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 Explorers Program have included Juno I, Juno II, various Thor, Scout, Delta and Pegasus launch vehicles, and Falcon 9.
The program has four classes: Probe Explorers, 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
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. The Juno I 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 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.
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). 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. +List of Interplanetary Monitoring Platform (IMP) missionsMissionPhotoSatelliteLaunch dateDecay dateNotesExplorerIMPIMP-1Explorer 18IMP-A27 November 1963, 02:30 UTC30 December 1965First use of integrated circuits in a spacecraft. First satellite in IMP-A/-B/-C design series.IMP-2Explorer 21IMP-B4 October 1964, 03:45 UTC1 January 1966Second satellite in IMP-A/-B/-C design series.IMP-3Explorer 28IMP-C29 May 1965, 12:00 UTC4 July 1968Third satellite in IMP-A/-B/-C design series.AIMP-1Explorer 33IMP-D1 July 1966, 16:02 UTCIn orbitFirst use of MOSFET integrated circuits in a spacecraft, similar design to IMP-E. Originally intended to orbit the Moon, but placed in an elliptical high orbit instead.IMP-4Explorer 34IMP-F24 May 1967, 14:05 UTC3 May 1969Similar design to IMP-G.AIMP-2Explorer 35IMP-E19 July 1967, 14:19 UTCAfter 24 June 1973Similar design to IMP-D. Positioned in Selenocentric orbit.IMP-5Explorer 41IMP-G21 June 1969, 08:47 UTC23 December 1972Similar design to IMP-F.IMP-6Explorer 43IMP-I13 March 1971, 16:15 UTC2 October 1974First spacecraft in IMP-I/-H/-J series.IMP-7Explorer 47IMP-H23 September 1972, 01:20 UTCIn orbitSecond spacecraft in IMP-I/-H/-J series.IMP-8Explorer 50IMP-J26 October 1973, 02:26 UTCIn orbitThird spacecraft in IMP-I/-H/-J series, remained in service until 2006 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.
SMEX, MIDEX, and Student Explorers Programs 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. 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 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. 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 (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. Later, NASA established the University-Class Explorer (UNEX) program for much cheaper missions, which is regarded as a successor to STEDI.
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. 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.
Excluding the launches, the MIDEX class has a current mission cap cost of US$250 million in 2018, with future MIDEX missions being capped at US$350 million. The cost cap for SMEX missions in 2017 was US$165 million. UNEX missions are capped at US$15 million. 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.
Following the 2020 Astronomy and Astrophysics Decadal Survey, NASA created the Probe Explorers program as a new class with the intention of filling the gap between flagship missions and the other classes of the Explorers Program. This class has a mission cap cost of US$1 billion. The first mission selected in this class was the far-infrared telescope PRIMA, targeted for a 2033 launch.
Classes
Probe Explorers +List of Probe Explorers missions Name PE number Explorernumber Launch (UTC) Status PRIMA 2033 Medium-Class Explorers (MIDEX) +List of MIDEX missions Name MIDEXnumber Explorernumber Launch (UTC) Status RXTE Explorer-69 30 December 1995 Ended in 2012 / Reentered on 30 April 2018 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 WISE / NEOWISE MIDEX-6 Explorer-92 14 December 2009 Mission operations completed on 31 July 2024. Reentered on 2 November 2024. TESS MIDEX-7 Explorer-95 18 April 2018 ICON MIDEX-8 Explorer-96 11 October 2019 Contact lost in November 2022 SPHEREx MIDEX-9 Explorer-102 12 March 2025 MUSE MIDEX-10 2027 HelioSwarm MIDEX-11 2029 UVEX MIDEX-12 2030
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 (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 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.
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). In June 2019 NASA selected TRACERS and PUNCH for flight.
+List of SMEX missions Name SMEXnumber Explorernumber Launch (UTC) End ofmission 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 Reentered on 18 July 2025 WIRE SMEX-5 Explorer-75 5 March 1999 Spacecraft equipment failure; reentered on 10 May 2011 RHESSI SMEX-6 Explorer-81 5 February 2002 April 2018 Deorbited on 20 April 2023 GALEX SMEX-7 Explorer-83 28 April 2003 May 2012 Decommissioned on 28 June 2013 SPIDR SMEX-8 Scheduled for 2005 Cancelled in 2003 due to poor instrument sensitivity Aeronomy of Ice in the Mesosphere (AIM) SMEX-9 Explorer-90 25 April 2007 19 August 2024 Reentered on 19 August 2024 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 Imaging X-ray Polarimetry Explorer (IXPE) SMEX-14 9 December 2021 PUNCH-NFI SMEX-15A Explorer-98 12 March 2025 PUNCH-WFI 1 SMEX-15B Explorer-99 12 March 2025 PUNCH-WFI 3 SMEX-15D Explorer-100 12 March 2025 PUNCH-WFI 2 SMEX-15C Explorer-101 12 March 2025 TRACERS A SMEX-16A Explorer-106 23 July 2025 TRACERS B SMEX-16B Explorer-107 23 July 2025 COSI SMEX-17 August 2027
University-Class Explorers (UNEX)