Explorer 1

Infobox Spacecraft
Name = Explorer 1 small|Satellite 1958 Alpha


Organization = Army Ballistic Missile Agency
Major_Contractors = Jet Propulsion Laboratory
Mission_Type = Earth science
Satellite_Of = Earth
Launch = February 1, 1958 at 03:48 UTC
Launch_Vehicle = Juno I
Decay = March 31, 1970
Mission_Duration = 111 days
Mass = 13.97 kg (30.80 lb) cite web
url = http://www.jpl.nasa.gov/explorer/facts/
title = Explorer 1 First U.S. Satellite - Fast Facts
accessdate = 2008-02-06
publisher = JPL, NASA ] 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 ]
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 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 ]
Inclination = 33.24°
Orbital_Period = 114.8 minutes
Apoapsis = 2,550 km (1,585 miles) 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 ]
Periapsis = 358 km (222 miles)
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 UTC from 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 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 Eisenhower administration in favor of the U.S. Navy's Project Vanguard.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)] 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.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]

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, 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 reentry tests for the Jupiter 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. [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]

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 mi) and an apogee of 2,550 kilometers (1,585 mi) having a period of 114.8 minutes.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 fourth-stage rocket casing orbited as a single unit, spinning around its long axis at 750 revolutions per minute.

Spacecraft design

Explorer 1 was designed and built by the Jet Propulsion Laboratory of the California Institute of Technology under the direction of Dr. 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: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]
* an Anton 314 omnidirectional Geiger-Müller tube, designed by Dr. George Ludwig of Iowa's Cosmic Ray Laboratory, to detect cosmic rays. It could detect protons with E > 30 MeV and electrons with E > 3 MeV. Most of the time the instrument was saturated;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]
* five temperature sensors (one internal, three external and one on the nose cone);
* an acoustic detector (crystal transducer and solid-state amplifier) to detect 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 m² and the average threshold sensitivity was 2.5 × 10^-3 g cm/s;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] 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 ] 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 nickel alloy wire with a diameter of 17 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.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 chemical batteries that made up approximately 40 % of the payload weight.

Data from the scientific instruments was transmitted to the ground by two antennas. A 60 milliwatt dipole antenna consisting of two fiberglass slot antennas in the body of the satellite operating on 108.03 MHz, and four flexible whips forming a 10 milliwatt turnstile antenna operating on 108.00 MHz.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 = 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]

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.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] 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

[
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] ] To the surprise of mission experts, satellite Explorer 1 changed 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 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 (this being the maximal-inertia axis). This motivated the first further development of the Eulerian theory of rigid body dynamics after nearly 200 years to address dissipation. [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] [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]

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 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^-3 impacts m^-2 s^-1.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]

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,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] when its batteries died, but remained in orbit for more than 12 years. It made a fiery reentry over the Pacific Ocean on March 31, 1970. Explorer 1 was the first of the long-running Explorer program, which as of April 2007 has launched 90 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

External links

* [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