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The Explorers Programcite-ref-1[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 Explorers 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

• Early Explorer satellites

• Continuation of the Explorers Program

• SMEX, MIDEX, and Student Explorers Programs

• Classes

• Medium-Class (MIDEX)

• Small Explorers (SMEX)

• University-Class Explorers (UNEX)

• Missions of Opportunity (MO)

• Beacon Explorers

• GEOS series

• Launched spacecraft

• Cancelled missions

• Launch statistics

• See also

• References

• External links

The Explorers Program began as a U.S. Army proposal (Project Orbiter) to place a "civilian" artificial satellite into orbit during the International Geophysical Year (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.cite-ref-boehm-nasa-2-0[2] The Juno I vehicle was replaced by the Juno II in 1959.

With the establishment of NASA in 1958, the Explorers 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.cite-ref-3[3]cite-ref-4[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).cite-ref-butrica-5-0[5]cite-ref-nasa-6-0[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.cite-ref-butrica-5-1[5]

| Mission | Photo | Satellite | | Launch date | Decay date |

|---|---|---|---|---|---|

| Mission | Photo | Explorer | IMP | Launch date | Decay date |

| IMP-1 | | Explorer 18 | IMP-A | 27 November 1963, 02:30 UTC | December 30, 1965 |

| IMP-2 | | Explorer 21 | IMP-B | 4 October 1964, 03:45 UTC | January 1, 1966 |

| IMP-3 | | Explorer 28 | IMP-C | 29 May 1965, 12:00 UTC | 4 July 1968 |

| AIMP-1 | | Explorer 33 | IMP-D | 1 July 1966, 16:02 UTC | In orbit |

| IMP-4 | | Explorer 34 | IMP-F | 24 May 1967, 14:05 UTC | May 3, 1969 |

| AIMP-2 | | Explorer 35 | IMP-E | 19 July 1967, 14:19 UTC | After June 24, 1973 |

| IMP-5 | | Explorer 41 | IMP-G | 21 June 1969, 08:47 UTC | December 23, 1972 |

| IMP-6 | | Explorer 43 | IMP-I | 13 March 1971, 16:15 UTC | October 2, 1974 |

| IMP-7 | | Explorer 47 | IMP-H | 23 September 1972, 01:20 UTC | In orbit |

| IMP-8 | | Explorer 50 | IMP-J | 26 October 1973, 02:26 UTC | In orbit |

| Mission | Notes |

|---|---|

| Mission | Notes |

| IMP-1 | First use of integrated circuits in a s… |

| IMP-2 | Second satellite in IMP-A/-B/-C design… |

| IMP-3 | Third satellite in IMP-A/-B/-C design s… |

| AIMP-1 | First use of MOSFET integrated circuits… |

| IMP-4 | Similar design to IMP-G. |

| AIMP-2 | Similar design to IMP-D. Positioned in… |

| IMP-5 | Similar design to IMP-F. |

| IMP-6 | First spacecraft in IMP-I/-H/-J series. |

| IMP-7 | Second spacecraft in IMP-I/-H/-J series. |

| IMP-8 | Third spacecraft in IMP-I/-H/-J series,… |

Over the following two decades, NASA has launched over 50 Explorer missions,cite-ref-nssdc-list-8-0[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.

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.cite-ref-nasahistory-9-0[9]cite-ref-10[10] The first three SMEX missions were chosen in April 1989 out of 51 candidates, and launched in 1992, 1996 and 1998cite-ref-ieee-11-0[11] The second set of two missions were announced in September 1994 and launched in 1998 and 1999.cite-ref-nasahistory-9-1[9]

In the mid-1990s, NASA initiated the Medium-class Explorers (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.cite-ref-nasahistory-9-2[9] This new launch vehicle was not developed and instead, these missions were flown on a modified Delta II rocket.cite-ref-12[12]cite-ref-13[13] The first announcement opportunity for MIDEX was issued in March 1995, and the first launch under this new class was FUSE in 1999.cite-ref-nasahistory-9-3[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.cite-ref-nasahistory-9-4[9] Later, NASA established the University-Class Explorer (UNEX) program for much cheaper missions, which is regarded as a successor to STEDI.cite-ref-14[14]

The Explorer missions were at first managed by the Small Explorers 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.cite-ref-welcome-15-0[15] The Explorers 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.cite-ref-16[16]

Excluding the launches, the MIDEX class has a current mission cap cost of US$250 million in 2018,cite-ref-midex19-17-0[17] with future MIDEX missions being capped at US$350 million.cite-ref-18[18] The cost cap for SMEX missions in 2017 was US$165 million.cite-ref-19[19] UNEX missions are capped at US$15 million.cite-ref-missions-20-0[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.cite-ref-midex19-17-1[17]cite-ref-missions-20-1[20]

| Name | MIDEX number | Explorer number | Launch (UTC) |

|---|---|---|---|

| RXTE | | Explorer-69 | 30 December 1995 |

| ACE | | Explorer-71 | 25 August 1997 |

| FUSE | MIDEX-0 | Explorer-77 | 23 June 1999 |

| IMAGE | MIDEX-1 | Explorer-78 | 25 March 2000 |

| WMAP | MIDEX-2 | Explorer-80 | 30 June 2001 |

| Swift | MIDEX-3 | Explorer-84 | 20 November 2004 |

| FAME | MIDEX-4 | — | Scheduled for 2004 |

| 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 |

| WISE / NEOWISE | MIDEX-6 | Explorer-92 | 14 December 2009 |

| TESS | MIDEX-7 | Explorer-95 | 18 April 2018 |

| ICON | MIDEX-8 | Explorer-96 | 11 October 2019 |

| SPHEREx | MIDEX-9 | Explorer-102 | 12 March 2025 |

| MUSE | MIDEX-10 | | 2027 |

| HelioSwarm | MIDEX-11 | | 2028 |

| UVEX | MIDEX-12 | | 2030 |

| Name | Status |

|---|---|

| RXTE | Ended in 2012 / Reentered on 30 April 2… |

| ACE | Operational |

| FUSE | Ended in 2007 |

| IMAGE | Lost contact in 2005. Partial contact r… |

| WMAP | Ended in 2010 |

| Swift | Operational |

| FAME | Cancelled in 2002 (cost) |

| THEMIS A | Operational |

| THEMIS B | Operational |

| THEMIS C | Operational |

| THEMIS D | Operational |

| THEMIS E | Operational |

| WISE / NEOWISE | Mission operations Completed on 31 July… |

| TESS | Operational |

| ICON | Operational |

| SPHEREx | Operational |

| MUSE | In development |

| HelioSwarm | In development |

| UVEX | In development |

The Small Explorers class was implemented in 1989 specifically to fund space exploration missions that cost no more than US$120 million.cite-ref-missions-20-2[20]cite-ref-ieee-11-1[11] The missions are managed by the Explorers Project at the Goddard Space Flight Center (GSFC).cite-ref-welcome-15-1[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 Explorers 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.cite-ref-welcome-15-2[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).cite-ref-25[25]cite-ref-26[26]cite-ref-27[27] In June 2019 NASA selected TRACERS and PUNCH for flight.cite-ref-28[28]

| Name | SMEX number | Explorer number | Launch (UTC) |

|---|---|---|---|

| SAMPEX | SMEX-1 | Explorer-68 | 3 July 1992 |

| FAST | SMEX-2 | Explorer-70 | 21 August 1996 |

| SWAS | SMEX-3 | Explorer-74 | 6 December 1998 |

| TRACE | SMEX-4 | Explorer-73 | 2 April 1998 |

| WIRE | SMEX-5 | Explorer-75 | 5 March 1999 |

| RHESSI | SMEX-6 | Explorer-81 | 5 February 2002 |

| GALEX | SMEX-7 | Explorer-83 | 28 April 2003 |

| SPIDR | SMEX-8 | — | Scheduled for 2005 |

| 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 | SMEX-12 | Explorer-94 | 28 June 2013 |

| GEMS | SMEX-13 | — | Scheduled for 2014 |

| Imaging X-ray Polarimetry Explorer (IXP… | SMEX-14 | Explorer-97 | 9 December 2021 |

| PUNCH | SMEX-15 | | 12 March 2025 |

| TRACERS | SMEX-16 | | 22 July 2025 |

| COSI | SMEX-17 | | August 2027 |

| Name | End of mission |

|---|---|

| SAMPEX | 30 June 2004 |

| FAST | 4 May 2009 |

| SWAS | 21 July 2004 |

| TRACE | 21 June 2010 |

| WIRE | — |

| RHESSI | April 2018 |

| GALEX | May 2012 |

| SPIDR | — |

| Aeronomy of Ice in the Mesosphere (AIM) | 19 August 2024 |

| GEMS | — |

| Name | Status |

|---|---|

| SAMPEX | Reentered on 13 November 2012 |

| TRACE | Reentered on 18 July 2025 |

| WIRE | Spacecraft equipment failure; reentered… |

| RHESSI | Deorbited on 20 April 2023 |

| GALEX | Decommissioned on 28 June 2013 |

| SPIDR | Cancelled in 2003 due to poor instrumen… |

| Aeronomy of Ice in the Mesosphere (AIM) | Reentered on 19 August 2024 |

| IBEX | Operational |

| NuSTAR | Operational |

| IRIS | Operational |

| GEMS | Cancelled in 2012 due to expected cost… |

| Imaging X-ray Polarimetry Explorer (IXP… | Operational |

| PUNCH | Operational |

| TRACERS | Pending launch |

| COSI | In development |

| 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) |

| 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.cite-ref-38[38]

| Name | Launcher (mission) | Launch (UTC) |

|---|---|---|

| HETE-2 | NASA (Explorer-79) | 9 October 2000 |

| INTEGRAL | ESA | 17 October 2002 |

| Suzaku (Astro-E2) | JAXA | 10 July 2005 |

| TWINS | NRO ( USA-184 ; USA-200 ) | TWINS-1: 28 June 2006 TWINS-2: 13 March… |

| CINDI | DoD ( C/NOFS ) | 16 April 2008 |

| Hitomi (Astro-H) | JAXA | 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 |

| SunRISE | NASA ( Maxar satellite) | 2024 |

| EZIE | NASA , JHUAPL | 2024 |

| Solar-C EUVST | JAXA | July 2028 |

| CASE | ESA ( Cosmic Vision M4 ) | 2029 |

| Name | Status |

|---|---|

| HETE-2 | Ended in 2008 |

| INTEGRAL | Operational |

| Suzaku (Astro-E2) | Ended in 2015 |

| TWINS | Operational |

| CINDI | Ended in 2015 |

| Hitomi (Astro-H) | Failed |

| NICER | Operational |

| GOLD | Operational |

| XRISM | Operational |

| AWE | Operational |

| GUSTO | Ended on 26 February 2024 |

| SunRISE | In development |

| EZIE | In development |

| Solar-C EUVST | In development |

| CASE | In development |

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.

Explorers Program 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.cite-ref-54[54]

| No. | Name(s) | Launch date (UTC) | Rocket |

|---|---|---|---|

| 1 | Explorer 1 | 1 February 1958 | Juno I |

| 2 | Explorer 2 | 5 March 1958 | Juno I |

| 3 | Explorer 3 | 26 March 1958 | Juno I |

| 4 | Explorer 4 | 26 July 1958 | Juno I |

| 5 | Explorer 5 | 24 August 1958 | Juno I |

| — | Explorer S-1 (7X) | 16 July 1959 | Juno II |

| 6 | Explorer 6 (S-2, Able 3) | 7 August 1959 | Thor-Able |

| 7 | Explorer 7 (S-1A) | 13 October 1959 | Juno II |

| – | S-46A (IE-B) | 23 March 1960 | Juno II |

| 8 | Explorer 8 (S-30) | 3 November 1960 | Juno II |

| – | S-56 | 4 December 1960 | Scout X-1 |

| 9 | Explorer 9 (S-56A) | 16 February 1961 | Scout X-1 |

| – | S-45 | 24 February 1961 | Juno II |

| 10 | Explorer 10 (P 14) | 25 March 1961 | Thor-Delta |

| 11 | Explorer 11 (S 15) | 27 April 1961 | Juno II |

| – | S-45A | 25 May 1961 | Juno II |

| – | S-55 (satellite) (Meteoroid Satellite-A… | 30 June 1961 | Scout X-1 |

| 12 | EPE-A (S 3, Energetic Particle Explorer… | 16 August 1961 | Thor-Delta |

| 13 | Explorer 13 (S-55A) | 25 August 1961 | Scout X-1 |

| 14 | EPE-B (Energetic Particle Explorer-B) | 2 October 1962 | Delta A |

| 15 | EPE-C (S-3B, Energetic Particle Explore… | 27 October 1962 | Delta A |

| 16 | S-55B | 16 December 1962 | Scout X-3 |

| 17 | AE-A (S-6, Atmosphere Explorer-A) | 3 April 1963 | Delta B |

| 18 | IMP-A (IMP 1, Interplanetary Monitoring… | 27 November 1963 | Delta C |

| 19 | AD-A (Atmospheric Density-A) | 19 December 1963 | Scout X-4 |

| – | BE-A ( Beacon Explorer-A , S-66A) | 19 March 1964 | Delta B |

| 20 | IE-A (S 48, TOPSI, Ionosphere Explorer-… | 25 August 1964 | Scout X-4 |

| 21 | IMP-B (IMP 2, Interplanetary Monitoring… | 4 October 1964 | Delta C |

| 22 | BE-B (Beacon Explorer-B, S-66B) | 10 October 1964 | Scout X-4 |

| 23 | S 55C | 6 November 1964 | Scout X-4 |

| 24 | AD-B (Atmospheric Density-B) | 21 November 1964 | Scout X-4 |

| 25 | Injun 4 (IE-B, Ionosphere Explorer-B) | 21 November 1964 | Scout X-4 |

| 26 | EPE-D (Energetic Particle Explorer-D) | 21 December 1964 | Delta C |

| 27 | BE-C (Beacon Explorer-C, S-66C) | 29 April 1965 | Scout X-4 |

| 28 | IMP-C (IMP 3, Interplanetary Monitoring… | 29 May 1965 | Delta C |

| 29 | GEOS 1 (GEOS-A, Geodetic Earth Orbiting… | 6 November 1965 | Delta E |

| 30 | SOLRAD 8 (SE-A) | 19 November 1965 | Scout X-4 |

| 31 | DME-A (Direct Measurements Explorer) | 29 November 1965 | Thor-Agena B |

| 32 | AE-B (Atmosphere Explorer-B) | 25 May 1966 | Delta C 1 |

| 33 | IMP-D (AIMP 1, Interplanetary Monitorin… | 1 July 1966 | Delta E 1 |

| 34 | IMP-F (IMP 4, Interplanetary Monitoring… | 24 May 1967 | Delta E 1 |

| 35 | IMP-E (AIMP 2, Interplanetary Monitorin… | 19 July 1967 | Delta E 1 |

| 36 | GEOS 2 (GEOS-B, Geodetic Earth Orbiting… | 11 January 1968 | Delta E 1 |

| 37 | SOLRAD 9 (SE B) | 5 March 1968 | Scout B |

| 38 | RAE-A (RAE 1, Radio Astronomy Explorer-… | 4 July 1968 | Delta J |

| 39 | AD-C (Atmospheric Density-C) | 8 August 1968 | Scout B |

| 40 | Injun 5 (Injun C, IE-C, Ionosphere Expl… | 8 August 1968 | Scout B |

| 41 | IMP-G (IMP 5, Interplanetary Monitoring… | 21 June 1969 | Delta E 1 |

| 42 | Uhuru (SAS-A, SAS 1) | 12 December 1970 | Scout B |

| 43 | IMP-H (IMP 7, Interplanetary Monitoring… | 13 March 1971 | Delta M 6 |

| 44 | SOLRAD 10 (SE-C, SOLRAD-C) | 8 July 1971 | Scout B |

| 45 | SSS-A (S-Cubed A) | 15 November 1971 | Scout B |

| 46 | MTS ( Meteoroid Technology Satellite ,… | 13 August 1972 | Scout D-1 |

| 47 | IMP-I (IMP 6, Interplanetary Monitoring… | 23 September 1972 | Delta 1604 |

| 48 | SAS-B (Small Astronomy Satellite-B, SAS… | 15 November 1972 | Scout D-1 |

| 49 | RAE-B (RAE 2, Radio Astronomy Explorer-… | 10 June 1973 | Delta 1913 |

| 50 | IMP-J (IMP 8, Interplanetary Monitoring… | 26 October 1973 | Delta 1604 |

| 51 | AE-C (Atmosphere Explorer-C) | 16 December 1973 | Delta 1900 |

| 52 | Hawkeye 1 (Injun-F, Injun 6, IE-D, Iono… | 3 June 1974 | Scout E-1 |

| 53 | SAS-C (Small Astronomy Satellite-C, SAS… | 7 May 1975 | Scout F-1 |

| 54 | AE-D (Atmosphere Explorer-D) | 6 October 1975 | Delta 2910 |

| 55 | AE-E (Atmosphere Explorer-E) | 20 November 1975 | Delta 2910 |

| — | DADE-A (Dual Air Density Explorer-A) | 5 December 1975 | Scout F-1 |

| — | DADE-B (Dual Air Density Explorer-B) | 5 December 1975 | Scout F-1 |

| 56 | ISEE-1 (ISEE-A) | 22 October 1977 | Delta 2914 |

| 57 | IUE | 26 January 1978 | Delta 2914 |

| 58 | HCMM (AEM-A) | 26 April 1978 | Scout F |

| 59 | ICE (ISEE 3, ISEE-C) | 12 August 1978 | Delta 2914 |

| 60 | SAGE (AEM-B) | 18 February 1979 | Scout D-1 |

| 61 | MAGSAT (AEM-C) | 30 October 1979 | Scout G-1 |

| 62 | Dynamics Explorer 1 (DE-1) | 3 August 1981 | Delta 3913 |

| 63 | Dynamics Explorer 2 (DE-2) | 3 August 1981 | Delta 3913 |

| 64 | SME | 6 October 1981 | Delta 2310 |

| 65 | AMPTE-CCE | 16 August 1984 | Delta 3924 |

| 66 | COBE | 18 November 1989 | Delta 5920 |

| 67 | EUVE (BERKSAT) | 7 June 1992 | Delta II 6920-X |

| 68 | SAMPEX | 3 July 1992 | Scout G-1 |

| 69 | RXTE | 30 December 1995 | Delta II 7920 |

| 70 | FAST | 21 August 1996 | Pegasus XL |

| — | HETE 1 | 4 November 1996 | Pegasus XL |

| 71 | ACE | 25 August 1997 | Delta II 7920 |

| 72 | SNOE | 26 February 1998 | Pegasus XL |

| 73 | TRACE | 2 April 1998 | Pegasus XL |

| 74 | SWAS | 6 December 1998 | Pegasus XL |

| 75 | WIRE | 5 March 1999 | Pegasus XL |

| 76 | TERRIERS | 18 May 1999 | Pegasus XL |

| 77 | FUSE | 23 June 1999 | Delta II 7320 |

| 78 | IMAGE | 25 March 2000 | Delta II 7326 |

| 79 | HETE-2 | 9 October 2000 | Pegasus-H |

| 80 | WMAP | 30 June 2001 | Delta II 7425-10 |

| 81 | RHESSI | 5 February 2002 | Pegasus XL |

| — | INTEGRAL | 17 October 2002 | Proton-K Blok DM-2 |

| 82 | CHIPSat | 13 January 2003 | Delta II 7320-10 |

| 83 | GALEX | 28 April 2003 | Pegasus XL |

| 84 | Swift | 20 November 2004 | Delta II 7320-10C |

| — | Suzaku (Astro E2) | 10 July 2005 | M-V |

| — | TWINS A | 28 June 2006 | Delta IV M+(4,2) |

| 85 | THEMIS A | 17 February 2007 | Delta II 7925 |

| 86 | THEMIS B (ARTEMIS P1) | 17 February 2007 | Delta II 7925 |

| 87 | THEMIS C (ARTEMIS P2) | 17 February 2007 | Delta II 7925 |

| 88 | THEMIS D | 17 February 2007 | Delta II 7925 |

| 89 | THEMIS E | 17 February 2007 | Delta II 7925 |

| 90 | AIM | 25 April 2007 | Pegasus XL |

| — | TWINS B | 13 March 2008 | Atlas V 411 |

| — | CINDI | 16 April 2008 | Pegasus XL |

| 91 | IBEX | 19 October 2008 | Pegasus XL |

| 92 | WISE | 14 December 2009 | Delta II 7320 |

| 93 | NuSTAR | 13 June 2012 | Pegasus XL |

| 94 | IRIS | 27 June 2013 | Pegasus XL |

| — | Hitomi (NeXT, ASTRO-H) | 7 February 2016 | H-2A -202 |

| — | NICER | 3 May 2017 | Falcon 9 FT |

| — | GOLD | 25 January 2018 | Ariane 5 ECA |

| 95 | TESS | 18 April 2018 | Falcon 9 FT |

| 96 | ICON | 11 October 2019 | Pegasus XL |

| 97 | IXPE | 9 December 2021 | Falcon 9 Block 5 |

| — | XRISM | 6 September 2023 | H-IIA 202 |

| No. | Mass (kg) | Orbit regime | End of data | Re-entry |

|---|---|---|---|---|

| 1 | 14 | MEO | 23 May 1958 | 31 March 1970 |

| 2 | 15 | Failed | — | — |

| 3 | 14 | MEO | 27 June 1958 | 27 June 1958 |

| 4 | 26 | MEO | 5 October 1958 | 23 October 1959 |

| 5 | 17 | failed | — | — |

| — | 42 | failed | — | — |

| 6 | 64 | HEO | 6 October 1959 | 12 July 1961 |

| 7 | 42 | LEO | 24 August 1961 | In orbit |

| – | 16 | failed | — | — |

| 8 | 41 | LEO | 27 December 1960 | 27 March 2012 |

| – | 6 | failed | — | — |

| 9 | 36 | LEO | 9 April 1964 | 9 April 1964 |

| – | 34 | failed | — | — |

| 10 | 79 | HEO | 25 March 1961 | 1 June 1968 |

| 11 | 37 | LEO | 17 November 1961 | In orbit |

| – | 34 | failed | — | — |

| – | 85 | failed | — | — |

| 12 | 38 | HEO | 6 December 1961 | 1 September 1963 |

| 13 | 86 | LEO | 28 August 1961 | 28 August 1961 |

| 14 | 40 | HEO | 11 August 1963 | 1 July 1966 |

| 15 | 44 | HEO | 30 January 1963 | 15 January 1978 |

| 16 | 101 | LEO | 22 July 1963 | In orbit |

| 17 | 184 | LEO | 10 July 1963 | 24 November 1966 |

| 18 | 138 | HEO | 10 May 1965 | 30 December 1965 |

| 19 | 8 | LEO | 10 May 1981 | 10 May 1981 |

| – | 114 | failed | — | — |

| 20 | 45 | LEO | 29 December 1965 | In orbit |

| 21 | 135 | HEO | 13 October 1965 | 30 January 1966 |

| 22 | 53 | LEO | February 1970 | In orbit |

| 23 | 134 | LEO | 7 November 1965 | 29 June 1983 |

| 24 | 9 | MEO | 18 October 1968 | 18 October 1968 |

| 25 | 40 | LEO | December 1966 | In orbit |

| 26 | 46 | MEO | 27 December 1967 | 23 August 2021 |

| 27 | 61 | LEO | 20 July 1973 | In orbit |

| 28 | 128 | HEO | 12 May 1967 | 4 July 1968 |

| 29 | 387 | LEO | 23 June 1978 | In orbit |

| 30 | 57 | LEO | 5 November 1967 | In orbit |

| 31 | 99 | LEO | 1 October 1969 | In orbit |

| 32 | 225 | LEO | March 1967 | 22 February 1985 |

| 33 | 212 | HEO | 21 September 1971 | In orbit |

| 34 | 163 | MEO | 3 May 1969 | 3 May 1969 |

| 35 | 230 | Lunar | 24 June 1973 | Lunar orbit |

| 36 | 469 | LEO | 1 July 1982 | In orbit |

| 37 | 198 | LEO | 30 April 1974 | 16 November 1990 |

| 38 | 602 | MEO | (~1969) | In orbit |

| 39 | 9 | LEO | 23 June 1971 | 22 June 1981 |

| 40 | 71 | LEO | June 1971 | In orbit |

| 41 | 145 | HEO | 23 December 1972 | 23 December 1972 |

| 42 | 142 | LEO | 4 January 1975 | 5 April 1979 |

| 43 | 635 | MEO | 2 October 1974 | 2 October 1974 |

| 44 | 260 | LEO | 30 June 1973 | 15 December 1979 |

| 45 | 52 | MEO | 30 September 1974 | 10 January 1992 |

| 46 | 90 | LEO | 4 November 1974 | 2 November 1979 |

| 47 | 635 | HEO | 31 October 1978 | In orbit |

| 48 | 166 | LEO | 8 June 1973 | 20 August 1980 |

| 49 | 328 | Lunar | 26 April 1977 | Presumed crashed into Moon sometime aft… |

| 50 | 371 | HEO | 7 October 2006 | In orbit |

| 51 | 658 | LEO | (12 December 1978) | 12 December 1978 |

| 52 | 23 | HEO | 28 April 1978 | 28 April 1978 |

| 53 | 197 | LEO | 7 April 1979 | 9 April 1979 |

| 54 | 681 | LEO | 29 January 1976 | 12 March 1976 |

| 55 | 735 | LEO | 25 September 1980 | 10 June 1981 |

| — | 40 | failed | — | — |

| — | 43 | failed | — | — |

| 56 | 340 | HEO | 26 September 1987 | 26 September 1987 |

| 57 | 669 | MEO | 30 September 1996 | In orbit |

| 58 | 117 | LEO | 30 September 1980 | December 22, 1981 |

| 59 | 390 | Sun–Earth L 1 | 16 September 2014 | Heliocentric orbit |

| 60 | 149 | LEO | 7 January 1982 | 11 April 1989 |

| 61 | 158 | LEO | 6 May 1980 | 11 June 1980 |

| 62 | 424 | MEO | 28 February 1991 | In orbit |

| 63 | 420 | LEO | 19 February 1983 | 19 February 1983 |

| 64 | 145 | LEO | 4 April 1989 | 5 March 1991 |

| 65 | 242 | MEO | 12 July 1989 | In orbit |

| 66 | 2,206 | LEO | 23 December 1993 | In orbit |

| 67 | 3,275 | LEO | 31 January 2001 | 30 January 2002 |

| 68 | 158 | LEO | 30 June 2004 | 13 November 2012. |

| 69 | 3,200 | LEO | 3 January 2012 | 30 April 2018 |

| 70 | 187 | LEO | 4 May 2009 | In orbit |

| — | 128 | LEO | — | 7 April 2002 |

| 71 | 596 | Sun–Earth L 1 | Operational | In L 1 orbit |

| 72 | 120 | LEO | 13 December 2003 | 13 December 2003 |

| 73 | 250 | LEO | 21 June 2010 | In orbit |

| 74 | 288 | LEO | 1 September 2005 | In orbit |

| 75 | 250 | SSO | 30 September 2000 | 10 May 2011 |

| 76 | 120 | Polar LEO | 18 May 1999 | In orbit |

| 77 | 1,400 | LEO | 18 October 2007 | In orbit |

| 78 | 536 | Polar MEO | 18 December 2005 | In orbit |

| 79 | 124 | LEO | 28 March 2007 | In orbit |

| 80 | 840 | Sun–Earth L 2 | October 2010 | Heliocentric orbit |

| 81 | 230 | LEO | 16 August 2018 | Deorbited |

| — | 4,000 | HEO | Operational | In orbit |

| 82 | 60 | LEO | 11 April 2008 | In orbit |

| 83 | 280 | LEO | 28 June 2013 | In orbit |

| 84 | 1,470 | LEO | Operational | In orbit |

| — | 1,706 | LEO | 2 September 2015 | In orbit |

| — | classified | Molniya | Operational | In orbit |

| 85 | 77 | HEO | Operational | In orbit |

| 86 | 77 | Lunar | Operational | Lunar orbit |

| 87 | 77 | Lunar | Operational | Lunar orbit |

| 88 | 77 | HEO | Operational | In orbit |

| 89 | 77 | HEO | Operational | In orbit |

| 90 | 197 | SSO | March 2023 | 19 August 2024 |

| — | classified | Molniya | Operational | In orbit |

| — | 395 | LEO | 28 November 2015 | 28 November 2015 |

| 91 | 107 | MEO | Operational | In orbit |

| 92 | 661 | LEO | August 2024 | 2 November 2024 |

| 93 | 350 | LEO | Operational | In orbit |

| 94 | 183 | SSO | Operational | In orbit |

| — | 2,700 | LEO | 26 March 2016 | In orbit |

| — | 372 | ISS | Operational | ISS |

| — | 37 | GEO | Operational | In orbit |

| 95 | 362 | HEO | Operational | In orbit |

| 96 | 287 | LEO | Operational | In orbit |

| 97 | 330 | LEO | Operational | In orbit |

| — | 2,300 | LEO | Operational | In orbit |

| No. | Mission/Notes |

|---|---|

| 1 | First American satellite, third satelli… |

| 2 | Failed to achieve orbit. |

| 3 | Energetic particle studies helped confi… |

| 4 | Monitor charged particles inside Van Al… |

| 5 | Planned in conjunction with Explorer 4,… |

| — | Planned to measure Earth's radiation ba… |

| 6 | Magnetosphere research and digital tele… |

| 7 | Micrometeoroids and energetic particle… |

| – | Analyze electron and proton radiation e… |

| 8 | Measured atmospheric composition of the… |

| – | Atmosphere density measurement, but fai… |

| 9 | Atmospheric density measurements, first… |

| – | Ionosphere research, but failed to achi… |

| 10 | Investigated the magnetic field between… |

| 11 | Gamma ray astronomy |

| – | Ionosphere research, failed to achieve… |

| – | Micrometeoroid research, failed to achi… |

| 12 | Energetic particle research |

| 13 | Micrometeoroid research; partial failure |

| 14 | Energetic particle research |

| 15 | Energetic particle research |

| 16 | Micrometeoroid research |

| 17 | Atmospheric research |

| 18 | Magnetospheric research |

| 19 | Atmospheric density measurements |

| – | Launch failure |

| 20 | Ionosphere research |

| 21 | Magnetospheric research |

| 22 | Ionospheric and geodetic research |

| 23 | Micrometeoric research |

| 24 | Atmospheric density measurements |

| 25 | Ionospheric research |

| 26 | High energy particle observations |

| 27 | Magnetospheric research |

| 28 | Magnetospheric research |

| 29 | Geodetic Earth monitoring |

| 30 | Solar radiation monitoring (Cover for c… |

| 31 | Ionospheric research |

| 32 | Atmospheric research |

| 33 | Magnetospheric research |

| 34 | Magnetospheric research |

| 35 | Magnetospheric research |

| 36 | Geodetic Earth monitoring |

| 37 | Solar radiation monitoring (Cover for c… |

| 38 | Radio astronomy |

| 39 | Atmospheric density measurements |

| 40 | Magnetospheric Research |

| 41 | Magnetospheric research |

| 42 | X-ray astronomy |

| 43 | Magnetospheric research |

| 44 | Solar radiation monitoring (Cover for c… |

| 45 | Magnetospheric research |

| 46 | Micrometeoroids research |

| 47 | Magnetospheric research |

| 48 | X-ray astronomy |

| 49 | Radio astronomy |

| 50 | Magnetospheric research |

| 51 | Atmospheric research |

| 52 | Magnetospheric research |

| 53 | X-ray astronomy |

| 54 | Atmospheric research |

| 55 | Atmospheric research |

| — | Atmospheric research; failed during lau… |

| — | Atmospheric research; failed during lau… |

| 56 | Magnetospheric research; launched with… |

| 57 | Ultraviolet astronomy |

| 58 | Thermal mapping of the Earth |

| 59 | Magnetospheric research; heliocentric m… |

| 60 | Stratospheric aerosol and ozone data |

| 61 | Mapped the near surface magnetic field… |

| 62 | Magnetospheric research |

| 63 | Magnetospheric research |

| 64 | Atmospheric research |

| 65 | Magnetosphere research |

| 66 | Microwave astronomy |

| 67 | Ultraviolet astronomy |

| 68 | SMEX: magnetospheric research |

| 69 | MIDEX: X-ray astronomy |

| 70 | SMEX: auroral phenomena |

| — | Separation failure, mission relaunched… |

| 71 | MIDEX: solar/interplanetary/interstella… |

| 72 | STEDI, UNEX: atmospheric research |

| 73 | SMEX: solar observatory |

| 74 | SMEX: submillimeter astronomy |

| 75 | SMEX, Infrared astronomy, primary missi… |

| 76 | STEDI: atmospheric research, satellite… |

| 77 | MIDEX: ultraviolet astronomy |

| 78 | MIDEX: magnetospheric research |

| 79 | MO: UV, X-ray, and gamma ray astronomy |

| 80 | MIDEX: microwave astronomy |

| 81 | SMEX: X-ray and gamma ray solar flare i… |

| — | International: space telescope for obse… |

| 82 | UNEX: ultraviolet spectroscopy and astr… |

| 83 | SMEX: ultraviolet astronomy |

| 84 | MIDEX: gamma ray astronomy |

| — | MO: instrument on JAXA's Suzaku mission |

| — | MO: payload on Trumpet-F/O-1 1 ( USA-18… |

| 85 | MIDEX: magnetospheric research |

| 86 | MIDEX; Magnetospheric research |

| 87 | MIDEX: magnetospheric research |

| 88 | MIDEX: magnetospheric research |

| 89 | MIDEX: magnetospheric research |

| 90 | SMEX: noctilucent cloud observation |

| — | MO: payload on Trumpet-F/O-1 2 ( USA-20… |

| — | MO: instruments on C/NOFS |

| 91 | SMEX: mapping the boundary between the… |

| 92 | MIDEX: infrared astronomy, NEOWISE exte… |

| 93 | SMEX: high-energy X-ray astronomy |

| 94 | SMEX: solar UV astronomy |

| — | MO: X-ray instrument on JAXA's Hitomi,… |

| — | MO: instrument on ISS for neutron star… |

| — | MO: instrument on SES-14 comsat for stu… |

| 95 | MIDEX: survey for transiting exoplanets |

| 96 | MIDEX: ionospheric studies |

| 97 | SMEX: X-ray studies |

| — | MO: instruments on JAXA 's XRISM x-ray… |

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.cite-ref-181[181] 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:cite-ref-skyrocket-57-1[57]

• Owl 1 and 2 (cost, 1965)cite-ref-182[182]

• MSS A (Magnetic Storm Satellite, Explorer-A, 1970)cite-ref-183[183]cite-ref-184[184]

• CATSAT (STEDI 3) (cost)cite-ref-nap-37-1[37]

• IMEX (UNEX 2) (cost)cite-ref-nap-37-2[37]

• FAME (MIDEX 4)

• SPIDR (SMEX 8) (technical, 2003)cite-ref-nap-37-3[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 - 2010

• WISE - 2011 (extended in 2013 as NEOWISE mission)

• RXTE - 2012

• Galaxy Evolution Explorer - 2013

Number of launches per decade:cite-ref-185[185]

• Cosmic Vision, a European Space Agency (ESA) programme

• Cosmic Vision S-class missions, the European Space Agency equivalent to the Small Explorers Program

• Discovery program – Solar system exploration program by NASAPages displaying short descriptions of redirect targets

• New Frontiers program – Series of space exploration missions being conducted by NASA

cite-note-11. ↑ "Explorers Program". Explorers Program Home Page at NASA Goddard. NASA. Retrieved 3 May 2022. This article incorporates text from this source, which is in the public domain.

cite-note-boehm-nasa-22. ↑ This article incorporates text from this source, which is in the public domain: J. Boehm, H.J. Fichtner and Otto A. Hoberg, EXPLORER SATELLITES LAUNCHED BY JUNO 1 AND JUNO 2 VEHICLES NASA Report

cite-note-33. ↑ citerefportree2013Portree, David S. F. (22 May 2013). "Cometary Explorer (1973)". Wired. Retrieved 24 June 2019.

cite-note-44. ↑ "Goddard Missions". Goddard Space Flight Center. NASA. 5 June 2018. Retrieved 24 June 2019. This article incorporates text from this source, which is in the public domain.

cite-note-butrica-55. ↑ citerefbutrica2015Butrica, Andrew J. (2015). "Chapter 3: NASA's Role in the Manufacture of Integrated Circuits" (PDF). In Dick, Steven J. (ed.). Historical Studies in the Societal Impact of Spaceflight. NASA. pp. 149-250 (237-242). ISBN 978-1-62683-027-1. This article incorporates text from this source, which is in the public domain.

cite-note-nasa-66. ↑ citerefbutler1989Butler, P. M. (29 August 1989). Interplanetary Monitoring Platform (PDF). NASA. pp. 1, 11, 134. Retrieved 12 August 2019. This article incorporates text from this source, which is in the public domain.

cite-note-jonathan-71. citerefmcdowellMcDowell, Jonathan. "Launch Log". Jonathan's Space Page. Retrieved 24 June 2018.

cite-note-nssdc-list-88. ↑ "NASA's Explorers Program Satellites". National Space Science Data Center. Retrieved 20 April 2018. This article incorporates text from this source, which is in the public domain.

cite-note-nasahistory-99. ↑ citerefrumerman2009Rumerman, Judy A. (2009). NASA Historical Data Book, Vol. VII: NASA Launch Systems, Space Transportation, Human Spaceflight, and Space Science, 1989-1998 (PDF). NASA. Retrieved 24 June 2019. This article incorporates text from this source, which is in the public domain.

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cite-note-ieee-1111. ↑ citerefmasonbakerblakeboughner1998Mason, G. M.; Baker, D. N.; Blake, J. B.; Boughner, R. E.; Callis, L. B.; et al. (1998). SAMPEX: NASA's First Small Explorer Satellite. IEEE Aerospace Conference 21–28 March 1998 Aspen, Colorado. Vol. 5. pp. 389–412. doi:10.1109/AERO.1998.685848.

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cite-note-1818. ↑ citerefjeff-foust2018Jeff Foust (5 January 2018). "Earth science decadal report recommends mix of large and small missions". SpaceNews. Retrieved 28 April 2018.

cite-note-1919. ↑ citerefwumanuelsalas2016Wu, Chauncey; Manuel, Greg; Salas, Andrea (15 August 2016). "2016 Heliophysics Small Explorers (SMEX) & Mission of Opportunity (MO) Solicitations Pre-Proposal Conference" (PDF). NASA. Retrieved 24 June 2019. This article incorporates text from this source, which is in the public domain.

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cite-note-212. "Explorers Program". explorers.gsfc.nasa.gov. Archived from the original on 23 March 2016. Retrieved 8 April 2016. This article incorporates text from this source, which is in the public domain.

cite-note-0-223. "Explorers Program". space.skyrocket.de. Retrieved 8 April 2016.

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cite-note-245. "NASA's NEOWISE Infrared Heritage Will Live On - NASA". 1 July 2024.

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cite-note-296. "Operational Small Explorers (SMEX) Missions". Explorers Program. Archived from the original on 25 March 2016. Retrieved 8 April 2016. This article incorporates text from this source, which is in the public domain.

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cite-note-1-3613. "Explorers Program". explorers.gsfc.nasa.gov. Archived from the original on 22 March 2016. Retrieved 8 April 2016. This article incorporates text from this source, which is in the public domain.

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