Explorer 1
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2008-06-24T23:30:43Z
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gen fixes + format *date= in cite templates - full explanation [[User:Rjwilmsi#My_correction_of_dates_in_templates|here]] using [[Project:AutoWikiBrowser|AWB]]
{{otheruse|this=the first US satellite|use1=the first Iranian space rocket called 'Explorer-1'|page1=Kavoshgar-1}}
{{Infobox Spacecraft
| Name = Explorer 1<br/>{{small|Satellite 1958 Alpha}}
| Image = [[Image:Explorer1.jpg|250px]]
| Organization = [[Army Ballistic Missile Agency]]
| Major_Contractors = [[Jet Propulsion Laboratory]]
| Mission_Type = [[Earth science]]
| Satellite_Of = [[Earth]]
| Launch = [[February 1]], [[1958]] at 03:48 [[Coordinated Universal Time|UTC]]
| Launch_Vehicle = [[Juno I]]
| Decay = [[March 31]], [[1970]]
| Mission_Duration = 111 days
| Mass = 13.97 kg (30.80 lb)<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="NSSDC">
{{cite web
| url = http://nssdc.gsfc.nasa.gov/nmc/masterCatalog.do?sc=1958-001A
| title = Explorer 1
| accessdate = 2008-02-06
| work = NSSDC Master Catalog
| publisher = [[NASA]]
}}</ref>
| NSSDC_ID = 1958-001A
| Webpage = [http://nssdc.gsfc.nasa.gov/nmc/masterCatalog.do?sc=1958-001A NASA NSSDC Master Catalog]
| Semimajor_Axis = 7,832.2 km (4,866.6 miles)
| Eccentricity = .139849<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>
| Inclination = 33.24°<ref name="trajectory"/>
| Orbital_Period = 114.8 minutes<ref name="facts"/><ref name="trajectory"/>
| Apoapsis = 2,550 km (1,585 miles)<ref name="trajectory"/><ref name="solarsystem">
{{cite web
| url = http://solarsystem.nasa.gov/missions/profile.cfm?MCode=Explorer_01
| title = Explorer 1
| accessdate = 2008-02-06
| work = Solar System Exploration
| publisher = [[NASA]]
}}</ref>
| Periapsis = 358 km (222 miles)<ref name="trajectory"/><ref name="solarsystem"/>
| Orbits = ~56,000
}}
'''Explorer 1''' (officially titled at NASA as '''satellite 1958 Alpha''') was the first Earth [[satellite]] of the [[United States]], launched on [[February 1]], [[1958]] at 03:48 [[Coordinated Universal Time|UTC]] from [[Cape Canaveral Air Force Station Launch Complex 26|LC-26]] at [[Cape Canaveral Air Force Station]] as part of the United States program for the [[International Geophysical Year]] and in response to the launch of the [[Soviet Union|Soviet]] satellite [[Sputnik 1]]. It was the first spacecraft to detect the [[Van Allen radiation belt]].
== Mission ==
The U.S. Earth satellite program began in [[1954]] as a joint [[United States Army]] and [[United States Navy]] proposal, called [[Project Orbiter]], to put a scientific satellite into [[orbit]] during the International Geophysical Year. However, this proposal was rejected in [[1955]] by the [[Presidency of Dwight D. Eisenhower|Eisenhower administration]] in favor of the U.S. Navy's [[Project Vanguard]].<ref name="time">
{{cite web
| 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, beginning the [[Space Race]].<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>
Explorer 1 was designed and built by the [[Jet Propulsion Laboratory]] (JPL), while the [[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 becoming 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 the 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 bring 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
| coauthors = Lomask, Milton
| year = 1970
| work = Vanguard - A History
| publisher = [[NASA]]
}}</ref>
== Orbit ==
On [[February 1]], [[1958]] at 03:48 UTC the Juno I rocket was launched putting Explorer 1 into orbit, which made Explorer 1 the first Earth satellite of the United States. The orbit had a [[perigee]] of 358 kilometers (222 [[mile|mi]]) and an [[apsis|apogee]] of 2,550 kilometers (1,585 mi) having a period of 114.8 minutes.<ref name="facts"/><ref name="trajectory"/><ref name="solarsystem"/>
The total weight of the satellite was 13.97 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.
[[Image:Explorer1 sketch.jpg|frame|center|Specifics of the Explorer 1 satellite.]]
== Spacecraft design ==
[[Image:Explorer1 preparations.jpg|thumb|250px|Preparations of Explorer 1.]]
Explorer 1 was designed and built by the Jet Propulsion Laboratory of the [[California Institute of Technology]] 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). The scientific instrumentation of the Explorer 1 satellite was designed and built under the direction of Dr. [[James Van Allen]] of the [[University of Iowa]]. It consisted of:<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>
* an Anton 314 omnidirectional [[Geiger-Müller tube]], designed by 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 [[saturation|saturated]];<ref name="cosmic ray detector">
{{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>
* five [[thermometer|temperature sensors]] (one internal, three external and one on the nose cone);
* an 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 × 10<sup>-3</sup> 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]]
}}</ref><ref name="manring">
{{cite journal
| last = Manring
| first = Edward R.
| year = 1959
| month = January
| title = Micrometeorite Measurements from 1958 Alpha and Gamma Satellites
| journal = Planetary and Space Science
| volume = 1
| pages = 27–31
| publisher = [[Pergamon Press]]
| location = Great Britain
| url = http://adsabs.harvard.edu/abs/1959P&SS....1...27M
| format = fee required
| accessdate = 2008-02-11
| doi = 10.1016/0032-0633(59)90019-4
}}</ref> and
* a 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"/>
Because of the limited space available and the requirements for low weight, the Explorer 1 instrumentation was designed and built with simplicity and high reliability in mind. Electrical power was provided by [[mercury battery|mercury chemical batteries]] that made up approximately 40 % of the payload weight.
Data from the scientific instruments was transmitted to the ground by two [[antenna (radio)|antennas]]. A 60 milliwatt [[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 10 milliwatt [[turnstile antenna]] operating on 108.00 MHz.<ref name="data sheet"/><ref name="Williams">
{{cite journal
| last = Williams, Jr.
| first = W.E.
| year = 1960
| month = April
| 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]]
| issn = 0096-8390
| 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>
The Explorer 1 instrumentation payload used [[transistor]] electronics, consisting of both [[germanium]] and [[silicon]] devices. This was a very early time frame in the development of transistor technology, and represents 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.
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 [[inclination]].
== Mission results ==
[[Image:Launch of Jupiter C with Explorer 1.jpg|thumb|200px|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>]]
To the surprise of mission experts, satellite Explorer 1 changed [[rotation|rotation axis]] after launch. The elongated body of the spacecraft had been supposed 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]] (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 to address dissipation.<ref>
{{cite journal
| last = Efroimsky
| first = Michael
| year = 2001
| month = August
| 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
}}</ref><ref>
{{cite journal
| last = Efroimsky
| first = Michael
| year = 2002
| month = March
| 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
}}</ref>
Sometimes the instrumentation would report the expected cosmic ray count (approximately thirty counts per second) but sometimes it would show a peculiar zero counts per second. The [[University of Iowa|Iowa group]] (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 500 km (310 miles) would show the expected level of cosmic rays. Later, after [[Explorer 3]], it was concluded that the original Geiger counter had been overwhelmed 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 of the Van Allen Belts by the Explorer satellites 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 leads for a twelve-day period to an impact rate of 8.0 × 10<sup>-3</sup> impacts m<sup>-2</sup> s<sup>-1</sup>.<ref name="dubin">
{{cite journal
| last = Dubin
| first = Maurice
| year = 1960
| month = January
| 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>
The mercury batteries provided power that operated 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 paper
| first = Pedro E.
| last = Zadunaisky
| title = The Orbit of Satellite 1958 Alpha (Explorer I) during the First 10500 Revolutions
| publisher = [[Smithsonian Astrophysical Observatory]]
| date = October 1960
| url = http://adsabs.harvard.edu/abs/1960SAOSR..50.....Z
| format = fee required
| accessdate = 2008-02-04
}}</ref>
when its batteries died, but remained in orbit for more than 12 years. It made a fiery [[orbital decay|reentry]] over the Pacific Ocean on [[March 31]], [[1970]]. Explorer 1 was the first of the long-running [[Explorer program]], which as of November [[2004]] has launched 83 Explorer probes.
An identically-constructed flight backup of Explorer 1 is currently located in the [[Smithsonian Institution]]'s [[National Air and Space Museum]], Milestones of Flight Gallery.
== References ==
{{reflist}}
== External links ==
{{commonscat|Explorer 1}}
* [http://www.ispyspace.com/Explorer_1.html Explorer 1]
* [http://solarsystem.nasa.gov/missions/profile.cfm?MCode=Explorer_01 Explorer 1 Mission Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
* [http://www.nasa.gov/externalflash/SpaceAge/index.html NASA's 50th Anniversary of the Space Age including Explorer 1 - Interactive Media]
* [http://history.nasa.gov/sputnik/expinfo.html Data Sheet], Department of Astronautics, [[National Air and Space Museum]], [[Smithsonian Institution]]
{{Explorer program}}
[[Category:1958 in space exploration]]
[[Category:1958 in the United States]]
[[Category:Artificial satellites formerly orbiting Earth]]
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