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{{about|the US satellite|the Iranian Explorer-1 rocket|Kavoshgar-1}} |
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{{Infobox spaceflight |
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| name = ''Explorer 1'' |
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| image = Explorer1.jpg |
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| image_caption = Photograph of the ''Explorer 1'' |
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| mission_type = [[Earth science]] |
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| operator = [[Army Ballistic Missile Agency]] |
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| Harvard_designation = 1958 Alpha 1 |
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| SATCAT = 4 |
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| mission_duration = 111 days |
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| spacecraft_bus = |
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| manufacturer = [[Jet Propulsion Laboratory]] |
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| dry_mass = |
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| launch_mass = {fucjkn1u`b |
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| launch_site = [[Cape Canaveral Air Force Station|Cape Canaveral]] [[Cape Canaveral Air Force Station Launch Complex 26|LC-26A]] |
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| last_contact = {{end-date|May 23, 1958}} |
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| decay_date = March 31, 1970 |
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| orbit_epoch = 31 March 1970 00:50:24 UTC |
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| orbit_reference = [[geocentric orbit|Geocentric]] |
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| orbit_regime = [[Medium Earth orbit|MEO]] |
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| orbit_periapsis = {{convert|358|km|mi|sp=us}} |
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| orbit_apoapsis = {{convert|2550|km|mi|sp=us}} |
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| orbit_inclination = 33.2400016784668° |
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| orbit_semimajor = {{convert|7832.2|km|mi|sp=us}} |
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| orbit_eccentricity = 0.1398490071296692 |
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| orbit_period = 114.80000305175781 minutes |
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| orbit_RAAN = 334.6171 degrees |
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| orbit_arg_periapsis = 311.5310 degrees |
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| orbit_mean_anomaly = 48.3249 degrees |
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| orbit_mean_motion = 16.27546304 |
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| orbit_rev_number = 58402 |
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| apsis = gee |
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}} |
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'''''Explorer 1''''' was the first [[satellite]] of the [[United States]], launched as part of its participation in the [[International Geophysical Year]]. The mission followed the first two satellites the previous year; the [[Soviet Union]]'s ''[[Sputnik 1]]'' and ''[[Sputnik 2|2]]'', beginning the [[Cold War]] [[Space Race]] between the two nations. |
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''Explorer 1'' was launched on January 31, 1958 at 22:48 Eastern Time (equal to February 1, 03:48 UTC) atop the first [[Juno I|Juno]] booster from [[Cape Canaveral Air Force Station Launch Complex 26|LC-26]] at the [[Cape Canaveral Air Force Station|Cape Canaveral Missile Annex]], 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), 190.''</ref> returning data until its batteries were exhausted after nearly four months. It remained in orbit until 1970, and has been followed by more than 90 scientific spacecraft in the [[Explorer program|Explorer series]]. |
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''Explorer 1'' was given [[Satellite Catalog Number]] 4, and the [[Harvard designation]] 1958 Alpha 1,<ref name="Yost">{{Cite book | last=Yost | first=Charles W. | date=1963-09-06 | title=Registration data for United States Space Launches | publisher=[[United Nations Office for Outer Space Affairs]] | url=http://www.oosa.unvienna.org/pdf/inf044E.pdf | format=PDF | accessdate=2009-02-19}}</ref> the forerunner to the modern [[International Designator]]. |
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==Background== |
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The U.S. Earth satellite program began in 1954 as a joint [[United States Army|U.S. Army]] and [[United States Navy|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 (rocket)|Redstone]] missile, was rejected in 1955 by the [[Presidency of Dwight D. Eisenhower|Eisenhower administration]] in favor of the Navy's [[Project Vanguard]], using a booster produced for civilian space launches.<ref name="time">{{cite news | url=http://www.time.com/time/magazine/article/0,9171,937919-1,00.html | title=Project Vanguard—Why It Failed to Live Up to Its Name | accessdate=2008-02-12 | date=October 21, 1957 | publisher=[[Time (magazine)]]}}</ref> Following the launch of the Soviet satellite Sputnik 1 on October 4, 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=http://history.nasa.gov/sputnik/ | title=Sputnik and the Dawn of the Space Age | accessdate=2008-02-13 | work=NASA History | publisher=[[NASA]] }}</ref> |
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''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 (air and space craft)|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]] [[Atmospheric reentry|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 November 3, 1957. The U.S. Navy's attempt to put the first U.S. satellite into orbit failed with the launch of the [[Vanguard TV3]] on December 6, 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 | accessdate=2008-02-13 | last=McLaughlin Green | first=Constance |author2=Lomask, Milton | year=1970 | work=Vanguard—A History | publisher=[[NASA]] }}</ref> |
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==Spacecraft design== |
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[[File:Explorer1 preparations.jpg|thumb|left|''Explorer 1'' is mated to its booster at LC-26]] |
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''Explorer 1'' was designed and built by the [[California Institute of Technology]]'s JPL under the direction of Dr. [[William Hayward Pickering|William H. Pickering]]. It was the second satellite to carry a mission payload (''Sputnik 2'' was the first). |
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The total weight of the satellite was 13.37 kilograms (30.80 lb), of which 8.3 kg (18.3 lb) were instrumentation. In comparison the first Soviet satellite ''Sputnik 1'' weighed 83.6 kg (184 lb). 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, spinning around its long axis at 750 revolutions per minute. |
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Data from the scientific instruments was transmitted to the ground by two [[antenna (radio)|antennas]]. A 60 milliwatt transmitter fed a [[dipole antenna]] consisting of two fiberglass slot antennas 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, Jr. | first=W.E. |date=April 1960 | title=Space Telemetry Systems | journal=Proceedings of the Institute of Radio Engineers | volume=48 | issue=4 | pages=685–690 | publisher=[[Institute of Electrical and Electronics Engineers|IEEE]] | doi=10.1109/JRPROC.1960.287448 | url=http://ieeexplore.ieee.org/xpls/abs_all.jsp?isnumber=4066036&arnumber=4066076&count=63&index=39 | format=fee required | accessdate=2008-02-05 }}</ref> |
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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 [[transistor]] electronics, consisting of both [[germanium]] and [[silicon]] devices. This was a very early time frame in the development of transistor technology, and was the first documented use of transistors in the U.S. Earth satellite program.<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 | accessdate=2008-02-25 | work=A Transistor Museum Interview with Dr. George Ludwig | publisher=The Transistor Museum}}</ref> A total of 29 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 percent of the payload weight. |
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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 coloration was determined by studies of shadow–sunlight intervals based on firing time, [[trajectory]], [[orbit]], and [[inclination]].[[File:Explorer1 sketch.jpg|frame|center|300 px|Explorer 1 schematic]] |
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==Science payload== |
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[[File:1958-02-03 First US Satellite Launched.ogv|thumb|thumbtime=3|[[Universal Newsreel]] about the satellite]] |
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The ''Explorer 1'' payload consisted of the Iowa [[Cosmic Ray]] Instrument without a 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. |
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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=http://history.nasa.gov/sputnik/expinfo.html | title=Explorer-I and Jupiter-C | accessdate=2008-02-09 | work=Data Sheet | publisher=Department of Astronautics, [[National Air and Space Museum]], [[Smithsonian Institution]]}}</ref> |
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* Anton 314 omnidirectional [[Geiger-Müller tube]], designed by [[George H. Ludwig|Dr. George Ludwig]] of Iowa's Cosmic Ray Laboratory, to detect [[cosmic ray]]s. It could detect [[proton]]s with E > 30 [[electronvolt|MeV]] and [[electron]]s with E > 3 MeV. Most of the time the instrument was [[wikt:Transwiki:Saturation (telecommunications)|saturated]];<ref name="cosmic ray detector"> |
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{{cite web | url=http://nssdc.gsfc.nasa.gov/nmc/experimentDisplay.do?id=1958-001A-01 | title=Cosmic-Ray Detector | accessdate=2008-02-09 | work=NSSDC Master Catalog | publisher=[[NASA]]}}</ref> |
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* Five [[thermometer|temperature sensors]] (one internal, three external and one on the nose cone); |
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* 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 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=http://nssdc.gsfc.nasa.gov/nmc/experimentDisplay.do?id=1958-001A-02 | title=Micrometeorite Detector | accessdate=2008-02-09 | work=NSSDC Master Catalog | publisher=[[NASA]] |
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}}</ref><ref name="manring"> |
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{{cite journal | last=Manring | first=Edward R. |date=January 1959 | title=Micrometeorite Measurements from 1958 Alpha and Gamma Satellites | journal=Planetary and Space Science | volume=1 | issue=1 | pages=27–31 | publisher=[[Pergamon Press]] | location=Great Britain | bibcode=1959P&SS....1...27M | doi=10.1016/0032-0633(59)90019-4 }}</ref> |
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* 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 wire|enameled]] [[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 1 cm by 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"/> |
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==Flight== |
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The Juno I rocket was launched January 31, 1958, putting ''Explorer 1'' into orbit with a [[perigee]] of {{convert|358|km|mi|sp=us}} and an [[apsis|apogee]] of {{convert|2,550|km|mi|sp=us}} having a period of 114.8 minutes.<ref name="facts">{{cite web | url=http://www.jpl.nasa.gov/explorer/facts/ | title=Explorer 1 First U.S. Satellite—Fast Facts | accessdate=2008-02-06 | publisher=[[Jet Propulsion Laboratory|JPL]], [[NASA]] }}</ref><ref name="trajectory">{{cite web | url=http://nssdc.gsfc.nasa.gov/nmc/spacecraftOrbit.do?id=1958-001A | title=Trajectory Details | accessdate=2008-02-06 | work=NSSDC Master Catalog | publisher=[[NASA]] }}</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 | accessdate=2008-02-06}}</ref> |
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At about 1:30 a.m. ET, after confirming that ''Explorer 1'' was indeed in orbit, a news conference was held in the Great Hall at the [[United States National Academy of Sciences|National Academy of Sciences]] in Washington, DC to announce it to the world.<ref name="Discovering Earth's Radiation Belts: Remembering Explorer 1 and 3">{{cite journal | url=http://www.agu.org/eos_elec/ | title=Discovering Earth's Radiation Belts: Remembering Explorer 1 and 3 | accessdate=2008-10-14 | work=NASA History | publisher=American Geological Union | author=McDonald, Naugle | year=2008 | volume=89 | number=39 }}</ref> |
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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 May 23, 1958<ref name="Zadunaisky">{{cite journal | first=Pedro E. | last=Zadunaisky | title=The Orbit of Satellite 456 Alpha (Explorer I) during the First 10500 Revolutions | publisher=[[Smithsonian Astrophysical Observatory]] | date=October 1960 | volume=50 | bibcode=1960SAOSR..50.....Z | journal=SAO Special Report #50 (1960)}}</ref> when its batteries died, but remained in orbit for more than 12 years. It [[orbital decay|reentered]] the atmosphere over the Pacific Ocean on March 31, 1970 after more than 58,000 orbits. |
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==Results== |
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[[File:Explorer 1 conference.jpg|thumb|[[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, DC after confirmation the satellite was in orbit.]] |
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To the surprise of mission experts{{who|date=May 2013}}, ''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 energy|kinetic]] [[rotational energy]] for a fixed angular momentum (this being the maximal-inertia axis). This motivated the first further development of the [[Leonhard Euler|Eulerian]] theory of [[rigid body kinetics|rigid body dynamics]] after nearly 200 years{{mdash}}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 | publisher=[[Elsevier]] | doi=10.1016/S0032-0633(01)00051-4 | url=http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V6T-43GH08G-5&_user=499905&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000024538&_version=1&_urlVersion=0&_userid=499905&md5=8414f735d59d3603c9ee3dd8baa09db2 | format=fee required | accessdate=2008-02-08 | bibcode=2001P&SS...49..937E|arxiv = astro-ph/9911072 }}</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 | publisher=[[Elsevier]] | doi=10.1016/S0273-1177(02)00017-0 | url=http://www.ingentaconnect.com/content/els/02731177/2002/00000029/00000005/art00017 | format=fee required | accessdate=2008-02-05 |arxiv = astro-ph/0112054 |bibcode = 2002AdSpR..29..725E }}</ref> |
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Sometimes the instrumentation would report the expected cosmic ray count (approximately 30 counts per second) but sometimes it would show a peculiar zero counts per second. The University of Iowa (under Van Allen) noted that all of the zero counts per second reports were from an altitude of 2,000+ km (1,250+ miles) over South America, while passes at {{convert|500|km|mi|abbr=on}} 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. |
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The acoustic micrometeorite detector detected 145 impacts of cosmic dust in 78,750 seconds. This calculates to an average impact rate of 8.0{{e|−3}} impacts per m<sup>−2</sup> s<sup>−1</sup> over the twelve-day period (29 impacts per hour per square meter).<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 | publisher=North-Holland Publishing Company | location=Amsterdam | url=http://stinet.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA285101 | format=fee required | accessdate=2008-02-11 }}</ref> |
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==Legacy== |
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''Explorer 1'' was the first of the long-running [[Explorer program]]. A follow-up to the first mission, [[Explorer-1'|Explorer-1 [PRIME]]], was successfully launched aboard a Delta II rocket in late October, 2011. |
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The PRIME was built using modern satellite construction techniques. The orbiting satellite was a backup, since the initial Explorer-1 PRIME, launched on March 4, 2011, failed to reach orbit due to a problem in the launch vehicle.<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 Oct. 28 on NASA rocket|accessdate=1 October 2013|date=23 October 2011|work=Space Science and Engineering Laboratory|author=Evelyn Boswell}}</ref> |
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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, DC. |
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==Gallery== |
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{{Gallery |
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|width=300 |
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|File:Explorer I Characteristics.jpg|''Explorer 1'' statistics and orbital diagram |
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|File:Explorer I 02.jpg|Officials with full-scale ''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]] |
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|File:Explorer 1 During the Installation to Jupiter-C.jpg|''Explorer 1'' mated to Juno I booster |
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|File:Jupier c explorer1 pad.jpg|''Explorer 1'' and Juno I booster in gantry at LC-26 |
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|File:Explorer 1 in Gantry.jpg| Close-up of ''Explorer 1'' atop Juno I booster |
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|File:Launch of Jupiter C with Explorer 1.jpg|Launch of ''Explorer 1'' on January 31, 1958<ref>{{cite web |url=http://antwrp.gsfc.nasa.gov/apod/ap080201.html | title=Astronomy Picture of the Day on January 31, 2008 | accessdate=2008-02-03| date=2008-01-31 | publisher=[[Marshall Space Flight Center]], [[NASA]] }}</ref> |
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|File:207457main antenna-331-2426a-516.jpg|Preliminary satellite tracking tests in a field near JPL<ref>{{cite web|url=http://www.nasa.gov/mission_pages/explorer/antenna.html |title=NASA—JPL Ground Antenna |publisher=Nasa.gov |accessdate=2012-03-30}}</ref> |
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|File:207469main_computers-p-163-500.jpg|Trajectory calculations were done by hand by this group of young women. |
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}} |
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==References== |
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{{Reflist|30em}} |
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==External links== |
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{{Commons category|Explorer 1}} |
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* [http://solarsystem.nasa.gov/missions/profile.cfm?MCode=Explorer_01 Explorer 1 Mission Profile on NASA's Solar System Exploration site] |
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* [http://www.nasa.gov/externalflash/SpaceAge/index.html NASA's 50th Anniversary of the Space Age including Explorer 1 - Interactive Media] |
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* [http://www.youtube.com/watch?feature=player_embedded&v=Osqro0m9clQ newsreel footage of the Explorer 1 launch] |
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* [http://history.nasa.gov/sputnik/expinfo.html Data Sheet], Department of Astronautics, [[National Air and Space Museum]], [[Smithsonian Institution]] |
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* {{Internet Archive film clip|id=arspace1958|description="Army Explorers in Space (1958)"}} |
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* {{Internet Archive film clip|id=1958-12-22_Atlas_In_Orbit|description="Atlas In Orbit. Radios Ike's Message Of Peace To World, 1958/12/22 (1958)"}} |
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* {{Internet Archive short film|id=gov.archives.arc.2569665|name=Big Picture: Army Satellites}} |
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{{Explorer program}} |
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{{NASA navbox}} |
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{{Orbital launches in 1958}} |
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{{Use American English|date=January 2014}} |
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{{DEFAULTSORT:Explorer 01}} |
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[[Category:Spacecraft launched in 1958]] |
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[[Category:1958 in the United States]] |
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[[Category:Artificial satellites formerly orbiting Earth]] |
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[[Category:Explorer program]] |
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[[Category:Individual spacecraft in the collection of the Smithsonian Institution]] |
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[[Category:First artificial satellite of a country]] |
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[[Category:Spacecraft which reentered in 1970]] |