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The Explorer program is a United States space exploration program that provides flight opportunities for physics, heliophysics, and astrophysics investigations from space. Over 90 space missions have been launched since 1958, and it is still active. Starting with Explorer 6, it has been a NASA program, and they have worked with a variety of other institutions and business, including many international partners.
The Explorer program was the United States's first successful attempt to launch an artificial satellite. It began as a U.S. Army proposal to place a scientific satellite into orbit during the International Geophysical Year; however, that proposal was rejected in favor of the U.S. Navy's Project Vanguard. The Explorer program was later reestablished to catch up with the Soviet Union after that nation's launch of Sputnik 1 on October 4, 1957. (See: Sputnik crisis) Explorer 1 was launched January 31, 1958. Besides being the first U.S. satellite, it is known for discovering the Van Allen radiation belt.
The Explorer program was transferred to NASA, which continued to use the name for an ongoing series of relatively small space missions, typically an artificial satellite with a science focus. Over the years, NASA has launched a series of Explorer spacecraft carrying a wide variety of scientific investigations.
Explorer satellites have made important discoveries: Earth's magnetosphere and the shape of its gravity field; the solar wind; properties of micrometeoroids raining down on the Earth; much about ultraviolet, cosmic, and X-rays from the solar system and universe beyond; ionospheric physics; Solar plasma; solar energetic particles; and atmospheric physics. These missions have also investigated air density, radio astronomy, geodesy, and gamma ray astronomy. Various space telescopes have made a variety of discoveries, including the first known Earth Trojan asteroid.
The Explorers Program Office at Goddard Space Flight Center in Greenbelt, Maryland, provides management of the multiple scientific exploration missions in the Explorer space flight program. The missions are characterized by relatively moderate cost, and by small to medium sized missions that are capable of being built, tested and launched in a short time interval compared to the large observatories.
Spacecraft by year 
Explorer name numbers can be found in the NSSDC master catalog, typically assigned to each spacecraft in a mission. However, one source says they were not officially assigned after 1975.
|#||Name(s)||Launch Date||Mission||End of Data||Re-Entry|
|1||Explorer 1||January 31, 1958||Energetic particle studies, discovered the Van Allen radiation belt||May 23, 1958||March 31, 1970|
|2||Explorer 2||March 5, 1958||Failed to achieve orbit||–||–|
|3||Explorer 3 (Gamma 1)||March 26, 1958||Energetic particle studies||June 27, 1958||June 27, 1958|
|4||Explorer 4||July 26, 1958||nuclear test studies||October 5, 1958||October 23, 1959|
|5||Explorer 5||August 24, 1958||Failed to achieve orbit||–||–|
|7x||Explorer 7x (S 1)||July 16, 1959||Measure earth's radiation balance, destroyed by range safety||–||–|
|6||Explorer 6 (S-2 (Able 3))||August 7, 1959||Magnetosphere research||October 6, 1959||July 1, 1961|
|7||Explorer 7 (S 1A)||October 13, 1959||Energetic particle studies||August 24, 1961||In orbit|
|–||S 46||March 23, 1960||Analyze electron and proton radiation energies, failed to achieve orbit||–||–|
|8||Explorer 8 (S 30)||November 3, 1960||Measured atmospheric composition of the ionosphere||December 27, 1960||March 27, 2012|
|–||S 56||December 4, 1960||Atmospheric density measurements, failed to achieve orbit||–||–|
|9||Explorer 9 (S 56A)||February 16, 1961||Atmospheric density measurements||April 9, 1964||April 9, 1964|
|–||S 45||February 24, 1961||Ionosphere research, failed to achieve orbit||–||–|
|10||Explorer 10 (P 14)||March 25, 1961||Investigated field magnetic field between the Earth and Moon||March 25, 1961||June 1, 1968|
|11||Explorer 11 (S 15)||April 27, 1961||Gamma ray astronomy||November 17, 1961||In orbit|
|–||S 45A||May 25, 1961||Ionosphere research, failed to achieve orbit||–||–|
|–||S 55 (Meteoroid Satellite-A, Micrometeorite Explorer)||June 30, 1961||Micrometeoroid research, failed to achieve orbit||–||–|
|12||EPE-A (S 3, Energetic Particle Explorer-A)||August 16, 1961||Energetic particle research||December 6, 1961||September 1, 1963|
|13||S 55A||August 25, 1961||Micrometeoroid research||August 28, 1961||August 28, 1961|
|14||EPE-B (Energetic Particle Explorer-B)||October 2, 1962||Energetic particle research||August 11, 1963||July 1, 1966|
|15||EPE-C (Energetic Particle Explorer-C)||October 27, 1962||Energetic particle research||January 30, 1963||January 15, 1978|
|16||S 55B||December 16, 1962||Micrometeoroid research||July 22, 1963||In orbit|
|17||AE-A (Atmosphere Explorer-A)||April 3, 1963||Atmospheric research||July 10, 1963||November 24, 1966|
|18||IMP-A (IMP 1, Interplanetary Monitoring Platform-A)||November 27, 1963||Magnetospheric research||May 10, 1965||December 30, 1965|
|19||AD-A (Atmospheric Density-A)||December 19, 1963||Atmospheric density measurements||May 10, 1981||May 10, 1981|
|20||IE-A (S 48, TOPSI, Ionosphere Explorer-A)||August 25, 1964||Ionosphere research||December 29, 1965||In orbit|
|21||IMP-B (IMP 2, Interplanetary Monitoring Platform-B)||October 4, 1964||Magnetospheric research||October 13, 1965||January 30, 1966|
|22||BE-B (S 66, Beacon Explorer-B)||October 10, 1964||Ionospheric and geodetic research||February 1970||In orbit|
|23||S 55C (Explorer 23)||November 6, 1964||Micrometeoric research||November 7, 1965||June 29, 1983|
|24||AD-B (Atmospheric Density-B)||November 21, 1964||Atmospheric density measurements||October 18, 1968||October 18, 1968|
|25||Injun 4 (IE-B, Ionosphere Explorer-B)||November 21, 1964||Ionospheric research||December 1966||In orbit|
|26||EPE-D (Energetic Particle Explorer-D)||December 21, 1964||High energy particle observations||December 27, 1967||In orbit|
|27||BE-C (Beacon Explorer-C)||April 29, 1965||Magnetospheric research||July 20, 1973||In orbit|
|28||IMP-C (IMP 3, Interplanetary Monitoring Platform-C)||May 29, 1965||Magnetospheric research||May 12, 1967||July 4, 1968|
|29||GEOS 1 (GEOS-A)||November 6, 1965||Geodetic earth monitoring||June 23, 1978||In orbit|
|30||SOLRAD 8 (SE-A)||November 19, 1965||Solar radiation monitoring
(Cover for covert ELINT mission)
|November 5, 1967||In orbit|
|31||DME A||November 29, 1965||Ionospheric research||October 1, 1969||In orbit|
|32||AE-B (Atmosphere Explorer-B)||May 25, 1966||Atmospheric research||March 1967||February 22, 1985|
|33||IMP-D (AIMP 1, Anchored IMP 1, Interplanetary Monitoring Platform-D)||July 1, 1966||Magnetospheric research||September 21, 1971||In orbit|
|34||IMP-F (IMP 4, Interplanetary Monitoring Platform-F)||May 24, 1967||Magnetospheric research||May 3, 1969||May 3, 1969|
|35||IMP-E (AIMP-E, AIMP 2, Anchored IMP 2, Interplanetary Monitoring Platform-E)||July 19, 1967||Magnetospheric research||June 24, 1973||In Lunar orbit|
|36||GEOS 2 (GEOS-B)||January 11, 1968||Geodetic earth monitoring||July 1, 1982||In orbit|
|37||Solrad 9 (SE B)||March 5, 1968||Solar radiation monitoring
(Cover for covert ELINT mission)
|April 30, 1974||November 16, 1990|
|38||RAE-A (RAE 1, Radio Astronomy Explorer-A)||July 4, 1968||Radio astronomy||?||In orbit|
|39||AD-C (Atmospheric Density-C)||August 8, 1968||Atmospheric density measurements||June 23, 1971||June 22, 1981|
|40||Injun 5 (Injun C, IE-C, Ionosphere Explorer-C)||August 8, 1968||Magnetospheric Research||June 1971||In orbit|
|41||IMP-G (IMP 5, Interplanetary Monitoring Platform-G)||June 21, 1969||Magnetospheric research||December 23, 1972||December 23, 1972|
|42||SAS-A (Small Astronomy Satellite-A, SAS 1)||December 12, 1970||X-Ray Astronomy||January 4, 1975||April 5, 1979|
|43||IMP-H (IMP 6, Interplanetary Monitoring Platform-H)||March 13, 1971||Magnetospheric research||October 2, 1974||October 2, 1974|
|44||Solrad 10 (SE-C, SOLRAD-C)||July 8, 1971||Solar radiation monitoring
(Cover for covert ELINT mission)
|June 30, 1973||December 15, 1979|
|45||SSS-A (S-Cubed A)||November 15, 1971||Magnetospheric research||September 30, 1974||January 10, 1992|
|46||Meteoroid Technology Satellite (MTS, METEC)||August 13, 1972||Micrometeoroids research||November 4, 1974||November 2, 1979|
|47||IMP-I (IMP 7, Interplanetary Monitoring Platform-I)||September 23, 1972||Magnetospheric research||October 31, 1978||In orbit|
|48||SAS-B (Small Astronomy Satellite-B, SAS 2)||November 15, 1972||X-Ray Astronomy||June 8, 1973||August 20, 1980|
|49||RAE-B (RAE 2, Radio Astronomy Explorer-B)||June 10, 1973||Radio astronomy||April 26, 1977||In Lunar orbit|
|50||IMP J (IMP 8, Interplanetary Monitoring Platform-J)||October 26, 1973||Magnetospheric research||October 7, 2006||In orbit|
|51||AE-C (Atmosphere Explorer-C)||December 16, 1973||Atmospheric research||?||December 12, 1978|
|52||Hawkeye 1, Injun 6 (IE-D, Ionosphere Explorer-D)||June 3, 1974||Magnetospheric research||April 28, 1978||April 28, 1978|
|53||SAS-C (Small Astronomy Satellite-C, SAS 3)||May 7, 1975||X-Ray Astronomy||April 7, 1979||April 9, 1979|
|54||AE-D (Atmosphere Explorer-D)||October 6, 1975||Atmospheric research||January 29, 1976||March 12, 1976|
|55||AE-E (Atmosphere Explorer-E)||November 20, 1975||Atmospheric research||September 25, 1980||June 10, 1981|
|56||ISEE 1 & 2 (International Sun-Earth Explorer-A & B)||October 22, 1977||Magnetospheric research||September 26, 1987||September 26, 1987|
|57||IUE||January 26, 1978||Ultraviolet astronomy||September 30, 1996||In orbit|
|58||HCMM (AEM-A, Applications Explorer Mission-A, Heat Capacity Mapping Mission)||April 26, 1978||Thermal mapping of the earth||September 30, 1980||December 22, 1981|
|59||ISEE 3 (International Sun-Earth Explorer-C, ICE)||August 12, 1978||Magnetospheric research||Hibernation||In heliocentric orbit|
|60||SAGE (AEM-B, Applications Explorer Mission-B, Stratospheric Aerosol and Gas Experiment)||February 18, 1979||Stratospheric aerosol and ozone data||January 7, 1982||April 11, 1989|
|61||MAGSAT (AEM-C, Applications Explorer Mission-C, Magnetic Field Satellite)||October 30, 1979||Mapped the near surface magnetic field of the Earth||May 6, 1980||June 11, 1980|
|62||DE 1 (DE-A, Dynamics Explorer-A)||August 3, 1981||Magnetospheric research||February 28, 1991||In orbit|
|63||DE 2 (DE-B, Dynamics Explorer-B)||August 3, 1981||Magnetospheric research||1983||February 19, 1983|
|64||SME (Solar Mesosphere Explorer)||October 6, 1981||Atmospheric research||April 4, 1989||March 5, 1991|
|65||AMTPE/CCE (Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer)||August 16, 1984||Magnetospheric research||July 12, 1989||In orbit|
|66||COBE||November 18, 1989||Microwave astronomy||December 23, 1993||In orbit|
|67||EUVE||June 7, 1992||Ultraviolet astronomy||January 30, 2002||January 30, 2002|
|68||SAMPEX||July 3, 1992||Magnetospheric research||June 30, 2004||November 13, 2012.|
|69||RXTE||December 30, 1995||X-ray astronomy||January 3, 2012
Operated for 16 years
|70||FAST||August 21, 1996||Auroral phenomena||2009?||In orbit|
|71||ACE||August 25, 1997||Solar/interplanetary/interstellar particle research||Operational||In L1 orbit|
|72||SNOE||February 26, 1998||Atmospheric research||December 13, 2003||December 13, 2003|
|73||TRACE||April 2, 1998||Solar observatory||June 21, 2010 (Hibernation)||In orbit|
|74||SWAS||December 6, 1998||Submillimeter astronomy||July 2004 hibernation
August 2005 after Deep Impact sub-mission
|75||WIRE||March 5, 1999||Infrared astronomy, primary mission failed due to loss of coolant||No Survey, limited secondary mission||May 10, 2011|
|76||TERRIERS||May 18, 1999||Atmospheric research, satellite failed shortly after achieving orbit||May 18, 1999||In orbit|
|77||FUSE||June 23, 1999||Ultraviolet astronomy||October 18, 2007||In orbit|
|78||IMAGE||March 25, 2000||Magnetospheric research||December 18, 2005||Contact lost|
|79||HETE-2||October 9, 2000||UV, X-Ray, and gamma ray astronomy||Past||In orbit|
|80||WMAP||June 30, 2001||Microwave astronomy||October 2010||Formerly in L2 orbit. Solar Orbit|
|81||RHESSI||February 5, 2002||X-ray and gamma ray solar flare imaging||Operational||In orbit|
|82||CHIPSat||January 13, 2003||Ultraviolet spectroscopy and astronomy||April 11, 2008 ||In orbit|
|83||GALEX||April 28, 2003||Ultraviolet astronomy||Operational||In orbit|
|84||SWIFT||November 20, 2004||Gamma ray astronomy||Operational||In orbit|
|85||THEMIS A||February 17, 2007||Magnetospheric research||Operational||In orbit|
|86||THEMIS B (ARTEMIS P1)||February 17, 2007||Magnetospheric research||Operational||In orbit|
|87||THEMIS C (ARTEMIS P2)||February 17, 2007||Magnetospheric research||Operational||In orbit|
|88||THEMIS D||February 17, 2007||Magnetospheric research||Operational||In orbit|
|89||THEMIS E||February 17, 2007||Magnetospheric research||Operational||In orbit|
|90||AIM||April 25, 2007||Noctilucent cloud observation||Operational||In orbit|
|91||IBEX||October 19, 2008||Mapping the boundary between the Solar System and interstellar space.||Operational||In orbit|
|92||WISE||December 14, 2009||Infrared astronomy||February 17, 2011
|93||NuSTAR||June 13, 2012||High-energy x-ray astronomy||Operational||In orbit|
Spacecraft by name 
Explorer missions in the NSSDC master catalog listing.  This does not include Missions of Opportunity (MO), but does include some spacecraft with launch failures or were cancelled.
Other missions 
Examples of cancelled missions, include:
- Owl 1
- Owl 2
- MSS A
- CATSAT (STEDI 3)
- IMEX (UNEX 2)
- FAME (MIDEX 4)
- SPIDR (SMEX 8)
Many more missions are proposed, but not selected. For example in 2011, the overall explorer program received 22 full missions solicitations, 20 Missions of Opportunity, and 8 USPI.
Another type of program called missions of opportunity (MO), which adds small missions to other spacecraft, such as an additional instrument. Examples of this include Astro-H, CINDI, TWINS, and HETE-2.
Recently ended missions 
Many explorers have been turned off due to tight funding.
See also 
- "Explorers Program". Goddard Space Flight Center. NASA. 2009. Retrieved 2009-12-05.
- Explorer Program History
- Goddard Space Flight Center: Explorers
- The NASA Explorer program of scientific satellites
- "Explorer 33 (NSSDC ID: 1966-058A)". NASA / National Space Science Data Center. 2008-04-02. Retrieved 2008-07-04.
- SAMPEX Data Center
- Caltech: WIRE
- NASA IMAGE
- NASA - HETE-2
- "Mission Complete! WMAP fires its thrusters for the last time".
- CHIPSat quietly shut down
- Gunter's Space Page - Explorer Program
- NASA - EX
- NASA Explorer program missions page
- NSSDC List of Explorer missions from 1 to 77
- Explorer Program Profile by NASA's Solar System Exploration
- 1957 Video (30 minutes) on launch and impact of Explorer
- Master list of Names, Initialisms, and Abbreviations for un-manned satellites
- U.S. Space Objects Registry
- Explorer Program at Gunters Space Page (detailed list of explorer missions
- NASA Science - Explorers