View source for Explorer 3

{{Short description|Satellite launched by the United States (1958)}}
{{For|International Sun/Earth Explorer 3 (ISEE-3) satellite|International Cometary Explorer}}
{{Use American English|date=February 2021}}
{{Use dmy dates|date=February 2021}}
{{Infobox spaceflight
| name               = Explorer 3
| names_list         = Explorer III<br/>1958 Gamma
| image              = Explorer III.jpg
| image_caption      = Juno I rocket with Explorer 3 satellite
| image_size         = 
| mission_type       = [[Earth science]]
| operator           = [[Jet Propulsion Laboratory|JPL]] / [[United States Army Ordnance Corps|U.S. Army Ordnance]]
| Harvard_designation = 1958 Gamma
| COSPAR_ID          = 1958-003A
| SATCAT             = 00006
| mission_duration   = 93 days (achieved)<br/>120 days (planned)
| spacecraft         = Explorer III
| spacecraft_type    = Science Explorer
| spacecraft_bus     = [[Explorer 1]]
| manufacturer       = [[Jet Propulsion Laboratory]]
| launch_mass        = {{cvt|14.1|kg}}
| payload_mass       = {{cvt|8.4|kg}}
| dimensions         = {{cvt|203|cm}} length<br/>{{cvt|15.2|cm}} diameter
| power              = 60 [[watt]]s
| launch_date        = 26 March 1958, 17:38:01 [[Greenwich Mean Time|GMT]]
| launch_rocket      = [[Juno I]] (RS-24)
| launch_site        = [[Eastern Range|Atlantic Missile Range]], [[Cape Canaveral Launch Complex 5|LC-5]]
| launch_contractor  = [[Army Ballistic Missile Agency]]
| entered_service    = 26 March 1958
| last_contact       = 28 June 1958<ref name="Trajectory"/>
| destroyed          = 
| decay_date         = 28 June 1958<ref name="Trajectory"/>
| orbit_reference    = [[Geocentric orbit]]<ref name="Trajectory"/>
| orbit_regime       = [[Medium Earth orbit]]
| orbit_periapsis    = {{cvt|186|km}}
| orbit_apoapsis     = {{cvt|2799|km}}
| orbit_inclination  = 33.38°
| orbit_period       = 115.70 minutes
| apsis              = gee
| instruments        = Cosmic Ray counter<br/>Micrometeorite detector
| programme          = '''Explorer Program'''
| previous_mission   = [[Explorer 2]]
| next_mission       = [[Explorer 4]]
}}

'''Explorer 3''' (Harvard designation '''1958 Gamma''') was an American artificial satellite launched into [[medium Earth orbit]] in 1958. It was the second successful launch in the [[Explorer program]], and was nearly identical to the first U.S. satellite [[Explorer 1]] in its design and mission.

==Background==
Explorer 3 was the third satellite in the Explorer small satellite series, which started with [[Explorer 1]], America's first artificial satellite.<ref name="Vanguard">{{cite book|title=''Vanguard — a History''|author=Constance Green and Milton Lomask|publisher=National Aeronautics and Space Administration|location=Washington D.C.|date=1970|id=SP-4202|isbn=978-1-97353-209-5|oclc=747307569|pages=17–18|url=https://www.hq.nasa.gov/office/pao/History/SP-4202/begin.html|access-date=21 February 2021|archive-date=3 March 2016|archive-url=https://web.archive.org/web/20160303213422/http://www.hq.nasa.gov/office/pao/History/SP-4202/begin.html|url-status=live}}</ref>{{rp|288}} The Explorer program was a direct successor to the [[Army Ballistic Missile Agency]] (ABMA)'s [[Project Orbiter]], initiated in November 1954 to use a slightly modified [[PGM-11 Redstone|Redstone]] missile<ref name=redstone53>{{cite web|url=https://history.redstone.army.mil/ihist-1953.html|title=Installation History 1953 - 1955|publisher=U.S. Army Aviation and Missile Life Cycle Management Command|date=2017|access-date=1 February 2021|archive-date=2 February 2023|archive-url=https://web.archive.org/web/20230202195242/https://history.redstone.army.mil/ihist-1953.html|url-status=live}}</ref> combined with solid-propellant rocket cluster upper stage to put a satellite into orbit.<ref name="Vanguard"/>{{rp|17-18, 43}}

In 1955, the "Stewart Committee", under the chairmanship of [[Homer J. Stewart]] of [[Jet Propulsion Laboratory]] (JPL), chose a [[Naval Research Laboratory]] (NRL) satellite plan using a rocket based on its [[Viking (rocket)|Viking]] rocket ([[Project Vanguard]]) for the [[International Geophysical Year]], which would start 1 July 1957.<ref name="Vanguard"/>{{rp|43, 51-56}} Nevertheless, ABMA hoped Redstone-Orbiter could still be used as a backup orbital system. Reentry tests that year conducted with the newly developed, Redstone-based [[Jupiter-C]], further strengthened ABMA confidence in their vehicle as an orbital launcher. 

Following the launch of the Soviet satellite [[Sputnik 1]] on 4 October 1957, Project Orbiter was revived,<ref name="space age">{{cite web|url=https://www.nasa.gov/history/sputnik/expinfo.html|title=Explorer Info|work=NASA History |publisher=NASA|access-date=20 Oct 2024}}</ref>  with two shots authorized as a back-up to Vanguard in early November.<ref name="Vanguard"/>{{rp|238}} The failure of America's [[Vanguard TV-3|first attempted Vanguard launch]] on 6 December 1957,<ref name = "Kaputnik">
{{Cite news
| last1          = Wilder
| first1         = Paul
| title          = Tribune Reporter Writes Eyewitness Story of Fiery Death Of Satellite Rocket 
| work           = [[Tampa Morning Tribune]]
| publisher      = The Tribune Company
| issn           = 1042-3761

| date           = 7 December 1957
| page           = 1
| url            = https://www.newspapers.com/article/the-tampa-tribune-tribune-reporter-write/168923658/

| access-date    = 26 March 2025
| via            = [[Newspapers.com]]
}}</ref> cleared the way for an "Explorer" (as the crash program was dubbed) to be the first American satellite.<ref name="Vanguard"/>{{rp|74,199-200, 212-213}}

Working closely together, ABMA and JPL completed the job of modifying the Jupiter-C to the Juno 1 and building Explorer 1 in 84 days.<ref name="space age">{{cite web|url=https://www.nasa.gov/history/sputnik/expinfo.html|title=Explorer Info|work=NASA History |publisher=NASA|access-date=20 Oct 2024}}</ref> An experiment developed for Vanguard by George Ludwig,<ref name="Vanguard"/>{{rp|238}} comprising an Anton 314 omnidirectional Geiger tube detector for measuring the flux of high energy charged protons and electrons, was adapted for Explorer 1.<ref name=explorer1exp>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1958-001A-01|title=Cosmic Ray Detector|accessdate=22 October 2024|publisher=NASA}}</ref> Because of the high spin rate of the Explorer 1 rocket, the experiment's tape recorder had to be omitted, which meant that data could only be collected when the satellite was in sight and range of a ground station.<ref name="Vanguard"/>{{rp|238}}

Explorer 1 took off 31 January 1958, becoming America's first satellite. Its Geiger tube worked properly, but acted contrary to expectations. As the satellite ascended in its orbit, the radiation count increased, then abruptly dropped to zero. When the satellite was descending, the tube abruptly began detecting charged particles again. As data could only be received about 15% of the time, it was yet impossible to determine the phenomenon Explorer had detected.<ref name="Vanguard"/>{{rp|241-242}}

==Spacecraft==
[[File:Explorer 3 Flight Recorder.jpg|thumb|upright=1.0|right|Explorer 3 Tape Recorder]]

The objective of this spacecraft was a continuation of experiments started with Explorer 1. The [[Payload (air and space craft)|payload]] consisted of a [[micrometeorite]] detector (a wire grid array and acoustic detector) and the same [[cosmic ray]] counter (a [[Geiger-Müller tube]]) experiment included on Explorer 1, but this time with an on-board [[tape recorder]] to provide a complete [[radiation]] history for each orbit, Ludwig having had time to accommodate for the [[spin-stabilized satellite|spin-stabilization]] of the satellite.<ref name="Trajectory">{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/displayTrajectory.action?id=1958-003A|title=Trajectory: Explorer-3 1958-003A|publisher=NASA|date=14 May 2020|access-date=15 February 2021}} {{PD-notice}}</ref>

Its total weight was {{cvt|14.1|kg}}, of which {{cvt|8.4|kg}} was instrumentation. The instrument section at the front end of the satellite and the empty scaled-down [[MGM-29 Sergeant|fourth-stage rocket casing]] orbited as a single unit, to be spun around its long axis at 750 [[revolutions per minute]]. Data from these instruments would be transmitted to the ground by a 60 [[Watt|milliwatt]] transmitter operating on 108.03 [[hertz|MHz]] and a 10 milliwatt transmitter operating on 108.00 MHz.<ref name="Trajectory"/>

Transmitting [[antenna (radio)|antennas]] consisted of two [[fiberglass]] slot antennas in the body of the satellite itself. The four flexible whip antennas of Explorer 1 were removed from the design.<ref>{{cite web|url=https://ntrs.nasa.gov/search.jsp?R=19650073999|title=Vehicle Motions as Inferred from Radio-signal – Strength Records|author=Pilkington, W. C.|date=September 5, 1958|publisher=NASA|access-date=October 18, 2018}} {{PD-notice}}</ref> The external skin of the instrument section was painted in alternate strips of white and dark green to provide passive temperature control of the satellite. The proportions of the light and dark strips were determined by studies of shadow-sunlight intervals based on firing time, [[trajectory]], orbit, and [[orbital inclination]].<ref name="Trajectory"/>

Electrical power was provided by Mallory type RM Mercury batteries that made up approximately 40% of the payload weight. These provided power that operated the high power transmitter for 31 days and the low-power transmitter for 105 days. Because of the limited space available and the requirements for low weight, the Explorer 3 instrumentation was designed and built with simplicity and high reliability in mind.<ref name="Trajectory"/>

==Launch and orbit==
Explorer 3 was launched at 17:38:03 [[Greenwich Mean Time|GMT]] on 26 March 1958 from [[Cape Canaveral Launch Complex 5]]<ref name=log>{{Cite web|url=https://planet4589.org/space/gcat/data/derived/launchlog.html|title=Launch Log|last=McDowell|first=Jonathan|publisher=Jonathan's Space Report|access-date=20 October 2024}}</ref> via the same type of modified Jupiter-C ([[Juno 1]]) as the prior two Explorers. A guidance system orbit placed the satellite into an orbit with a higher [[Apsis|apogee]] and lower [[Apsis|perigee]] than planned: {{cvt|1735|mi}} and {{cvt|125|mi}}, respectively. The ensuing [[orbital decay]] made for a comparatively short lifespan: initial estimates placed it at 4.6 months.<ref name=avweek1958a>{{cite magazine|date=31 March 1958|title=Explorer III Orbit|url=http://archive.aviationweek.com/issue/19580331#!&pid=26|magazine=[[Aviation Week & Space Technology | Aviation Week]]|publisher=[[McGraw Hill Education | McGraw Hill Publishing Company]]|access-date=20 October 2024|page=27|url-access=subscription | volume = 68 | number = 13}}</ref> In fact, the satellite reentered the atmosphere on 28 June 1958, after just 93 days of operation;<ref name="Trajectory"/> by the week before reentry, Explorer 3's apogee had dropped to {{cvt|375|mi}} and the perigee to {{cvt|99|mi}}.<ref name=avweek1958b>{{cite magazine|date=30 June 1958|title=Army Evaluates Data From Explorer III|url=http://archive.aviationweek.com/issue/19580630#!&pid=18|magazine=Aviation Week|publisher=McGraw Hill Publishing Company|access-date=19 October 2024|page=19|url-access=subscription | volume = 68 | number = 26}}</ref>

==Mission results==
Explorer 3 largely confirmed the findings of Explorer 1, with the same zero count returned above an altitude of around {{cvt|1000|km}} to {{cvt|1200|km}}. Because of the consistent results, Van Allen hypothesized that the satellites' equipment might have been saturated by unexpectedly high radiation concentrations, trapped in a belt of charged particles by the Earth's magnetic field. [[Explorer 4]], equipped with a lead-shielded counter, flew in July and confirmed the existence of the radiation fields subsequently known as the [[Van Allen Belts]].<ref name=Vanguard/>{{rp|242-243}}

On 7 May 1958, micrometeorites associated with the [[Eta Aquariids]] meteor shower ruptured two of Explorer 3's micrometeorite erosion gauges.<ref name=avweek1958b/>

==Status==

A replica of the spacecraft is currently located in the [[Smithsonian Institution]]'s [[National Air and Space Museum]], Milestones of Flight Gallery.

==See also==
{{Portal|Spaceflight}}
*[[Timeline of artificial satellites and space probes]]
*[[Explorer program]]

==References==
{{Reflist|30em}}

==External links==
*{{youtube|kjlr6v0Psvo|Big Picture: Army Satellites – Explorer 1, Explorer 2 and Explorer 3}}
*[https://history.nasa.gov/sputnik/expinfo.html Data Sheet], Department of Astronautics, [[National Air and Space Museum]], [[Smithsonian Institution]].

{{Explorer program}}
{{Orbital launches in 1958}}

[[Category:Spacecraft launched in 1958]]
[[Category:Spacecraft which reentered in 1958]]
[[Category:Explorers Program]]
[[Category:1958 in the United States]]