Editing Explorer 1

{{Short description|First satellite launched by the United States (1958)}}
{{About|the U.S. satellite|other uses|Explorer One (disambiguation)}}
{{Other uses|Explorer (disambiguation)}}
{{Use American English|date=February 2021}}
{{Use dmy dates|date=February 2021}}
{{Infobox spaceflight
| name                = Explorer 1
| names_list          = Explorer I<br/>1958 Alpha 1
| image               = Explorer1.jpg
| image_caption       = Explorer 1 in its orbital configuration, with the launch vehicle's fourth stage attached
| image_size          = 

| mission_type        = [[Earth science]]
| operator            = [[Jet Propulsion Laboratory|JPL]] / [[Army Ballistic Missile Agency|ABMA]]
| Harvard_designation = 1958 Alpha 1  
| COSPAR_ID           = 1958-001A
| SATCAT              = 00004
| mission_duration    = 120 days (planned)<br/>111 days (achieved)

| spacecraft          = Explorer I
| spacecraft_type     = Science Explorer
| spacecraft_bus      = Explorer 1
| manufacturer        = [[Jet Propulsion Laboratory]]
| launch_mass         = {{cvt|13.97|kg}}
| dimensions          = {{cvt|203|cm}} length<br/>{{cvt|15.2|cm}} diameter
| power               = 60 [[watt]]s

| launch_date         = 1 February 1958, 03:47:56 [[Greenwich Mean Time|GMT]]
| launch_rocket       = [[Juno I]] (RS-29)
| launch_site         = [[Eastern Range|Atlantic Missile Range]], [[Cape Canaveral Launch Complex 26|LC-26A]]
| launch_contractor   = [[Army Ballistic Missile Agency]]

| entered_service     = 1 February 1958
| last_contact        = 23 May 1958
| destroyed           = 
| decay_date          = 31 March 1970

| orbit_reference     = [[Geocentric orbit]]<ref name="Trajectory"/>
| orbit_regime        = [[Medium Earth orbit]]
| orbit_periapsis     = {{cvt|358|km}}
| orbit_apoapsis      = {{cvt|2550|km}}
| orbit_inclination   = 33.24°
| orbit_period        = 114.80 minutes
| orbit_rev_number    = 58402
| apsis               = gee

| instruments         = Cosmic-Ray Detector<br/>Micrometeorite Detector<br/>Resistance Thermometers<br/>Satellite Drag Atmospheric Density

| programme           = '''[[Explorer program]]'''
| previous_mission    = 
| next_mission        = [[Explorer 2]]
}}

'''Explorer 1''' was the first satellite launched by the [[United States]] in 1958 and was part of the U.S. participation in the [[International Geophysical Year]] (IGY). The mission followed the first two satellites, both launched by the [[Soviet Union]] during the previous year, [[Sputnik 1]] and [[Sputnik 2]]. This began a [[Space Race]] during the [[Cold War]] between the two nations.

Explorer 1 was launched on 1 February 1958 at 03:47:56 [[Greenwich Mean Time|GMT]] (or 31 January 1958 at 22:47:56 Eastern Time) atop the first [[Juno I]] booster from [[Cape Canaveral Launch Complex 26|LC-26A]] at the [[Cape Canaveral Space Force Station|Cape Canaveral Missile Test Center]] of the [[Eastern Range|Atlantic Missile Range]] (AMR), in [[Florida]]. It was the first spacecraft to detect the [[Van Allen radiation belt]],<ref>Paul Dickson, ''Sputnik: The Launch of the Space Race'', Toronto: MacFarlane Walter & Ross, 2001, p. 190.</ref> returning data until its batteries were exhausted after nearly four months. It remained in orbit until 1970.

Explorer 1 was given Satellite Catalog Number 00004 and the Harvard designation 1958 Alpha 1,<ref name="Yost">{{cite book |url=http://www.oosa.unvienna.org/pdf/inf044E.pdf |title=Registration data for United States Space Launches |last=Yost |first=Charles W. |publisher=United Nations Office for Outer Space Affairs |date=6 September 1963 |access-date=30 July 2024}}</ref> the forerunner to the modern [[International Designator]].

== Background ==
The U.S. Earth satellite program began in 1954 as a joint U.S. Army and U.S. Navy proposal, called Project Orbiter, to put a scientific satellite into orbit during the [[International Geophysical Year]]. The proposal, using a military Redstone missile, was rejected in 1955 by the Eisenhower administration in favor of the Navy's [[Project Vanguard]], using a booster advertised as more civilian in nature.<ref>Matt Bille and Erika Lishock, The First Space Race: Launching the World's First Satellites, Texas A&M University Press, 2004, Chapter 5.</ref><ref name="time">{{cite magazine |url=http://www.time.com/time/magazine/article/0,9171,937919-1,00.html |archive-url=https://web.archive.org/web/20080515162755/http://www.time.com/time/magazine/article/0,9171,937919-1,00.html |archive-date=15 May 2008 |title=Project Vanguard – Why It Failed to Live Up to Its Name |access-date=30 July 2024 |date=21 October 1957 |magazine=[[Time (magazine)|Time]]}}</ref> Following the launch of the Soviet satellite Sputnik 1 on 4 October 1957, the initial [[Project Orbiter]] program was revived as the Explorer program to catch up with the Soviet Union.<ref name="space age">{{cite web |url=https://history.nasa.gov/sputnik/ |title=Sputnik and the Dawn of the Space Age |work=NASA History |publisher=NASA |date=2 February 2005 |access-date=30 July 2024}} {{PD-notice}}</ref>

Explorer 1 was designed and built by the [[Jet Propulsion Laboratory]] (JPL), while a [[Jupiter-C]] rocket was modified by the [[Army Ballistic Missile Agency]] (ABMA) to accommodate a satellite payload; the resulting rocket known as the Juno I. The Jupiter-C design used for the launch had already been flight-tested in nose cone reentry tests for the [[PGM-19 Jupiter|Jupiter]] [[intermediate-range ballistic missile]] (IRBM) and was modified into Juno I. Working closely together, ABMA and JPL completed the job of modifying the Jupiter-C and building Explorer 1 in 84 days. However, before work was completed, the Soviet Union launched a second satellite, [[Sputnik 2]], on 3 November 1957. The [[U.S. Navy]] attempted to put the first U.S. satellite into orbit but failed with the launch of the [[Vanguard TV-3]] on 6 December 1957.<ref>{{cite web |url=http://www.hq.nasa.gov/office/pao/History/SP-4202/cover.htm |title=Chapter 11: from Sputnik I to TV-3 |access-date=7 October 2018 |last=McLaughlin Green |first=Constance |author2=Lomask, Milton |year=1970 |work=Vanguard, A History |publisher=NASA |archive-date=7 October 2018 |archive-url=https://web.archive.org/web/20181007015322/https://www.hq.nasa.gov/office/pao/History/SP-4202/cover.htm}} {{PD-notice}}</ref>

== Spacecraft ==
[[File:Explorer1 preparations.jpg|thumb|upright=1.0|left|The satellite Explorer 1 is mated to its booster at LC-26.]]

Explorer 1 was designed and built by [[California Institute of Technology]]'s JPL under the direction of Dr. [[William Hayward Pickering]]. It was the second satellite to carry a mission payload (Sputnik 2 was the first).

The total mass of the satellite was {{cvt|13.97|kg}}, of which {{cvt|8.3|kg}} were instrumentation. In comparison, the mass of the first Soviet satellite Sputnik 1 was {{cvt|83.6|kg}}. The instrument section at the front end of the satellite and the empty scaled-down fourth-stage rocket casing orbited as a single unit, spinning around its long axis at 750 revolutions per minute.

Data from the scientific instruments was transmitted to the ground by two antennas. A 60 [[Watt|milliwatt]] transmitter fed a [[dipole antenna]] consisting of two fiberglasses [[slot antenna]]s in the body of the satellite operating on 108.03 [[Hertz|MHz]], and four flexible whips forming a [[turnstile antenna]] were fed by a 10 milliwatt transmitter operating on 108.00 MHz.<ref name="data sheet"/><ref name="Williams">{{cite journal |last=Williams |first=W. E. Jr. |date=April 1960 |title=Space Telemetry Systems |url=https://ieeexplore.ieee.org/document/4066076 |journal=Proceedings of the Institute of Radio Engineers |volume=48 |issue=4 |pages=685–690 |doi=10.1109/JRPROC.1960.287448 |s2cid=51646193 |access-date=30 July 2024|url-access=subscription }}</ref>

Because of the limited space available and the requirements for low weight, the payload instrumentation was designed and built with simplicity and high reliability in mind, using [[germanium]] and [[silicon]] transistors in its electronics.<ref name="semiconductormuseum">{{cite web |url=http://semiconductormuseum.com/Transistors/LectureHall/Ludwig/Ludwig_Index.htm |title=The First Transistors in Space – Personal Reflections by the Designer of the Cosmic Ray Instrumentation Package for the Explorer I Satellite |access-date=2008-02-25 |work=A Transistor Museum Interview with Dr. George Ludwig |publisher=The Transistor Museum}}</ref> A total of 20 transistors were used in Explorer 1, plus additional ones in the Army's micrometeorite amplifier. Electrical power was provided by [[Mercury battery|mercury chemical batteries]] that made up approximately 40% of the payload weight.

The external skin of the instrument section was sandblasted [[stainless steel]] with white stripes. Several other color schemes had been tested, resulting in backup articles, models, and photographs showing different configurations, including alternate white and green striping and blue stripes alternating with copper. The final color scheme was determined by studies of shadow–sunlight intervals based on firing time, trajectory, orbit and inclination.

[[File:Explorer1 sketch.jpg|thumb|upright=1.0|left|''Explorer 1 schematic'']]

== Science payload ==
[[File:1958-02-03 First US Satellite Launched.ogv|thumb|upright=1.0|right|thumbtime=3|[[Universal Newsreel]] about the satellite]]

The Explorer 1 payload consisted of the Iowa [[Cosmic ray|Cosmic Ray]] Instrument without a [[tape recorder|tape data recorder]] which was not modified in time to make it onto the spacecraft. The real-time data received on the ground was therefore very sparse and puzzling showing normal counting rates and no counts at all. The later [[Explorer 3]] mission, which included a tape data recorder in the payload, provided the additional data for confirmation of the earlier Explorer 1 data.

The scientific instrumentation of Explorer 1 was designed and built under the direction of Dr. [[James Van Allen]] of the [[University of Iowa]] containing:<ref name="data sheet">{{cite web |url=https://www.nasa.gov/history/sputnik/expinfo.html |title=Explorer-I and Jupiter-C |publisher=NASA |access-date=30 July 2024}} {{PD-notice}}</ref>

* Anton 314 omnidirectional [[Geiger–Müller tube]], designed by Dr. [[George H. Ludwig]] of Iowa's Cosmic Ray Laboratory, to detect [[cosmic ray]]s. It could detect protons with E>30 MeV and electrons with E>3 MeV. Most of the time the instrument was saturated;<ref name="cosmic ray detector">{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1958-001A-01 |title=Cosmic-Ray Detector |publisher=NASA |date=14 May 2020 |access-date=12 February 2021}} {{PD-notice}}</ref>

* Five temperature sensors (one internal, three external and one on the nose cone);

* Acoustic detector (crystal [[transducer]] and solid-state [[amplifier]]) to detect [[micrometeoroid|micrometeorite]] ([[cosmic dust]]) impacts. It responded to micrometeorite impacts on the spacecraft skin in such a way that each impact would be a function of mass and velocity. Its effective area was 0.075 m<sup>2</sup> and the average threshold sensitivity was 2.5{{e|−3}} g cm/s;<ref name="micrometeorite detector">{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1958-001A-02 |title=Micrometeorite Detector |publisher=NASA |date=28 October 2022 |access-date=30 July 2024}} {{PD-notice}}</ref><ref name="manring">{{cite journal |last=Manring |first=Edward R. |date=January 1959 |title=Micrometeorite Measurements from 1958 Alpha and Gamma Satellites |url=https://ui.adsabs.harvard.edu/abs/1959P%26SS....1...27M/abstract |journal=Planetary and Space Science |volume=1 |issue=1 |pages=27–31 |bibcode=1959P&SS....1...27M |doi=10.1016/0032-0633(59)90019-4}}</ref>

* Wire grid detector, also to detect micrometeorite impacts. It consisted of 12 parallel connected cards mounted in a fiberglass supporting ring. Each card was wound with two layers of enameled [[Cupronickel|nickel alloy]] wire with a diameter of 17 [[Micrometre|μm]] (21 μm with the enamel insulation included) in such way that a total area of {{cvt|1|xx|1|cm}} was completely covered. If a [[micrometeorite]] of about 10 μm impacted, it would fracture the wire, destroy the electrical connection, and thus record the event.<ref name="micrometeorite detector"/><ref name="manring"/>

== Flight ==
[[File:Explorer 1 launch control console Huntsville Space museum.jpg|thumb|right|Explorer 1 launch control console on display at Huntsville Space and Rocket Center. The red arrow points to the manually turned launch key switch.]]

After a jet stream-related delay on 28 January 1958, at 03:47:56 GMT on 1 February 1958 <ref name="ley196810">{{cite magazine |url=https://archive.org/details/Galaxy_v27n03_1968-10/page/n93/mode/2up?view=theater |title=The Orbit of Explorer 1 |magazine=[[Galaxy Science Fiction]] |last=Ley |first=Willy |pages=93–102 |date=October 1968 |access-date=30 July 2024}}</ref> the Juno I rocket was launched, putting Explorer 1 into orbit with a [[Apsis|perigee]] of {{cvt|358|km}} and an [[Apsis|apogee]] of {{cvt|2550|km}} having a period of 114.80 minutes, and an [[Orbital inclination|inclination]] of 33.24°.<ref name="Trajectory">{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/displayTrajectory.action?id=1958-001A |title=Trajectory: Explorer-1 1958-001A |publisher=NASA |date=14 May 2020 |access-date=12 February 2021}} {{PD-notice}}</ref><ref name="solarsystem">{{cite web |url=http://solarsystem.nasa.gov/missions/profile.cfm?MCode=Explorer_01 |title=Solar System Exploration Explorer 1 |publisher=NASA |access-date=2008-02-06 |archive-url=https://web.archive.org/web/20080108105558/http://solarsystem.nasa.gov/missions/profile.cfm?MCode=Explorer_01 |archive-date=2008-01-08 |url-status=dead}} {{PD-notice}}</ref> [[Goldstone Deep Space Communications Complex|Goldstone Tracking Station]] could not report after 90 minutes as planned whether the launch had succeeded because the orbit was larger than expected.{{r|ley196810}} At about 06:30 GMT, after confirming that Explorer 1 was indeed in orbit, a news conference was held in the Great Hall at the [[National Academy of Sciences]] in [[Washington, D.C.]] to announce it to the world.<ref name="Discovering Earth's Radiation Belts: Remembering Explorer 1 and 3">{{cite journal |title=Discovering Earth's Radiation Belts: Remembering Explorer 1 and 3 |journal=NASA History |author=McDonald, Naugle |year=2008 |volume=89 |pages=361–363 |number=39 |doi=10.1029/2008EO390001 |bibcode=2008EOSTr..89..361M |publisher=NASA}} {{PD-notice}}</ref>

[[File:Explorer 1 launch control manual mission plot Huntsville Space museum.jpg|thumb|right|Hand drawn Explorer 1 mission plot]]

The original expected lifetime of the satellite before [[orbital decay]] was three years.{{r|ley196810}} [[Mercury batteries]] powered the high-power transmitter for 31 days and the low-power transmitter for 105 days. Explorer 1 stopped transmission of data on 23 May 1958,<ref name="Zadunaisky">{{cite journal |first=Pedro E. |last=Zadunaisky |title=The Orbit of Satellite 456 Alpha (Explorer 1) during the First 10500 Revolutions |date=October 1960 |bibcode=1960SAOSR..50.....Z |journal=SAO Special Report |volume=50}}</ref> when its batteries died, but remained in orbit for more than 12 years.<ref>{{Cite magazine |date=October 1968 |url=http://archive.org/details/Galaxy_v27n03_1968-10 |author-link=Willy Ley |last=Ley |first=Willy |title=The Orbit of Explorer-1 |department=For Your Information |magazine=Galaxy Science Fiction |pages=[https://archive.org/details/Galaxy_v27n03_1968-10/page/n93/mode/2up?view=theater 93]-102 |volume=27 |issue=3 |quote=The original estimate of the lifetime of Explorer-1, made a week or so after firing, was three years. It has been orbiting for ten years by now and the estimate of its remaining lifetime is again three years, but this time surrounded by careful explanations about the factors we don't know.}}</ref> It reentered the atmosphere over the [[Pacific Ocean]] on 31 March 1970 after more than 58,400 orbits.

== Results ==
Explorer 1 changed [[Rotation|rotation axis]] after launch. The elongated body of the spacecraft had been designed to spin about its long (least-inertia) axis but refused to do so, and instead started [[precession|precessing]] due to energy [[dissipation]] from flexible structural elements. Later it was understood that on general grounds, the body ends up in the spin state that minimizes the kinetic rotational energy for a fixed angular momentum (this being the maximal-inertia axis). This motivated the first further development of the [[List of things named after Leonhard Euler|Eulerian]] theory of rigid body dynamics after nearly 200 years – to address this kind of momentum-preserving energy dissipation.<ref>{{cite journal |last=Efroimsky |first=Michael |date=August 2001 |title=Relaxation of wobbling asteroids and comets – theoretical problems, perspectives of experimental observation |journal=Planetary and Space Science |volume=49 |issue=9 |pages=937–955 |doi=10.1016/S0032-0633(01)00051-4 |bibcode=2001P&SS...49..937E |arxiv=astro-ph/9911072 |citeseerx=10.1.1.256.6140 |s2cid=14114765}}</ref><ref>{{cite journal |last=Efroimsky |first=Michael |date=March 2002 |title=Euler, Jacobi, and missions to comets and asteroids |journal=Advances in Space Research |volume=29 |issue=5 |pages=725–734 |doi=10.1016/S0273-1177(02)00017-0 |arxiv=astro-ph/0112054 |bibcode=2002AdSpR..29..725E |citeseerx=10.1.1.192.380 |s2cid=1110286}}</ref>

Sometimes the instrumentation reported the expected cosmic ray count (approximately 30 counts per second) but other times it would show a peculiar zero counts per second. The University of Iowa (under James Van Allen) observed that all of the zero counts per second reports were from an altitude of more than {{cvt|2000|km}} over [[South America]], while passes at {{cvt|500|km}} would show the expected level of cosmic rays. Later, after Explorer 3, it was concluded that the original Geiger counter had been overwhelmed ("saturated") by strong radiation coming from a belt of charged particles trapped in space by the Earth's magnetic field. This belt of charged particles is now known as the [[Van Allen radiation belt]]. The discovery was considered to be one of the outstanding discoveries of the International Geophysical Year.

The acoustic micrometeorite detector detected 145 impacts of cosmic dust in 78,750 seconds. This calculates to an average impact rate of 8.0<sup>−3</sup> impacts per second per square meter, or 29 impacts per hour per square meter, over the twelve-day period.<ref name="dubin">{{cite journal |last=Dubin |first=Maurice |date=January 1960 |title=IGY Micrometeorite Measurements |journal=Space Research – Proceedings of the First International Space Science Symposium |volume=1 |issue=1 |pages=1042–1058 |bibcode=1960spre.conf.1042D |url=https://apps.dtic.mil/sti/pdfs/ADA285101.pdf |access-date=2023-02-09}}</ref>

== Legacy ==
Explorer 1 was the first of the long-running Explorers program. Four follow-up satellites of the Explorer series were launched by the Juno I launch vehicle in 1958. Of these, Explorer 3 and [[Explorer 4|4]] were successful, while [[Explorer 2]] and [[Explorer 5|5]] failed to reach orbit. The final flight of the Juno I booster, the satellite [[Project Beacon|Beacon-1]], also failed.<ref>J. Boehm, H.J. Fichtner and Otto A. Hoberg, [https://www.nasa.gov/sites/default/files/atoms/files/explorer_i_boehm_document.pdf EXPLORER SATELLITES LAUNCHED BY JUNO 1 AND JUNO 2 VEHICLES], NASA. {{PD-notice}}</ref> The Juno I vehicle was replaced by the [[Juno II]] launch vehicle in 1959.

A follow-up to the first mission, [[M-Cubed|Explorer-1 Prime Unit 2]], was successfully launched aboard a [[Delta II]] launch vehicle in late October 2011. The Prime was built using modern satellite construction techniques. The orbiting satellite was a backup, because the initial [[Explorer-1 Prime]], launched on 4 March 2011, did not reach orbit due to a launch vehicle failure.<ref>{{cite news |url=https://ssel.montana.edu/msus-twin-satellite-to-launch-oct-28-on-nasa-rocket/ |title=MSU's twin satellite to launch October 28 on NASA rocket |access-date=5 October 2013 |date=23 October 2011 |work=Space Science and Engineering Laboratory |author=Evelyn Boswell |archive-url=https://web.archive.org/web/20131005021134/https://ssel.montana.edu/msus-twin-satellite-to-launch-oct-28-on-nasa-rocket/ |archive-date=5 October 2013}}</ref>

An identically constructed flight backup of Explorer 1 is on display in the [[Smithsonian Institution]]'s [[National Air and Space Museum]], Milestones of Flight Gallery in Washington, D.C., [[Cape Canaveral Space Force Station|LC-26A]] was deactivated in 1963 and was designated for use as a museum in 1964, the [[Cape Canaveral Space Force Museum|Air Force Space and Missile Museum]].<ref>[http://www.afspacemuseum.org/displays/LC26blockhouse/ Launch Complex 26 Blockhouse] {{Webarchive |url=https://web.archive.org/web/20160425064750/http://www.afspacemuseum.org/displays/LC26blockhouse/ |date=25 April 2016}} {{PD-notice}}</ref> Here too, a full-scale Explorer 1 is on display, but this one is a mock-up.<ref>[http://www.afspacemuseum.org/displays/ExplorerI/ Explorer I] {{Webarchive |url=https://web.archive.org/web/20160526205044/http://afspacemuseum.org/displays/ExplorerI/ |date=26 May 2016}} {{PD-notice}}</ref>

== Gallery ==
<gallery mode=packed heights=180px>
Explorer I Characteristics.jpg |Explorer 1 statistics and orbital diagram
Explorer I 02.jpg|Officials with Explorer 1 model at [[Redstone Arsenal]], including Maj. Gen. [[John Bruce Medaris|John Medaris]] (3rd from left), [[Walter Haeussermann]], [[Wernher von Braun]] and [[Ernst Stuhlinger]]
Explorer1 people.jpg|[[Bill Pickering (rocket scientist)|William Hayward Pickering]], [[James Van Allen]], and [[Wernher von Braun]] display a full-scale model of Explorer 1 at a crowded news conference in Washington, D.C. after confirmation the satellite was in orbit.
Explorer 1 During the Installation to Jupiter-C.jpg|Explorer 1 in spin test facility
Jupier c explorer1 pad.jpg|Explorer 1 and Juno I booster in gantry at LC-26A
Explorer 1 in Gantry.jpg|Close-up of Explorer 1 atop Juno I booster
Launch of Jupiter C with Explorer 1.jpg|Launch of Explorer 1 on 1 February 1958 <ref>{{cite APOD |title=The First Explorer |access-date=2008-02-03 |date=2008-01-31}} {{PD-notice}}</ref>
207457main antenna-331-2426a-516.jpg|Preliminary satellite tracking tests in a field near [[Jet Propulsion Laboratory]]<ref>{{cite web |url=http://www.nasa.gov/mission_pages/explorer/antenna.html |title=NASA / JPL - Ground Antenna |publisher=NASA |access-date=2012-03-30}} {{PD-notice}}</ref>
207469main_computers-p-163-500.jpg|Trajectory calculations were done by hand by this group of women.
</gallery>

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

== References ==
{{Reflist}}

== Bibliography ==
{{cite book |last=West |first=Doug |title=Dr Wernher von Braun: A Short Biography |location=U.S. |year=2017 |isbn=978-1-9779279-1-0}}

== External links ==
{{Commons category|Explorer 1}}
* [https://www.nasa.gov/mission_pages/explorer/index.html NASA images and videos of Explorer 1 and other early satellites] {{Webarchive |url=https://web.archive.org/web/20211114184520/https://www.nasa.gov/mission_pages/explorer/index.html |date=14 November 2021}}
* [https://history.nasa.gov/sputnik/expinfo.html Data Sheet], Department of Astronautics, [[National Air and Space Museum]], [[Smithsonian Institution]]
* [http://libarchstor.uah.edu:8081/repositories/2/resources/161 Explorer I Collection, The University of Alabama in Huntsville Archives and Special Collections]
* {{Internet Archive film clip|id=arspace1958|description="Army Explorers in Space (1958)"}}
* {{Internet Archive short film|id=gov.archives.arc.2569665|name=Big Picture: Army Satellites}}
* {{YouTube|aGxPSlCcIjI|X-minus 80 Days - JPL-Army Ballistic Missile Agency}}
* [https://www.youtube.com/watch?v=RdDJtUxLwqQ Lecture with detailed evaluation of the Explorer 1 rotation anomaly]

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