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Ongoing United States space exploration program

The Explorers program 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.

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. + Explorers Program satellites No. Name(s) Launch date (UTC) Rocket Mass (kg) Orbit regime End of data Re entry 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 2 Explorer 2 5 March 1958 Juno I 15 Failed — — Failed to achieve orbit. 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 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) 5 Explorer 5 August 24, 1958 Juno I 17 failed — — Planned in conjunction with Explorer 4, but launch failed — 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 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 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 – S 46A (IE B) March 23, 1960 Juno II 16 failed — — Analyze electron and proton radiation energies, failed to achieve orbit 8 Explorer 8 (S 30) 3 November 1960 Juno II 41 LEO 27 December 1960 27 March 2012 Measured atmospheric composition of the ionosphere – S 56 December 4, 1960 Scout X 1 6 failed — — Atmosphere density measurement, but failed to achieve orbit 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 – S 45 February 24, 1961 Juno II 34 failed — — Ionosphere research, but failed to achieve orbit 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 11 Explorer 11 (S 15) April 27, 1961 Juno II 37 LEO November 17, 1961 In orbit Gamma ray astronomy – S 45A May 25, 1961 Juno II 34 failed — — Ionosphere research, failed to achieve orbit. Last Juno II launch. – S 55 (satellite) (Meteoroid Satellite A, Micrometeorite Explorer) June 30, 1961 Scout X 1 85 failed — — Micrometeoroid research, failed to achieve orbit 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 13 S 55A August 25, 1961 Scout X 1 86 LEO August 28, 1961 August 28, 1961 Micrometeoroid research; partial failure 14 EPE B (Energetic Particle Explorer B) October 2, 1962 Delta A 40 HEO August 11, 1963 July 1, 1966 Energetic particle research 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 16 S 55B December 16, 1962 Scout X 3 101 LEO July 22, 1963 In orbit Micrometeoroid research 17 AE A (S 6, Atmosphere Explorer A) April 3, 1963 Delta B 184 LEO July 10, 1963 November 24, 1966 Atmospheric research 18 IMP A (IMP 1, Interplanetary Monitoring Platform A) November 27, 1963 Delta C 138 HEO May 10, 1965 December 30, 1965 Magnetospheric research 19 AD A (Atmospheric Density A) December 19, 1963 Scout X 4 8 LEO May 10, 1981 May 10, 1981 Atmospheric density measurements – BE A (Beacon Explorer A, S 66A) March 19, 1964 Delta B 114 failed — — Launch failure 20 IE A (S 48, TOPSI, Ionosphere Explorer A) August 25, 1964 Scout X 4 45 LEO December 29, 1965 In orbit Ionosphere research 21 IMP B (IMP 2, Interplanetary Monitoring Platform B) October 4, 1964 Delta C 135 HEO October 13, 1965 January 30, 1966 Magnetospheric research 22 BE B (Beacon Explorer B, S 66B) October 10, 1964 Scout X 4 53 LEO February 1970 In orbit Ionospheric and geodetic research 23 S 55C November 6, 1964 Scout X 4 134 LEO November 7, 1965 June 29, 1983 Micrometeoric research 24 AD B (Atmospheric Density B) November 21, 1964 Scout X 4 9 MEO October 18, 1968 October 18, 1968 Atmospheric density measurements 25 Injun 4 (IE B, Ionosphere Explorer B) November 21, 1964 Scout X 4 40 LEO December 1966 In orbit Ionospheric research 26 EPE D (Energetic Particle Explorer D) December 21, 1964 Delta C 46 MEO December 27, 1967 August 23, 2021 High energy particle observations 27 BE C (Beacon Explorer C,S 66C) April 29, 1965 Scout X 4 61 LEO July 20, 1973 In orbit Magnetospheric research 28 IMP C (IMP 3, Interplanetary Monitoring Platform C) May 29, 1965 Delta C 128 HEO May 12, 1967 July 4, 1968 Magnetospheric research 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 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) 31 DME A (Direct Measurements Explorer) November 29, 1965 Thor Agena B 99 LEO October 1, 1969 In orbit Ionospheric research 32 AE B (Atmosphere Explorer B) May 25, 1966 Delta C1 225 LEO March 1967 February 22, 1985 Atmospheric research 33 IMP D (AIMP 1, Interplanetary Monitoring Platform D) July 1, 1966 Delta E1 212 HEO September 21, 1971 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 35 IMP E (AIMP 2, Interplanetary Monitoring Platform E) July 19, 1967 Delta E1 230 Lunar June 24, 1973 Lunar orbit Magnetospheric research 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 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) 38 RAE A (RAE 1, Radio Astronomy Explorer A) July 4, 1968 Delta J 602 MEO (~1969) In orbit Radio astronomy 39 AD C (Atmospheric Density C) August 8, 1968 Scout B 9 LEO June 23, 1971 June 22, 1981 Atmospheric density measurements 40 Injun 5 (Injun C, IE C, Ionosphere Explorer C) August 8, 1968 Scout B 71 LEO June 1971 In orbit Magnetospheric Research 41 IMP G (IMP 5, Interplanetary Monitoring Platform G) June 21, 1969 Delta E1 145 HEO December 23, 1972 December 23, 1972 Magnetospheric research 42 Uhuru (SAS A, SAS 1) December 12, 1970 Scout B 142 LEO January 4, 1975 April 5, 1979 X ray astronomy 43 IMP H (IMP 7, Interplanetary Monitoring Platform H) March 13, 1971 Delta M6 635 MEO October 2, 1974 October 2, 1974 Magnetospheric research 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) 45 SSS A (S Cubed A) November 15, 1971 Scout B 52 MEO September 30, 1974 January 10, 1992 Magnetospheric research 46 MTS (Meteoroid Technology Satellite, METEC) August 13, 1972 Scout D 1 90 LEO November 4, 1974 November 2, 1979 Micrometeoroids research 47 IMP I (IMP 6, Interplanetary Monitoring Platform I) September 23, 1972 Delta 1604 635 HEO October 31, 1978 In orbit Magnetospheric research 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 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 50 IMP J (IMP 8, Interplanetary Monitoring Platform J) October 26, 1973 Delta 1604 371 HEO October 7, 2006 In orbit Magnetospheric research 51 AE C (Atmosphere Explorer C) December 16, 1973 Delta 1900 658 LEO (December 12, 1978) December 12, 1978 Atmospheric research 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 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 54 AE D (Atmosphere Explorer D) October 6, 1975 Delta 2910 681 LEO January 29, 1976 March 12, 1976 Atmospheric research 55 AE E (Atmosphere Explorer E) November 20, 1975 Delta 2910 735 LEO September 25, 1980 June 10, 1981 Atmospheric research — DADE A (Dual Air Density Explorer A) December 5, 1975 Scout F 1 40 failed — — Atmospheric research; failed during launch — DADE B (Dual Air Density Explorer B) December 5, 1975 Scout F 1 43 failed — — Atmospheric research; failed during launch 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 57 IUE January 26, 1978 Delta 2914 669 MEO September 30, 1996 In orbit Ultraviolet astronomy 58 HCMM (AEM A) April 26, 1978 Scout F 117 LEO September 30, 1980 December 22, 1981 Thermal mapping of the Earth 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. 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 62 Dynamics Explorer 1 (DE 1) 3 August 1981 Delta 3913 424 MEO 28 February 1991 In orbit Magnetospheric research 63 Dynamics Explorer 2 (DE 2) 3 August 1981 Delta 3913 420 LEO 19 February 1983 19 February 1983 Magnetospheric research 64 SME October 6, 1981 Delta 2310 145 LEO April 4, 1989 March 5, 1991 Atmospheric research 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 67 EUVE (BERKSAT) June 7, 1992 Delta II 6920 X 3,275 LEO January 31, 2001 January 30, 2002 Ultraviolet astronomy 68 SAMPEX July 3, 1992 Scout G 1 158 LEO June 30, 2004 November 13, 2012. SMEX: magnetospheric research 69 RXTE December 30, 1995 Delta II 7920 3,200 LEO January 3, 2012 April 30, 2018 MIDEX: X ray astronomy 70 FAST August 21, 1996 Pegasus XL 187 LEO May 4, 2009 In orbit SMEX: auroral phenomena — HETE 1 November 4, 1996 Pegasus XL 128 LEO — April 7, 2002 Separation failure, mission relaunched as HETE 2 71 ACE August 25, 1997 Delta II 7920 596 Sun–Earth L1 Operational In L1 orbit MIDEX: solar/interplanetary/interstellar particle research 72 SNOE February 26, 1998 Pegasus XL 120 LEO December 13, 2003 December 13, 2003 STEDI, UNEX: atmospheric research 73 TRACE April 2, 1998 Pegasus XL 250 LEO June 21, 2010 In orbit SMEX: solar observatory 74 SWAS December 6, 1998 Pegasus XL 288 LEO September 1, 2005 In orbit SMEX: submillimeter astronomy 75 WIRE March 5, 1999 Pegasus XL 250 SSO September 30, 2000 May 10, 2011 SMEX, Infrared astronomy, primary mission failed due to loss of coolant 76 TERRIERS May 18, 1999 Pegasus XL 120 Polar LEO May 18, 1999 In orbit STEDI: atmospheric research, satellite failed shortly after achieving orbit 77 FUSE June 23, 1999 Delta II 7320 1,400 LEO October 18, 2007 In orbit MIDEX: ultraviolet astronomy 78 IMAGE March 25, 2000 Delta II 7326 536 Polar MEO December 18, 2005 In orbit MIDEX: magnetospheric research 79 HETE 2 October 9, 2000 Pegasus H 124 LEO March 28, 2007 In orbit MO: UV, X ray, and gamma ray astronomy 80 WMAP June 30, 2001 Delta II 7425 10 840 Sun–Earth L2 October 2010 Heliocentric orbit MIDEX: microwave astronomy 81 RHESSI February 5, 2002 Pegasus XL 230 LEO August 16, 2018 In orbit SMEX: X ray and gamma ray solar flare imaging — INTEGRAL October 17, 2002 Proton K Blok DM 2 4,000 HEO Operational In orbit International: space telescope for observing gamma rays 82 CHIPSat January 13, 2003 Delta II 7320 10 60 LEO April 11, 2008 In orbit UNEX: ultraviolet spectroscopy and astronomy 83 GALEX April 28, 2003 Pegasus XL 280 LEO June 28, 2013 In orbit SMEX: ultraviolet astronomy 84 Swift November 20, 2004 Delta II 7320 10C 1,470 LEO Operational In orbit MIDEX: gamma ray astronomy — Suzaku (Astro E2) July 10, 2005 M V 1,706 LEO September 2, 2015 In orbit MO: instrument on JAXA's Suzaku mission — 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 86 THEMIS B (ARTEMIS P1) February 17, 2007 Delta II 7925 77 Lunar Operational Lunar orbit MIDEX; Magnetospheric research 87 THEMIS C (ARTEMIS P2) February 17, 2007 Delta II 7925 77 Lunar Operational Lunar orbit MIDEX: magnetospheric research 88 THEMIS D February 17, 2007 Delta II 7925 77 HEO Operational In orbit MIDEX: magnetospheric research 89 THEMIS E February 17, 2007 Delta II 7925 77 HEO Operational In orbit MIDEX: magnetospheric research 90 AIM April 25, 2007 Pegasus XL 197 SSO Operational In orbit SMEX: noctilucent cloud observation — 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. 92 WISE December 14, 2009 Delta II 7320 661 LEO Operational In orbit MIDEX: infrared astronomy, NEOWISE extension. Discovered first Earth trojan. 93 NuSTAR June 13, 2012 Pegasus XL 350 LEO Operational In orbit SMEX: high energy X ray astronomy 94 IRIS June 27, 2013 Pegasus XL 183 SSO Operational In orbit SMEX: solar UV astronomy — 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 — 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