View source for GALEX

{{Short description|NASA UV space telescope of the Explorer program, operated 2003-2013}}
{{About|the ultraviolet space telescope|the neutrino detector|GALLEX|other uses|GALEX (disambiguation)}}
{{Use American English|date=June 2021}}
{{Use dmy dates|date=June 2021}}
{{Infobox spaceflight
| name              = Galaxy Evolution Explorer
| names_list        = Explorer-83<br/>SMEX-7
| image             = GALEX spacecraft model.png
| image_caption     = GALEX spacecraft
| image_size        = 300px

| mission_type      = [[Ultraviolet astronomy]]
| operator          = [[NASA]]{{\}}[[Jet Propulsion Laboratory|JPL]] (2003-2012)<br/>[[California Institute of Technology|Caltech]] (2012-2013)
| COSPAR_ID         = 2003-017A
| SATCAT            = 27783
| website           = https://www.galex.caltech.edu/
| mission_duration  = 29 months (planned) <ref name="Display"/><br/>{{time interval|28 April 2003|28 June 2013|show=ym|sep=,}} (achieved) <ref name="JPL"/><ref name="Decommission"/>

| spacecraft        = Explorer LXXXIII
| spacecraft_type   = Galaxy Evolution Explorer
| spacecraft_bus    = [[GeoEye|Orbview 4]]
| manufacturer      = [[Orbital Sciences Corporation]]
| launch_mass       = {{cvt|280|kg}}
| dimensions        = {{cvt|1|xx|2.5|m}}
| power             = 290 [[watt]]s

| launch_date       = 28 April 2003, 11:59:57 [[Coordinated Universal Time|UTC]]<ref name="Display"/>
| launch_rocket     = [[Pegasus (rocket)|Pegasus XL]] (F33)
| launch_site       = [[Cape Canaveral Space Force Station|Cape Canaveral]], ''[[Stargazer (aircraft)|Stargazer]]''
| launch_contractor = [[Orbital Sciences Corporation]]

| entered_service   = 28 May 2003 <ref name="JPL"/>
| deactivated       = 28 June 2013, 19:09 UTC <ref name="Decommission"/>
| last_contact      = 28 June 2013
| decay_date        = 2068 (planned)

| orbit_reference   = [[Geocentric orbit]]<ref name="Trajectory"/>
| orbit_regime      = [[Low Earth orbit]]
| orbit_periapsis   = {{cvt|691|km}}
| orbit_apoapsis    = {{cvt|697|km}}
| orbit_inclination = 29.00°
| orbit_period      = 98.60 minutes
| orbit_rev_number  = 85423
| apsis             = gee

| instruments       = Ultraviolet telescope

| telescope_type         = [[Ritchey–Chrétien telescope|Ritchey–Chrétien]]<ref name="Display"/>
| telescope_diameter     = {{cvt|50|cm}}
| telescope_focal_length = f/6.0
| telescope_wavelength   = 135–280 [[Nanometre|nm]] ([[Ultraviolet]])

| insignia          = 
| insignia_caption  = 
| insignia_size     = 200px

| programme         = '''Explorer program'''
| previous_mission  = [[CHIPS (satellite)|CHIPS]] (Explorer 82)
| next_mission      = [[Neil Gehrels Swift Observatory]] (Explorer 84)
}}

'''Galaxy Evolution Explorer''' ('''GALEX''' or '''Explorer 83''' or '''SMEX-7''') was a [[NASA]] orbiting space telescope designed to observe the [[universe]] in [[ultraviolet]] [[wavelength]]s to measure the history of [[star formation]] in the universe. In addition to paving the way for future ultraviolet missions, the space telescope allowed astronomers to uncover mysteries about the early universe and how it evolved, as well as better characterize phenomena like [[black hole]]s and [[dark matter]]. The mission was extended three times over a period of 10 years before it was decommissioned in June 2013. GALEX was launched on 28 April 2003 and decommissioned in June 2013.<ref name="Display">{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2003-017A|title=Display: GALEX (Explorer 83) 2003-017A |publisher=NASA|date=28 October 2021|access-date=3 December 2021}} {{PD-notice}}</ref>

== Spacecraft ==
The spacecraft was [[Spacecraft attitude control|three-axis stabilized]], with power coming from four fixed [[solar panel]]s. The [[satellite bus]] is from [[Orbital Sciences Corporation]] based on [[GeoEye|OrbView 4]]. The telescope was a {{cvt|50|cm}} [[Ritchey–Chrétien telescope|Modified Ritchey–Chrétien]] with a rotating [[grism]]. GALEX used the first ever UV light dichroic beam-splitter flown in space to direct [[photon]]s to the [[Ultraviolet|Near UV]] (175–280 nanometers) and Far UV (135–174 nanometers) [[microchannel plate detector]]s. Each of the two detectors has a {{cvt|65|mm}} diameter. The target orbit is {{cvt|670|km}} circular and [[Orbital inclination|inclined]] at 29.00° to the [[equator]].

== Launch ==
An air launched [[Northrop Grumman Pegasus|Pegasus]] [[launch vehicle]], launched on 28 April 2003 at 11:59:57 [[Coordinated Universal Time|UTC]], placed the craft into a nearly circular orbit at an [[altitude]] of {{cvt|697|km}} and an [[orbital inclination]] to the Earth's [[equator]] of 29.00°.<ref name="Trajectory">{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/displayTrajectory.action?id=2003-017A|title=Trajectory: GALEX (Explorer 83) 2003-017A|publisher=NASA|date=28 October 2021|access-date=3 December 2021}} {{PD-notice}}</ref>

== Mission ==
The Galaxy Evolution Explorer (GALEX) which explored the origin and evolution of galaxies, and the origins of stars and heavy elements over the [[redshift]] range of Z between 0 and 2. GALEX conducted an all-sky imaging survey, a deep imaging survey, and a survey of 200 [[Galaxy|galaxies]] nearest to the [[Milky Way]] galaxy. As well, GALEX performed three spectroscopic surveys over the 135–300 [[nanometre]] band. GALEX had a planned 29-month mission, and is a part of the Small Explorer (SMEX) program.

The [[First light (astronomy)|first observation]] was dedicated to the crew of the [[Space Shuttle Columbia]], and was images in the [[constellation]] of [[Hercules (astronomy)|Hercules]] taken on 21 May 2003. This region was selected because it had been directly overhead the shuttle at the time of its last contact with the [[Christopher C. Kraft Jr. Mission Control Center|NASA Mission Control Center]], [[Houston]], [[Texas]].

After its primary mission of 29 months, observation operations were extended. In 2009, one of its detectors, which observed in far-ultraviolet light, stopped functioning.<ref name="SFN20120210"/> Late in the mission, observations of more intense UV sources were allowed, including the Kepler field.<ref name="SFN20120210"/>

Observation operations were extended to almost 9 years, with NASA placing it into standby mode on 7 February 2012.<ref name=UT>[http://www.universetoday.com/93409/galex-mission-comes-to-an-end/ ''GALEX Mission Comes to an End'' 7 February 2012 Universe Today]</ref> NASA cut off financial support for operations of GALEX in early February 2011 as it was ranked lower than other projects which were seeking a limited supply of funding. The mission's life-cycle cost to NASA was US$150.6 million. The [[California Institute of Technology]] (Caltech) negotiated to transfer control of GALEX and its associated ground control equipment to the California Institute of Technology in keeping with the [[Stevenson-Wydler Technology Innovation Act of 1980|Stevenson-Wydler Technology Innovation Act]]. Under this Act, excess research equipment owned by the [[Federal government of the United States|U.S. government]] can be transferred to educational institutions and [[Nonprofit organization|non-profit organizations]].<ref name="SFN20120210">{{cite news|author=Stephen Clark|title=NASA, Caltech mull over unique satellite donation|url=https://www.spaceflightnow.com/news/n1202/10galex/|publisher=Spaceflight Now|date=10 February 2012|access-date=8 June 2021}}</ref> On 17 May 2012, GALEX operations were transferred to Caltech.<ref name="Phys.org">{{cite web|url=https://phys.org/news/2012-05-nasa-ultraviolet-space-telescope-caltech.html|title=NASA lends ultraviolet space telescope to Caltech|publisher=Phys.org|date=17 June 2012|access-date=3 December 2021}}</ref>

On 28 June 2013, NASA decommissioned GALEX. It is expected that the spacecraft will remain in orbit until at least 2068 before it will re-enter the atmosphere.<ref name="Decommission">{{cite web |url=http://www.galex.caltech.edu/newsroom/glx2013-03r.html|title=NASA Decommissions Its Galaxy Hunter Spacecraft|publisher=CalTech|date=28 June 2013|access-date=3 December 2021}}</ref><ref name="JPL">{{cite web|url=https://www.jpl.nasa.gov/missions/galaxy-evolution-explorer-galex/|title=Mission to Universe: Galaxy Evolution Explorer|publisher=NASA|access-date=3 December 2021}} {{PD-notice}}</ref>

== Science mission ==
[[File:GALEX Field of View.jpg|thumb|upright=1.0|left|GALEX field of view compared to a full Moon]]

The telescope made observations in ultraviolet wavelengths to measure the history of [[star]] formation in the [[universe]] 80% of the way back to the [[Big Bang]]. Since scientists have evidence that the Universe to be about 13.8 billion years old, the mission studied galaxies and stars across about 10 billion years of cosmic history.<ref>{{cite web |url=http://www.esa.int/Our_Activities/Space_Science/Cosmic_detectives|title=Cosmic Detectives|publisher=European Space Agency (ESA)|date=2013-04-02|access-date=2013-04-26}}</ref>

The [[space observatory|spacecraft's]] mission was to observe hundreds of thousands of galaxies, with the goal of determining the distance of each galaxy from [[Earth]] and the rate of [[star formation]] in each galaxy. Near-UV (NUV) and Far-UV (FUV) emissions as measured by GALEX can indicate the presence of young stars, but may also originate from old stellar populations (e.g. [[sdB stars]]).

Partnering with the NASA [[Jet Propulsion Laboratory]] (JPL) on the mission were the [[California Institute of Technology]], [[Orbital Sciences Corporation]], [[University of California, Berkeley]], [[Yonsei University]], [[Johns Hopkins University]], [[Columbia University]], and [[Marseille Observatory|Laboratoire d'Astrophysique de Marseille]], [[France]].

The observatory participated in GOALS with [[Spitzer Space Telescope]], [[Chandra X-ray Observatory]], and [[Hubble Space Telescope]].<ref name=goals>[https://arxiv.org/abs/0904.4498 GOALS: The Great Observatories All-Sky LIRG Survey]</ref> GOALS stands for ''Great Observatories All-sky LIRG Survey'', and [[Luminous Infrared Galaxies]] were studied at the multiple wavelengths allowed by the telescopes.<ref name=goals/>

== Science objectives ==
The primary objective of the Galaxy Evolution Explorer was to learn what factors trigger star formation inside galaxies; how quickly stars form, evolve and die; and how heavy chemical elements form in stars. Additional goals include:<ref name="galex-presskit">{{cite web|url=http://www.jpl.nasa.gov/news/press_kits/galex.pdf|title=Galaxy Evolution Explorer Launch|publisher=NASA|date=April 2003|access-date=3 December 2021}} {{PD-notice}}</ref>

* Determining how fast stars are forming inside each galaxy
* Determining when and how the stars we see today formed
* Creating the first map of the ultraviolet universe
* Helping scientists find and understand ultraviolet bright quasars. These objects can serve as background sources for the Hubble Space Telescope and FUSE as it probes the gases from which galaxies form stars

To accomplish its objectives, the Galaxy Evolution Explorer conducted eight surveys, grouped into two broad categories – a local universe investigation and a star formation history investigation. The local universe investigation includes the following four surveys:<ref name="galex-presskit"/>

* All-sky imaging survey – will look at the entire sky and develop a comprehensive catalogue of ultraviolet galaxy images, useful to map the distribution of star formation within the local universe
* Nearby galaxy survey – will study about 150 nearby galaxies that are familiar to scientists to understand how stars formed in individual galaxies
* Wide-field spectroscopic survey – will analyze the light wavelengths of galaxies in a wide swath of the sky
* Medium spectroscopic survey – will examine the light properties of galaxies within a narrower portion of the sky

The star formation history investigation will take information gathered by the local universe investigation and apply it to more distant galaxies by looking further back in time. It includes the following four surveys:<ref name="galex-presskit"/>

* Deep imaging survey – will look at a portion of the sky to study the distribution of star formation in the deep universe
* Deep spectroscopic survey – will look for the most distant galaxies
* Ultra-deep imaging survey – will look as deep as possible at a very small portion of the sky
* Medium imaging survey – will study star formation in galaxies beyond our local cosmic neighborhood, but not as deep as the deep imaging survey

== Telescope specifications ==
The telescope had a {{cvt|50|cm}} diameter aperture primary, in a [[Ritchey–Chrétien telescope]] f/6.0 configuration. It can see light wavelengths from 135 nanometres to 280-nm, with a field of view of 1.2° wide (larger than a full Moon). It had [[gallium arsenide]] (GaAs) [[solar cell]]s which supply nearly 300 [[watt]]s to the spacecraft.<ref name=astro>[http://www.astronautix.com/craft/galex.htm Encyclopedia Astronautica – GALEX] {{webarchive|url=https://web.archive.org/web/20080706050843/http://astronautix.com/craft/galex.htm|date=2008-07-06}}</ref>

== Experiment ==
=== Ultraviolet telescope ===
GALEX carries a single f/6.0, Ritchey–Chrétien telescope, with a {{cvt|50|cm}} diameter primary, and a {{cvt|22|cm}} secondary mirror. Beam-splitters direct the Near UV (NUV) and Far UV (FUV) components to separate photoelectric detectors of diameter {{cvt|6.5|cm}}. In each, the photoelectrons are multiplied by a microchannel plate, and detected by the anode grid. The grid enables determination of the exact position of [[electron]] impact, by the time delay of each pulse at the two ends. The telescope has a [[field of view]] (FoV) of 1.2°, and a resolution of five [[Minute and second of arc|arcseconds]], and enables either imaging or spectral composition of a single star/galaxy, by a rotatable wheel containing a clear window and a [[grism]] (a cross between a grating and a prism).<ref name="Experiment1">{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=2003-017A-01|title=Experiment: Ultraviolet Telescope|publisher=NASA|date=28 October 2021|access-date=3 December 2021}} {{PD-notice}}</ref>

== Pre-launch images ==
{{Gallery|width=200
|File:Galex PIA04264.jpg|GALEX at the pre-launch tests
|File:GALEX Pegasus.jpg|GALEX being mated to a [[Pegasus XL]] launch vehicle
|File:GALEXRollout.jpg|GALEX's Pegasus XL being attached to the [[Lockheed Martin|Lockheed]] [[Lockheed L-1011 TriStar|L-1011]] ''[[Stargazer (aircraft)|Stargazer]]''
|File:GALEXLaunch.jpg|The L-1011 ''Stargazer'' take-off with GALEX attached under-belly
}}

== See also ==
{{Portal|Spaceflight}}
* {{annotated link|Explorer program}}
* [[Ultraviolet astronomy]]
* [[GALEX Arecibo SDSS Survey]]
* [[Arecibo Observatory]]
* [[Galaxy And Mass Assembly survey]], with GALEX participation

== References ==
{{Reflist}}

== External links ==
{{Commons category|GALEX}}
* [http://www.galex.caltech.edu GALEX website] by the [[California Institute of Technology]]
* [https://www.jpl.nasa.gov/missions/galaxy-evolution-explorer-galex GALEX website] by the [[Jet Propulsion Laboratory]]
* [http://galex.stsci.edu GALEX data archive] by the [[Space Telescope Science Institute|STScI]]{{\}}MAST
* [http://galex.stsci.edu/GalexView/ GALEXView Search Tool] by the STScI{{\}}MAST
* [http://wikisky.org/?object=M31&img_source=GALEX GALEX Ultraviolet Sky Survey] at [[Wikisky.org]]

{{Explorer program}}
{{Space observatories}}
{{Jet Propulsion Laboratory}}
{{Orbital launches in 2003}}

[[Category:Satellites orbiting Earth]]
[[Category:Explorers Program]]
[[Category:Space telescopes]]
[[Category:Ultraviolet telescopes]]
[[Category:Spacecraft launched in 2003]]
[[Category:Spacecraft launched by Pegasus rockets]]