Explorers Program
The Explorers Program is a United States space exploration program that provides flight opportunities for physics, geophysics, heliophysics, and astrophysics investigations from space. Launched in 1958, Explorer 1 was USA's first spacecraft 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 Jupiter C, Juno II, various Thor, Scout, Delta and Pegasus rockets, and Falcon 9.
Currently, the program has three classes: Medium-Class Explorers, Small Explorers, and University-Class Explorers, with select Missions of Opportunity operated with other agencies.
History
Early Explorer Satellites
The Explorers Program began as a U.S. Army proposal to place a "civilian" artificial satellite into orbit during the International Geophysical Year. 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 October 4, 1957 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 January 31, 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 rocket in 1958, of which Explorer 3 and Explorer 4 were successful, while Explorer 2 and Explorer 5 failed to reach orbit. The Juno-1 vehicle was replaced by the Juno II in 1959.
Continuation of the Explorers program
With the establishment of NASA in 1958, the Explorers Program was transferred to NASA from the US 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.
The Interplanetary Monitoring Platform 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 chips and MOSFETs. The IMP A in 1963 was the first spacecraft to use IC chips, and the IMP D in 1966 was the first to use MOSFETs.
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 drops in NASA's budget, Explorer missions became infrequent in the early 1980s.
SMEX, MIDEX, and Student Explorer programs
In 1988, the Small Explorer 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. The first three SMEX missions were chosen in April 1989 out of 51 candidates, and launched in 1992, 1996 and 1998. The second set of two missions were announced in September 1994 and launched in 1998 and 1999.In the mid 1990s, NASA initiated the Medium-class Explorer 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. This new launch vehicle was not developed and instead, these missions were flown on a modified Delta II rocket. The first announcement opportunity for MIDEX was issued in March 1995, and the first launch under this new class was FUSE in 1999.
In May 1994, NASA started the Student Explorer Demonstration Initiative 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. Later, NASA established the University-Class Explorers program for much cheaper missions, which is regarded as a successor to STEDI.
The Explorer missions were at first managed by the Small Explorer Project Office at NASA's 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 SMEX class so that each mission was managed by its Principal Investigator, with oversight by the GSFC Explorers Project. The Explorers Program Office at Goddard Space Flight Center in Greenbelt, Maryland, 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.
Excluding the launches, the MIDEX class has a current mission cap cost of in 2018, with future MIDEX missions being capped at $350 million. The cost cap for SMEX missions in 2017 was $165 million. UNEX missions are capped at $15 million. A sub-project called Missions of Opportunity has funded science instruments or hardware components of onboard non-NASA space missions, and have a total NASA cost cap of $70 million.
Classes
Medium-Class Explorers (MIDEX)
| Name | MIDEX number | Explorer number | Launch | Status |
| RXTE | Explorer-69 | December 30, 1995 | Ended in 2012 | |
| ACE | Explorer-71 | August 25, 1997 | ||
| FUSE | MIDEX-0 | Explorer-77 | June 23, 1999 | Ended in 2007 |
| IMAGE | MIDEX-1 | Explorer-78 | March 25, 2000 | Lost contact in 2005. Partial contact reestablished in January 2018 |
| WMAP | MIDEX-2 | Explorer-80 | June 30, 2001 | Ended in 2010 |
| SWIFT | MIDEX-3 | Explorer-84 | November 20, 2004 | |
| FAME | MIDEX-4 | Scheduled for 2004 | Cancelled in 2002 | |
| THEMIS A | MIDEX-5A | Explorer-85 | February 17, 2007 | |
| THEMIS B | MIDEX-5B | Explorer-86 | February 17, 2007 | |
| THEMIS C | MIDEX-5C | Explorer-87 | February 17, 2007 | |
| THEMIS D | MIDEX-5D | Explorer-88 | February 17, 2007 | |
| THEMIS E | MIDEX-5E | Explorer-89 | February 17, 2007 | |
| WISE / NEOWISE | MIDEX-6 | Explorer-92 | December 14, 2009 | |
| TESS | MIDEX-7 | Explorer-95 | April 18, 2018 | |
| ICON | MIDEX-8 | Explorer-96 | 11 October 2019 | |
| SPHEREx | MIDEX-9 | 2024 |
Small Explorers (SMEX)
The Small Explorers class was implemented in 1989 specifically to fund space exploration missions that cost no more than. The missions are managed by the Explorers Project at the Goddard Space Flight Center.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, Focusing Optics X-ray Solar Imager, Multi-Slit Solar Explorer, Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites, and Polarimeter to Unify the Corona and Heliosphere. In June 2019 NASA selected TRACERS and PUNCH for flight.
| Name | SMEX number | Explorer number | Launch | 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 | Decommissioned on 16 August 2018 |
| 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 | ||
| 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 | Cancelled in 2012 due to expected cost overruns | ||
| IXPE | SMEX-14 | 2021 | |||
| PUNCH | 2023 | ||||
| TRACERS | 2023 |
University-Class Explorers (UNEX)
| Name | UNEX number | Explorer number | Launch | Status |
| SNOE | UNEX-1 | Explorer-72 | 26 February 1998 | Ended in 2000 |
| IMEX | UNEX-2 | Scheduled for 2003 | Cancelled before 2005 | |
| CHIPS | UNEX-3 | Explorer-82 | 12 January 2003 | Ended in 2008 |
Missions of Opportunity (MO)
Missions of Opportunity 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.| Name | Launcher | Launch | Status |
| HETE-2 | NASA | 9 October 2000 | Ended in 2008 |
| INTEGRAL | ESA | October 17, 2002 | |
| Suzaku | JAXA | 10 July 2005 | Ended in 2015 |
| TWINS | NRO | TWINS-1: 28 June 2006 TWINS-2: 13 March 2008 | |
| CINDI | DoD | 16 April 2008 | Ended in 2015 |
| Hitomi | JAXA | 17 February 2016 | Failed |
| NICER | ISS | 3 June 2017 | |
| GOLD | SES | 25 January 2018 | |
| GUSTO | NASA, high-altitude balloon | December 2021 | |
| XRISM | JAXA | January 2022 | |
| AWE | ISS | August 2022 | |
| SunRISE | NASA | July 2023 | |
| CASE | ESA | 2028 |
Launched spacecraft
Explorers 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.| # | Name | Launch date | Rocket | Mass | Orbit regime | End of data | Re-entry | Mission/Notes |
| 1 | Explorer 1 | January 31, 1958 | Juno I | 14 | MEO | May 23, 1958 | March 31, 1970 | First American satellite, third satellite to achieve orbit; discovered the Van Allen radiation belt; launched by the US Army |
| 2 | Explorer 2 | March 5, 1958 | Juno I | 15 | failed | — | — | Failed to achieve orbit |
| 3 | Explorer 3 | March 26, 1958 | Juno I | 14 | MEO | June 27, 1958 | June 27, 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 |
| 5 | Explorer 5 | August 24, 1958 | Juno I | 17 | failed | — | — | Planned in conjunction with Explorer 4, but launch failed |
| — | Explorer 7x | July 16, 1959 | Juno I | 42 | failed | — | — | Planned to measure Earth's radiation balance, but destroyed within seconds by range safety |
| 6 | Explorer 6 | 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 | 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 | March 23, 1960 | Juno II | 16 | failed | — | — | Analyze electron and proton radiation energies, failed to achieve orbit |
| 8 | Explorer 8 | November 3, 1960 | Juno II | 41 | LEO | December 27, 1960 | March 27, 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 | 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 | 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 | 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 | June 30, 1961 | Scout X-1 | 85 | failed | — | — | Micrometeoroid research, failed to achieve orbit |
| 12 | EPE-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 | October 2, 1962 | Delta A | 40 | HEO | August 11, 1963 | July 1, 1966 | Energetic particle research |
| 15 | EPE-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 | April 3, 1963 | Delta B | 184 | LEO | July 10, 1963 | November 24, 1966 | Atmospheric research |
| 18 | IMP-A | November 27, 1963 | Delta C | 138 | HEO | May 10, 1965 | December 30, 1965 | Magnetospheric research |
| 19 | AD-A | December 19, 1963 | Scout X-4 | 8 | LEO | May 10, 1981 | May 10, 1981 | Atmospheric density measurements |
| – | BE-A | March 19, 1964 | Delta B | 114 | failed | — | — | Launch failure |
| 20 | IE-A | August 25, 1964 | Scout X-4 | 45 | LEO | December 29, 1965 | In orbit | Ionosphere research |
| 21 | IMP-B | October 4, 1964 | Delta C | 135 | HEO | October 13, 1965 | January 30, 1966 | Magnetospheric research |
| 22 | BE-B | 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 | November 21, 1964 | Scout X-4 | 9 | MEO | October 18, 1968 | October 18, 1968 | Atmospheric density measurements |
| 25 | Injun 4 | November 21, 1964 | Scout X-4 | 40 | LEO | December 1966 | In orbit | Ionospheric research |
| 26 | EPE-D | December 21, 1964 | Delta C | 46 | MEO | December 27, 1967 | In orbit | High energy particle observations |
| 27 | BE-C | April 29, 1965 | Scout X-4 | 61 | LEO | July 20, 1973 | In orbit | Magnetospheric research |
| 28 | IMP-C | May 29, 1965 | Delta C | 128 | HEO | May 12, 1967 | July 4, 1968 | Magnetospheric research |
| 29 | GEOS 1 | November 6, 1965 | Delta E | 387 | LEO | June 23, 1978 | In orbit | Geodetic Earth monitoring |
| 30 | SOLRAD 8 | November 19, 1965 | Scout X-4 | 57 | LEO | November 5, 1967 | In orbit | Solar radiation monitoring |
| 31 | DME-A | November 29, 1965 | Thor-Agena B | 99 | LEO | October 1, 1969 | In orbit | Ionospheric research |
| 32 | AE-B | May 25, 1966 | Delta C1 | 225 | LEO | March 1967 | February 22, 1985 | Atmospheric research |
| 33 | IMP-D | July 1, 1966 | Delta E1 | 212 | HEO | September 21, 1971 | In orbit | Magnetospheric research |
| 34 | IMP-F | May 24, 1967 | Delta E1 | 163 | MEO | May 3, 1969 | May 3, 1969 | Magnetospheric research |
| 35 | IMP-E | July 19, 1967 | Delta E1 | 230 | Lunar | June 24, 1973 | Lunar orbit | Magnetospheric research |
| 36 | GEOS 2 | January 11, 1968 | Delta E1 | 469 | LEO | July 1, 1982 | In orbit | Geodetic Earth monitoring |
| 37 | SOLRAD 9 | March 5, 1968 | Scout B | 198 | LEO | April 30, 1974 | November 16, 1990 | Solar radiation monitoring |
| 38 | RAE-A | July 4, 1968 | Delta J | 602 | MEO | In orbit | Radio astronomy | |
| 39 | AD-C | August 8, 1968 | Scout B | 9 | LEO | June 23, 1971 | June 22, 1981 | Atmospheric density measurements |
| 40 | Injun 5 | August 8, 1968 | Scout B | 71 | LEO | June 1971 | In orbit | Magnetospheric Research |
| 41 | IMP-G | June 21, 1969 | Delta E1 | 145 | HEO | December 23, 1972 | December 23, 1972 | Magnetospheric research |
| 42 | Uhuru | December 12, 1970 | Scout B | 142 | LEO | January 4, 1975 | April 5, 1979 | X-ray astronomy |
| 43 | IMP-H | March 13, 1971 | Delta M6 | 635 | MEO | October 2, 1974 | October 2, 1974 | Magnetospheric research |
| 44 | SOLRAD 10 | July 8, 1971 | Scout B | 260 | LEO | June 30, 1973 | December 15, 1979 | Solar radiation monitoring |
| 45 | SSS-A | November 15, 1971 | Scout B | 52 | MEO | September 30, 1974 | January 10, 1992 | Magnetospheric research |
| 46 | MTS | August 13, 1972 | Scout D-1 | 90 | LEO | November 4, 1974 | November 2, 1979 | Micrometeoroids research |
| 47 | IMP-I | September 23, 1972 | Delta 1604 | 635 | HEO | October 31, 1978 | In orbit | Magnetospheric research |
| 48 | SAS-B | November 15, 1972 | Scout D-1 | 166 | LEO | June 8, 1973 | August 20, 1980 | X-ray astronomy |
| 49 | RAE-B | June 10, 1973 | Delta 1913 | 328 | Lunar | April 26, 1977 | Lunar orbit | Radio astronomy |
| 50 | IMP-J | October 26, 1973 | Delta 1604 | 371 | HEO | October 7, 2006 | In orbit | Magnetospheric research |
| 51 | AE-C | December 16, 1973 | Delta 1900 | 658 | LEO | December 12, 1978 | Atmospheric research | |
| 52 | Hawkeye 1 | June 3, 1974 | Scout E-1 | 23 | HEO | April 28, 1978 | April 28, 1978 | Magnetospheric research |
| 53 | SAS-C | May 7, 1975 | Scout F-1 | 197 | LEO | April 7, 1979 | April 9, 1979 | X-ray astronomy |
| 54 | AE-D | October 6, 1975 | Delta 2910 | 681 | LEO | January 29, 1976 | March 12, 1976 | Atmospheric research |
| 55 | AE-E | November 20, 1975 | Delta 2910 | 735 | LEO | September 25, 1980 | June 10, 1981 | Atmospheric research |
| — | DADE-A | December 5, 1975 | Scout F-1 | 40 | failed | — | — | Atmospheric research; failed during launch |
| — | DADE-B | December 5, 1975 | Scout F-1 | 43 | failed | — | — | Atmospheric research; failed during launch |
| 56 | ISEE-1 | 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 | April 26, 1978 | Scout F | 117 | LEO | September 30, 1980 | December 22, 1981 | Thermal mapping of the Earth |
| 59 | ICE | 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. First spacecraft to be placed at a libration point, and first one to perform a flyby of a comet. |
| 60 | SAGE | February 18, 1979 | Scout D-1 | 149 | LEO | January 7, 1982 | April 11, 1989 | Stratospheric aerosol and ozone data |
| 61 | MAGSAT | October 30, 1979 | Scout G-1 | 158 | LEO | May 6, 1980 | June 11, 1980 | Mapped the near surface magnetic field of the Earth |
| 62 | DE 1 | August 3, 1981 | Delta 3913 | 424 | MEO | February 28, 1991 | In orbit | Magnetospheric research |
| 63 | DE 2 | August 3, 1981 | Delta 3913 | 420 | LEO | February 19, 1983 | February 19, 1983 | Magnetospheric research |
| 64 | SME | October 6, 1981 | Delta 2310 | 145 | LEO | April 4, 1989 | March 5, 1991 | Atmospheric research |
| 65 | AMTPE | August 16, 1984 | Delta 3924 | 242 | MEO | July 12, 1989 | In orbit | Magnetospheric research |
| 66 | COBE | November 18, 1989 | Delta 5920 | 2,206 | LEO | December 23, 1993 | In orbit | Microwave astronomy |
| 67 | EUVE | 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 | August 21, 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 | 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+ | classified | Molniya | Operational | In orbit | MO: payload on Trumpet-F/O-1 1 |
| 85 | THEMIS A | February 17, 2007 | Delta II 7925 | 77 | HEO | Operational | In orbit | MIDEX: magnetospheric research |
| 86 | THEMIS B | February 17, 2007 | Delta II 7925 | 77 | Lunar | Operational | Lunar orbit | MIDEX; Magnetospheric research |
| 87 | THEMIS C | 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: moctilucent cloud observation |
| — | TWINS B | March 13, 2008 | Atlas V 411 | classified | Molniya | Operational | In orbit | MO: payload on Trumpet-F/O-1 2 |
| — | 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 | 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 |
Cancelled missions
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. 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:
- Owl 1 and 2
- MSS A
- CATSAT
- IMEX
- FAME
- SPIDR
- GEMS
- FAST - 2009
- TRACE - 2010
- Wilkinson MAP - 2010
- WISE - 2011
- Rossi XTE - 2012
- Galaxy Evolution Explorer - 2013
Launch statistics