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Explorers Program

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

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.

History

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Early Explorer satellites

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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

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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]

List of Interplanetary Monitoring Platform (IMP) missions
Mission Photo Satellite Launch Date Decay Date Notes
Explorer IMP
IMP-1 Explorer 18 IMP-A 27 November 1963, 02:30 UTC[7] December 30, 1965 First use of integrated circuits in a spacecraft. First satellite in IMP-A/-B/-C design series.
IMP-2 Explorer 21 IMP-B 4 October 1964, 03:45 UTC[7] January 1, 1966 Second satellite in IMP-A/-B/-C design series.
IMP-3 Explorer 28 IMP-C 29 May 1965, 12:00 UTC[7] 4 July 1968 Third satellite in IMP-A/-B/-C design series.
AIMP-1 Explorer 33 IMP-D 1 July 1966, 16:02 UTC[7] In orbit First use of MOSFET integrated circuits in a spacecraft, similar design to IMP-E. Originally intended to orbit the Moon, but placed in an elliptical high orbit instead.
IMP-4 Explorer 34 IMP-F 24 May 1967, 14:05 UTC[7] May 3, 1969 Similar design to IMP-G.
AIMP-2 Explorer 35 IMP-E 19 July 1967, 14:19 UTC[7] After June 24, 1973 Similar design to IMP-D. Positioned in Selenocentric orbit.
IMP-5 Explorer 41 IMP-G 21 June 1969, 08:47 UTC[7] December 23, 1972 Similar design to IMP-F.
IMP-6 Explorer 43 IMP-I 13 March 1971, 16:15 UTC[7] October 2, 1974 First spacecraft in IMP-I/-H/-J series.
IMP-7 Explorer 47 IMP-H 23 September 1972, 01:20 UTC[7] In orbit Second spacecraft in IMP-I/-H/-J series.
IMP-8 Explorer 50 IMP-J 26 October 1973, 02:26 UTC[7] In orbit Third spacecraft in IMP-I/-H/-J series, remained in service until 2006

Over the following two decades, NASA has launched over 50 Explorer missions,[8] 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.[citation needed]

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

SMEX, MIDEX, and Student Explorer programs

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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.[9][10] The first three SMEX missions were chosen in April 1989 out of 51 candidates, and launched in 1992, 1996 and 1998[11] The second set of two missions were announced in September 1994 and launched in 1998 and 1999.[9]

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 MIDEX-3 Swift, which can also detect X-rays, and has contributed to the study of gamma-ray bursts and other topics.

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.[9] This new launch vehicle was not developed and instead, these missions were flown on a modified Delta II rocket.[12][13] The first announcement opportunity for MIDEX was issued in March 1995, and the first launch under this new class was FUSE in 1999.[9]

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.[9] Later, NASA established the University-Class Explorer (UNEX) program for much cheaper missions, which is regarded as a successor to STEDI.[14]

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.[15] 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.[16]

Excluding the launches, the MIDEX class has a current mission cap cost of US$250 million in 2018,[17] with future MIDEX missions being capped at US$350 million.[18] The cost cap for SMEX missions in 2017 was US$165 million.[19] UNEX missions are capped at US$15 million.[20] 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.[17][20]

Classes

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Medium-Class Explorers (MIDEX)

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List of MIDEX missions[21][22][23]
Name MIDEX
number 		Explorer
number 		Launch (UTC) Status
RXTE Explorer-69 30 December 1995 Ended in 2012 / Reentered on 30 April 2018
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 30 June 2001 Ended in 2010
Swift MIDEX-3 Explorer-84 20 November 2004 Operational
FAME MIDEX-4
—
 Scheduled for 2004 Cancelled in 2002 (cost)
THEMIS A MIDEX-5A Explorer-85 17 February 2007 Operational
THEMIS B MIDEX-5B Explorer-86 17 February 2007 Operational
THEMIS C MIDEX-5C Explorer-87 17 February 2007 Operational
THEMIS D MIDEX-5D Explorer-88 17 February 2007 Operational
THEMIS E MIDEX-5E Explorer-89 17 February 2007 Operational
WISE / NEOWISE MIDEX-6 Explorer-92 14 December 2009 Mission operations Completed on 31 July 2024. [24] Reentered on 02 November 2024
TESS MIDEX-7 Explorer-95 18 April 2018 Operational
ICON MIDEX-8 Explorer-96 11 October 2019 Operational
SPHEREx MIDEX-9 27 February 2025 In development[25]
MUSE MIDEX-10 2027 In development
HelioSwarm MIDEX-11 2028 In development
UVEX MIDEX-12 2030 In development

Small Explorers (SMEX)

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The Small Explorers class was implemented in 1989 specifically to fund space exploration missions that cost no more than US$120 million.[20][11] The missions are managed by the Explorers Project at the Goddard Space Flight Center (GSFC).[15]

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.[15]

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).[26][27][28] In June 2019 NASA selected TRACERS and PUNCH for flight.[29]

List of SMEX missions [22][30]
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 20 April 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 [31]
Aeronomy of Ice in the Mesosphere (AIM) SMEX-9 Explorer-90 25 April 2007 Reentered on 19 August 2024
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[32]
Imaging X-ray Polarimetry Explorer (IXPE) SMEX-14 Explorer-97 9 December 2021 Operational
PUNCH SMEX-15 27 February 2025[33] In development[34]
TRACERS SMEX-16 April 2025[35] In development
COSI SMEX-17 August 2027[36] In development
  • SAMPEX
    SAMPEX
  • RHESSI
    RHESSI
  • IBEX
    IBEX
  • IXPE
    IXPE

University-Class Explorers (UNEX)

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List of UNEX missions[37]
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)[38]
CHIPS UNEX-3 Explorer-82 12 January 2003 Ended in 2008

Missions of Opportunity (MO)

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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.[39]

List of MO[37]
Name Launcher (mission) Launch (UTC) Status
HETE-2 NASA (Explorer-79) 9 October 2000 Ended in 2008[40]
INTEGRAL ESA[41] 17 October 2002 Operational
Suzaku (Astro-E2) JAXA[42] 10 July 2005 Ended in 2015
TWINS NRO (USA-184;[43]
USA-200[44]) 		TWINS-1: 28 June 2006
TWINS-2: 13 March 2008 		Operational
CINDI DoD (C/NOFS)[45] 16 April 2008 Ended in 2015
Hitomi (Astro-H) JAXA[46] 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) 31 December 2023 Ended on 26 February 2024[47]
SunRISE NASA (Maxar satellite) 2024 In development[48][49]
EZIE NASA, JHUAPL 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

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

GEOS series

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

Launched spacecraft

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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[8][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 26 July 1958 Juno I 26 MEO 5 October 1958 23 October 1959 Monitor charged particles inside Van Allen belts from nuclear detonations (during Operation Argus)[63]
5 Explorer 5 24 August 1958 Juno I 17 failed — — Planned in conjunction with Explorer 4, but launch failed[64]
— Explorer S-1 (7X) 16 July 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) 7 August 1959 Thor-Able 64 HEO 6 October 1959 12 July 1961 Magnetosphere research and digital telemetry; first NASA launch, first Earth photo from orbit[66][67]
7 Explorer 7 (S-1A) 13 October 1959 Juno II 42 LEO 24 August 1961 In orbit Micrometeoroids and energetic particle studies, first satellite to measure Earth's climate[68][69][70]
– S-46A (IE-B) 23 March 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 4 December 1960 Scout X-1 6 failed — — Atmosphere density measurement, but failed to achieve orbit[73]
9 Explorer 9 (S-56A) 16 February 1961 Scout X-1 36 LEO 9 April 1964 9 April 1964 Atmospheric density measurements, first spacecraft placed in orbit by a solid-fuel rocket[74]
– S-45 24 February 1961 Juno II 34 failed — — Ionosphere research, but failed to achieve orbit[75]
10 Explorer 10 (P 14) 25 March 1961 Thor-Delta 79 HEO 25 March 1961 1 June 1968 Investigated the magnetic field between the Earth and Moon[76]
11 Explorer 11 (S 15) 27 April 1961 Juno II 37 LEO 17 November 1961 In orbit Gamma ray astronomy[77]
– S-45A 25 May 1961 Juno II 34 failed — — Ionosphere research, failed to achieve orbit. Last Juno II launch.[78]
– S-55 (satellite) (Meteoroid Satellite-A, Micrometeorite Explorer) 30 June 1961 Scout X-1 85 failed — — Micrometeoroid research, failed to achieve orbit[79]
12 EPE-A (S 3, Energetic Particle Explorer-A) 16 August 1961 Thor-Delta 38 HEO 6 December 1961 1 September 1963 Energetic particle research[80]
13 Explorer 13 (S-55A) 25 August 1961 Scout X-1 86 LEO 28 August 1961 28 August 1961 Micrometeoroid research; partial failure[81]
14 EPE-B (Energetic Particle Explorer-B) 2 October 1962 Delta A 40 HEO 11 August 1963 1 July 1966 Energetic particle research[82]
15 EPE-C (S-3B, Energetic Particle Explorer-C) 27 October 1962 Delta A 44 HEO 30 January 1963 15 January 1978 Energetic particle research[83]
16 S-55B 16 December 1962 Scout X-3 101 LEO 22 July 1963 In orbit Micrometeoroid research[84]
17 AE-A (S-6, Atmosphere Explorer-A) 3 April 1963 Delta B 184 LEO 10 July 1963 24 November 1966 Atmospheric research[85]
18 IMP-A (IMP 1, Interplanetary Monitoring Platform-A) 27 November 1963 Delta C 138 HEO 10 May 1965 30 December 1965 Magnetospheric research[86]
19 AD-A (Atmospheric Density-A) 19 December 1963 Scout X-4 8 LEO 10 May 1981 10 May 1981 Atmospheric density measurements[87]
– BE-A (Beacon Explorer-A, S-66A) 19 March 1964 Delta B 114 failed — — Launch failure[88]
20 IE-A (S 48, TOPSI, Ionosphere Explorer-A) 25 August 1964 Scout X-4 45 LEO 29 December 1965 In orbit Ionosphere research[89]
21 IMP-B (IMP 2, Interplanetary Monitoring Platform-B) 4 October 1964 Delta C 135 HEO 13 October 1965 30 January 1966 Magnetospheric research[90]
22 BE-B (Beacon Explorer-B, S-66B) 10 October 1964 Scout X-4 53 LEO February 1970 In orbit Ionospheric and geodetic research[91]
23 S 55C 6 November 1964 Scout X-4 134 LEO 7 November 1965 29 June 1983 Micrometeoric research[92]
24 AD-B (Atmospheric Density-B) 21 November 1964 Scout X-4 9 MEO 18 October 1968 18 October 1968 Atmospheric density measurements[93]
25 Injun 4 (IE-B, Ionosphere Explorer-B) 21 November 1964 Scout X-4 40 LEO December 1966 In orbit Ionospheric research[94]
26 EPE-D (Energetic Particle Explorer-D) 21 December 1964 Delta C 46 MEO 27 December 1967 23 August 2021 High energy particle observations[95]
27 BE-C (Beacon Explorer-C, S-66C) 29 April 1965 Scout X-4 61 LEO 20 July 1973 In orbit Magnetospheric research[96]
28 IMP-C (IMP 3, Interplanetary Monitoring Platform-C) 29 May 1965 Delta C 128 HEO 12 May 1967 4 July 1968 Magnetospheric research[97]
29 GEOS 1 (GEOS-A, Geodetic Earth Orbiting Satellite-1) 6 November 1965 Delta E 387 LEO 23 June 1978 In orbit Geodetic Earth monitoring[98]
30 SOLRAD 8 (SE-A) 19 November 1965 Scout X-4 57 LEO 5 November 1967 In orbit Solar radiation monitoring (Cover for covert ELINT mission)[99]
31 DME-A (Direct Measurements Explorer) 29 November 1965 Thor-Agena B 99 LEO 1 October 1969 In orbit Ionospheric research[100]
32 AE-B (Atmosphere Explorer-B) 25 May 1966 Delta C1 225 LEO March 1967 22 February 1985 Atmospheric research[101]
33 IMP-D (AIMP 1, Interplanetary Monitoring Platform-D) 1 July 1966 Delta E1 212 HEO 21 September 1971[102] In orbit Magnetospheric research
34 IMP-F (IMP 4, Interplanetary Monitoring Platform-F) 24 May 1967 Delta E1 163 MEO 3 May 1969 3 May 1969 Magnetospheric research[103]
35 IMP-E (AIMP 2, Interplanetary Monitoring Platform-E) 19 July 1967 Delta E1 230 Lunar 24 June 1973 Lunar orbit Magnetospheric research[104]
36 GEOS 2 (GEOS-B, Geodetic Earth Orbiting Satellite-2) 11 January 1968 Delta E1 469 LEO 1 July 1982 In orbit Geodetic Earth monitoring[105]
37 SOLRAD 9 (SE B) 5 March 1968 Scout B 198 LEO 30 April 1974 16 November 1990 Solar radiation monitoring
(Cover for covert ELINT mission)[106]
38 RAE-A (RAE 1, Radio Astronomy Explorer-A) 4 July 1968 Delta J 602 MEO (~1969) In orbit Radio astronomy[107]
39 AD-C (Atmospheric Density-C) 8 August 1968 Scout B 9 LEO 23 June 1971 22 June 1981 Atmospheric density measurements[108]
40 Injun 5 (Injun C, IE-C, Ionosphere Explorer-C) 8 August 1968 Scout B 71 LEO June 1971 In orbit Magnetospheric Research[109]
41 IMP-G (IMP 5, Interplanetary Monitoring Platform-G) 21 June 1969 Delta E1 145 HEO 23 December 1972 23 December 1972 Magnetospheric research[110]
42 Uhuru (SAS-A, SAS 1) 12 December 1970 Scout B 142 LEO 4 January 1975 5 April 1979 X-ray astronomy[111]
43 IMP-H (IMP 7, Interplanetary Monitoring Platform-H) 13 March 1971 Delta M6 635 MEO 2 October 1974 2 October 1974 Magnetospheric research[112]
44 SOLRAD 10 (SE-C, SOLRAD-C) 8 July 1971 Scout B 260 LEO 30 June 1973 15 December 1979 Solar radiation monitoring
(Cover for covert ELINT mission)[113]
45 SSS-A (S-Cubed A) 15 November 1971 Scout B 52 MEO 30 September 1974 10 January 1992 Magnetospheric research[114]
46 MTS (Meteoroid Technology Satellite, METEC)[115] 13 August 1972 Scout D-1 90 LEO 4 November 1974 2 November 1979 Micrometeoroids research[116]
47 IMP-I (IMP 6, Interplanetary Monitoring Platform-I) 23 September 1972 Delta 1604 635 HEO 31 October 1978 In orbit Magnetospheric research[117]
48 SAS-B (Small Astronomy Satellite-B, SAS 2) 15 November 1972 Scout D-1 166 LEO 8 June 1973 20 August 1980 X-ray astronomy[118]
49 RAE-B (RAE 2, Radio Astronomy Explorer-B) 10 June 1973 Delta 1913 328 Lunar 26 April 1977 Presumed crashed into Moon sometime after August 1977[119] Radio astronomy[119]
50 IMP-J (IMP 8, Interplanetary Monitoring Platform-J) 26 October 1973 Delta 1604 371 HEO 7 October 2006 In orbit Magnetospheric research[120]
51 AE-C (Atmosphere Explorer-C) 16 December 1973 Delta 1900 658 LEO (12 December 1978) 12 December 1978 Atmospheric research[121]
52 Hawkeye 1 (Injun-F, Injun 6, IE-D, Ionosphere Explorer-D) 3 June 1974 Scout E-1 23 HEO 28 April 1978 28 April 1978 Magnetospheric research[122]
53 SAS-C (Small Astronomy Satellite-C, SAS 3) 7 May 1975 Scout F-1 197 LEO 7 April 1979 9 April 1979 X-ray astronomy[123]
54 AE-D (Atmosphere Explorer-D) 6 October 1975 Delta 2910 681 LEO 29 January 1976 12 March 1976 Atmospheric research[124]
55 AE-E (Atmosphere Explorer-E) 20 November 1975 Delta 2910 735 LEO 25 September 1980 10 June 1981 Atmospheric research[125]
— DADE-A (Dual Air Density Explorer-A) 5 December 1975 Scout F-1 40 failed — — Atmospheric research; failed during launch[126]
— DADE-B (Dual Air Density Explorer-B) 5 December 1975 Scout F-1 43 failed — — Atmospheric research; failed during launch[127]
56 ISEE-1 (ISEE-A) 22 October 1977 Delta 2914 340 HEO 26 September 1987 26 September 1987 Magnetospheric research; launched with ESA's ISEE-2; co-mission with ISEE 3[128]
57 IUE 26 January 1978 Delta 2914 669 MEO 30 September 1996 In orbit Ultraviolet astronomy[129]
58 HCMM (AEM-A) 26 April 1978 Scout F 117 LEO 30 September 1980 December 22, 1981 Thermal mapping of the Earth[130]
59 ICE (ISEE 3, ISEE-C) 12 August 1978 Delta 2914 390 Sun–Earth L1 16 September 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) 18 February 1979 Scout D-1 149 LEO 7 January 1982 11 April 1989 Stratospheric aerosol and ozone data[132]
61 MAGSAT (AEM-C) 30 October 1979 Scout G-1 158 LEO 6 May 1980 11 June 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 6 October 1981 Delta 2310 145 LEO 4 April 1989 5 March 1991 Atmospheric research[136]
65 AMPTE-CCE 16 August 1984 Delta 3924 242 MEO 12 July 1989 In orbit Magnetosphere research[137]
66 COBE 18 November 1989 Delta 5920 2,206 LEO 23 December 1993 In orbit Microwave astronomy[138]
67 EUVE (BERKSAT) 7 June 1992 Delta II 6920-X 3,275 LEO 31 January 2001 30 January 2002 Ultraviolet astronomy[139]
68 SAMPEX 3 July 1992 Scout G-1 158 LEO 30 June 2004[140] 13 November 2012.[141] SMEX: magnetospheric research[142]
69 RXTE 30 December 1995 Delta II 7920 3,200 LEO 3 January 2012 30 April 2018[143] MIDEX: X-ray astronomy[144]
70 FAST 21 August 1996 Pegasus XL 187 LEO 4 May 2009 In orbit SMEX: auroral phenomena[145]
— HETE 1 4 November 1996 Pegasus XL 128 LEO — 7 April 2002 Separation failure, mission relaunched as HETE 2[146]
71 ACE 25 August 1997 Delta II 7920 596 Sun–Earth L1 Operational In L1 orbit MIDEX: solar/interplanetary/interstellar particle research[147]
72 SNOE 26 February 1998 Pegasus XL 120 LEO 13 December 2003 13 December 2003 STEDI, UNEX: atmospheric research[148]
73 TRACE 2 April 1998 Pegasus XL 250 LEO 21 June 2010[149] In orbit SMEX: solar observatory[150]
74 SWAS 6 December 1998 Pegasus XL 288 LEO 1 September 2005 In orbit SMEX: submillimeter astronomy[151]
75 WIRE 5 March 1999 Pegasus XL 250 SSO 30 September 2000[152] 10 May 2011[153] SMEX, Infrared astronomy, primary mission failed due to loss of coolant[154]
76 TERRIERS 18 May 1999 Pegasus XL 120 Polar LEO 18 May 1999 In orbit STEDI: atmospheric research, satellite failed shortly after achieving orbit[155]
77 FUSE 23 June 1999 Delta II 7320 1,400 LEO 18 October 2007 In orbit MIDEX: ultraviolet astronomy[156]
78 IMAGE 25 March 2000 Delta II 7326 536 Polar MEO 18 December 2005[157] In orbit MIDEX: magnetospheric research[158]
79 HETE-2 9 October 2000 Pegasus-H 124 LEO 28 March 2007[159] In orbit MO: UV, X-ray, and gamma ray astronomy[160]
80 WMAP 30 June 2001 Delta II 7425-10 840 Sun–Earth L2 October 2010[161] Heliocentric orbit MIDEX: microwave astronomy[162]
81 RHESSI 5 February 2002 Pegasus XL 230 LEO 16 August 2018 Deorbited SMEX: X-ray and gamma ray solar flare imaging[163]
— INTEGRAL 17 October 2002 Proton-K Blok DM-2 4,000 HEO Operational In orbit International: space telescope for observing gamma rays[164]
82 CHIPSat 13 January 2003 Delta II 7320-10 60 LEO 11 April 2008[165] In orbit UNEX: ultraviolet spectroscopy and astronomy[166]
83 GALEX 28 April 2003 Pegasus XL 280 LEO 28 June 2013[167] In orbit SMEX: ultraviolet astronomy[168]
84 Swift 20 November 2004 Delta II 7320-10C 1,470 LEO Operational In orbit MIDEX: gamma ray astronomy[169]
— Suzaku (Astro E2) 10 July 2005 M-V 1,706 LEO 2 September 2015 In orbit MO: instrument on JAXA's Suzaku mission[170]
— TWINS A 28 June 2006 Delta IV M+(4,2) classified Molniya Operational In orbit MO: payload on Trumpet-F/O-1 1 (USA-184)
85 THEMIS A 17 February 2007 Delta II 7925 77 HEO Operational In orbit MIDEX: magnetospheric research[171]
86 THEMIS B (ARTEMIS P1) 17 February 2007 Delta II 7925 77 Lunar Operational Lunar orbit MIDEX; Magnetospheric research[172]
87 THEMIS C (ARTEMIS P2) 17 February 2007 Delta II 7925 77 Lunar Operational Lunar orbit MIDEX: magnetospheric research[173]
88 THEMIS D 17 February 2007 Delta II 7925 77 HEO Operational In orbit MIDEX: magnetospheric research[174]
89 THEMIS E 17 February 2007 Delta II 7925 77 HEO Operational In orbit MIDEX: magnetospheric research[175]
90 AIM 25 April 2007 Pegasus XL 197 SSO March 2023 19 August 2024 SMEX: noctilucent cloud observation[176]
— TWINS B 13 March 2008 Atlas V 411 classified Molniya Operational In orbit MO: payload on Trumpet-F/O-1 2 (USA-200)
— CINDI 16 April 2008 Pegasus XL 395 LEO 28 November 2015 28 November 2015 MO: instruments on C/NOFS
91 IBEX 19 October 2008 Pegasus XL 107 MEO Operational In orbit SMEX: mapping the boundary between the Solar System and interstellar space.[177]
92 WISE 14 December 2009 Delta II 7320 661 LEO August 2024 02 November 2024 MIDEX: infrared astronomy, NEOWISE extension. Discovered first Earth trojan.[178]
93 NuSTAR 13 June 2012 Pegasus XL 350 LEO Operational In orbit SMEX: high-energy X-ray astronomy[179]
94 IRIS 27 June 2013 Pegasus XL 183 SSO Operational In orbit SMEX: solar UV astronomy[180]
— Hitomi (NeXT, ASTRO-H) 7 February 2016 H-2A-202 2,700 LEO 26 March 2016 In orbit MO: X-ray instrument on JAXA's Hitomi, but spacecraft failed after initial checkouts[181]
— NICER 3 May 2017 Falcon 9 FT 372 ISS Operational ISS MO: instrument on ISS for neutron star observations
— GOLD 25 January 2018 Ariane 5 ECA 37 GEO Operational In orbit MO: instrument on SES-14 comsat for studying Earth–space boundary
95 TESS 18 April 2018 Falcon 9 FT 362 HEO Operational In orbit MIDEX: survey for transiting exoplanets
96 ICON 11 October 2019 Pegasus XL 287 LEO Operational In orbit MIDEX: ionospheric studies
97 IXPE 9 December 2021 Falcon 9 Block 5 330 LEO Operational In orbit SMEX: X-ray studies
— XRISM 6 September 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.[citation needed]

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)[38]
  • IMEX (UNEX 2) (cost)[38]
  • FAME (MIDEX 4)
  • SPIDR (SMEX 8) (technical, 2003)[38]
  • 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

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