The Explorers program[1] 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 Explorer 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.
The Explorer 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.
With the establishment of NASA in 1958, the Explorer 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.[3][4]
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).[5][6] 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.[5]
Over the following two decades, NASA has launched over 50 Explorer missions,[7] 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 drops in NASA's budget, Explorer missions became infrequent in the early 1980s.
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.[8][9] The first three SMEX missions were chosen in April 1989 out of 51 candidates, and launched in 1992, 1996 and 1998[10] The second set of two missions were announced in September 1994 and launched in 1998 and 1999.[8]
In the mid 1990s, NASA initiated the Medium-class Explorer (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.[8] This new launch vehicle was not developed and instead, these missions were flown on a modified Delta II rocket.[11][12] The first announcement opportunity for MIDEX was issued in March 1995, and the first launch under this new class was FUSE in 1999.[8]
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.[8] Later, NASA established the University-Class Explorer (UNEX) program for much cheaper missions, which is regarded as a successor to STEDI.[13]
The Explorer missions were at first managed by the Small Explorer 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.[14] The Explorer 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.[15]
Excluding the launches, the MIDEX class has a current mission cap cost of US$250 million in 2018,[16] with future MIDEX missions being capped at US$350 million.[17] The cost cap for SMEX missions in 2017 was US$165 million.[18] UNEX missions are capped at US$15 million.[19] 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.[16][19]
Name
MIDEX
number
Explorer
number
Launch (UTC)
Status
Explorer-69
30 December 1995
Ended in 2012
Explorer-71
25 August 1997
Operational
MIDEX-0
Explorer-77
23 June 1999
Ended in 2007
MIDEX-1
Explorer-78
25 March 2000
Lost contact in 2005. Partial contact reestablished in January 2018
MIDEX-2
Explorer-80
June 30, 2001
Ended in 2010
MIDEX-3
Explorer-84
November 20, 2004
Operational
MIDEX-4
Scheduled for 2004
Cancelled in 2002 (cost)
MIDEX-5A
Explorer-85
February 17, 2007
Operational
MIDEX-5B
Explorer-86
February 17, 2007
Operational
MIDEX-5C
Explorer-87
February 17, 2007
Operational
MIDEX-5D
Explorer-88
February 17, 2007
Operational
MIDEX-5E
Explorer-89
February 17, 2007
Operational
MIDEX-6
Explorer-92
December 14, 2009
Operational[23]
MIDEX-7
Explorer-95
April 18, 2018
Operational
MIDEX-8
Explorer-96
11 October 2019
Operational
MIDEX-9
April 2025
In development[24]
2030
In development
The Small Explorers class was implemented in 1989 specifically to fund space exploration missions that cost no more than US$120 million.[19][10] The missions are managed by the Explorers Project at the Goddard Space Flight Center (GSFC).[14]
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 Explorer 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.[14]
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).[25][26][27] In June 2019 NASA selected TRACERS and PUNCH for flight.[28]
Name
SMEX
number
Explorer
number
Launch (UTC)
End of
mission
Status
SMEX-1
Explorer-68
3 July 1992
30 June 2004
Reentered on 13 November 2012
SMEX-2
Explorer-70
21 August 1996
4 May 2009
SMEX-3
Explorer-74
6 December 1998
21 July 2004
SMEX-4
Explorer-73
2 April 1998
21 June 2010
SMEX-5
Explorer-75
5 March 1999
—
Spacecraft equipment failure; reentered on 10 May 2011
SMEX-6
Explorer-81
5 February 2002
April 2018
Deorbited on April 20, 2023
SMEX-7
Explorer-83
28 April 2003
May 2012
Decommissioned on 28 June 2013
SPIDR
SMEX-8
Scheduled for 2005
—
Cancelled in 2003 due to poor instrument sensitivity [30]
SMEX-9
Explorer-90
25 April 2007
Operational
SMEX-10
Explorer-91
19 October 2008
Operational
SMEX-11
Explorer-93
13 June 2012
Operational
SMEX-12
Explorer-94
28 June 2013
Operational
SMEX-13
Scheduled for 2014
—
Cancelled in 2012 due to expected cost overruns[31]
SMEX-14
Explorer-97
9 December 2021
Operational
April 2025[32]
In development
April 2025[33]
In development
August 2027[34]
In development
List of UNEX missions[35]
Name
UNEX number
Explorer
number
Launch (UTC)
Status
UNEX-1
Explorer-72
26 February 1998
Ended in 2000
IMEX
UNEX-2
Scheduled for 2003
Cancelled before 2005 (cost)[36]
UNEX-3
Explorer-82
12 January 2003
Ended in 2008
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.[37]
List of MO[35]
Name
Launcher (mission)
Launch (UTC)
Status
NASA (Explorer-79)
9 October 2000
Ended in 2008[38]
17 October 2002
Operational
Suzaku (Astro-E2)
10 July 2005
Ended in 2015
TWINS-1: 28 June 2006
TWINS-2: 13 March 2008
Operational
16 April 2008
Ended in 2015
Hitomi (Astro-H)
17 February 2016
Failed
3 June 2017
Operational
25 January 2018
Operational
6 September 2023
Operational
December 2023
Operational
NASA, high-altitude balloon
December 2023
April 2024
June 2024
In development[49]
July 2028
2029
Three satellites were planned in this series: Beacon Explorer-A, Beacon Explorer-B, Beacon Explorer-C.
A series of three Geodetic Earth Orbiting Satellite (GEOS) were put in orbit: GEOS 1, GEOS 2, GEOS 3.
Explorer 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.[54]
No.
Name(s)
Launch date (UTC)
Rocket
Mass (kg)
Orbit regime
End of data
Re-entry
[58]
Mission/Notes
1
1 February 1958
14
MEO
23 May 1958
31 March 1970
First American satellite, third satellite to achieve orbit; discovered the Van Allen radiation belt; launched by the U.S. Army[59]
2
5 March 1958
15
Failed
—
—
Failed to achieve orbit.[60]
3
26 March 1958
14
MEO
27 June 1958
27 June 1958
Energetic particle studies helped confirm the presence of Van Allen radiation belt[61]
4
July 26, 1958
26
MEO
October 5, 1958
October 23, 1959
Monitor charged particles inside Van Allen belts from nuclear detonations (during Operation Argus)[62]
5
August 24, 1958
17
failed
—
—
Planned in conjunction with Explorer 4, but launch failed[63]
—
Explorer S-1 (7X)
July 16, 1959
42
failed
—
—
Planned to measure Earth's radiation balance, but destroyed within seconds by range safety[64]
6
Explorer 6 (S-2, Able 3)
August 7, 1959
64
HEO
October 6, 1959
July 1, 1961
Magnetosphere research and digital telemetry; first NASA launch, first Earth photo from orbit[65][66]
7
Explorer 7 (S-1A)
October 13, 1959
42
LEO
August 24, 1961
In orbit
Micrometeoroids and energetic particle studies, first satellite to measure Earth's climate[67][68][69]
–
S-46A (IE-B)
March 23, 1960
16
failed
—
—
Analyze electron and proton radiation energies, failed to achieve orbit[70]
8
Explorer 8 (S-30)
3 November 1960
41
LEO
27 December 1960
27 March 2012
Measured atmospheric composition of the ionosphere[71]
–
December 4, 1960
6
failed
—
—
Atmosphere density measurement, but failed to achieve orbit[72]
9
Explorer 9 (S-56A)
February 16, 1961
36
LEO
April 9, 1964
April 9, 1964
Atmospheric density measurements, first spacecraft placed in orbit by a solid-fuel rocket[73]
–
February 24, 1961
34
failed
—
—
Ionosphere research, but failed to achieve orbit[74]
10
Explorer 10 (P 14)
March 25, 1961
79
HEO
March 25, 1961
June 1, 1968
Investigated the magnetic field between the Earth and Moon[75]
11
Explorer 11 (S 15)
April 27, 1961
37
LEO
November 17, 1961
In orbit
Gamma ray astronomy[76]
–
May 25, 1961
34
failed
—
—
Ionosphere research, failed to achieve orbit. Last Juno II launch.[77]
–
S-55 (satellite) (Meteoroid Satellite-A, Micrometeorite Explorer)
June 30, 1961
85
failed
—
—
Micrometeoroid research, failed to achieve orbit[78]
12
EPE-A (S 3, Energetic Particle Explorer-A)
August 16, 1961
38
HEO
December 6, 1961
September 1, 1963
Energetic particle research[79]
13
S-55A
August 25, 1961
86
LEO
August 28, 1961
August 28, 1961
Micrometeoroid research; partial failure[80]
14
EPE-B (Energetic Particle Explorer-B)
October 2, 1962
40
HEO
August 11, 1963
July 1, 1966
Energetic particle research[81]
15
EPE-C (S-3B, Energetic Particle Explorer-C)
October 27, 1962
44
HEO
January 30, 1963
January 15, 1978
Energetic particle research[82]
16
S-55B
December 16, 1962
101
LEO
July 22, 1963
In orbit
Micrometeoroid research[83]
17
AE-A (S-6, Atmosphere Explorer-A)
April 3, 1963
184
LEO
July 10, 1963
November 24, 1966
Atmospheric research[84]
18
IMP-A (IMP 1, Interplanetary Monitoring Platform-A)
November 27, 1963
138
HEO
May 10, 1965
December 30, 1965
Magnetospheric research[85]
19
December 19, 1963
8
LEO
May 10, 1981
May 10, 1981
Atmospheric density measurements[86]
–
BE-A (Beacon Explorer-A, S-66A)
March 19, 1964
114
failed
—
—
Launch failure[87]
20
IE-A (S 48, TOPSI, Ionosphere Explorer-A)
August 25, 1964
45
LEO
December 29, 1965
In orbit
Ionosphere research[88]
21
IMP-B (IMP 2, Interplanetary Monitoring Platform-B)
October 4, 1964
135
HEO
October 13, 1965
January 30, 1966
Magnetospheric research[89]
22
BE-B (Beacon Explorer-B, S-66B)
October 10, 1964
53
LEO
February 1970
In orbit
Ionospheric and geodetic research[90]
23
S 55C
November 6, 1964
134
LEO
November 7, 1965
June 29, 1983
Micrometeoric research[91]
24
AD-B (Atmospheric Density-B)
November 21, 1964
9
MEO
October 18, 1968
October 18, 1968
Atmospheric density measurements[92]
25
Injun 4 (IE-B, Ionosphere Explorer-B)
November 21, 1964
40
LEO
December 1966
In orbit
Ionospheric research[93]
26
EPE-D (Energetic Particle Explorer-D)
December 21, 1964
46
MEO
December 27, 1967
August 23, 2021
High energy particle observations[94]
27
BE-C (Beacon Explorer-C,S-66C)
April 29, 1965
61
LEO
July 20, 1973
In orbit
Magnetospheric research[95]
28
IMP-C (IMP 3, Interplanetary Monitoring Platform-C)
May 29, 1965
128
HEO
May 12, 1967
July 4, 1968
Magnetospheric research[96]
29
GEOS 1 (GEOS-A, Geodetic Earth Orbiting Satellite-1)
November 6, 1965
387
LEO
June 23, 1978
In orbit
Geodetic Earth monitoring[97]
30
SOLRAD 8 (SE-A)
November 19, 1965
57
LEO
November 5, 1967
In orbit
Solar radiation monitoring (Cover for covert ELINT mission)[98]
31
DME-A (Direct Measurements Explorer)
November 29, 1965
99
LEO
October 1, 1969
In orbit
Ionospheric research[99]
32
May 25, 1966
225
LEO
March 1967
February 22, 1985
Atmospheric research[100]
33
IMP-D (AIMP 1, Interplanetary Monitoring Platform-D)
July 1, 1966
212
HEO
September 21, 1971[101]
In orbit
Magnetospheric research
34
IMP-F (IMP 4, Interplanetary Monitoring Platform-F)
May 24, 1967
163
MEO
May 3, 1969
May 3, 1969
Magnetospheric research[102]
35
IMP-E (AIMP 2, Interplanetary Monitoring Platform-E)
July 19, 1967
230
June 24, 1973
Lunar orbit
Magnetospheric research[103]
36
GEOS 2 (GEOS-B, Geodetic Earth Orbiting Satellite-2)
January 11, 1968
469
LEO
July 1, 1982
In orbit
Geodetic Earth monitoring[104]
37
SOLRAD 9 (SE B)
March 5, 1968
198
LEO
April 30, 1974
November 16, 1990
Solar radiation monitoring
(Cover for covert ELINT mission)[105]
38
RAE-A (RAE 1, Radio Astronomy Explorer-A)
July 4, 1968
602
MEO
(~1969)
In orbit
Radio astronomy[106]
39
AD-C (Atmospheric Density-C)
August 8, 1968
9
LEO
June 23, 1971
June 22, 1981
Atmospheric density measurements[107]
40
Injun 5 (Injun C, IE-C, Ionosphere Explorer-C)
August 8, 1968
71
LEO
June 1971
In orbit
Magnetospheric Research[108]
41
IMP-G (IMP 5, Interplanetary Monitoring Platform-G)
June 21, 1969
145
HEO
December 23, 1972
December 23, 1972
Magnetospheric research[109]
42
Uhuru (SAS-A, SAS 1)
December 12, 1970
142
LEO
January 4, 1975
April 5, 1979
X-ray astronomy[110]
43
IMP-H (IMP 7, Interplanetary Monitoring Platform-H)
March 13, 1971
635
MEO
October 2, 1974
October 2, 1974
Magnetospheric research[111]
44
SOLRAD 10 (SE-C, SOLRAD-C)
July 8, 1971
260
LEO
June 30, 1973
December 15, 1979
Solar radiation monitoring
(Cover for covert ELINT mission)[112]
45
SSS-A (S-Cubed A)
November 15, 1971
52
MEO
September 30, 1974
January 10, 1992
Magnetospheric research[113]
46
MTS (Meteoroid Technology Satellite, METEC)[114]
August 13, 1972
90
LEO
November 4, 1974
November 2, 1979
Micrometeoroids research[115]
47
IMP-I (IMP 6, Interplanetary Monitoring Platform-I)
September 23, 1972
635
HEO
October 31, 1978
In orbit
Magnetospheric research[116]
48
SAS-B (Small Astronomy Satellite-B, SAS 2)
November 15, 1972
166
LEO
June 8, 1973
August 20, 1980
X-ray astronomy[117]
49
RAE-B (RAE 2, Radio Astronomy Explorer-B)
June 10, 1973
328
April 26, 1977
Presumed crashed into Moon sometime after August 1977[118]
Radio astronomy[118]
50
IMP-J (IMP 8, Interplanetary Monitoring Platform-J)
October 26, 1973
371
HEO
October 7, 2006
In orbit
Magnetospheric research[119]
51
AE-C (Atmosphere Explorer-C)
December 16, 1973
658
LEO
(December 12, 1978)
December 12, 1978
Atmospheric research[120]
52
Hawkeye 1 (Injun-F, Injun 6, IE-D, Ionosphere Explorer-D)
June 3, 1974
23
HEO
April 28, 1978
April 28, 1978
Magnetospheric research[121]
53
SAS-C (Small Astronomy Satellite-C, SAS 3)
May 7, 1975
197
LEO
April 7, 1979
April 9, 1979
X-ray astronomy[122]
54
AE-D (Atmosphere Explorer-D)
October 6, 1975
681
LEO
January 29, 1976
March 12, 1976
Atmospheric research[123]
55
AE-E (Atmosphere Explorer-E)
November 20, 1975
735
LEO
September 25, 1980
June 10, 1981
Atmospheric research[124]
—
DADE-A (Dual Air Density Explorer-A)
December 5, 1975
40
failed
—
—
Atmospheric research; failed during launch[125]
—
DADE-B (Dual Air Density Explorer-B)
December 5, 1975
43
failed
—
—
Atmospheric research; failed during launch[126]
56
ISEE-1 (ISEE-A)
October 22, 1977
340
HEO
September 26, 1987
September 26, 1987
Magnetospheric research; launched with ESA's ISEE-2; co-mission with ISEE 3[127]
57
January 26, 1978
669
MEO
September 30, 1996
In orbit
Ultraviolet astronomy[128]
58
HCMM (AEM-A)
April 26, 1978
117
LEO
September 30, 1980
December 22, 1981
Thermal mapping of the Earth[129]
59
ICE (ISEE 3, ISEE-C)
August 12, 1978
390
September 16, 2014
Magnetospheric 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.[130][66]
60
SAGE (AEM-B)
February 18, 1979
149
LEO
January 7, 1982
April 11, 1989
Stratospheric aerosol and ozone data[131]
61
MAGSAT (AEM-C)
October 30, 1979
158
LEO
May 6, 1980
June 11, 1980
Mapped the near surface magnetic field of the Earth[132]
62
Dynamics Explorer 1 (DE-1)
3 August 1981
424
MEO
28 February 1991
In orbit
Magnetospheric research[133]
63
Dynamics Explorer 2 (DE-2)
3 August 1981
420
LEO
19 February 1983
19 February 1983
Magnetospheric research[134]
64
October 6, 1981
145
LEO
April 4, 1989
March 5, 1991
Atmospheric research[135]
65
16 August 1984
242
MEO
12 July 1989
In orbit
66
November 18, 1989
2,206
LEO
December 23, 1993
In orbit
Microwave astronomy[137]
67
EUVE (BERKSAT)
June 7, 1992
Delta II 6920-X
3,275
LEO
January 31, 2001
January 30, 2002
Ultraviolet astronomy[138]
68
July 3, 1992
158
LEO
June 30, 2004[139]
November 13, 2012.[140]
SMEX: magnetospheric research[141]
69
December 30, 1995
Delta II 7920
3,200
LEO
January 3, 2012
April 30, 2018[142]
MIDEX: X-ray astronomy[143]
70
August 21, 1996
187
LEO
May 4, 2009
In orbit
SMEX: auroral phenomena[144]
—
November 4, 1996
128
LEO
—
April 7, 2002
Separation failure, mission relaunched as HETE 2[145]
71
August 25, 1997
Delta II 7920
596
Operational
InL1 orbit
MIDEX: solar/interplanetary/interstellar particle research[146]
72
February 26, 1998
120
LEO
December 13, 2003
December 13, 2003
STEDI, UNEX: atmospheric research[147]
73
April 2, 1998
250
LEO
June 21, 2010[148]
In orbit
SMEX: solar observatory[149]
74
December 6, 1998
288
LEO
September 1, 2005
In orbit
SMEX: submillimeter astronomy[150]
75
March 5, 1999
250
September 30, 2000[151]
May 10, 2011[152]
SMEX, Infrared astronomy, primary mission failed due to loss of coolant[153]
76
TERRIERS
May 18, 1999
120
Polar LEO
May 18, 1999
In orbit
STEDI: atmospheric research, satellite failed shortly after achieving orbit[154]
77
June 23, 1999
Delta II 7320
1,400
LEO
October 18, 2007
In orbit
MIDEX: ultraviolet astronomy[155]
78
March 25, 2000
Delta II 7326
536
Polar MEO
December 18, 2005[156]
In orbit
MIDEX: magnetospheric research[157]
79
October 9, 2000
124
LEO
March 28, 2007[158]
In orbit
MO: UV, X-ray, and gamma ray astronomy[159]
80
June 30, 2001
Delta II 7425-10
840
October 2010[160]
Heliocentric orbit
MIDEX: microwave astronomy[161]
81
February 5, 2002
230
LEO
August 16, 2018
In orbit
SMEX: X-ray and gamma ray solar flare imaging[162]
—
October 17, 2002
4,000
HEO
Operational
In orbit
International: space telescope for observing gamma rays[163]
82
January 13, 2003
Delta II 7320-10
60
LEO
April 11, 2008[164]
In orbit
UNEX: ultraviolet spectroscopy and astronomy[165]
83
April 28, 2003
280
LEO
June 28, 2013[166]
In orbit
SMEX: ultraviolet astronomy[167]
84
November 20, 2004
Delta II 7320-10C
1,470
LEO
Operational
In orbit
MIDEX: gamma ray astronomy[168]
—
Suzaku (Astro E2)
July 10, 2005
1,706
LEO
September 2, 2015
In orbit
MO: instrument on JAXA's Suzaku mission[169]
—
June 28, 2006
Delta IV M+(4,2)
classified
Operational
In orbit
MO: payload on Trumpet-F/O-1 1 (USA-184)
85
February 17, 2007
Delta II 7925
77
HEO
Operational
In orbit
MIDEX: magnetospheric research[170]
86
THEMIS B (ARTEMIS P1)
February 17, 2007
Delta II 7925
77
Operational
Lunar orbit
MIDEX; Magnetospheric research[171]
87
THEMIS C (ARTEMIS P2)
February 17, 2007
Delta II 7925
77
Operational
Lunar orbit
MIDEX: magnetospheric research[172]
88
February 17, 2007
Delta II 7925
77
HEO
Operational
In orbit
MIDEX: magnetospheric research[173]
89
February 17, 2007
Delta II 7925
77
HEO
Operational
In orbit
MIDEX: magnetospheric research[174]
90
April 25, 2007
197
Operational
In orbit
SMEX: noctilucent cloud observation[175]
—
March 13, 2008
Atlas V 411
classified
Operational
In orbit
MO: payload on Trumpet-F/O-1 2 (USA-200)
—
April 16, 2008
395
LEO
November 28, 2015
November 28, 2015
MO: instruments on C/NOFS
91
October 19, 2008
107
MEO
Operational
In orbit
SMEX: mapping the boundary between the Solar System and interstellar space.[176]
92
December 14, 2009
Delta II 7320
661
LEO
Operational
In orbit
MIDEX: infrared astronomy, NEOWISE extension. Discovered first Earth trojan.[177]
93
June 13, 2012
350
LEO
Operational
In orbit
SMEX: high-energy X-ray astronomy[178]
94
June 27, 2013
183
Operational
In orbit
SMEX: solar UV astronomy[179]
—
Hitomi (NeXT, ASTRO-H)
February 7, 2016
H-2A-202
2,700
LEO
March 26, 2016
In orbit
MO: X-ray instrument on JAXA's Hitomi, but spacecraft failed after initial checkouts[180]
—
May 3, 2017
Falcon 9FT
372
Operational
ISS
MO: instrument on ISS for neutron star observations
—
January 25, 2018
Ariane 5 ECA
37
Operational
In orbit
MO: instrument on SES-14 comsat for studying Earth–space boundary
95
April 18, 2018
Falcon 9FT
362
HEO
Operational
In orbit
MIDEX: survey for transiting exoplanets
96
October 11, 2019
PegasusXL
287
LEO
Operational
In orbit
MIDEX: ionospheric studies
97
December 9, 2021
Falcon 9 Block 5
330
LEO
Operational
In orbit
SMEX: X-ray studies
—
September 6, 2023
H-IIA 202
2,300
LEO
Operational
In orbit
MO: instruments on JAXA's XRISM x-ray space telescope
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.[181] 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:[57]
Recent examples of conclusions of launched missions, cancelled due to budgetary constraints:
Number of launches per decade:[185]
Number of Explorer launches by decade
Decade
1950s
1960s
1970s
1980s
1990s
2000s
2010s
2020s
This article incorporates text from this source, which is in the public domain: J. Boehm, H.J. Fichtner, and Otto A. Hoberg, EXPLORER SATELLITES LAUNCHED BY JUNO 1 AND JUNO 2 VEHICLES NASA Report
This article incorporates text from this source, which is in the public domain: "Goddard Missions - Past". Goddard Space Flight Center. NASA. 5 June 2018. Retrieved 24 June 2019.
This article incorporates text from this source, which is in the public domain: Butrica, Andrew J. (2015). "Chapter 3: NASA's Role in the Manufacture of Integrated Circuits" (PDF). In Dick, Steven J. (ed.). Historical Studies in the Societal Impact of Spaceflight. NASA. pp. 149-250 (237-42). ISBN 978-1-62683-027-1.
This article incorporates text from this source, which is in the public domain: Butler, P. M. (29 August 1989). Interplanetary Monitoring Platform (PDF). NASA. pp. 1, 11, 134. Retrieved 12 August 2019.
{{cite web}}: CS1 maint: unfit URL (link)
This article incorporates text from this source, which is in the public domain: "Welcome to the Small Explorer's Web Site". NASA. 18 February 2000. Archived from the original on 17 August 2000.
This article incorporates text from this source, which is in the public domain: "Explorers Program". Explorers.gsfc.nasa.gov. 31 January 1958. Archived from the original on 4 March 2016. Retrieved 24 February 2016.
This article incorporates text from this source, which is in the public domain: "NASA Selects Proposals to Study Galaxies, Stars, Planets". NASA. 9 August 2017. Retrieved 28 April 2018.
This article incorporates text from this source, which is in the public domain: Wu, Chauncey; Manuel, Greg; Salas, Andrea (15 August 2016). "2016 Heliophysics Small Explorer (SMEX) & Mission of Opportunity (MO) Solicitations Pre-Proposal Conference" (PDF). NASA. Retrieved 24 June 2019.
This article incorporates text from this source, which is in the public domain: "Explorer Missions". NASA. Archived from the original on 23 March 2010. Retrieved 28 August 2015.
This article incorporates text from this source, which is in the public domain: "Operational Small Explorers (SMEX) Missions". Explorer Program. Archived from the original on 25 March 2016. Retrieved 8 April 2016.
This article incorporates text from this source, which is in the public domain: "The HETE-2 Satellite". NASA. Retrieved 28 April 2018.
This article incorporates text from this source, which is in the public domain: "NASA - NSSDCA - Spacecraft". NASA. Retrieved 20 April 2018.
This article incorporates text from this source, which is in the public domain: "NASA - NSSDCA - Spacecraft". NASA. 21 March 2017. Retrieved 20 April 2018.
This article incorporates text from this source, which is in the public domain: "NASA - NSSDCA - Spacecraft". NASA. Retrieved 20 April 2018.
This article incorporates text from this source, which is in the public domain: "NASA - NSSDCA - Spacecraft". NASA. Retrieved 20 April 2018.
This article incorporates text from this source, which is in the public domain: "NASA - NSSDCA - Spacecraft". NASA. 21 March 2017. Retrieved 20 April 2018.
This article incorporates text from this source, which is in the public domain: "NASA - NSSDCA - Spacecraft". NASA. 21 March 2017. Retrieved 20 April 2018.
This article incorporates text from this source, which is in the public domain: "Explorer 1 (1958-001A)". NASA. 7 January 2022. Retrieved 12 February 2022.
This article incorporates text from this source, which is in the public domain: "Explorer 1 (EXPLR2)". NASA. 7 January 2022. Retrieved 12 February 2022.
This article incorporates text from this source, which is in the public domain: "Explorer 3 (1958-003A)". NASA. 7 January 2022. Retrieved 12 February 2022.
This article incorporates text from this source, which is in the public domain: "AMPTE-CEE (1984-088A)". NSSDCA. Retrieved 26 November 2021.
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