The Explorers program[1] is a NASA exploration program that provides flight opportunities for physics, geophysics, heliophysics, and astrophysics investigations from space. Launched in 1958, Explorer 1 was the first spacecraft of the United States to achieve orbit. Over 90 space missions have been launched since. Starting with Explorer 6, it has been operated by NASA, with regular collaboration with a variety of other institutions, including many international partners.
Launchers for the Explorer program have included Juno I, Juno II, various Thor, Scout, Delta and Pegasus launch vehicles, and Falcon 9.
The program has three classes: Medium-Class Explorers (MIDEX), Small Explorers (SMEX), and University-Class Explorers (UNEX), with select Missions of Opportunity operated with other agencies.
The Explorer program began as a U.S. Army proposal (Project Orbiter) to place a "civilian" artificial satellite into orbit during the International Geophysical Year (IGY). Although that proposal was rejected in favor of the U.S. Navy's Project Vanguard, which made the first sub-orbital flight Vanguard TV0 in December 1956, the Soviet Union's launch of Sputnik 1 on 4 October 1957 (and the resulting "Sputnik crisis") and the failure of the Vanguard 1 launch attempt resulted in the Army program being funded to match the Soviet space achievements. Explorer 1 was launched on the Juno I on 1 February 1958, becoming the first U.S. satellite, as well as discovering the Van Allen radiation belt.
Four follow-up satellites of the Explorer series were launched by the Juno I launch vehicle in 1958, of which Explorer 3 and Explorer 4 were successful, while Explorer 2 and Explorer 5 failed to reach orbit. The Juno I vehicle was replaced by the Juno II in 1959.
With the establishment of NASA in 1958, the Explorer program was transferred to NASA from the U.S. Army. NASA continued to use the name for an ongoing series of relatively small space missions, typically an artificial satellite with a specific science focus. Explorer 6 in 1959 was the first scientific satellite under the project direction of NASA's Goddard Space Flight Center (GSFC) in Greenbelt, Maryland.[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, some in conjunction to military programs, usually of an exploratory or survey nature or had specific objectives not requiring the capabilities of a major space observatory. Explorer satellites have made many important discoveries on: Earth's magnetosphere and the shape of its gravity field; the solar wind; properties of micrometeoroids raining down on the Earth; ultraviolet, cosmic and X-rays from the Solar System and beyond; ionospheric physics; Solar plasma; solar energetic particles; and atmospheric physics. These missions have also investigated air density, radio astronomy, geodesy, and gamma-ray astronomy.
With decreases in NASA's budget, Explorer missions became infrequent in the early 1980s.
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.[6] [7] The first three SMEX missions were chosen in April 1989 out of 51 candidates, and launched in 1992, 1996 and 1998[8] The second set of two missions were announced in September 1994 and launched in 1998 and 1999.[6]
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.[6] This new launch vehicle was not developed and instead, these missions were flown on a modified Delta II rocket.[9] [10] The first announcement opportunity for MIDEX was issued in March 1995, and the first launch under this new class was FUSE in 1999.[6]
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.[6] Later, NASA established the University-Class Explorer (UNEX) program for much cheaper missions, which is regarded as a successor to STEDI.[11]
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.[12] 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.[13]
Excluding the launches, the MIDEX class has a current mission cap cost of US$250 million in 2018,[14] with future MIDEX missions being capped at US$350 million.[15] The cost cap for SMEX missions in 2017 was US$165 million.[16] UNEX missions are capped at US$15 million.[17] 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.[14] [17]
| Name | MIDEX number | Explorer number | Launch (UTC) | Status | |
|---|---|---|---|---|---|
| RXTE | Explorer-69 | 30 December 1995 | Ended in 2012 | ||
| ACE | Explorer-71 | 25 August 1997 | |||
| 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 | ||
| FAME | MIDEX-4 | Scheduled for 2004 | Cancelled in 2002 (cost) | ||
| THEMIS A | MIDEX-5A | Explorer-85 | 17 February 2007 | ||
| THEMIS B | MIDEX-5B | Explorer-86 | 17 February 2007 | ||
| THEMIS C | MIDEX-5C | Explorer-87 | 17 February 2007 | ||
| THEMIS D | MIDEX-5D | Explorer-88 | 17 February 2007 | ||
| THEMIS E | MIDEX-5E | Explorer-89 | 17 February 2007 | ||
| MIDEX-6 | Explorer-92 | 14 December 2009 | [20] | ||
| TESS | MIDEX-7 | Explorer-95 | 18 April 2018 | ||
| ICON | MIDEX-8 | Explorer-96 | 11 October 2019 | ||
| SPHEREx | MIDEX-9 | 27 February 2025 | |||
| UVEX | 2030 |
The Small Explorers class was implemented in 1989 specifically to fund space exploration missions that cost no more than .[17] The missions are managed by the Explorers Project at the Goddard Space Flight Center (GSFC).
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.
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).[21] [22] [23] In June 2019 NASA selected TRACERS and PUNCH for flight.[24]
| 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 | |
| TOMS-EP | 2 July 1996 | [27] --> | ||||
| 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 [28] | |||
| Aeronomy of Ice in the Mesosphere (AIM) | SMEX-9 | Explorer-90 | 25 April 2007 | |||
| IBEX | SMEX-10 | Explorer-91 | 19 October 2008 | |||
| NuSTAR | SMEX-11 | Explorer-93 | 13 June 2012 | |||
| IRIS | 28 June 2013 | |||||
| GEMS | SMEX-13 | Scheduled for 2014 | Cancelled in 2012 due to expected cost overruns[29] | |||
| Imaging X-ray Polarimetry Explorer (IXPE) | SMEX-14 | 9 December 2021 | ||||
| PUNCH | 27 February 2025[30] | [31] | ||||
| TRACERS | April 2025[32] | |||||
| COSI | August 2027[33] |
| 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) 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]
| Name | Launcher (mission) | Launch (UTC) | Status | |
|---|---|---|---|---|
| HETE-2 | NASA (Explorer-79) | 9 October 2000 | Ended in 2008[37] | |
| INTEGRAL | ESA[38] | 17 October 2002 | ||
| 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 | ||
| CINDI | DoD (C/NOFS)[42] | 16 April 2008 | Ended in 2015 | |
| Hitomi (Astro-H) | JAXA[43] | 17 February 2016 | ||
| NICER | ISS (CRS-11) | 3 June 2017 | ||
| GOLD | SES (SES-14) | 25 January 2018 | ||
| XRISM | JAXA | 6 September 2023 | ||
| AWE | ISS (CRS-29) | December 2023 | ||
| GUSTO | NASA (high-altitude balloon) | 31 December 2023 | Ended on 26 February 2024[44] | |
| SunRISE | NASA (Maxar satellite) | 2024 | ||
| EZIE | NASA, JHUAPL | 2024 | ||
| Solar-C EUVST | JAXA | July 2028 | ||
| CASE | ESA (Cosmic Vision M4) | 2029 |
Three satellites were planned in this series: Beacon Explorer-A, Beacon Explorer-B, Beacon Explorer-C.
A series of three Geodetic Earth Orbiting Satellite (GEOS) were put in orbit: GEOS 1, GEOS 2, GEOS 3.
Explorer name numbers can be found in the NSSDC master catalog, typically assigned to each spacecraft in a mission. These numbers were not officially assigned until after 1975.[45]
| No. | Name(s) | Launch date (UTC) | Rocket | Mass (kg) | Orbit regime | End of data | Re-entry [50] | 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[51] | |
| 2 | Explorer 2 | 5 March 1958 | Juno I | 15 | Failed | — | — | Failed to achieve orbit.[52] | |
| 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[53] | |
| 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)[54] | |
| 5 | Explorer 5 | 24 August 1958 | Juno I | 17 | failed | — | — | Planned in conjunction with Explorer 4, but launch failed[55] | |
| — | 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[56] | |
| 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[57] [58] | |
| 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[59] [60] [61] | |
| – | S-46A (IE-B) | 23 March 1960 | Juno II | 16 | failed | — | — | Analyze electron and proton radiation energies, failed to achieve orbit[62] | |
| 8 | Explorer 8 (S-30) | 3 November 1960 | Juno II | 41 | LEO | 27 December 1960 | 27 March 2012 | Measured atmospheric composition of the ionosphere[63] | |
| – | S-56 | 4 December 1960 | Scout X-1 | 6 | failed | — | — | Atmosphere density measurement, but failed to achieve orbit[64] | |
| 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[65] | |
| – | S-45 | 24 February 1961 | Juno II | 34 | failed | — | — | Ionosphere research, but failed to achieve orbit[66] | |
| 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[67] | |
| 11 | Explorer 11 (S 15) | April 27, 1961 | Juno II | 37 | LEO | November 17, 1961 | In orbit | Gamma ray astronomy[68] | |
| – | S-45A | May 25, 1961 | Juno II | 34 | failed | — | — | Ionosphere research, failed to achieve orbit. Last Juno II launch.[69] | |
| – | S-55 (satellite) (Meteoroid Satellite-A, Micrometeorite Explorer) | June 30, 1961 | Scout X-1 | 85 | failed | — | — | Micrometeoroid research, failed to achieve orbit[70] | |
| 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[71] | |
| 13 | Explorer 13 (S-55A) | August 25, 1961 | Scout X-1 | 86 | LEO | August 28, 1961 | August 28, 1961 | Micrometeoroid research; partial failure[72] | |
| 14 | EPE-B (Energetic Particle Explorer-B) | October 2, 1962 | Delta A | 40 | HEO | August 11, 1963 | July 1, 1966 | Energetic particle research[73] | |
| 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[74] | |
| 16 | S-55B | December 16, 1962 | Scout X-3 | 101 | LEO | July 22, 1963 | In orbit | Micrometeoroid research[75] | |
| 17 | AE-A (S-6, Atmosphere Explorer-A) | April 3, 1963 | Delta B | 184 | LEO | July 10, 1963 | November 24, 1966 | Atmospheric research[76] | |
| 18 | IMP-A (IMP 1, Interplanetary Monitoring Platform-A) | November 27, 1963 | Delta C | 138 | HEO | May 10, 1965 | December 30, 1965 | Magnetospheric research[77] | |
| 19 | AD-A (Atmospheric Density-A) | December 19, 1963 | Scout X-4 | 8 | LEO | May 10, 1981 | May 10, 1981 | Atmospheric density measurements[78] | |
| – | BE-A (Beacon Explorer-A, S-66A) | March 19, 1964 | Delta B | 114 | failed | — | — | Launch failure[79] | |
| 20 | IE-A (S 48, TOPSI, Ionosphere Explorer-A) | August 25, 1964 | Scout X-4 | 45 | LEO | December 29, 1965 | In orbit | Ionosphere research[80] | |
| 21 | IMP-B (IMP 2, Interplanetary Monitoring Platform-B) | October 4, 1964 | Delta C | 135 | HEO | October 13, 1965 | January 30, 1966 | Magnetospheric research[81] | |
| 22 | BE-B (Beacon Explorer-B, S-66B) | October 10, 1964 | Scout X-4 | 53 | LEO | February 1970 | In orbit | Ionospheric and geodetic research[82] | |
| 23 | S 55C | November 6, 1964 | Scout X-4 | 134 | LEO | November 7, 1965 | June 29, 1983 | Micrometeoric research[83] | |
| 24 | AD-B (Atmospheric Density-B) | November 21, 1964 | Scout X-4 | 9 | MEO | October 18, 1968 | October 18, 1968 | Atmospheric density measurements[84] | |
| 25 | Injun 4 (IE-B, Ionosphere Explorer-B) | November 21, 1964 | Scout X-4 | 40 | LEO | December 1966 | In orbit | Ionospheric research[85] | |
| 26 | EPE-D (Energetic Particle Explorer-D) | December 21, 1964 | Delta C | 46 | MEO | December 27, 1967 | August 23, 2021 | High energy particle observations[86] | |
| 27 | BE-C (Beacon Explorer-C, S-66C) | April 29, 1965 | Scout X-4 | 61 | LEO | July 20, 1973 | In orbit | Magnetospheric research[87] | |
| 28 | IMP-C (IMP 3, Interplanetary Monitoring Platform-C) | May 29, 1965 | Delta C | 128 | HEO | May 12, 1967 | July 4, 1968 | Magnetospheric research[88] | |
| 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[89] | |
| 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)[90] | |
| 31 | DME-A (Direct Measurements Explorer) | November 29, 1965 | Thor-Agena B | 99 | LEO | October 1, 1969 | In orbit | Ionospheric research[91] | |
| 32 | AE-B (Atmosphere Explorer-B) | May 25, 1966 | Delta C1 | 225 | LEO | March 1967 | February 22, 1985 | Atmospheric research[92] | |
| 33 | IMP-D (AIMP 1, Interplanetary Monitoring Platform-D) | July 1, 1966 | Delta E1 | 212 | HEO | September 21, 1971[93] | 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[94] | |
| 35 | IMP-E (AIMP 2, Interplanetary Monitoring Platform-E) | July 19, 1967 | Delta E1 | 230 | Lunar | June 24, 1973 | Lunar orbit | Magnetospheric research[95] | |
| 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[96] | |
| 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)[97] | |
| 38 | RAE-A (RAE 1, Radio Astronomy Explorer-A) | July 4, 1968 | Delta J | 602 | MEO | (~1969) | In orbit | Radio astronomy[98] | |
| 39 | AD-C (Atmospheric Density-C) | August 8, 1968 | Scout B | 9 | LEO | June 23, 1971 | June 22, 1981 | Atmospheric density measurements[99] | |
| 40 | Injun 5 (Injun C, IE-C, Ionosphere Explorer-C) | August 8, 1968 | Scout B | 71 | LEO | June 1971 | In orbit | Magnetospheric Research[100] | |
| 41 | IMP-G (IMP 5, Interplanetary Monitoring Platform-G) | June 21, 1969 | Delta E1 | 145 | HEO | December 23, 1972 | December 23, 1972 | Magnetospheric research[101] | |
| 42 | Uhuru (SAS-A, SAS 1) | December 12, 1970 | Scout B | 142 | LEO | January 4, 1975 | April 5, 1979 | X-ray astronomy[102] | |
| 43 | IMP-H (IMP 7, Interplanetary Monitoring Platform-H) | March 13, 1971 | Delta M6 | 635 | MEO | October 2, 1974 | October 2, 1974 | Magnetospheric research[103] | |
| 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)[104] | |
| 45 | SSS-A (S-Cubed A) | November 15, 1971 | Scout B | 52 | MEO | September 30, 1974 | January 10, 1992 | Magnetospheric research[105] | |
| 46 | MTS (Meteoroid Technology Satellite, METEC)[106] | August 13, 1972 | Scout D-1 | 90 | LEO | November 4, 1974 | November 2, 1979 | Micrometeoroids research[107] | |
| 47 | IMP-I (IMP 6, Interplanetary Monitoring Platform-I) | September 23, 1972 | Delta 1604 | 635 | HEO | October 31, 1978 | In orbit | Magnetospheric research[108] | |
| 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[109] | |
| 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 | Radio astronomy[110] | |
| 50 | IMP-J (IMP 8, Interplanetary Monitoring Platform-J) | October 26, 1973 | Delta 1604 | 371 | HEO | October 7, 2006 | In orbit | Magnetospheric research[111] | |
| 51 | AE-C (Atmosphere Explorer-C) | December 16, 1973 | Delta 1900 | 658 | LEO | (December 12, 1978) | December 12, 1978 | Atmospheric research[112] | |
| 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[113] | |
| 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[114] | |
| 54 | AE-D (Atmosphere Explorer-D) | October 6, 1975 | Delta 2910 | 681 | LEO | January 29, 1976 | March 12, 1976 | Atmospheric research[115] | |
| 55 | AE-E (Atmosphere Explorer-E) | November 20, 1975 | Delta 2910 | 735 | LEO | September 25, 1980 | June 10, 1981 | Atmospheric research[116] | |
| — | DADE-A (Dual Air Density Explorer-A) | December 5, 1975 | Scout F-1 | 40 | failed | — | — | Atmospheric research; failed during launch[117] | |
| — | DADE-B (Dual Air Density Explorer-B) | December 5, 1975 | Scout F-1 | 43 | failed | — | — | Atmospheric research; failed during launch[118] | |
| 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[119] | |
| 57 | IUE | January 26, 1978 | Delta 2914 | 669 | MEO | September 30, 1996 | In orbit | Ultraviolet astronomy[120] | |
| 58 | HCMM (AEM-A) | April 26, 1978 | Scout F | 117 | LEO | September 30, 1980 | December 22, 1981 | Thermal mapping of the Earth[121] | |
| 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.[122] | |
| 60 | SAGE (AEM-B) | February 18, 1979 | Scout D-1 | 149 | LEO | January 7, 1982 | April 11, 1989 | Stratospheric aerosol and ozone data[123] | |
| 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[124] | |
| 62 | Dynamics Explorer 1 (DE-1) | 3 August 1981 | Delta 3913 | 424 | MEO | 28 February 1991 | In orbit | Magnetospheric research[125] | |
| 63 | Dynamics Explorer 2 (DE-2) | 3 August 1981 | Delta 3913 | 420 | LEO | 19 February 1983 | 19 February 1983 | Magnetospheric research[126] | |
| 64 | SME | October 6, 1981 | Delta 2310 | 145 | LEO | April 4, 1989 | March 5, 1991 | Atmospheric research[127] | |
| 65 | AMPTE-CCE | 16 August 1984 | Delta 3924 | 242 | MEO | 12 July 1989 | In orbit | Magnetosphere research | |
| 66 | COBE | November 18, 1989 | Delta 5920 | 2,206 | LEO | December 23, 1993 | In orbit | Microwave astronomy[128] | |
| 67 | EUVE (BERKSAT) | June 7, 1992 | Delta II 6920-X | 3,275 | LEO | January 31, 2001 | January 30, 2002 | Ultraviolet astronomy[129] | |
| 68 | SAMPEX | July 3, 1992 | Scout G-1 | 158 | LEO | June 30, 2004[130] | November 13, 2012.[131] | SMEX: magnetospheric research[132] | |
| 69 | RXTE | December 30, 1995 | Delta II 7920 | 3,200 | LEO | January 3, 2012 | April 30, 2018[133] | MIDEX: X-ray astronomy[134] | |
| 70 | FAST | August 21, 1996 | Pegasus XL | 187 | LEO | May 4, 2009 | In orbit | SMEX: auroral phenomena[135] | |
| — | HETE 1 | November 4, 1996 | Pegasus XL | 128 | LEO | — | April 7, 2002 | Separation failure, mission relaunched as HETE 2[136] | |
| 71 | ACE | August 25, 1997 | Delta II 7920 | 596 | Sun–Earth L1 | Operational | In L1 orbit | MIDEX: solar/interplanetary/interstellar particle research[137] | |
| 72 | SNOE | February 26, 1998 | Pegasus XL | 120 | LEO | December 13, 2003 | December 13, 2003 | STEDI, UNEX: atmospheric research[138] | |
| 73 | TRACE | April 2, 1998 | Pegasus XL | 250 | LEO | June 21, 2010[139] | In orbit | SMEX: solar observatory[140] | |
| 74 | SWAS | December 6, 1998 | Pegasus XL | 288 | LEO | September 1, 2005 | In orbit | SMEX: submillimeter astronomy[141] | |
| 75 | WIRE | March 5, 1999 | Pegasus XL | 250 | SSO | September 30, 2000[142] | May 10, 2011[143] | SMEX, Infrared astronomy, primary mission failed due to loss of coolant[144] | |
| 76 | TERRIERS | May 18, 1999 | Pegasus XL | 120 | Polar LEO | May 18, 1999 | In orbit | STEDI: atmospheric research, satellite failed shortly after achieving orbit[145] | |
| 77 | FUSE | June 23, 1999 | Delta II 7320 | 1,400 | LEO | October 18, 2007 | In orbit | MIDEX: ultraviolet astronomy[146] | |
| 78 | IMAGE | March 25, 2000 | Delta II 7326 | 536 | Polar MEO | December 18, 2005[147] | In orbit | MIDEX: magnetospheric research[148] | |
| 79 | HETE-2 | October 9, 2000 | Pegasus-H | 124 | LEO | March 28, 2007[149] | In orbit | MO: UV, X-ray, and gamma ray astronomy[150] | |
| 80 | WMAP | June 30, 2001 | Delta II 7425-10 | 840 | Sun–Earth L2 | October 2010[151] | Heliocentric orbit | MIDEX: microwave astronomy[152] | |
| 81 | RHESSI | February 5, 2002 | Pegasus XL | 230 | LEO | August 16, 2018 | In orbit | SMEX: X-ray and gamma ray solar flare imaging[153] | |
| — | INTEGRAL | October 17, 2002 | Proton-K Blok DM-2 | 4,000 | HEO | Operational | In orbit | International: space telescope for observing gamma rays[154] | |
| 82 | CHIPSat | January 13, 2003 | Delta II 7320-10 | 60 | LEO | April 11, 2008[155] | In orbit | UNEX: ultraviolet spectroscopy and astronomy[156] | |
| 83 | GALEX | April 28, 2003 | Pegasus XL | 280 | LEO | June 28, 2013[157] | In orbit | SMEX: ultraviolet astronomy[158] | |
| 84 | Swift | November 20, 2004 | Delta II 7320-10C | 1,470 | LEO | Operational | In orbit | MIDEX: gamma ray astronomy[159] | |
| — | Suzaku (Astro E2) | July 10, 2005 | M-V | 1,706 | LEO | September 2, 2015 | In orbit | MO: instrument on JAXA's Suzaku mission[160] | |
| — | 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[161] | |
| 86 | THEMIS B (ARTEMIS P1) | February 17, 2007 | Delta II 7925 | 77 | Lunar | Operational | Lunar orbit | MIDEX; Magnetospheric research[162] | |
| 87 | THEMIS C (ARTEMIS P2) | February 17, 2007 | Delta II 7925 | 77 | Lunar | Operational | Lunar orbit | MIDEX: magnetospheric research[163] | |
| 88 | THEMIS D | February 17, 2007 | Delta II 7925 | 77 | HEO | Operational | In orbit | MIDEX: magnetospheric research[164] | |
| 89 | THEMIS E | February 17, 2007 | Delta II 7925 | 77 | HEO | Operational | In orbit | MIDEX: magnetospheric research[165] | |
| 90 | AIM | April 25, 2007 | Pegasus XL | 197 | SSO | Operational | In orbit | SMEX: noctilucent cloud observation[166] | |
| — | TWINS B | March 13, 2008 | Atlas V 411 | classified | Molniya | Operational | In orbit | MO: payload on Trumpet-F/O-1 2 (USA-200) | |
| — | 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.[167] | |
| 92 | WISE | December 14, 2009 | Delta II 7320 | 661 | LEO | Operational | In orbit | MIDEX: infrared astronomy, NEOWISE extension. Discovered first Earth trojan.[168] | |
| 93 | NuSTAR | June 13, 2012 | Pegasus XL | 350 | LEO | Operational | In orbit | SMEX: high-energy X-ray astronomy[169] | |
| 94 | IRIS | June 27, 2013 | Pegasus XL | 183 | SSO | Operational | In orbit | SMEX: solar UV astronomy[170] | |
| — | 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[171] | |
| — | May 3, 2017 | Falcon 9 FT | 372 | ISS | Operational | ISS | MO: instrument on ISS for neutron star observations | ||
| — | 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 |
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.[172] 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:[49]
Recent examples of conclusions of launched missions, cancelled due to budgetary constraints:
Number of launches per decade:[176]