |access-date=April 3, 2024 |archive-date=April 3, 2024 |archive-url=https://web.archive.org/web/20240403185901/https://europa.nasa.gov/news/mission-updates/ |url-status=live }}</ref>

===''Europa Lander''===
A [[Lander (spacecraft)|lander]], simply called ''[[Europa Lander (NASA)|Europa Lander]]'' was assessed by the Jet Propulsion Laboratory.<ref>{{cite web |url=https://www.jpl.nasa.gov/missions/web/absscicon/02-AbsSciCon-Mission-Overview-13Jun2019-no-BU.pdf |title=Europa Lander Mission Concept Overview |first1=Grace |last1=Tan-Wang |first2=Steve |last2=Sell |publisher=NASA/Jet Propulsion Laboratory |date=June 26, 2019 |access-date=December 5, 2020 |archive-date=January 31, 2021 |archive-url=https://web.archive.org/web/20210131175314/https://www.jpl.nasa.gov/missions/web/absscicon/02-AbsSciCon-Mission-Overview-13Jun2019-no-BU.pdf |url-status=dead }}</ref> {{As of|2024}}, this mission remains a concept, although some funds were released for instrument development and maturation.<ref>{{cite web |url=https://www.jpl.nasa.gov/missions/europa-lander/ |title=Europa Lander |publisher=NASA/Jet Propulsion Laboratory |access-date=April 4, 2024 |archive-date=March 18, 2021 |archive-url=https://web.archive.org/web/20210318001710/https://www.jpl.nasa.gov/missions/europa-lander/ |url-status=live }}</ref>

==Notes==
{{notelist}}

==Citations==
{{Reflist|30em}}

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* {{cite journal |last1=D'Amario |first1=Louis A. |last2=Bright |first2=Larry E. |last3=Wolf |first3=Aron A. |title=Galileo Trajectory Design |journal=[[Space Science Reviews]] |issn=0038-6308 |volume=60 |issue=1–4 |pages=23–78 |date=May 1992 |doi=10.1007/BF00216849 |bibcode=1992SSRv...60...23D|s2cid=122388506 }}
* {{cite book |last1=Dawson |first1=Virginia |first2=Mark |last2=Bowles |title=Taming Liquid Hydrogen: The Centaur Upper Stage Rocket |series=The NASA History Series |id=SP-4230 |url=https://history.nasa.gov/SP-4230.pdf |location=Washington, DC |year=2004 |publisher=NASA |access-date=October 1, 2020 |archive-date=September 29, 2006 |archive-url=https://web.archive.org/web/20060929004415/https://history.nasa.gov/SP-4230.pdf |url-status=live }}
* {{cite journal |last1=Fieseler |first1=P. D. |last2=Ardalan |first2=S. M. |last3=Frederickson |first3=A. R. |title=The Radiation Effects on Galileo Spacecraft Systems at Jupiter |journal=[[IEEE Transactions on Nuclear Science]] |issn=0018-9499 |volume=49 |issue=6 |pages=2739–2758 |date=December 2002 |doi=10.1109/TNS.2002.805386 |bibcode=2002ITNS...49.2739F }}
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* {{cite report |last=Finley |first=Harry R. |title=Space Exploration: Cost, Schedule, and Performance of NASA's Galileo Mission to Jupiter |publisher=General Accounting Office |location=Washington, DC |id=GAO/NSIAD-88-138FS |date=May 1988 |url=https://www.gao.gov/assets/nsiad-88-138fs.pdf |access-date=April 9, 2024 |archive-date=June 12, 2024 |archive-url=https://web.archive.org/web/20240612144942/https://www.gao.gov/assets/nsiad-88-138fs.pdf |url-status=live }}
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* {{cite book |last=Greenberg |first=Richard |title=Europa - The Ocean Moon: Search for an Alien Biosphere |publisher=Springer-Praxis |location=Berlin |year=2005 |isbn=3-540-22450-5 |oclc=488990934}}
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* {{cite book |last1=Janson |first1=Bette R. |first2=Eleanor H. |last2=Ritchie |title=Astronautics and Aeronautics, 1979-1984: A Chronology |publisher=NASA |id=SP-4025 |location=Washington, DC |date=1990 |url=https://history.nasa.gov/AAchronologies/1979-1984.pdf |access-date=October 11, 2020 |oclc=21925765 |archive-date=January 8, 2021 |archive-url=https://web.archive.org/web/20210108015022/https://history.nasa.gov/AAchronologies/1979-1984.pdf |url-status=live }}
* {{cite book |last= Jenkins |first= Dennis R. |title= Space Shuttle: Developing an Icon – 1972–2013 |volume=II: Technical Description |isbn=978-1-58007-249-6 |location=Forest Lake, Minnesota |publisher= Specialty Press |date= 2016}}
* {{cite journal |last1=Johnson |first1=Torrence V. |last2=Yeates |first2=Clayne M. |last3=Young |first3=Richard |last4=Dunne |first4=James |title=The Galileo Venus Encounter |journal=[[Science (journal)|Science]] |series=New Series |issn=0036-8075 |volume=253 |issue=5027 |date=September 27, 1991 |pages=1516–1518 |doi=10.1126/science.253.5027.1516 |jstor=2884985 |pmid=17784091 |bibcode=1991Sci...253.1516J |s2cid=37156782 }}
* {{cite conference |last=Johnson |first=Michael R. |title=The Galileo High Gain Antenna Deployment Anomaly |date=May 1, 1994 |publisher=NASA |conference=The 28th Aerospace Mechanisms Symposium |location=Cleveland, Ohio |url=https://ntrs.nasa.gov/api/citations/19940028813/downloads/19940028813.pdf |pages=359–377 |access-date=November 15, 2011 |archive-date=November 21, 2020 |archive-url=https://web.archive.org/web/20201121085741/https://ntrs.nasa.gov/api/citations/19940028813/downloads/19940028813.pdf |url-status=live }}
* {{cite journal |last1=Kivelson |first1=M. G. |author-link=Margaret G. Kivelson |last2=Khurana |first2=K. K. |last3=Russell |first3=C. T. |last4=Walker |first4=R. J. |title=Discovery of Ganymede's magnetic field by the Galileo spacecraft |journal=[[Nature (journal)|Nature]] |issn=0028-0836 |volume=384 |issue=6609 |date=December 12, 1996 |pages=537–541 |doi=10.1038/384537a0 |bibcode=1996Natur.384..537K |s2cid=4246607 }}
* {{cite conference |last=Marr |first=James C. |title=Performing the Galileo mission using the S-band low-gain antenna |conference=1994 IEEE Aerospace Applications Conference |date=5–12 February 1994 |location=Vail, Colorado |publisher=IEEE |isbn=0-7803-1831-5 |doi=10.1109/AERO.1994.291197 }}
* {{cite book |last=Meltzer |first=Michael |title=Mission to Jupiter: A History of the ''Galileo'' Project |year=2007 |id=SP-4231 |series=The NASA History Series |publisher=NASA |location=Washington, DC |oclc=124150579 |url=https://history.nasa.gov/sp4231.pdf |access-date=19 January 2021 |archive-url=https://web.archive.org/web/20240112074922/https://history.nasa.gov/sp4231.pdf |archive-date=January 12, 2024 |url-status=deviated }}
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* {{cite book |author1=National Research Council |author2=European Space Science Committee |chapter-url=https://www.nap.edu/read/5981/chapter/5 |chapter=Case Studies of U.S.-European Missions |title=U.S.-European Collaboration in Space Science |publisher=National Academies Press |location=Washington, DC |pages=42–100 |date=1998 |isbn=978-0-309-05984-8 |doi=10.17226/5981 |bibcode=1998nap..book.5981N |access-date=November 12, 2017 |archive-date=November 7, 2018 |archive-url=https://web.archive.org/web/20181107043640/https://www.nap.edu/read/5981/chapter/5 |url-status=live }}
* {{cite report |author=Office of Space Science and Applications |date=May 1, 1989 |location=Washington |publisher=NASA |title=Final Environmental Impact Statement for the Galileo Mission (Tier 2) |oclc=24780391 |url=https://ntrs.nasa.gov/api/citations/19900002786/downloads/19900002786.pdf |access-date=November 4, 2020 |archive-date=April 11, 2023 |archive-url=https://web.archive.org/web/20230411232109/https://ntrs.nasa.gov/api/citations/19900002786/downloads/19900002786.pdf |url-status=live }}
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* {{cite report |last=Rogers |first=William P. |author-link=William P. Rogers |title=Report to the President by the Presidential Commission on the Space Shuttle Challenger Accident |date=June 6, 1986 |location=Washington, DC |publisher=NASA |url=https://spaceflight.nasa.gov/outreach/SignificantIncidents/assets/rogers_commission_report.pdf |archive-url=https://web.archive.org/web/20161229201146/http://spaceflight.nasa.gov/outreach/SignificantIncidents/assets/rogers_commission_report.pdf |url-status=dead |archive-date=December 29, 2016 |access-date=October 18, 2020 }}
* {{cite journal |last1=Sagan |first1=C. |author-link=Carl Sagan |first2=W. R. |last2=Thompson |first3=R. |last3=Carlson |first4=D. |last4=Gurnett |first5=C. |last5=Hord |title=A Search for Life on Earth from the ''Galileo'' Spacecraft |journal=[[Nature (journal)|Nature]] |issn=0028-0836 |date=1993 |volume=365 |pages=715–721 |doi=10.1038/365715a0 |pmid=11536539 |issue=6448 |bibcode=1993Natur.365..715S |s2cid=4269717 |ref=CITEREFSagan et al.1993}}
* {{cite journal |last1=Sarid |first1=Alyssa Rose |last2=Greenberg |first2=Richard |last3=Hoppa |first3=Gregory V. |last4=Hurford |first4=Terry A. |last5=Geissler |first5=Paul |title = Polar Wander and Surface Convergence of Europa's Ice Shell: Evidence from a Survey of Strike-Slip Displacement |journal = [[Icarus (journal)|Icarus]] |issn=0019-1035 |volume = 158 |issue=1 |pages=24–41 |year=2002 |doi = 10.1006/icar.2002.6873 |bibcode=2002Icar..158...24S |ref=CITEREFSarid et al.2002 }}
* {{cite book |last1=Siddiqi |first1=Asif A. |author-link=Asif Azam Siddiqi |title=Beyond Earth: A Chronicle of Deep Space Exploration, 1958–2016 |lccn=2017059404 |isbn=978-1-62683-042-4 |publisher=NASA History Program Office |edition=second |year=2018 |id=SP-4041 |series=The NASA History Series |location=Washington, DC |url=https://www.nasa.gov/sites/default/files/atoms/files/beyond-earth-tagged.pdf |access-date=October 29, 2020 |ref=none |archive-date=April 24, 2019 |archive-url=https://web.archive.org/web/20190424211923/https://www.nasa.gov/sites/default/files/atoms/files/beyond-earth-tagged.pdf |url-status=live }}
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* {{cite journal |last=Young |first=Richard E. |title=The Galileo Probe Mission to Jupiter: Science Overview |journal=[[Journal of Geophysical Research]] |issn=0148-0227 |volume=103 |issue=El0 |pages=22,775–22,790 |date=September 25, 1998 |doi=10.1029/98JE01051 |bibcode=1998JGR...10322775Y }}
* {{cite journal |last1=Zahnle |first1=Kevin |last2=Schenk |first2=Paul |last3=Levison |first3=Harold |last4=Dones |first4=Luke |title=Cratering rates in the outer Solar System |journal=[[Icarus (journal)|Icarus]] |issn=0019-1035 |volume=163 |issue=2 |date=June 2003 |pages=263–289 |doi=10.1016/S0019-1035(03)00048-4 |bibcode=2003Icar..163..263Z }}
{{refend}}

==External links==
{{Commons category|Galileo mission}}
* [https://web.archive.org/web/20151114055422/http://solarsystem.nasa.gov/missions/Galileo ''Galileo'' mission site] by NASA's Solar System Exploration
* [http://solarsystem.nasa.gov/galileo/ ''Galileo'' legacy site] {{Webarchive|url=https://web.archive.org/web/20041103173530/http://solarsystem.nasa.gov/galileo/ |date=November 3, 2004 }} by NASA's Solar System Exploration
* [http://rpif.asu.edu/Galileo/ ''Galileo'' Satellite Image Mosaics] by Arizona State University
* [https://www.flickr.com/photos/kevinmgill/albums/72157651267182078 Galileo image album] by Kevin M. Gill

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{{DEFAULTSORT:Galileo (Spacecraft)}}
[[Category:Galileo Galilei]]
[[Category:Galileo program]]
[[Category:Missions to Jupiter]]
[[Category:Missions to main-belt asteroids]]
[[Category:Articles containing video clips]]