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Explorers Program

April 29, 2024

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.

An Explorer mission observes Sagittarius A*, the Milky Way's central black hole, flaring.

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.

History edit

Early Explorer satellites edit

 
Launch of Explorer 1 on the Juno I launch vehicle.
 
Explorer 1, the first Earth satellite orbited by the United States

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.

Continuation of the Explorer program edit

 
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 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.

SMEX, MIDEX, and Student Explorer programs edit

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]

Classes edit

Medium-Class Explorers (MIDEX) edit

List of MIDEX missions[20][21][22]
Name MIDEX
number 		Explorer
number 		Launch (UTC) Status
RXTE Explorer-69 30 December 1995 Ended in 2012
ACE Explorer-71 25 August 1997 Operational
FUSE MIDEX-0 Explorer-77 23 June 1999 Ended in 2007
IMAGE MIDEX-1 Explorer-78 25 March 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[23]
TESS MIDEX-7 Explorer-95 April 18, 2018 Operational
ICON MIDEX-8 Explorer-96 11 October 2019 Operational
SPHEREx MIDEX-9 April 2025 In development[24]
UVEX 2030 In development

Small Explorers (SMEX) edit

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]

List of SMEX missions [21][29]
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 — Spacecraft equipment failure; reentered on 10 May 2011
RHESSI SMEX-6 Explorer-81 5 February 2002 April 2018 Deorbited on April 20, 2023
GALEX 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]
Aeronomy of Ice in the Mesosphere (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 — Cancelled in 2012 due to expected cost overruns[31]
Imaging X-ray Polarimetry Explorer (IXPE) SMEX-14 Explorer-97 9 December 2021 Operational
TRACERS April 2025[32] In development
PUNCH April 2025[33] In development
COSI 2027[34][35] In development
  •  
    SAMPEX
  •  
    RHESSI
  •  
    IBEX
  •  
    IXPE

University-Class Explorers (UNEX) edit

List of UNEX missions[36]
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)[37]
CHIPS UNEX-3 Explorer-82 12 January 2003 Ended in 2008

Missions of Opportunity (MO) edit

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.[38]

List of MO[36]
Name Launcher (mission) Launch (UTC) Status
HETE-2 NASA (Explorer-79) 9 October 2000 Ended in 2008[39]
INTEGRAL ESA[40] 17 October 2002 Operational
Suzaku (Astro-E2) JAXA[41] 10 July 2005 Ended in 2015
TWINS NRO (USA-184;[42]
USA-200[43]) 		TWINS-1: 28 June 2006
TWINS-2: 13 March 2008 		Operational
CINDI DoD (C/NOFS)[44] 16 April 2008 Ended in 2015
Hitomi (Astro-H) JAXA[45] 17 February 2016 Failed
NICER ISS (CRS-11) 3 June 2017 Operational
GOLD SES (SES-14) 25 January 2018 Operational
XRISM JAXA 6 September 2023 Operational
AWE ISS (CRS-29) December 2023 Operational
GUSTO NASA, high-altitude balloon December 2023 In development[46][47]
SunRISE NASA (Maxar satellite) April 2024 In development[48][49]
EZIE NASA, JHUAPL June 2024 In development[50]
Solar-C EUVST JAXA July 2028 In development[51][52]
CASE ESA (Cosmic Vision M4) 2029 In development[53][54]

Beacon Explorers edit

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

GEOS series edit

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

Launched spacecraft edit

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.[55]

Explorers Program satellites[7][56][57][58]
No. Name(s) Launch date (UTC) Rocket Mass (kg) Orbit regime End of data Re-entry
[59] 		Mission/Notes
1 Explorer 1 1 February 1958 Juno I 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[60]
2 Explorer 2 5 March 1958 Juno I 15 Failed — — Failed to achieve orbit.[61]
3 Explorer 3 26 March 1958 Juno I 14 MEO 27 June 1958 27 June 1958 Energetic particle studies helped confirm the presence of Van Allen radiation belt[62]
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)[63]
5 Explorer 5 August 24, 1958 Juno I 17 failed — — Planned in conjunction with Explorer 4, but launch failed[64]
— Explorer S-1 (7X) July 16, 1959 Juno II 42 failed — — Planned to measure Earth's radiation balance, but destroyed within seconds by range safety[65]
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[66][67]
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[68][69][70]
– S-46A (IE-B) March 23, 1960 Juno II 16 failed — — Analyze electron and proton radiation energies, failed to achieve orbit[71]
8 Explorer 8 (S-30) 3 November 1960 Juno II 41 LEO 27 December 1960 27 March 2012 Measured atmospheric composition of the ionosphere[72]
– S-56 December 4, 1960 Scout X-1 6 failed — — Atmosphere density measurement, but failed to achieve orbit[73]
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[74]
– S-45 February 24, 1961 Juno II 34 failed — — Ionosphere research, but failed to achieve orbit[75]
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[76]
11 Explorer 11 (S 15) April 27, 1961 Juno II 37 LEO November 17, 1961 In orbit Gamma ray astronomy[77]
– S-45A May 25, 1961 Juno II 34 failed — — Ionosphere research, failed to achieve orbit. Last Juno II launch.[78]
– S-55 (satellite) (Meteoroid Satellite-A, Micrometeorite Explorer) June 30, 1961 Scout X-1 85 failed — — Micrometeoroid research, failed to achieve orbit[79]
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[80]
13 S-55A August 25, 1961 Scout X-1 86 LEO August 28, 1961 August 28, 1961 Micrometeoroid research; partial failure[81]
14 EPE-B (Energetic Particle Explorer-B) October 2, 1962 Delta A 40 HEO August 11, 1963 July 1, 1966 Energetic particle research[82]
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[83]
16 S-55B December 16, 1962 Scout X-3 101 LEO July 22, 1963 In orbit Micrometeoroid research[84]
17 AE-A (S-6, Atmosphere Explorer-A) April 3, 1963 Delta B 184 LEO July 10, 1963 November 24, 1966 Atmospheric research[85]
18 IMP-A (IMP 1, Interplanetary Monitoring Platform-A) November 27, 1963 Delta C 138 HEO May 10, 1965 December 30, 1965 Magnetospheric research[86]
19 AD-A (Atmospheric Density-A) December 19, 1963 Scout X-4 8 LEO May 10, 1981 May 10, 1981 Atmospheric density measurements[87]
– BE-A (Beacon Explorer-A, S-66A) March 19, 1964 Delta B 114 failed — — Launch failure[88]
20 IE-A (S 48, TOPSI, Ionosphere Explorer-A) August 25, 1964 Scout X-4 45 LEO December 29, 1965 In orbit Ionosphere research[89]
21 IMP-B (IMP 2, Interplanetary Monitoring Platform-B) October 4, 1964 Delta C 135 HEO October 13, 1965 January 30, 1966 Magnetospheric research[90]
22 BE-B (Beacon Explorer-B, S-66B) October 10, 1964 Scout X-4 53 LEO February 1970 In orbit Ionospheric and geodetic research[91]
23 S 55C November 6, 1964 Scout X-4 134 LEO November 7, 1965 June 29, 1983 Micrometeoric research[92]
24 AD-B (Atmospheric Density-B) November 21, 1964 Scout X-4 9 MEO October 18, 1968 October 18, 1968 Atmospheric density measurements[93]
25 Injun 4 (IE-B, Ionosphere Explorer-B) November 21, 1964 Scout X-4 40 LEO December 1966 In orbit Ionospheric research[94]
26 EPE-D (Energetic Particle Explorer-D) December 21, 1964 Delta C 46 MEO December 27, 1967 August 23, 2021 High energy particle observations[95]
27 BE-C (Beacon Explorer-C,S-66C) April 29, 1965 Scout X-4 61 LEO July 20, 1973 In orbit Magnetospheric research[96]
28 IMP-C (IMP 3, Interplanetary Monitoring Platform-C) May 29, 1965 Delta C 128 HEO May 12, 1967 July 4, 1968 Magnetospheric research[97]
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[98]
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)[99]
31 DME-A (Direct Measurements Explorer) November 29, 1965 Thor-Agena B 99 LEO October 1, 1969 In orbit Ionospheric research[100]
32 AE-B (Atmosphere Explorer-B) May 25, 1966 Delta C1 225 LEO March 1967 February 22, 1985 Atmospheric research[101]
33 IMP-D (AIMP 1, Interplanetary Monitoring Platform-D) July 1, 1966 Delta E1 212 HEO September 21, 1971[102] 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[103]
35 IMP-E (AIMP 2, Interplanetary Monitoring Platform-E) July 19, 1967 Delta E1 230 Lunar June 24, 1973 Lunar orbit Magnetospheric research[104]
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[105]
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)[106]
38 RAE-A (RAE 1, Radio Astronomy Explorer-A) July 4, 1968 Delta J 602 MEO (~1969) In orbit Radio astronomy[107]
39 AD-C (Atmospheric Density-C) August 8, 1968 Scout B 9 LEO June 23, 1971 June 22, 1981 Atmospheric density measurements[108]
40 Injun 5 (Injun C, IE-C, Ionosphere Explorer-C) August 8, 1968 Scout B 71 LEO June 1971 In orbit Magnetospheric Research[109]
41 IMP-G (IMP 5, Interplanetary Monitoring Platform-G) June 21, 1969 Delta E1 145 HEO December 23, 1972 December 23, 1972 Magnetospheric research[110]
42 Uhuru (SAS-A, SAS 1) December 12, 1970 Scout B 142 LEO January 4, 1975 April 5, 1979 X-ray astronomy[111]
43 IMP-H (IMP 7, Interplanetary Monitoring Platform-H) March 13, 1971 Delta M6 635 MEO October 2, 1974 October 2, 1974 Magnetospheric research[112]
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)[113]
45 SSS-A (S-Cubed A) November 15, 1971 Scout B 52 MEO September 30, 1974 January 10, 1992 Magnetospheric research[114]
46 MTS (Meteoroid Technology Satellite, METEC)[115] August 13, 1972 Scout D-1 90 LEO November 4, 1974 November 2, 1979 Micrometeoroids research[116]
47 IMP-I (IMP 6, Interplanetary Monitoring Platform-I) September 23, 1972 Delta 1604 635 HEO October 31, 1978 In orbit Magnetospheric research[117]
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[118]
49 RAE-B (RAE 2, Radio Astronomy Explorer-B) June 10, 1973 Delta 1913 328 Lunar April 26, 1977 Presumed crashed into Moon sometime after August 1977[119] Radio astronomy[119]
50 IMP-J (IMP 8, Interplanetary Monitoring Platform-J) October 26, 1973 Delta 1604 371 HEO October 7, 2006 In orbit Magnetospheric research[120]
51 AE-C (Atmosphere Explorer-C) December 16, 1973 Delta 1900 658 LEO (December 12, 1978) December 12, 1978 Atmospheric research[121]
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[122]
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[123]
54 AE-D (Atmosphere Explorer-D) October 6, 1975 Delta 2910 681 LEO January 29, 1976 March 12, 1976 Atmospheric research[124]
55 AE-E (Atmosphere Explorer-E) November 20, 1975 Delta 2910 735 LEO September 25, 1980 June 10, 1981 Atmospheric research[125]
— DADE-A (Dual Air Density Explorer-A) December 5, 1975 Scout F-1 40 failed — — Atmospheric research; failed during launch[126]
— DADE-B (Dual Air Density Explorer-B) December 5, 1975 Scout F-1 43 failed — — Atmospheric research; failed during launch[127]
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[128]
57 IUE January 26, 1978 Delta 2914 669 MEO September 30, 1996 In orbit Ultraviolet astronomy[129]
58 HCMM (AEM-A) April 26, 1978 Scout F 117 LEO September 30, 1980 December 22, 1981 Thermal mapping of the Earth[130]
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.[131][67]
60 SAGE (AEM-B) February 18, 1979 Scout D-1 149 LEO January 7, 1982 April 11, 1989 Stratospheric aerosol and ozone data[132]
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[133]
62 Dynamics Explorer 1 (DE-1) 3 August 1981 Delta 3913 424 MEO 28 February 1991 In orbit Magnetospheric research[134]
63 Dynamics Explorer 2 (DE-2) 3 August 1981 Delta 3913 420 LEO 19 February 1983 19 February 1983 Magnetospheric research[135]
64 SME October 6, 1981 Delta 2310 145 LEO April 4, 1989 March 5, 1991 Atmospheric research[136]
65 AMPTE-CCE 16 August 1984 Delta 3924 242 MEO 12 July 1989 In orbit Magnetosphere research[137]
66 COBE November 18, 1989 Delta 5920 2,206 LEO December 23, 1993 In orbit Microwave astronomy[138]
67 EUVE (BERKSAT) June 7, 1992 Delta II 6920-X 3,275 LEO January 31, 2001 January 30, 2002 Ultraviolet astronomy[139]
68 SAMPEX July 3, 1992 Scout G-1 158 LEO June 30, 2004[140] November 13, 2012.[141] SMEX: magnetospheric research[142]
69 RXTE December 30, 1995 Delta II 7920 3,200 LEO January 3, 2012 April 30, 2018[143] MIDEX: X-ray astronomy[144]
70 FAST August 21, 1996 Pegasus XL 187 LEO May 4, 2009 In orbit SMEX: auroral phenomena[145]
— HETE 1 November 4, 1996 Pegasus XL 128 LEO — April 7, 2002 Separation failure, mission relaunched as HETE 2[146]
71 ACE August 25, 1997 Delta II 7920 596 Sun–Earth L1 Operational In L1 orbit MIDEX: solar/interplanetary/interstellar particle research[147]
72 SNOE February 26, 1998 Pegasus XL 120 LEO December 13, 2003 December 13, 2003 STEDI, UNEX: atmospheric research[148]
73 TRACE April 2, 1998 Pegasus XL 250 LEO June 21, 2010[149] In orbit SMEX: solar observatory[150]
74 SWAS December 6, 1998 Pegasus XL 288 LEO September 1, 2005 In orbit SMEX: submillimeter astronomy[151]
75 WIRE March 5, 1999 Pegasus XL 250 SSO September 30, 2000[152] May 10, 2011[153] SMEX, Infrared astronomy, primary mission failed due to loss of coolant[154]
76 TERRIERS May 18, 1999 Pegasus XL 120 Polar LEO May 18, 1999 In orbit STEDI: atmospheric research, satellite failed shortly after achieving orbit[155]
77 FUSE June 23, 1999 Delta II 7320 1,400 LEO October 18, 2007 In orbit MIDEX: ultraviolet astronomy[156]
78 IMAGE March 25, 2000 Delta II 7326 536 Polar MEO December 18, 2005[157] In orbit MIDEX: magnetospheric research[158]
79 HETE-2 October 9, 2000 Pegasus-H 124 LEO March 28, 2007[159] In orbit MO: UV, X-ray, and gamma ray astronomy[160]
80 WMAP June 30, 2001 Delta II 7425-10 840 Sun–Earth L2 October 2010[161] Heliocentric orbit MIDEX: microwave astronomy[162]
81 RHESSI February 5, 2002 Pegasus XL 230 LEO August 16, 2018 In orbit SMEX: X-ray and gamma ray solar flare imaging[163]
— INTEGRAL October 17, 2002 Proton-K Blok DM-2 4,000 HEO Operational In orbit International: space telescope for observing gamma rays[164]
82 CHIPSat January 13, 2003 Delta II 7320-10 60 LEO April 11, 2008[165] In orbit UNEX: ultraviolet spectroscopy and astronomy[166]
83 GALEX April 28, 2003 Pegasus XL 280 LEO June 28, 2013[167] In orbit SMEX: ultraviolet astronomy[168]
84 Swift November 20, 2004 Delta II 7320-10C 1,470 LEO Operational In orbit MIDEX: gamma ray astronomy[169]
— Suzaku (Astro E2) July 10, 2005 M-V 1,706 LEO September 2, 2015 In orbit MO: instrument on JAXA's Suzaku mission[170]
— 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[171]
86 THEMIS B (ARTEMIS P1) February 17, 2007 Delta II 7925 77 Lunar Operational Lunar orbit MIDEX; Magnetospheric research[172]
87 THEMIS C (ARTEMIS P2) February 17, 2007 Delta II 7925 77 Lunar Operational Lunar orbit MIDEX: magnetospheric research[173]
88 THEMIS D February 17, 2007 Delta II 7925 77 HEO Operational In orbit MIDEX: magnetospheric research[174]
89 THEMIS E February 17, 2007 Delta II 7925 77 HEO Operational In orbit MIDEX: magnetospheric research[175]
90 AIM April 25, 2007 Pegasus XL 197 SSO Operational In orbit SMEX: noctilucent cloud observation[176]
— 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.[177]
92 WISE December 14, 2009 Delta II 7320 661 LEO Operational In orbit MIDEX: infrared astronomy, NEOWISE extension. Discovered first Earth trojan.[178]
93 NuSTAR June 13, 2012 Pegasus XL 350 LEO Operational In orbit SMEX: high-energy X-ray astronomy[179]
94 IRIS June 27, 2013 Pegasus XL 183 SSO Operational In orbit SMEX: solar UV astronomy[180]
— 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[181]
— 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
97 IXPE December 9, 2021 Falcon 9 Block 5 330 LEO Operational In orbit SMEX: X-ray studies
— XRISM September 6, 2023 H-IIA 202 2,300 LEO Operational In orbit MO: instruments on JAXA's XRISM x-ray space telescope

Cancelled missions edit

 
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.[182] 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:[58]

  • Owl 1 and 2 (cost, 1965)[183]
  • MSS A (Magnetic Storm Satellite, Explorer-A, 1970)[184][185]
  • CATSAT (STEDI 3) (cost)[37]
  • IMEX (UNEX 2) (cost)[37]
  • FAME (MIDEX 4)
  • SPIDR (SMEX 8) (technical, 2003)[37]
  • GEMS (SMEX 13)

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

Launch statistics edit

Number of launches per decade:[186]

Number of Explorer launches by decade
Decade
1950s
8
1960s
34
1970s
20
1980s
5
1990s
11
2000s
15
2010s
7
2020s
2

See also edit

References edit

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External links edit

 
Wikimedia Commons has media related to Small Explorer program.
  • . Goddard Space Flight Center. NASA. 2009. Archived from the original on 31 October 2009. Retrieved 5 December 2009.
  • NSSDC updated list of Explorers missions
  • by NASA's Solar System Exploration
  • Small Explorers Missions by Goddard Space Flight Center
  • SMEX-series satellites by Colorado State University

April 29, 2024
explorers, program, explorers, program, nasa, exploration, program, that, provides, flight, opportunities, physics, geophysics, heliophysics, astrophysics, investigations, from, space, launched, 1958, explorer, first, spacecraft, united, states, achieve, orbit. 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 An Explorer mission observes Sagittarius A the Milky Way s central black hole flaring 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 Contents 1 History 1 1 Early Explorer satellites 1 2 Continuation of the Explorer program 1 3 SMEX MIDEX and Student Explorer programs 2 Classes 2 1 Medium Class Explorers MIDEX 2 2 Small Explorers SMEX 2 3 University Class Explorers UNEX 2 4 Missions of Opportunity MO 2 5 Beacon Explorers 2 6 GEOS series 3 Launched spacecraft 4 Cancelled missions 5 Launch statistics 6 See also 7 References 8 External linksHistory editEarly Explorer satellites edit nbsp Launch of Explorer 1 on the Juno I launch vehicle nbsp Explorer 1 the first Earth satellite orbited by the United States 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 Continuation of the Explorer program edit nbsp 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 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 SMEX MIDEX and Student Explorer programs edit 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 Classes editMedium Class Explorers MIDEX edit List of MIDEX missions 20 21 22 Name MIDEXnumber Explorernumber Launch UTC Status RXTE Explorer 69 30 December 1995 Ended in 2012 ACE Explorer 71 25 August 1997 Operational FUSE MIDEX 0 Explorer 77 23 June 1999 Ended in 2007 IMAGE MIDEX 1 Explorer 78 25 March 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 23 TESS MIDEX 7 Explorer 95 April 18 2018 Operational ICON MIDEX 8 Explorer 96 11 October 2019 Operational SPHEREx MIDEX 9 April 2025 In development 24 UVEX 2030 In development Small Explorers SMEX edit 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 List of SMEX missions 21 29 Name SMEXnumber Explorernumber Launch UTC End ofmission 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 Spacecraft equipment failure reentered on 10 May 2011 RHESSI SMEX 6 Explorer 81 5 February 2002 April 2018 Deorbited on April 20 2023 GALEX 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 Aeronomy of Ice in the Mesosphere 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 Cancelled in 2012 due to expected cost overruns 31 Imaging X ray Polarimetry Explorer IXPE SMEX 14 Explorer 97 9 December 2021 Operational TRACERS April 2025 32 In development PUNCH April 2025 33 In development COSI 2027 34 35 In development nbsp SAMPEX nbsp RHESSI nbsp IBEX nbsp IXPE University Class Explorers UNEX edit List of UNEX missions 36 Name UNEX number Explorernumber Launch UTC Status SNOE UNEX 1 Explorer 72 26 February 1998 Ended in 2000 IMEX UNEX 2 Scheduled for 2003 Cancelled before 2005 cost 37 CHIPS UNEX 3 Explorer 82 12 January 2003 Ended in 2008 Missions of Opportunity MO edit 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 38 List of MO 36 Name Launcher mission Launch UTC Status HETE 2 NASA Explorer 79 9 October 2000 Ended in 2008 39 INTEGRAL ESA 40 17 October 2002 Operational Suzaku Astro E2 JAXA 41 10 July 2005 Ended in 2015 TWINS NRO USA 184 42 USA 200 43 TWINS 1 28 June 2006TWINS 2 13 March 2008 Operational CINDI DoD C NOFS 44 16 April 2008 Ended in 2015 Hitomi Astro H JAXA 45 17 February 2016 Failed NICER ISS CRS 11 3 June 2017 Operational GOLD SES SES 14 25 January 2018 Operational XRISM JAXA 6 September 2023 Operational AWE ISS CRS 29 December 2023 Operational GUSTO NASA high altitude balloon December 2023 In development 46 47 SunRISE NASA Maxar satellite April 2024 In development 48 49 EZIE NASA JHUAPL June 2024 In development 50 Solar C EUVST JAXA July 2028 In development 51 52 CASE ESA Cosmic Vision M4 2029 In development 53 54 Beacon Explorers edit Three satellites were planned in this series Beacon Explorer A Beacon Explorer B Beacon Explorer C GEOS series edit A series of three Geodetic Earth Orbiting Satellite GEOS were put in orbit GEOS 1 GEOS 2 GEOS 3 Launched spacecraft editExplorer 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 55 Explorers Program satellites 7 56 57 58 No Name s Launch date UTC Rocket Mass kg Orbit regime End of data Re entry 59 Mission Notes 1 Explorer 1 1 February 1958 Juno I 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 60 2 Explorer 2 5 March 1958 Juno I 15 Failed Failed to achieve orbit 61 3 Explorer 3 26 March 1958 Juno I 14 MEO 27 June 1958 27 June 1958 Energetic particle studies helped confirm the presence of Van Allen radiation belt 62 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 63 5 Explorer 5 August 24 1958 Juno I 17 failed Planned in conjunction with Explorer 4 but launch failed 64 Explorer S 1 7X July 16 1959 Juno II 42 failed Planned to measure Earth s radiation balance but destroyed within seconds by range safety 65 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 66 67 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 68 69 70 S 46A IE B March 23 1960 Juno II 16 failed Analyze electron and proton radiation energies failed to achieve orbit 71 8 Explorer 8 S 30 3 November 1960 Juno II 41 LEO 27 December 1960 27 March 2012 Measured atmospheric composition of the ionosphere 72 S 56 December 4 1960 Scout X 1 6 failed Atmosphere density measurement but failed to achieve orbit 73 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 74 S 45 February 24 1961 Juno II 34 failed Ionosphere research but failed to achieve orbit 75 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 76 11 Explorer 11 S 15 April 27 1961 Juno II 37 LEO November 17 1961 In orbit Gamma ray astronomy 77 S 45A May 25 1961 Juno II 34 failed Ionosphere research failed to achieve orbit Last Juno II launch 78 S 55 satellite Meteoroid Satellite A Micrometeorite Explorer June 30 1961 Scout X 1 85 failed Micrometeoroid research failed to achieve orbit 79 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 80 13 S 55A August 25 1961 Scout X 1 86 LEO August 28 1961 August 28 1961 Micrometeoroid research partial failure 81 14 EPE B Energetic Particle Explorer B October 2 1962 Delta A 40 HEO August 11 1963 July 1 1966 Energetic particle research 82 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 83 16 S 55B December 16 1962 Scout X 3 101 LEO July 22 1963 In orbit Micrometeoroid research 84 17 AE A S 6 Atmosphere Explorer A April 3 1963 Delta B 184 LEO July 10 1963 November 24 1966 Atmospheric research 85 18 IMP A IMP 1 Interplanetary Monitoring Platform A November 27 1963 Delta C 138 HEO May 10 1965 December 30 1965 Magnetospheric research 86 19 AD A Atmospheric Density A December 19 1963 Scout X 4 8 LEO May 10 1981 May 10 1981 Atmospheric density measurements 87 BE A Beacon Explorer A S 66A March 19 1964 Delta B 114 failed Launch failure 88 20 IE A S 48 TOPSI Ionosphere Explorer A August 25 1964 Scout X 4 45 LEO December 29 1965 In orbit Ionosphere research 89 21 IMP B IMP 2 Interplanetary Monitoring Platform B October 4 1964 Delta C 135 HEO October 13 1965 January 30 1966 Magnetospheric research 90 22 BE B Beacon Explorer B S 66B October 10 1964 Scout X 4 53 LEO February 1970 In orbit Ionospheric and geodetic research 91 23 S 55C November 6 1964 Scout X 4 134 LEO November 7 1965 June 29 1983 Micrometeoric research 92 24 AD B Atmospheric Density B November 21 1964 Scout X 4 9 MEO October 18 1968 October 18 1968 Atmospheric density measurements 93 25 Injun 4 IE B Ionosphere Explorer B November 21 1964 Scout X 4 40 LEO December 1966 In orbit Ionospheric research 94 26 EPE D Energetic Particle Explorer D December 21 1964 Delta C 46 MEO December 27 1967 August 23 2021 High energy particle observations 95 27 BE C Beacon Explorer C S 66C April 29 1965 Scout X 4 61 LEO July 20 1973 In orbit Magnetospheric research 96 28 IMP C IMP 3 Interplanetary Monitoring Platform C May 29 1965 Delta C 128 HEO May 12 1967 July 4 1968 Magnetospheric research 97 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 98 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 99 31 DME A Direct Measurements Explorer November 29 1965 Thor Agena B 99 LEO October 1 1969 In orbit Ionospheric research 100 32 AE B Atmosphere Explorer B May 25 1966 Delta C1 225 LEO March 1967 February 22 1985 Atmospheric research 101 33 IMP D AIMP 1 Interplanetary Monitoring Platform D July 1 1966 Delta E1 212 HEO September 21 1971 102 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 103 35 IMP E AIMP 2 Interplanetary Monitoring Platform E July 19 1967 Delta E1 230 Lunar June 24 1973 Lunar orbit Magnetospheric research 104 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 105 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 106 38 RAE A RAE 1 Radio Astronomy Explorer A July 4 1968 Delta J 602 MEO 1969 In orbit Radio astronomy 107 39 AD C Atmospheric Density C August 8 1968 Scout B 9 LEO June 23 1971 June 22 1981 Atmospheric density measurements 108 40 Injun 5 Injun C IE C Ionosphere Explorer C August 8 1968 Scout B 71 LEO June 1971 In orbit Magnetospheric Research 109 41 IMP G IMP 5 Interplanetary Monitoring Platform G June 21 1969 Delta E1 145 HEO December 23 1972 December 23 1972 Magnetospheric research 110 42 Uhuru SAS A SAS 1 December 12 1970 Scout B 142 LEO January 4 1975 April 5 1979 X ray astronomy 111 43 IMP H IMP 7 Interplanetary Monitoring Platform H March 13 1971 Delta M6 635 MEO October 2 1974 October 2 1974 Magnetospheric research 112 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 113 45 SSS A S Cubed A November 15 1971 Scout B 52 MEO September 30 1974 January 10 1992 Magnetospheric research 114 46 MTS Meteoroid Technology Satellite METEC 115 August 13 1972 Scout D 1 90 LEO November 4 1974 November 2 1979 Micrometeoroids research 116 47 IMP I IMP 6 Interplanetary Monitoring Platform I September 23 1972 Delta 1604 635 HEO October 31 1978 In orbit Magnetospheric research 117 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 118 49 RAE B RAE 2 Radio Astronomy Explorer B June 10 1973 Delta 1913 328 Lunar April 26 1977 Presumed crashed into Moon sometime after August 1977 119 Radio astronomy 119 50 IMP J IMP 8 Interplanetary Monitoring Platform J October 26 1973 Delta 1604 371 HEO October 7 2006 In orbit Magnetospheric research 120 51 AE C Atmosphere Explorer C December 16 1973 Delta 1900 658 LEO December 12 1978 December 12 1978 Atmospheric research 121 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 122 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 123 54 AE D Atmosphere Explorer D October 6 1975 Delta 2910 681 LEO January 29 1976 March 12 1976 Atmospheric research 124 55 AE E Atmosphere Explorer E November 20 1975 Delta 2910 735 LEO September 25 1980 June 10 1981 Atmospheric research 125 DADE A Dual Air Density Explorer A December 5 1975 Scout F 1 40 failed Atmospheric research failed during launch 126 DADE B Dual Air Density Explorer B December 5 1975 Scout F 1 43 failed Atmospheric research failed during launch 127 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 3 128 57 IUE January 26 1978 Delta 2914 669 MEO September 30 1996 In orbit Ultraviolet astronomy 129 58 HCMM AEM A April 26 1978 Scout F 117 LEO September 30 1980 December 22 1981 Thermal mapping of the Earth 130 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 131 67 60 SAGE AEM B February 18 1979 Scout D 1 149 LEO January 7 1982 April 11 1989 Stratospheric aerosol and ozone data 132 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 133 62 Dynamics Explorer 1 DE 1 3 August 1981 Delta 3913 424 MEO 28 February 1991 In orbit Magnetospheric research 134 63 Dynamics Explorer 2 DE 2 3 August 1981 Delta 3913 420 LEO 19 February 1983 19 February 1983 Magnetospheric research 135 64 SME October 6 1981 Delta 2310 145 LEO April 4 1989 March 5 1991 Atmospheric research 136 65 AMPTE CCE 16 August 1984 Delta 3924 242 MEO 12 July 1989 In orbit Magnetosphere research 137 66 COBE November 18 1989 Delta 5920 2 206 LEO December 23 1993 In orbit Microwave astronomy 138 67 EUVE BERKSAT June 7 1992 Delta II 6920 X 3 275 LEO January 31 2001 January 30 2002 Ultraviolet astronomy 139 68 SAMPEX July 3 1992 Scout G 1 158 LEO June 30 2004 140 November 13 2012 141 SMEX magnetospheric research 142 69 RXTE December 30 1995 Delta II 7920 3 200 LEO January 3 2012 April 30 2018 143 MIDEX X ray astronomy 144 70 FAST August 21 1996 Pegasus XL 187 LEO May 4 2009 In orbit SMEX auroral phenomena 145 HETE 1 November 4 1996 Pegasus XL 128 LEO April 7 2002 Separation failure mission relaunched as HETE 2 146 71 ACE August 25 1997 Delta II 7920 596 Sun Earth L1 Operational In L1 orbit MIDEX solar interplanetary interstellar particle research 147 72 SNOE February 26 1998 Pegasus XL 120 LEO December 13 2003 December 13 2003 STEDI UNEX atmospheric research 148 73 TRACE April 2 1998 Pegasus XL 250 LEO June 21 2010 149 In orbit SMEX solar observatory 150 74 SWAS December 6 1998 Pegasus XL 288 LEO September 1 2005 In orbit SMEX submillimeter astronomy 151 75 WIRE March 5 1999 Pegasus XL 250 SSO September 30 2000 152 May 10 2011 153 SMEX Infrared astronomy primary mission failed due to loss of coolant 154 76 TERRIERS May 18 1999 Pegasus XL 120 Polar LEO May 18 1999 In orbit STEDI atmospheric research satellite failed shortly after achieving orbit 155 77 FUSE June 23 1999 Delta II 7320 1 400 LEO October 18 2007 In orbit MIDEX ultraviolet astronomy 156 78 IMAGE March 25 2000 Delta II 7326 536 Polar MEO December 18 2005 157 In orbit MIDEX magnetospheric research 158 79 HETE 2 October 9 2000 Pegasus H 124 LEO March 28 2007 159 In orbit MO UV X ray and gamma ray astronomy 160 80 WMAP June 30 2001 Delta II 7425 10 840 Sun Earth L2 October 2010 161 Heliocentric orbit MIDEX microwave astronomy 162 81 RHESSI February 5 2002 Pegasus XL 230 LEO August 16 2018 In orbit SMEX X ray and gamma ray solar flare imaging 163 INTEGRAL October 17 2002 Proton K Blok DM 2 4 000 HEO Operational In orbit International space telescope for observing gamma rays 164 82 CHIPSat January 13 2003 Delta II 7320 10 60 LEO April 11 2008 165 In orbit UNEX ultraviolet spectroscopy and astronomy 166 83 GALEX April 28 2003 Pegasus XL 280 LEO June 28 2013 167 In orbit SMEX ultraviolet astronomy 168 84 Swift November 20 2004 Delta II 7320 10C 1 470 LEO Operational In orbit MIDEX gamma ray astronomy 169 Suzaku Astro E2 July 10 2005 M V 1 706 LEO September 2 2015 In orbit MO instrument on JAXA s Suzaku mission 170 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 171 86 THEMIS B ARTEMIS P1 February 17 2007 Delta II 7925 77 Lunar Operational Lunar orbit MIDEX Magnetospheric research 172 87 THEMIS C ARTEMIS P2 February 17 2007 Delta II 7925 77 Lunar Operational Lunar orbit MIDEX magnetospheric research 173 88 THEMIS D February 17 2007 Delta II 7925 77 HEO Operational In orbit MIDEX magnetospheric research 174 89 THEMIS E February 17 2007 Delta II 7925 77 HEO Operational In orbit MIDEX magnetospheric research 175 90 AIM April 25 2007 Pegasus XL 197 SSO Operational In orbit SMEX noctilucent cloud observation 176 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 177 92 WISE December 14 2009 Delta II 7320 661 LEO Operational In orbit MIDEX infrared astronomy NEOWISE extension Discovered first Earth trojan 178 93 NuSTAR June 13 2012 Pegasus XL 350 LEO Operational In orbit SMEX high energy X ray astronomy 179 94 IRIS June 27 2013 Pegasus XL 183 SSO Operational In orbit SMEX solar UV astronomy 180 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 181 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 97 IXPE December 9 2021 Falcon 9 Block 5 330 LEO Operational In orbit SMEX X ray studies XRISM September 6 2023 H IIA 202 2 300 LEO Operational In orbit MO instruments on JAXA s XRISM x ray space telescopeCancelled missions edit nbsp WISE was restarted after it was turned off nbsp Explorer 6 on a Thor Able III launch in August 1959 nbsp 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 182 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 58 Owl 1 and 2 cost 1965 183 MSS A Magnetic Storm Satellite Explorer A 1970 184 185 CATSAT STEDI 3 cost 37 IMEX UNEX 2 cost 37 FAME MIDEX 4 SPIDR SMEX 8 technical 2003 37 GEMS SMEX 13 Recent examples of conclusions of launched missions cancelled due to budgetary constraints FAST 2009 TRACE 2010 Solar observatory see Solar Dynamics Observatory Wilkinson MAP 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