Explorers Program

The Explorers Program is a United States space exploration program that provides flight opportunities for physics, geophysics, heliophysics, and astrophysics investigations from space. Launched in 1958, Explorer 1 was USA's first spacecraft 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.

For other uses, see Explorer (disambiguation).
An Explorers mission observes Sagittarius A*, the Milky Way's central black hole, flaring.

Launchers for the Explorers Program have included Jupiter C (Juno I), Juno II, various Thor, Scout, Delta and Pegasus rockets, and Falcon 9.

Currently, 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.

History

Early Explorer Satellites
Launch of Explorer 1 on the Juno I rocket.
Explorer 1, the first Earth satellite orbited by the United States

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. 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 October 4, 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 January 31, 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 rocket in 1958, of which Explorer 3 and Explorer 4 were successful, while Explorer 2 and Explorer 5 failed to reach orbit.[1] The Juno-1 vehicle was replaced by the Juno II in 1959.

Continuation of the Explorers program
This artificially colored view of M101 maps ultraviolet light as blue while visible light is red since UV light does not have a "color" (the eye stopping at about violet). This view was taken by the Explorer SWIFT, which can also detect X-Rays, and has contributed to the study of Gamma-ray bursts and other topics

With the establishment of NASA in 1958, the Explorers Program was transferred to NASA from the US 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.[2][3]

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).[4][5] 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.[4]

Over the following two decades, NASA has launched over 50 Explorer missions,[6] 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.

SMEX, MIDEX, and Student Explorer 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.[7][8] The first three SMEX missions were chosen in April 1989 out of 51 candidates, and launched in 1992, 1996 and 1998.[9] The second set of two missions were announced in September 1994 and launched in 1998 and 1999.[7]

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.[7] This new launch vehicle was not developed and instead, these missions were flown on a modified Delta II rocket.[10][11] The first announcement opportunity for MIDEX was issued in March 1995, and the first launch under this new class was FUSE in 1999.[7]

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.[7] Later, NASA established the University-Class Explorers (UNEX) program for much cheaper missions, which is regarded as a successor to STEDI.[12]

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 Explorers Project.[13] The Explorers Program Office at Goddard Space Flight Center in Greenbelt, Maryland, 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.[14]

Excluding the launches, the MIDEX class has a current mission cap cost of US$250 million in 2018,[15] with future MIDEX missions being capped at $350 million.[16] The cost cap for SMEX missions in 2017 was $165 million.[17] UNEX missions are capped at $15 million.[18] 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 $70 million.[15][18]

Classes

Medium-Class Explorers (MIDEX)
List of MIDEX missions[19][20][21]
Name MIDEX
number		 Explorer
number		 Launch (UTC) Status
RXTE Explorer-69 December 30, 1995 Ended in 2012
ACE Explorer-71 August 25, 1997 Operational
FUSE MIDEX-0 Explorer-77 June 23, 1999 Ended in 2007
IMAGE MIDEX-1 Explorer-78 March 25, 2000 Lost contact in 2005. Partial contact reestablished in January 2018
WMAP MIDEX-2 Explorer-80 June 30, 2001 Ended in 2010
SWIFT MIDEX-3 Explorer-84 November 20, 2004 Operational
FAME MIDEX-4
—
 Scheduled for 2004 Cancelled in 2002 (cost)
THEMIS A MIDEX-5A Explorer-85 February 17, 2007 Operational
THEMIS B MIDEX-5B Explorer-86 February 17, 2007 Operational
THEMIS C MIDEX-5C Explorer-87 February 17, 2007 Operational
THEMIS D MIDEX-5D Explorer-88 February 17, 2007 Operational
THEMIS E MIDEX-5E Explorer-89 February 17, 2007 Operational
WISE / NEOWISE MIDEX-6 Explorer-92 December 14, 2009 Operational[22]
TESS MIDEX-7 Explorer-95 April 18, 2018 Operational
ICON MIDEX-8 Explorer-96 11 October 2019 Operational
SPHEREx MIDEX-9 2024 In development[23]

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.[18][9] The missions are managed by the Explorers Project at the Goddard Space Flight Center (GSFC).[13]

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.[13]

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).[24][25][26] In June 2019 NASA selected TRACERS and PUNCH for flight.[27]

List of SMEX missions[20][28]
Name SMEX
number		 Explorer
number		 Launch (UTC) End of
mission		 Status
SAMPEX SMEX-1 Explorer-68 3 July 1992 30 June 2004 Reentered on 13 November 2012
FAST SMEX-2 Explorer-70 21 August 1996 4 May 2009
SWAS SMEX-3 Explorer-74 6 December 1998 21 July 2004
TRACE SMEX-4 Explorer-73 2 April 1998 21 June 2010
WIRE SMEX-5 Explorer-75 5 March 1999 N/A Spacecraft equipment failure; reentered on 10 May 2011
RHESSI SMEX-6 Explorer-81 5 February 2002 April 2018 Decommissioned on 16 August 2018
GALEX SMEX-7 Explorer-83 28 April 2003 May 2012 Decommissioned on 28 June 2013
SPIDR SMEX-8
—
 Scheduled for 2005 N/A Cancelled in 2003 due to poor instrument sensitivity[29]
AIM SMEX-9 Explorer-90 25 April 2007 Operational
IBEX SMEX-10 Explorer-91 19 October 2008 Operational
NuSTAR SMEX-11 Explorer-93 13 June 2012 Operational
IRIS SMEX-12 Explorer-94 28 June 2013 Operational
GEMS SMEX-13
—
 Scheduled for 2014 N/A Cancelled in 2012 due to expected cost overruns[30]
IXPE SMEX-14 2021[31] In development[32]
PUNCH 2023[33] In development
TRACERS 2023[33] In development
  • SAMPEX
  • RHESSI
  • IBEX
  • IXPE

University-Class Explorers (UNEX)
List of UNEX missions[34]
Name UNEX number Explorer
number		 Launch (UTC) Status
SNOE UNEX-1 Explorer-72 26 February 1998 Ended in 2000
IMEX UNEX-2
—
 Scheduled for 2003 Cancelled before 2005 (cost)[35]
CHIPS UNEX-3 Explorer-82 12 January 2003 Ended 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.[36]

List of MO[34]
Name Launcher (mission) Launch (UTC) Status
HETE-2 NASA (Explorer-79) 9 October 2000 Ended in 2008[37]
INTEGRAL ESA[38] October 17, 2002 Operational
Suzaku (Astro-E2) JAXA[39] 10 July 2005 Ended in 2015
TWINS NRO (USA-184;[40]
USA-200[41])		 TWINS-1: 28 June 2006
TWINS-2: 13 March 2008		 Operational
CINDI DoD (C/NOFS)[42] 16 April 2008 Ended in 2015
Hitomi (Astro-H) JAXA[43] 17 February 2016 Failed
NICER ISS (CRS-11) 3 June 2017 Operational
GOLD SES (SES-14) 25 January 2018 Operational
GUSTO NASA, high-altitude balloon December 2021 In development[44]
XRISM JAXA January 2022 In development
AWE ISS (CRS-?) August 2022 In development[45]
SunRISE NASA (Maxar satellite) July 2023 In development[46]
CASE ESA (Cosmic Vision M4) 2028 In development[47][48]

Launched spacecraft
See also: List of Explorers Program missions

Explorers 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.[49]

Explorers Program satellites[6][50][51][52]
# Name(s) Launch date Rocket Mass (kg) Orbit regime End of data Re-entry
[53]		 Mission/Notes
1 Explorer 1 January 31, 1958 Juno I 14 MEO May 23, 1958 March 31, 1970 First American satellite, third satellite to achieve orbit; discovered the Van Allen radiation belt; launched by the US Army[54]
2 Explorer 2 March 5, 1958 Juno I 15 failed — — Failed to achieve orbit[55]
3 Explorer 3 (Gamma 1) March 26, 1958 Juno I 14 MEO June 27, 1958 June 27, 1958 Energetic particle studies helped confirm the presence of Van Allen radiation belt[56]
4 Explorer 4 July 26, 1958 Juno I 26 MEO October 5, 1958 October 23, 1959 Monitor charged particles inside Van Allen belts from nuclear detonations (during Operation Argus)[57]
5 Explorer 5 August 24, 1958 Juno I 17 failed — — Planned in conjunction with Explorer 4, but launch failed[58]
— Explorer 7x (S 1) July 16, 1959 Juno I 42 failed — — Planned to measure Earth's radiation balance, but destroyed within seconds by range safety[59]
6 Explorer 6 (S-2, Able 3) August 7, 1959 Thor-Able 64 HEO October 6, 1959 July 1, 1961 Magnetosphere research and digital telemetry; first NASA launch, first Earth photo from orbit[60][61]
7 Explorer 7 (S-1A) October 13, 1959 Juno II 42 LEO August 24, 1961 In orbit Micrometeoroids and energetic particle studies, first satellite to measure Earth's climate[62][63][64]
– S-46A (IE-B) March 23, 1960 Juno II 16 failed — — Analyze electron and proton radiation energies, failed to achieve orbit[65]
8 Explorer 8 (S-30) November 3, 1960 Juno II 41 LEO December 27, 1960 March 27, 2012 Measured atmospheric composition of the ionosphere[66]
– S-56 December 4, 1960 Scout X-1 6 failed — — Atmosphere density measurement, but failed to achieve orbit[67]
9 Explorer 9 (S-56A) February 16, 1961 Scout X-1 36 LEO April 9, 1964 April 9, 1964 Atmospheric density measurements, first spacecraft placed in orbit by a solid-fuel rocket[68]
– S-45 February 24, 1961 Juno II 34 failed — — Ionosphere research, but failed to achieve orbit[69]
10 Explorer 10 (P 14) March 25, 1961 Thor-Delta 79 HEO March 25, 1961 June 1, 1968 Investigated the magnetic field between the Earth and Moon[70]
11 Explorer 11 (S 15) April 27, 1961 Juno II 37 LEO November 17, 1961 In orbit Gamma ray astronomy[71]
– S-45A May 25, 1961 Juno II 34 failed — — Ionosphere research, failed to achieve orbit. Last Juno II launch.[72]
– S-55 (satellite) (Meteoroid Satellite-A, Micrometeorite Explorer) June 30, 1961 Scout X-1 85 failed — — Micrometeoroid research, failed to achieve orbit[73]
12 EPE-A (S 3, Energetic Particle Explorer-A) August 16, 1961 Thor-Delta 38 HEO December 6, 1961 September 1, 1963 Energetic particle research[74]
13 S-55A August 25, 1961 Scout X-1 86 LEO August 28, 1961 August 28, 1961 Micrometeoroid research; partial failure[75]
14 EPE-B (Energetic Particle Explorer-B) October 2, 1962 Delta A 40 HEO August 11, 1963 July 1, 1966 Energetic particle research[76]
15 EPE-C (S-3B, Energetic Particle Explorer-C) October 27, 1962 Delta A 44 HEO January 30, 1963 January 15, 1978 Energetic particle research[77]
16 S-55B December 16, 1962 Scout X-3 101 LEO July 22, 1963 In orbit Micrometeoroid research[78]
17 AE-A (S-6, Atmosphere Explorer-A) April 3, 1963 Delta B 184 LEO July 10, 1963 November 24, 1966 Atmospheric research[79]
18 IMP-A (IMP 1, Interplanetary Monitoring Platform-A) November 27, 1963 Delta C 138 HEO May 10, 1965 December 30, 1965 Magnetospheric research[80]
19 AD-A (Atmospheric Density-A) December 19, 1963 Scout X-4 8 LEO May 10, 1981 May 10, 1981 Atmospheric density measurements[81]
– BE-A (Beacon Explorer-A, S-66A) March 19, 1964 Delta B 114 failed — — Launch failure[82]
20 IE-A (S 48, TOPSI, Ionosphere Explorer-A) August 25, 1964 Scout X-4 45 LEO December 29, 1965 In orbit Ionosphere research[83]
21 IMP-B (IMP 2, Interplanetary Monitoring Platform-B) October 4, 1964 Delta C 135 HEO October 13, 1965 January 30, 1966 Magnetospheric research[84]
22 BE-B (Beacon Explorer-B, S-66B) October 10, 1964 Scout X-4 53 LEO February 1970 In orbit Ionospheric and geodetic research[85]
23 S 55C November 6, 1964 Scout X-4 134 LEO November 7, 1965 June 29, 1983 Micrometeoric research[86]
24 AD-B (Atmospheric Density-B) November 21, 1964 Scout X-4 9 MEO October 18, 1968 October 18, 1968 Atmospheric density measurements[87]
25 Injun 4 (IE-B, Ionosphere Explorer-B) November 21, 1964 Scout X-4 40 LEO December 1966 In orbit Ionospheric research[88]
26 EPE-D (Energetic Particle Explorer-D) December 21, 1964 Delta C 46 MEO December 27, 1967 In orbit High energy particle observations[89]
27 BE-C (Beacon Explorer-C,S-66C) April 29, 1965 Scout X-4 61 LEO July 20, 1973 In orbit Magnetospheric research[90]
28 IMP-C (IMP 3, Interplanetary Monitoring Platform-C) May 29, 1965 Delta C 128 HEO May 12, 1967 July 4, 1968 Magnetospheric research[91]
29 GEOS 1 (GEOS-A, Geodetic Earth Orbiting Satellite-1) November 6, 1965 Delta E 387 LEO June 23, 1978 In orbit Geodetic Earth monitoring[92]
30 SOLRAD 8 (SE-A) November 19, 1965 Scout X-4 57 LEO November 5, 1967 In orbit Solar radiation monitoring (Cover for covert ELINT mission)[93]
31 DME-A (Direct Measurements Explorer) November 29, 1965 Thor-Agena B 99 LEO October 1, 1969 In orbit Ionospheric research[94]
32 AE-B (Atmosphere Explorer-B) May 25, 1966 Delta C1 225 LEO March 1967 February 22, 1985 Atmospheric research[95]
33 IMP-D (AIMP 1, Interplanetary Monitoring Platform-D) July 1, 1966 Delta E1 212 HEO September 21, 1971[96] In orbit Magnetospheric research
34 IMP-F (IMP 4, Interplanetary Monitoring Platform-F) May 24, 1967 Delta E1 163 MEO May 3, 1969 May 3, 1969 Magnetospheric research[97]
35 IMP-E (AIMP 2, Interplanetary Monitoring Platform-E) July 19, 1967 Delta E1 230 Lunar June 24, 1973 Lunar orbit Magnetospheric research[98]
36 GEOS 2 (GEOS-B, Geodetic Earth Orbiting Satellite-2) January 11, 1968 Delta E1 469 LEO July 1, 1982 In orbit Geodetic Earth monitoring[99]
37 SOLRAD 9 (SE B) March 5, 1968 Scout B 198 LEO April 30, 1974 November 16, 1990 Solar radiation monitoring
(Cover for covert ELINT mission)[100]
38 RAE-A (RAE 1, Radio Astronomy Explorer-A) July 4, 1968 Delta J 602 MEO (~1969) In orbit Radio astronomy[101]
39 AD-C (Atmospheric Density-C) August 8, 1968 Scout B 9 LEO June 23, 1971 June 22, 1981 Atmospheric density measurements[102]
40 Injun 5 (Injun C, IE-C, Ionosphere Explorer-C) August 8, 1968 Scout B 71 LEO June 1971 In orbit Magnetospheric Research[103]
41 IMP-G (IMP 5, Interplanetary Monitoring Platform-G) June 21, 1969 Delta E1 145 HEO December 23, 1972 December 23, 1972 Magnetospheric research[104]
42 Uhuru (SAS-A, SAS 1) December 12, 1970 Scout B 142 LEO January 4, 1975 April 5, 1979 X-ray astronomy[105]
43 IMP-H (IMP 7, Interplanetary Monitoring Platform-H) March 13, 1971 Delta M6 635 MEO October 2, 1974 October 2, 1974 Magnetospheric research[106]
44 SOLRAD 10 (SE-C, SOLRAD-C) July 8, 1971 Scout B 260 LEO June 30, 1973 December 15, 1979 Solar radiation monitoring
(Cover for covert ELINT mission)[107]
45 SSS-A (S-Cubed A) November 15, 1971 Scout B 52 MEO September 30, 1974 January 10, 1992 Magnetospheric research[108]
46 MTS (Meteoroid Technology Satellite, METEC)[109] August 13, 1972 Scout D-1 90 LEO November 4, 1974 November 2, 1979 Micrometeoroids research[110]
47 IMP-I (IMP 6, Interplanetary Monitoring Platform-I) September 23, 1972 Delta 1604 635 HEO October 31, 1978 In orbit Magnetospheric research[111]
48 SAS-B (Small Astronomy Satellite-B, SAS 2) November 15, 1972 Scout D-1 166 LEO June 8, 1973 August 20, 1980 X-ray astronomy[112]
49 RAE-B (RAE 2, Radio Astronomy Explorer-B) June 10, 1973 Delta 1913 328 Lunar April 26, 1977 Lunar orbit Radio astronomy[113]
50 IMP-J (IMP 8, Interplanetary Monitoring Platform-J) October 26, 1973 Delta 1604 371 HEO October 7, 2006 In orbit Magnetospheric research[114]
51 AE-C (Atmosphere Explorer-C) December 16, 1973 Delta 1900 658 LEO (December 12, 1978) December 12, 1978 Atmospheric research[115]
52 Hawkeye 1 (Injun-F, Injun 6, IE-D, Ionosphere Explorer-D) June 3, 1974 Scout E-1 23 HEO April 28, 1978 April 28, 1978 Magnetospheric research[116]
53 SAS-C (Small Astronomy Satellite-C, SAS 3) May 7, 1975 Scout F-1 197 LEO April 7, 1979 April 9, 1979 X-ray astronomy[117]
54 AE-D (Atmosphere Explorer-D) October 6, 1975 Delta 2910 681 LEO January 29, 1976 March 12, 1976 Atmospheric research[118]
55 AE-E (Atmosphere Explorer-E) November 20, 1975 Delta 2910 735 LEO September 25, 1980 June 10, 1981 Atmospheric research[119]
— DADE-A (Dual Air Density Explorer-A) December 5, 1975 Scout F-1 40 failed — — Atmospheric research; failed during launch[120]
— DADE-B (Dual Air Density Explorer-B) December 5, 1975 Scout F-1 43 failed — — Atmospheric research; failed during launch[121]
56 ISEE-1 (ISEE-A) October 22, 1977 Delta 2914 340 HEO September 26, 1987 September 26, 1987 Magnetospheric research; launched with ESA's ISEE-2; co-mission with ISEE 3[122]
57 IUE January 26, 1978 Delta 2914 669 MEO September 30, 1996 In orbit Ultraviolet astronomy[123]
58 HCMM (AEM-A) April 26, 1978 Scout F 117 LEO September 30, 1980 December 22, 1981 Thermal mapping of the Earth[124]
59 ICE (ISEE 3, ISEE-C) August 12, 1978 Delta 2914 390 Sun–Earth L1 September 16, 2014 Heliocentric orbit 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.[125][61]
60 SAGE (AEM-B) February 18, 1979 Scout D-1 149 LEO January 7, 1982 April 11, 1989 Stratospheric aerosol and ozone data[126]
61 MAGSAT (AEM-C) October 30, 1979 Scout G-1 158 LEO May 6, 1980 June 11, 1980 Mapped the near surface magnetic field of the Earth[127]
62 DE 1 (DE-A) August 3, 1981 Delta 3913 424 MEO February 28, 1991 In orbit Magnetospheric research[128]
63 DE 2 (DE-B) August 3, 1981 Delta 3913 420 LEO February 19, 1983 February 19, 1983 Magnetospheric research[129]
64 SME October 6, 1981 Delta 2310 145 LEO April 4, 1989 March 5, 1991 Atmospheric research[130]
65 AMTPE (CCE) August 16, 1984 Delta 3924 242 MEO July 12, 1989 In orbit Magnetospheric research[131]
66 COBE November 18, 1989 Delta 5920 2,206 LEO December 23, 1993 In orbit Microwave astronomy[132]
67 EUVE (BERKSAT) June 7, 1992 Delta II 6920-X 3,275 LEO January 31, 2001 January 30, 2002 Ultraviolet astronomy[133]
68 SAMPEX July 3, 1992 Scout G-1 158 LEO June 30, 2004[134] November 13, 2012.[135] SMEX: magnetospheric research[136]
69 RXTE December 30, 1995 Delta II 7920 3,200 LEO January 3, 2012 April 30, 2018[137] MIDEX: X-ray astronomy[138]
70 FAST August 21, 1996 Pegasus XL 187 LEO May 4, 2009 In orbit SMEX: auroral phenomena[139]
— HETE 1 August 21, 1996 Pegasus XL 128 LEO — April 7, 2002 Separation failure, mission relaunched as HETE 2[140]
71 ACE August 25, 1997 Delta II 7920 596 Sun–Earth L1 Operational In L1 orbit MIDEX: solar/interplanetary/interstellar particle research[141]
72 SNOE February 26, 1998 Pegasus XL 120 LEO December 13, 2003 December 13, 2003 STEDI, UNEX: atmospheric research[142]
73 TRACE April 2, 1998 Pegasus XL 250 LEO June 21, 2010[143] In orbit SMEX: solar observatory[144]
74 SWAS December 6, 1998 Pegasus XL 288 LEO September 1, 2005 In orbit SMEX: submillimeter astronomy[145]
75 WIRE March 5, 1999 Pegasus XL 250 SSO September 30, 2000[146] May 10, 2011[147] SMEX, Infrared astronomy, primary mission failed due to loss of coolant[148]
76 TERRIERS May 18, 1999 Pegasus XL 120 Polar LEO May 18, 1999 In orbit STEDI: atmospheric research, satellite failed shortly after achieving orbit[149]
77 FUSE June 23, 1999 Delta II 7320 1,400 LEO October 18, 2007 In orbit MIDEX: ultraviolet astronomy[150]
78 IMAGE March 25, 2000 Delta II 7326 536 Polar MEO December 18, 2005[151] In orbit MIDEX: magnetospheric research[152]
79 HETE-2 October 9, 2000 Pegasus-H 124 LEO March 28, 2007[153] In orbit MO: UV, X-ray, and gamma ray astronomy[154]
80 WMAP June 30, 2001 Delta II 7425-10 840 Sun–Earth L2 October 2010[155] Heliocentric orbit MIDEX: microwave astronomy[156]
81 RHESSI February 5, 2002 Pegasus XL 230 LEO August 16, 2018 In orbit SMEX: X-ray and gamma ray solar flare imaging[157]
— INTEGRAL October 17, 2002 Proton-K Blok DM-2 4,000 HEO Operational In orbit International: space telescope for observing gamma rays[158]
82 CHIPSat January 13, 2003 Delta II 7320-10 60 LEO April 11, 2008[159] In orbit UNEX: ultraviolet spectroscopy and astronomy[160]
83 GALEX April 28, 2003 Pegasus XL 280 LEO June 28, 2013[161] In orbit SMEX: ultraviolet astronomy[162]
84 SWIFT November 20, 2004 Delta II 7320-10C 1,470 LEO Operational In orbit MIDEX: gamma ray astronomy[163]
— Suzaku (Astro E2) July 10, 2005 M-V 1,706 LEO September 2, 2015 In orbit MO: instrument on JAXA's Suzaku mission[164]
— TWINS A June 28, 2006 Delta IV M+(4,2) classified Molniya Operational In orbit MO: payload on Trumpet-F/O-1 1 (USA-184)
85 THEMIS A February 17, 2007 Delta II 7925 77 HEO Operational In orbit MIDEX: magnetospheric research[165]
86 THEMIS B (ARTEMIS P1) February 17, 2007 Delta II 7925 77 Lunar Operational Lunar orbit MIDEX; Magnetospheric research[166]
87 THEMIS C (ARTEMIS P2) February 17, 2007 Delta II 7925 77 Lunar Operational Lunar orbit MIDEX: magnetospheric research[167]
88 THEMIS D February 17, 2007 Delta II 7925 77 HEO Operational In orbit MIDEX: magnetospheric research[168]
89 THEMIS E February 17, 2007 Delta II 7925 77 HEO Operational In orbit MIDEX: magnetospheric research[169]
90 AIM April 25, 2007 Pegasus XL 197 SSO Operational In orbit SMEX: noctilucent cloud observation[170]
— TWINS B March 13, 2008 Atlas V 411 classified Molniya Operational In orbit MO: payload on Trumpet-F/O-1 2 (USA-200)
— CINDI April 16, 2008 Pegasus XL 395 LEO November 28, 2015 November 28, 2015 MO: instruments on C/NOFS
91 IBEX October 19, 2008 Pegasus XL 107 MEO Operational In orbit SMEX: mapping the boundary between the Solar System and interstellar space.[171]
92 WISE December 14, 2009 Delta II 7320 661 LEO Operational In orbit MIDEX: infrared astronomy, NEOWISE extension. Discovered first Earth trojan.[172]
93 NuSTAR June 13, 2012 Pegasus XL 350 LEO Operational In orbit SMEX: high-energy X-ray astronomy[173]
94 IRIS June 27, 2013 Pegasus XL 183 SSO Operational In orbit SMEX: solar UV astronomy[174]
— 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[175]
— NICER May 3, 2017 Falcon 9 FT 372 ISS Operational ISS MO: instrument on ISS for neutron star observations
— GOLD January 25, 2018 Ariane 5 ECA 37 GEO Operational In orbit MO: instrument on SES-14 comsat for studying Earth–space boundary
95 TESS April 18, 2018 Falcon 9 FT 362 HEO Operational In orbit MIDEX: survey for transiting exoplanets
96 ICON October 11, 2019 Pegasus XL 287 LEO Operational In orbit MIDEX: ionospheric studies

Cancelled missions
WISE was restarted after it was turned off
Explorer 6 on a Thor-Able III launch in August 1959
ISEE-C in a dynamic test chamber, 1978

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.[176] 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:[52]

  • Owl 1 and 2 (cost, 1965)[177]
  • MSS A (Magnetic Storm Satellite, Explorer-A, 1970)[178][179]
  • CATSAT (STEDI 3) (cost)[35]
  • IMEX (UNEX 2) (cost)[35]
  • FAME (MIDEX 4)
  • SPIDR (SMEX 8) (technical, 2003)[35]
  • GEMS (SMEX 13)

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

Launch statistics

Number of launches per decade:[180]

Number of Explorer launches by decade
Decade
    1950s
    8
    1960s
    34
    1970s
    20
    1980s
    5
    1990s
    11
    2000s
    15
    2010s
    7
    gollark: Okay, I fixed it so I'm not doxxing myself.
    gollark: One where the government can't really be trusted to do much sanely.
    gollark: Please hold on while I censor it.
    gollark: Okay, oops, accidentally released my details.
    gollark: I have pictures somewhere.

    See also

    References
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