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Explorers Program
United States space exploration program

The Explorers Program is a NASA exploration program that provides flight opportunities for physics, geophysics, heliophysics, and astrophysics investigations from space. Launched in 1958, Explorer 1 was the first spacecraft of the United States to achieve orbit. Over 90 space missions have been launched since. Starting with Explorer 6, it has been operated by NASA, with regular collaboration with a variety of other institutions, including many international partners.

Launchers for the Explorers Program have included Juno I, Juno II, various Thor, Scout, Delta and Pegasus launch vehicles, and Falcon 9.

The program has three classes: Medium-Class Explorers (MIDEX), Small Explorers (SMEX), and University-Class Explorers (UNEX), with select Missions of Opportunity operated with other agencies.

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History

Early Explorer satellites

The Explorers Program began as a U.S. Army proposal (Project Orbiter) to place a "civilian" artificial satellite into orbit during the International Geophysical Year (IGY). Although that proposal was rejected in favor of the U.S. Navy's Project Vanguard, which made the first sub-orbital flight Vanguard TV0 in December 1956, the Soviet Union's launch of Sputnik 1 on 4 October 1957 (and the resulting "Sputnik crisis") and the failure of the Vanguard 1 launch attempt resulted in the Army program being funded to match the Soviet space achievements. Explorer 1 was launched on the Juno I on 1 February 1958, becoming the first U.S. satellite, as well as discovering the Van Allen radiation belt.

Four follow-up satellites of the Explorer series were launched by the Juno I launch vehicle in 1958, of which Explorer 3 and Explorer 4 were successful, while Explorer 2 and Explorer 5 failed to reach orbit.2 The Juno I vehicle was replaced by the Juno II in 1959.

Continuation of the Explorers Program

With the establishment of NASA in 1958, the Explorers Program was transferred to NASA from the U.S. Army. NASA continued to use the name for an ongoing series of relatively small space missions, typically an artificial satellite with a specific science focus. Explorer 6 in 1959 was the first scientific satellite under the project direction of NASA's Goddard Space Flight Center (GSFC) in Greenbelt, Maryland.34

The Interplanetary Monitoring Platform (IMP) was launched in 1963 and involved a network of eleven Explorer satellites designed to collect data on space radiation in support of the Apollo program. The IMP program was a major step forward in spacecraft electronics design, as it was the first space program to use integrated circuit (IC) chips and MOSFETs (MOS transistors).56 The IMP-A (Explorer 18) in 1963 was the first spacecraft to use IC chips, and the IMP-D (Explorer 33) in 1966 was the first to use MOSFETs.7

List of Interplanetary Monitoring Platform (IMP) missions
MissionPhotoSatelliteLaunch DateDecay DateNotes
ExplorerIMP
IMP-1Explorer 18IMP-A27 November 1963, 02:30 UTC8December 30, 1965First use of integrated circuits in a spacecraft. First satellite in IMP-A/-B/-C design series.
IMP-2Explorer 21IMP-B4 October 1964, 03:45 UTC9January 1, 1966Second satellite in IMP-A/-B/-C design series.
IMP-3Explorer 28IMP-C29 May 1965, 12:00 UTC104 July 1968Third satellite in IMP-A/-B/-C design series.
AIMP-1Explorer 33IMP-D1 July 1966, 16:02 UTC11In orbitFirst use of MOSFET integrated circuits in a spacecraft, similar design to IMP-E. Originally intended to orbit the Moon, but placed in an elliptical high orbit instead.
IMP-4Explorer 34IMP-F24 May 1967, 14:05 UTC12May 3, 1969Similar design to IMP-G.
AIMP-2Explorer 35IMP-E19 July 1967, 14:19 UTC13After June 24, 1973Similar design to IMP-D. Positioned in Selenocentric orbit.
IMP-5Explorer 41IMP-G21 June 1969, 08:47 UTC14December 23, 1972Similar design to IMP-F.
IMP-6Explorer 43IMP-I13 March 1971, 16:15 UTC15October 2, 1974First spacecraft in IMP-I/-H/-J series.
IMP-7Explorer 47IMP-H23 September 1972, 01:20 UTC16In orbitSecond spacecraft in IMP-I/-H/-J series.
IMP-8Explorer 50IMP-J26 October 1973, 02:26 UTC17In orbitThird spacecraft in IMP-I/-H/-J series, remained in service until 2006

Over the following two decades, NASA has launched over 50 Explorer missions,18 some in conjunction to military programs, usually of an exploratory or survey nature or had specific objectives not requiring the capabilities of a major space observatory. Explorer satellites have made many important discoveries on: Earth's magnetosphere and the shape of its gravity field; the solar wind; properties of micrometeoroids raining down on the Earth; ultraviolet, cosmic and X-rays from the Solar System and 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.

With decreases in NASA's budget, Explorer missions became infrequent in the early 1980s.

SMEX, MIDEX, and Student Explorers Programs

In 1988, the Small Explorer (SMEX) class was established with a focus on frequent flight opportunities for highly focused and relatively inexpensive space science missions in the disciplines of astrophysics and space physics.1920 The first three SMEX missions were chosen in April 1989 out of 51 candidates, and launched in 1992, 1996 and 199821 The second set of two missions were announced in September 1994 and launched in 1998 and 1999.22

In the mid-1990s, NASA initiated the Medium-class Explorers (MIDEX) to enable more frequent flights. These are larger than SMEX missions and were to be launched aboard a new kind of medium-light class launch vehicle.23 This new launch vehicle was not developed and instead, these missions were flown on a modified Delta II rocket.2425 The first announcement opportunity for MIDEX was issued in March 1995, and the first launch under this new class was FUSE in 1999.26

In May 1994, NASA started the Student Explorer Demonstration Initiative (STEDI) pilot program, to demonstrate that high-quality space science can be carried out with small, low-cost missions. Of the three selected missions, SNOE was launched in 1998 and TERRIERS in 1999, but the latter failed after launch. The STEDI program was terminated in 2001.27 Later, NASA established the University-Class Explorer (UNEX) program for much cheaper missions, which is regarded as a successor to STEDI.28

The Explorer missions were at first managed by the Small Explorers Project Office at NASA's Goddard Space Flight Center (GSFC). In early 1999, that office was closed and with the announcement of opportunity for the third set of SMEX missions NASA converted the SMEX class so that each mission was managed by its principal investigator, with oversight by the GSFC Explorer Project.29 The Explorers Program Office at Goddard Space Flight Center, provides management of the many operational scientific exploration missions that are characterized by relatively moderate costs and small to medium-sized missions that are capable of being built, tested, and launched in a short time interval compared to larger observatories like NASA's Great Observatories.30

Excluding the launches, the MIDEX class has a current mission cap cost of US$250 million in 2018,31 with future MIDEX missions being capped at US$350 million.32 The cost cap for SMEX missions in 2017 was US$165 million.33 UNEX missions are capped at US$15 million.34 A sub-project called Missions of Opportunity (MO) has funded science instruments or hardware components of onboard non-NASA space missions, and have a total NASA cost cap of US$70 million.3536

Classes

Medium-Class (MIDEX)

List of MIDEX missions373839
NameMIDEXnumberExplorernumberLaunch (UTC)Status
RXTEExplorer-6930 December 1995Ended in 2012 / Reentered on 30 April 2018
ACEExplorer-7125 August 1997Operational
FUSEMIDEX-0Explorer-7723 June 1999Ended in 2007
IMAGEMIDEX-1Explorer-7825 March 2000Lost contact in 2005. Partial contact reestablished in January 2018
WMAPMIDEX-2Explorer-8030 June 2001Ended in 2010
SwiftMIDEX-3Explorer-8420 November 2004Operational
FAMEMIDEX-4—Scheduled for 2004Cancelled in 2002 (cost)
THEMIS AMIDEX-5AExplorer-8517 February 2007Operational
THEMIS BMIDEX-5BExplorer-8617 February 2007Operational
THEMIS CMIDEX-5CExplorer-8717 February 2007Operational
THEMIS DMIDEX-5DExplorer-8817 February 2007Operational
THEMIS EMIDEX-5EExplorer-8917 February 2007Operational
WISE / NEOWISEMIDEX-6Explorer-9214 December 2009Mission operations Completed on 31 July 2024.40 Reentered on 2 November 2024
TESSMIDEX-7Explorer-9518 April 2018Operational
ICONMIDEX-8Explorer-9611 October 2019Operational
SPHERExMIDEX-98 March 2025In development41
MUSEMIDEX-102027In development
HelioSwarmMIDEX-112028In development
UVEXMIDEX-122030In development

Small Explorers (SMEX)

The Small Explorers class was implemented in 1989 specifically to fund space exploration missions that cost no more than US$120 million.4243 The missions are managed by the Explorers Project at the Goddard Space Flight Center (GSFC).44

The first set of three SMEX missions were launched between 1992 and 1998. The second set of two missions were launched in 1998 and 1999. These early missions were managed by the Small Explorers Project Office at Goddard Space Flight Center. In early 1999, that office was closed and with the announcement of opportunity for the third set of SMEX missions NASA converted the program so that each mission was managed by its Principal Investigator, with oversight by the GSFC Explorers Project.45

NASA funded a competitive study of five candidate heliophysics Small Explorers missions for flight in 2022. The proposals were Mechanisms of Energetic Mass Ejection – eXplorer (MEME-X), Focusing Optics X-ray Solar Imager (FOXSI), Multi-Slit Solar Explorer (MUSE), Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites (TRACERS), and Polarimeter to Unify the Corona and Heliosphere (PUNCH).464748 In June 2019 NASA selected TRACERS and PUNCH for flight.49

List of SMEX missions 5051
NameSMEXnumberExplorernumberLaunch (UTC)End ofmissionStatus
SAMPEXSMEX-1Explorer-683 July 199230 June 2004Reentered on 13 November 2012
FASTSMEX-2Explorer-7021 August 19964 May 2009
SWASSMEX-3Explorer-746 December 199821 July 2004
TRACESMEX-4Explorer-732 April 199821 June 2010
WIRESMEX-5Explorer-755 March 1999—Spacecraft equipment failure; reentered on 10 May 2011
RHESSISMEX-6Explorer-815 February 2002April 2018Deorbited on 20 April 2023
GALEXSMEX-7Explorer-8328 April 2003May 2012Decommissioned on 28 June 2013
SPIDRSMEX-8—Scheduled for 2005—Cancelled in 2003 due to poor instrument sensitivity 52
Aeronomy of Ice in the Mesosphere (AIM)SMEX-9Explorer-9025 April 2007Reentered on 19 August 2024
IBEXSMEX-10Explorer-9119 October 2008Operational
NuSTARSMEX-11Explorer-9313 June 2012Operational
IRISSMEX-12Explorer-9428 June 2013Operational
GEMSSMEX-13—Scheduled for 2014—Cancelled in 2012 due to expected cost overruns53
Imaging X-ray Polarimetry Explorer (IXPE)SMEX-14Explorer-979 December 2021Operational
PUNCHSMEX-1512 March 202554Launched55
TRACERSSMEX-16April 202556In development
COSISMEX-17August 202757In development

University-Class Explorers (UNEX)

List of UNEX missions58
NameUNEX numberExplorernumberLaunch (UTC)Status
SNOEUNEX-1Explorer-7226 February 1998Ended in 2000
IMEXUNEX-2—Scheduled for 2003Cancelled before 2005 (cost)59
CHIPSUNEX-3Explorer-8212 January 2003Ended in 2008

Missions of Opportunity (MO)

Missions of Opportunity (MO) are investigations characterized by being part of a non-NASA space mission of any size and having a total NASA cost of under $55 million. These missions are conducted on a no-exchange-of-funds basis with the organization sponsoring the mission. NASA solicits proposals for Missions of Opportunity on SMEX, MIDEX and UNEX investigations.60

List of MO61
NameLauncher (mission)Launch (UTC)Status
HETE-2NASA (Explorer-79)9 October 2000Ended in 200862
INTEGRALESA6317 October 2002Operational
Suzaku (Astro-E2)JAXA6410 July 2005Ended in 2015
TWINSNRO (USA-184;65USA-20066)TWINS-1: 28 June 2006TWINS-2: 13 March 2008Operational
CINDIDoD (C/NOFS)6716 April 2008Ended in 2015
Hitomi (Astro-H)JAXA6817 February 2016Failed
NICERISS (CRS-11)3 June 2017Operational
GOLDSES (SES-14)25 January 2018Operational
XRISMJAXA6 September 2023Operational
AWEISS (CRS-29)December 2023Operational
GUSTONASA (high-altitude balloon)31 December 2023Ended on 26 February 202469
SunRISENASA (Maxar satellite)2024In development7071
EZIENASA, JHUAPL2024In development72
Solar-C EUVSTJAXAJuly 2028In development7374
CASEESA (Cosmic Vision M4)2029In development7576

Beacon Explorers

Three satellites were planned in this series: Beacon Explorer-A, Beacon Explorer-B, Beacon Explorer-C.

GEOS series

A series of three Geodetic Earth Orbiting Satellite (GEOS) were put in orbit: GEOS 1, GEOS 2, GEOS 3.

Launched spacecraft

Explorers Program name numbers can be found in the NSSDC master catalog, typically assigned to each spacecraft in a mission. These numbers were not officially assigned until after 1975.77

Explorers Program satellites78798081
No.Name(s)Launch date (UTC)RocketMass (kg)Orbit regimeEnd of dataRe-entry82Mission/Notes
1Explorer 11 February 1958Juno I14MEO23 May 195831 March 1970First American satellite, third satellite to achieve orbit; discovered the Van Allen radiation belt; launched by the U.S. Army83
2Explorer 25 March 1958Juno I15Failed——Failed to achieve orbit.84
3Explorer 326 March 1958Juno I14MEO27 June 195827 June 1958Energetic particle studies helped confirm the presence of Van Allen radiation belt85
4Explorer 426 July 1958Juno I26MEO5 October 195823 October 1959Monitor charged particles inside Van Allen belts from nuclear detonations (during Operation Argus)86
5Explorer 524 August 1958Juno I17failed——Planned in conjunction with Explorer 4, but launch failed87
—Explorer S-1 (7X)16 July 1959Juno II42failed——Planned to measure Earth's radiation balance, but destroyed within seconds by range safety88
6Explorer 6 (S-2, Able 3)7 August 1959Thor-Able64HEO6 October 195912 July 1961Magnetosphere research and digital telemetry; first NASA launch, first Earth photo from orbit8990
7Explorer 7 (S-1A)13 October 1959Juno II42LEO24 August 1961In orbitMicrometeoroids and energetic particle studies, first satellite to measure Earth's climate919293
–S-46A (IE-B)23 March 1960Juno II16failed——Analyze electron and proton radiation energies, failed to achieve orbit94
8Explorer 8 (S-30)3 November 1960Juno II41LEO27 December 196027 March 2012Measured atmospheric composition of the ionosphere95
–S-564 December 1960Scout X-16failed——Atmosphere density measurement, but failed to achieve orbit96
9Explorer 9 (S-56A)16 February 1961Scout X-136LEO9 April 19649 April 1964Atmospheric density measurements, first spacecraft placed in orbit by a solid-fuel rocket97
–S-4524 February 1961Juno II34failed——Ionosphere research, but failed to achieve orbit98
10Explorer 10 (P 14)25 March 1961Thor-Delta79HEO25 March 19611 June 1968Investigated the magnetic field between the Earth and Moon99
11Explorer 11 (S 15)27 April 1961Juno II37LEO17 November 1961In orbitGamma ray astronomy100
–S-45A25 May 1961Juno II34failed——Ionosphere research, failed to achieve orbit. Last Juno II launch.101
–S-55 (satellite) (Meteoroid Satellite-A, Micrometeorite Explorer)30 June 1961Scout X-185failed——Micrometeoroid research, failed to achieve orbit102
12EPE-A (S 3, Energetic Particle Explorer-A)16 August 1961Thor-Delta38HEO6 December 19611 September 1963Energetic particle research103
13Explorer 13 (S-55A)25 August 1961Scout X-186LEO28 August 196128 August 1961Micrometeoroid research; partial failure104
14EPE-B (Energetic Particle Explorer-B)2 October 1962Delta A40HEO11 August 19631 July 1966Energetic particle research105
15EPE-C (S-3B, Energetic Particle Explorer-C)27 October 1962Delta A44HEO30 January 196315 January 1978Energetic particle research106
16S-55B16 December 1962Scout X-3101LEO22 July 1963In orbitMicrometeoroid research107
17AE-A (S-6, Atmosphere Explorer-A)3 April 1963Delta B184LEO10 July 196324 November 1966Atmospheric research108
18IMP-A (IMP 1, Interplanetary Monitoring Platform-A)27 November 1963Delta C138HEO10 May 196530 December 1965Magnetospheric research109
19AD-A (Atmospheric Density-A)19 December 1963Scout X-48LEO10 May 198110 May 1981Atmospheric density measurements110
–BE-A (Beacon Explorer-A, S-66A)19 March 1964Delta B114failed——Launch failure111
20IE-A (S 48, TOPSI, Ionosphere Explorer-A)25 August 1964Scout X-445LEO29 December 1965In orbitIonosphere research112
21IMP-B (IMP 2, Interplanetary Monitoring Platform-B)4 October 1964Delta C135HEO13 October 196530 January 1966Magnetospheric research113
22BE-B (Beacon Explorer-B, S-66B)10 October 1964Scout X-453LEOFebruary 1970In orbitIonospheric and geodetic research114
23S 55C6 November 1964Scout X-4134LEO7 November 196529 June 1983Micrometeoric research115
24AD-B (Atmospheric Density-B)21 November 1964Scout X-49MEO18 October 196818 October 1968Atmospheric density measurements116
25Injun 4 (IE-B, Ionosphere Explorer-B)21 November 1964Scout X-440LEODecember 1966In orbitIonospheric research117
26EPE-D (Energetic Particle Explorer-D)21 December 1964Delta C46MEO27 December 196723 August 2021High energy particle observations118
27BE-C (Beacon Explorer-C, S-66C)29 April 1965Scout X-461LEO20 July 1973In orbitMagnetospheric research119
28IMP-C (IMP 3, Interplanetary Monitoring Platform-C)29 May 1965Delta C128HEO12 May 19674 July 1968Magnetospheric research120
29GEOS 1 (GEOS-A, Geodetic Earth Orbiting Satellite-1)6 November 1965Delta E387LEO23 June 1978In orbitGeodetic Earth monitoring121
30SOLRAD 8 (SE-A)19 November 1965Scout X-457LEO5 November 1967In orbitSolar radiation monitoring (Cover for covert ELINT mission)122
31DME-A (Direct Measurements Explorer)29 November 1965Thor-Agena B99LEO1 October 1969In orbitIonospheric research123
32AE-B (Atmosphere Explorer-B)25 May 1966Delta C1225LEOMarch 196722 February 1985Atmospheric research124
33IMP-D (AIMP 1, Interplanetary Monitoring Platform-D)1 July 1966Delta E1212HEO21 September 1971125In orbitMagnetospheric research
34IMP-F (IMP 4, Interplanetary Monitoring Platform-F)24 May 1967Delta E1163MEO3 May 19693 May 1969Magnetospheric research126
35IMP-E (AIMP 2, Interplanetary Monitoring Platform-E)19 July 1967Delta E1230Lunar24 June 1973Lunar orbitMagnetospheric research127
36GEOS 2 (GEOS-B, Geodetic Earth Orbiting Satellite-2)11 January 1968Delta E1469LEO1 July 1982In orbitGeodetic Earth monitoring128
37SOLRAD 9 (SE B)5 March 1968Scout B198LEO30 April 197416 November 1990Solar radiation monitoring(Cover for covert ELINT mission)129
38RAE-A (RAE 1, Radio Astronomy Explorer-A)4 July 1968Delta J602MEO(~1969)In orbitRadio astronomy130
39AD-C (Atmospheric Density-C)8 August 1968Scout B9LEO23 June 197122 June 1981Atmospheric density measurements131
40Injun 5 (Injun C, IE-C, Ionosphere Explorer-C)8 August 1968Scout B71LEOJune 1971In orbitMagnetospheric Research132
41IMP-G (IMP 5, Interplanetary Monitoring Platform-G)21 June 1969Delta E1145HEO23 December 197223 December 1972Magnetospheric research133
42Uhuru (SAS-A, SAS 1)12 December 1970Scout B142LEO4 January 19755 April 1979X-ray astronomy134
43IMP-H (IMP 7, Interplanetary Monitoring Platform-H)13 March 1971Delta M6635MEO2 October 19742 October 1974Magnetospheric research135
44SOLRAD 10 (SE-C, SOLRAD-C)8 July 1971Scout B260LEO30 June 197315 December 1979Solar radiation monitoring(Cover for covert ELINT mission)136
45SSS-A (S-Cubed A)15 November 1971Scout B52MEO30 September 197410 January 1992Magnetospheric research137
46MTS (Meteoroid Technology Satellite, METEC)13813 August 1972Scout D-190LEO4 November 19742 November 1979Micrometeoroids research139
47IMP-I (IMP 6, Interplanetary Monitoring Platform-I)23 September 1972Delta 1604635HEO31 October 1978In orbitMagnetospheric research140
48SAS-B (Small Astronomy Satellite-B, SAS 2)15 November 1972Scout D-1166LEO8 June 197320 August 1980X-ray astronomy141
49RAE-B (RAE 2, Radio Astronomy Explorer-B)10 June 1973Delta 1913328Lunar26 April 1977Presumed crashed into Moon sometime after August 1977142Radio astronomy143
50IMP-J (IMP 8, Interplanetary Monitoring Platform-J)26 October 1973Delta 1604371HEO7 October 2006In orbitMagnetospheric research144
51AE-C (Atmosphere Explorer-C)16 December 1973Delta 1900658LEO(12 December 1978)12 December 1978Atmospheric research145
52Hawkeye 1 (Injun-F, Injun 6, IE-D, Ionosphere Explorer-D)3 June 1974Scout E-123HEO28 April 197828 April 1978Magnetospheric research146
53SAS-C (Small Astronomy Satellite-C, SAS 3)7 May 1975Scout F-1197LEO7 April 19799 April 1979X-ray astronomy147
54AE-D (Atmosphere Explorer-D)6 October 1975Delta 2910681LEO29 January 197612 March 1976Atmospheric research148
55AE-E (Atmosphere Explorer-E)20 November 1975Delta 2910735LEO25 September 198010 June 1981Atmospheric research149
—DADE-A (Dual Air Density Explorer-A)5 December 1975Scout F-140failed——Atmospheric research; failed during launch150
—DADE-B (Dual Air Density Explorer-B)5 December 1975Scout F-143failed——Atmospheric research; failed during launch151
56ISEE-1 (ISEE-A)22 October 1977Delta 2914340HEO26 September 198726 September 1987Magnetospheric research; launched with ESA's ISEE-2; co-mission with ISEE 3152
57IUE26 January 1978Delta 2914669MEO30 September 1996In orbitUltraviolet astronomy153
58HCMM (AEM-A)26 April 1978Scout F117LEO30 September 1980December 22, 1981Thermal mapping of the Earth154
59ICE (ISEE 3, ISEE-C)12 August 1978Delta 2914390Sun–Earth L116 September 2014Heliocentric orbitMagnetospheric research; heliocentric mission, re-purposed in 1982 as a cometary probe (renamed International Cometary Explorer). First spacecraft to be placed at a libration point, and first one to perform a flyby of a comet.155156
60SAGE (AEM-B)18 February 1979Scout D-1149LEO7 January 198211 April 1989Stratospheric aerosol and ozone data157
61MAGSAT (AEM-C)30 October 1979Scout G-1158LEO6 May 198011 June 1980Mapped the near surface magnetic field of the Earth158
62Dynamics Explorer 1 (DE-1)3 August 1981Delta 3913424MEO28 February 1991In orbitMagnetospheric research159
63Dynamics Explorer 2 (DE-2)3 August 1981Delta 3913420LEO19 February 198319 February 1983Magnetospheric research160
64SME6 October 1981Delta 2310145LEO4 April 19895 March 1991Atmospheric research161
65AMPTE-CCE16 August 1984Delta 3924242MEO12 July 1989In orbitMagnetosphere research162
66COBE18 November 1989Delta 59202,206LEO23 December 1993In orbitMicrowave astronomy163
67EUVE (BERKSAT)7 June 1992Delta II 6920-X3,275LEO31 January 200130 January 2002Ultraviolet astronomy164
68SAMPEX3 July 1992Scout G-1158LEO30 June 200416513 November 2012.166SMEX: magnetospheric research167
69RXTE30 December 1995Delta II 79203,200LEO3 January 201230 April 2018168MIDEX: X-ray astronomy169
70FAST21 August 1996Pegasus XL187LEO4 May 2009In orbitSMEX: auroral phenomena170
—HETE 14 November 1996Pegasus XL128LEO—7 April 2002Separation failure, mission relaunched as HETE 2171
71ACE25 August 1997Delta II 7920596Sun–Earth L1OperationalIn L1 orbitMIDEX: solar/interplanetary/interstellar particle research172
72SNOE26 February 1998Pegasus XL120LEO13 December 200313 December 2003STEDI, UNEX: atmospheric research173
73TRACE2 April 1998Pegasus XL250LEO21 June 2010174In orbitSMEX: solar observatory175
74SWAS6 December 1998Pegasus XL288LEO1 September 2005In orbitSMEX: submillimeter astronomy176
75WIRE5 March 1999Pegasus XL250SSO30 September 200017710 May 2011178SMEX, Infrared astronomy, primary mission failed due to loss of coolant179
76TERRIERS18 May 1999Pegasus XL120Polar LEO18 May 1999In orbitSTEDI: atmospheric research, satellite failed shortly after achieving orbit180
77FUSE23 June 1999Delta II 73201,400LEO18 October 2007In orbitMIDEX: ultraviolet astronomy181
78IMAGE25 March 2000Delta II 7326536Polar MEO18 December 2005182In orbitMIDEX: magnetospheric research183
79HETE-29 October 2000Pegasus-H124LEO28 March 2007184In orbitMO: UV, X-ray, and gamma ray astronomy185
80WMAP30 June 2001Delta II 7425-10840Sun–Earth L2October 2010186Heliocentric orbitMIDEX: microwave astronomy187
81RHESSI5 February 2002Pegasus XL230LEO16 August 2018DeorbitedSMEX: X-ray and gamma ray solar flare imaging188
—INTEGRAL17 October 2002Proton-K Blok DM-24,000HEOOperationalIn orbitInternational: space telescope for observing gamma rays189
82CHIPSat13 January 2003Delta II 7320-1060LEO11 April 2008190In orbitUNEX: ultraviolet spectroscopy and astronomy191
83GALEX28 April 2003Pegasus XL280LEO28 June 2013192In orbitSMEX: ultraviolet astronomy193
84Swift20 November 2004Delta II 7320-10C1,470LEOOperationalIn orbitMIDEX: gamma ray astronomy194
—Suzaku (Astro E2)10 July 2005M-V1,706LEO2 September 2015In orbitMO: instrument on JAXA's Suzaku mission195
—TWINS A28 June 2006Delta IV M+(4,2)classifiedMolniyaOperationalIn orbitMO: payload on Trumpet-F/O-1 1 (USA-184)
85THEMIS A17 February 2007Delta II 792577HEOOperationalIn orbitMIDEX: magnetospheric research196
86THEMIS B (ARTEMIS P1)17 February 2007Delta II 792577LunarOperationalLunar orbitMIDEX; Magnetospheric research197
87THEMIS C (ARTEMIS P2)17 February 2007Delta II 792577LunarOperationalLunar orbitMIDEX: magnetospheric research198
88THEMIS D17 February 2007Delta II 792577HEOOperationalIn orbitMIDEX: magnetospheric research199
89THEMIS E17 February 2007Delta II 792577HEOOperationalIn orbitMIDEX: magnetospheric research200
90AIM25 April 2007Pegasus XL197SSOMarch 202319 August 2024SMEX: noctilucent cloud observation201
—TWINS B13 March 2008Atlas V 411classifiedMolniyaOperationalIn orbitMO: payload on Trumpet-F/O-1 2 (USA-200)
—CINDI16 April 2008Pegasus XL395LEO28 November 201528 November 2015MO: instruments on C/NOFS
91IBEX19 October 2008Pegasus XL107MEOOperationalIn orbitSMEX: mapping the boundary between the Solar System and interstellar space.202
92WISE14 December 2009Delta II 7320661LEOAugust 20242 November 2024MIDEX: infrared astronomy, NEOWISE extension. Discovered first Earth trojan.203
93NuSTAR13 June 2012Pegasus XL350LEOOperationalIn orbitSMEX: high-energy X-ray astronomy204
94IRIS27 June 2013Pegasus XL183SSOOperationalIn orbitSMEX: solar UV astronomy205
—Hitomi (NeXT, ASTRO-H)7 February 2016H-2A-2022,700LEO26 March 2016In orbitMO: X-ray instrument on JAXA's Hitomi, but spacecraft failed after initial checkouts206
—NICER3 May 2017Falcon 9 FT372ISSOperationalISSMO: instrument on ISS for neutron star observations
—GOLD25 January 2018Ariane 5 ECA37GEOOperationalIn orbitMO: instrument on SES-14 comsat for studying Earth–space boundary
95TESS18 April 2018Falcon 9 FT362HEOOperationalIn orbitMIDEX: survey for transiting exoplanets
96ICON11 October 2019Pegasus XL287LEOOperationalIn orbitMIDEX: ionospheric studies
97IXPE9 December 2021Falcon 9 Block 5330LEOOperationalIn orbitSMEX: X-ray studies
—XRISM6 September 2023H-IIA 2022,300LEOOperationalIn orbitMO: instruments on JAXA's XRISM x-ray space telescope

Cancelled missions

Many missions are proposed, but not selected. For example, in 2011, the Explorers Program received 22 full missions solicitations, 20 Missions of Opportunity, and 8 USPI.207 Sometimes mission are only partially developed but must be stopped for financial, technological, or bureaucratic reasons. Some missions failed upon reaching orbit including WIRE and TERRIERS.

Examples of missions that were not developed or cancelled were:208

  • Owl 1 and 2 (cost, 1965)209
  • MSS A (Magnetic Storm Satellite, Explorer-A, 1970)210211
  • CATSAT (STEDI 3) (cost)212
  • IMEX (UNEX 2) (cost)213
  • FAME (MIDEX 4)
  • SPIDR (SMEX 8) (technical, 2003)214
  • GEMS (SMEX 13)

Recent examples of conclusions of launched missions, cancelled due to budgetary constraints:

Launch statistics

Number of launches per decade:215

Number of Explorer launches by decade
Decade
1950s8
1960s34
1970s20
1980s5
1990s11
2000s15
2010s7
2020s2

See also

  • Spaceflight portal
Wikimedia Commons has media related to Explorer program.

References

  1. "Explorers Program". Explorers Program Home Page at NASA Goddard. NASA. Retrieved 3 May 2022. This article incorporates text from this source, which is in the public domain. https://explorers.gsfc.nasa.gov/ ↩

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