Explorers Program

The Explorers program^[1] is a NASA exploration program that provides flight opportunities for physics, geophysics, heliophysics, and astrophysics investigations from space. Launched in 1958, Explorer 1 was the first spacecraft of the United States to achieve orbit. Over 90 space missions have been launched since. Starting with Explorer 6, it has been operated by NASA, with regular collaboration with a variety of other institutions, including many international partners.

An Explorer mission observes Sagittarius A*, the Milky Way's central black hole, flaring.

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.

History

Early Explorer satellites

Launch of Explorer 1 on the Juno I launch vehicle.

Explorer 1, the first Earth satellite orbited by the United States

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.^[2] The Juno I vehicle was replaced by the Juno II in 1959.

Continuation of the Explorer program

With the establishment of NASA in 1958, the Explorer program was transferred to NASA from the U.S. Army. NASA continued to use the name for an ongoing series of relatively small space missions, typically an artificial satellite with a specific science focus. Explorer 6 in 1959 was the first scientific satellite under the project direction of NASA's Goddard Space Flight Center (GSFC) in Greenbelt, Maryland.^[3][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).^[5][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.^[5]

List of Interplanetary Monitoring Platform (IMP) missions

Mission	Photo	Satellite		Launch Date	Decay Date	Notes
		Explorer	IMP
IMP-1		Explorer 18	IMP-A	27 November 1963, 02:30 UTC[7]	December 30, 1965	First use of integrated circuits in a spacecraft. First satellite in IMP-A/-B/-C design series.
IMP-2		Explorer 21	IMP-B	4 October 1964, 03:45 UTC[7]	January 1, 1966	Second satellite in IMP-A/-B/-C design series.
IMP-3		Explorer 28	IMP-C	29 May 1965, 12:00 UTC[7]	4 July 1968	Third satellite in IMP-A/-B/-C design series.
AIMP-1		Explorer 33	IMP-D	1 July 1966, 16:02 UTC[7]	In orbit	First 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-4		Explorer 34	IMP-F	24 May 1967, 14:05 UTC[7]	May 3, 1969	Similar design to IMP-G.
AIMP-2		Explorer 35	IMP-E	19 July 1967, 14:19 UTC[7]	After June 24, 1973	Similar design to IMP-D. Positioned in Selenocentric orbit.
IMP-5		Explorer 41	IMP-G	21 June 1969, 08:47 UTC[7]	December 23, 1972	Similar design to IMP-F.
IMP-6		Explorer 43	IMP-I	13 March 1971, 16:15 UTC[7]	October 2, 1974	First spacecraft in IMP-I/-H/-J series.
IMP-7		Explorer 47	IMP-H	23 September 1972, 01:20 UTC[7]	In orbit	Second spacecraft in IMP-I/-H/-J series.
IMP-8		Explorer 50	IMP-J	26 October 1973, 02:26 UTC[7]	In orbit	Third spacecraft in IMP-I/-H/-J series, remained in service until 2006

Over the following two decades, NASA has launched over 50 Explorer missions,^[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.^{[citation needed]}

With decreases in NASA's budget, Explorer missions became infrequent in the early 1980s.^{[citation needed]}

SMEX, MIDEX, and Student Explorer 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.^[9][10] The first three SMEX missions were chosen in April 1989 out of 51 candidates, and launched in 1992, 1996 and 1998^[11] The second set of two missions were announced in September 1994 and launched in 1998 and 1999.^[9]

This artificially colored view of M101 maps ultraviolet light as blue while visible light is red since UV light does not have a "color" (the eye stopping at about violet). This view was taken by the MIDEX-3 Swift, which can also detect X-rays, and has contributed to the study of gamma-ray bursts and other topics.

In the mid-1990s, NASA initiated the Medium-class Explorer (MIDEX) to enable more frequent flights. These are larger than SMEX missions and were to be launched aboard a new kind of medium-light class launch vehicle.^[9] This new launch vehicle was not developed and instead, these missions were flown on a modified Delta II rocket.^[12][13] The first announcement opportunity for MIDEX was issued in March 1995, and the first launch under this new class was FUSE in 1999.^[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.^[9] Later, NASA established the University-Class Explorer (UNEX) program for much cheaper missions, which is regarded as a successor to STEDI.^[14]

The Explorer missions were at first managed by the Small Explorer Project Office at NASA's Goddard Space Flight Center (GSFC). In early 1999, that office was closed and with the announcement of opportunity for the third set of SMEX missions NASA converted the SMEX class so that each mission was managed by its principal investigator, with oversight by the GSFC Explorer Project.^[15] The Explorer program Office at Goddard Space Flight Center, provides management of the many operational scientific exploration missions that are characterized by relatively moderate costs and small to medium-sized missions that are capable of being built, tested, and launched in a short time interval compared to larger observatories like NASA's Great Observatories.^[16]

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

Classes

Medium-Class Explorers (MIDEX)

List of MIDEX missions^[21][22][23]

Name	MIDEX number	Explorer number	Launch (UTC)	Status
RXTE		Explorer-69	30 December 1995	Ended in 2012 / Reentered on 30 April 2018
ACE		Explorer-71	25 August 1997	Operational
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	Operational
FAME	MIDEX-4	—	Scheduled for 2004	Cancelled in 2002 (cost)
THEMIS A	MIDEX-5A	Explorer-85	17 February 2007	Operational
THEMIS B	MIDEX-5B	Explorer-86	17 February 2007	Operational
THEMIS C	MIDEX-5C	Explorer-87	17 February 2007	Operational
THEMIS D	MIDEX-5D	Explorer-88	17 February 2007	Operational
THEMIS E	MIDEX-5E	Explorer-89	17 February 2007	Operational
WISE / NEOWISE	MIDEX-6	Explorer-92	14 December 2009	Mission operations Completed on 31 July 2024. [24] Reentered on 02 November 2024
TESS	MIDEX-7	Explorer-95	18 April 2018	Operational
ICON	MIDEX-8	Explorer-96	11 October 2019	Operational
SPHEREx	MIDEX-9		27 February 2025	In development[25]
MUSE	MIDEX-10		2027	In development
HelioSwarm	MIDEX-11		2028	In development
UVEX	MIDEX-12		2030	In development

Small Explorers (SMEX)

The Small Explorers class was implemented in 1989 specifically to fund space exploration missions that cost no more than US$120 million.^[20][11] The missions are managed by the Explorers Project at the Goddard Space Flight Center (GSFC).^[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 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.^[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).^[26][27][28] In June 2019 NASA selected TRACERS and PUNCH for flight.^[29]

List of SMEX missions ^[22][30]

Name	SMEX number	Explorer number	Launch (UTC)	End of mission	Status
SAMPEX	SMEX-1	Explorer-68	3 July 1992	30 June 2004	Reentered on 13 November 2012
FAST	SMEX-2	Explorer-70	21 August 1996	4 May 2009
SWAS	SMEX-3	Explorer-74	6 December 1998	21 July 2004
TRACE	SMEX-4	Explorer-73	2 April 1998	21 June 2010
WIRE	SMEX-5	Explorer-75	5 March 1999	—	Spacecraft equipment failure; reentered on 10 May 2011
RHESSI	SMEX-6	Explorer-81	5 February 2002	April 2018	Deorbited on 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 [31]
Aeronomy of Ice in the Mesosphere (AIM)	SMEX-9	Explorer-90	25 April 2007	Reentered on 19 August 2024
IBEX	SMEX-10	Explorer-91	19 October 2008		Operational
NuSTAR	SMEX-11	Explorer-93	13 June 2012		Operational
IRIS	SMEX-12	Explorer-94	28 June 2013		Operational
GEMS	SMEX-13	—	Scheduled for 2014	—	Cancelled in 2012 due to expected cost overruns[32]
Imaging X-ray Polarimetry Explorer (IXPE)	SMEX-14	Explorer-97	9 December 2021		Operational
PUNCH	SMEX-15		27 February 2025[33]		In development[34]
TRACERS	SMEX-16		April 2025[35]		In development
COSI	SMEX-17		August 2027[36]		In development

• 
  SAMPEX
• 
  RHESSI
• 
  IBEX
• 
  IXPE

University-Class Explorers (UNEX)

List of UNEX missions^[37]

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)[38]
CHIPS	UNEX-3	Explorer-82	12 January 2003	Ended in 2008

Missions of Opportunity (MO)

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.^[39]

List of MO^[37]

Name	Launcher (mission)	Launch (UTC)	Status
HETE-2	NASA (Explorer-79)	9 October 2000	Ended in 2008[40]
INTEGRAL	ESA[41]	17 October 2002	Operational
Suzaku (Astro-E2)	JAXA[42]	10 July 2005	Ended in 2015
TWINS	NRO (USA-184;[43] USA-200[44])	TWINS-1: 28 June 2006 TWINS-2: 13 March 2008	Operational
CINDI	DoD (C/NOFS)[45]	16 April 2008	Ended in 2015
Hitomi (Astro-H)	JAXA[46]	17 February 2016	Failed
NICER	ISS (CRS-11)	3 June 2017	Operational
GOLD	SES (SES-14)	25 January 2018	Operational
XRISM	JAXA	6 September 2023	Operational
AWE	ISS (CRS-29)	December 2023	Operational
GUSTO	NASA (high-altitude balloon)	31 December 2023	Ended on 26 February 2024[47]
SunRISE	NASA (Maxar satellite)	2024	In development[48][49]
EZIE	NASA, JHUAPL	2024	In development[50]
Solar-C EUVST	JAXA	July 2028	In development[51][52]
CASE	ESA (Cosmic Vision M4)	2029	In development[53][54]

Beacon Explorers

Three satellites were planned in this series: Beacon Explorer-A, Beacon Explorer-B, Beacon Explorer-C.

GEOS series

A series of three Geodetic Earth Orbiting Satellite (GEOS) were put in orbit: GEOS 1, GEOS 2, GEOS 3.

Launched spacecraft

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.^[55]

Explorers Program satellites^{[8][56][57][58]}

No.	Name(s)	Launch date (UTC)	Rocket	Mass (kg)	Orbit regime	End of data	Re-entry [59]	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[60]
2	Explorer 2	5 March 1958	Juno I	15	Failed	—	—	Failed to achieve orbit.[61]
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[62]
4	Explorer 4	26 July 1958	Juno I	26	MEO	5 October 1958	23 October 1959	Monitor charged particles inside Van Allen belts from nuclear detonations (during Operation Argus)[63]
5	Explorer 5	24 August 1958	Juno I	17	failed	—	—	Planned in conjunction with Explorer 4, but launch failed[64]
—	Explorer S-1 (7X)	16 July 1959	Juno II	42	failed	—	—	Planned to measure Earth's radiation balance, but destroyed within seconds by range safety[65]
6	Explorer 6 (S-2, Able 3)	7 August 1959	Thor-Able	64	HEO	6 October 1959	12 July 1961	Magnetosphere research and digital telemetry; first NASA launch, first Earth photo from orbit[66][67]
7	Explorer 7 (S-1A)	13 October 1959	Juno II	42	LEO	24 August 1961	In orbit	Micrometeoroids and energetic particle studies, first satellite to measure Earth's climate[68][69][70]
–	S-46A (IE-B)	23 March 1960	Juno II	16	failed	—	—	Analyze electron and proton radiation energies, failed to achieve orbit[71]
8	Explorer 8 (S-30)	3 November 1960	Juno II	41	LEO	27 December 1960	27 March 2012	Measured atmospheric composition of the ionosphere[72]
–	S-56	4 December 1960	Scout X-1	6	failed	—	—	Atmosphere density measurement, but failed to achieve orbit[73]
9	Explorer 9 (S-56A)	16 February 1961	Scout X-1	36	LEO	9 April 1964	9 April 1964	Atmospheric density measurements, first spacecraft placed in orbit by a solid-fuel rocket[74]
–	S-45	24 February 1961	Juno II	34	failed	—	—	Ionosphere research, but failed to achieve orbit[75]
10	Explorer 10 (P 14)	25 March 1961	Thor-Delta	79	HEO	25 March 1961	1 June 1968	Investigated the magnetic field between the Earth and Moon[76]
11	Explorer 11 (S 15)	27 April 1961	Juno II	37	LEO	17 November 1961	In orbit	Gamma ray astronomy[77]
–	S-45A	25 May 1961	Juno II	34	failed	—	—	Ionosphere research, failed to achieve orbit. Last Juno II launch.[78]
–	S-55 (satellite) (Meteoroid Satellite-A, Micrometeorite Explorer)	30 June 1961	Scout X-1	85	failed	—	—	Micrometeoroid research, failed to achieve orbit[79]
12	EPE-A (S 3, Energetic Particle Explorer-A)	16 August 1961	Thor-Delta	38	HEO	6 December 1961	1 September 1963	Energetic particle research[80]
13	Explorer 13 (S-55A)	25 August 1961	Scout X-1	86	LEO	28 August 1961	28 August 1961	Micrometeoroid research; partial failure[81]
14	EPE-B (Energetic Particle Explorer-B)	2 October 1962	Delta A	40	HEO	11 August 1963	1 July 1966	Energetic particle research[82]
15	EPE-C (S-3B, Energetic Particle Explorer-C)	27 October 1962	Delta A	44	HEO	30 January 1963	15 January 1978	Energetic particle research[83]
16	S-55B	16 December 1962	Scout X-3	101	LEO	22 July 1963	In orbit	Micrometeoroid research[84]
17	AE-A (S-6, Atmosphere Explorer-A)	3 April 1963	Delta B	184	LEO	10 July 1963	24 November 1966	Atmospheric research[85]
18	IMP-A (IMP 1, Interplanetary Monitoring Platform-A)	27 November 1963	Delta C	138	HEO	10 May 1965	30 December 1965	Magnetospheric research[86]
19	AD-A (Atmospheric Density-A)	19 December 1963	Scout X-4	8	LEO	10 May 1981	10 May 1981	Atmospheric density measurements[87]
–	BE-A (Beacon Explorer-A, S-66A)	19 March 1964	Delta B	114	failed	—	—	Launch failure[88]
20	IE-A (S 48, TOPSI, Ionosphere Explorer-A)	25 August 1964	Scout X-4	45	LEO	29 December 1965	In orbit	Ionosphere research[89]
21	IMP-B (IMP 2, Interplanetary Monitoring Platform-B)	4 October 1964	Delta C	135	HEO	13 October 1965	30 January 1966	Magnetospheric research[90]
22	BE-B (Beacon Explorer-B, S-66B)	10 October 1964	Scout X-4	53	LEO	February 1970	In orbit	Ionospheric and geodetic research[91]
23	S 55C	6 November 1964	Scout X-4	134	LEO	7 November 1965	29 June 1983	Micrometeoric research[92]
24	AD-B (Atmospheric Density-B)	21 November 1964	Scout X-4	9	MEO	18 October 1968	18 October 1968	Atmospheric density measurements[93]
25	Injun 4 (IE-B, Ionosphere Explorer-B)	21 November 1964	Scout X-4	40	LEO	December 1966	In orbit	Ionospheric research[94]
26	EPE-D (Energetic Particle Explorer-D)	21 December 1964	Delta C	46	MEO	27 December 1967	23 August 2021	High energy particle observations[95]
27	BE-C (Beacon Explorer-C, S-66C)	29 April 1965	Scout X-4	61	LEO	20 July 1973	In orbit	Magnetospheric research[96]
28	IMP-C (IMP 3, Interplanetary Monitoring Platform-C)	29 May 1965	Delta C	128	HEO	12 May 1967	4 July 1968	Magnetospheric research[97]
29	GEOS 1 (GEOS-A, Geodetic Earth Orbiting Satellite-1)	6 November 1965	Delta E	387	LEO	23 June 1978	In orbit	Geodetic Earth monitoring[98]
30	SOLRAD 8 (SE-A)	19 November 1965	Scout X-4	57	LEO	5 November 1967	In orbit	Solar radiation monitoring (Cover for covert ELINT mission)[99]
31	DME-A (Direct Measurements Explorer)	29 November 1965	Thor-Agena B	99	LEO	1 October 1969	In orbit	Ionospheric research[100]
32	AE-B (Atmosphere Explorer-B)	25 May 1966	Delta C1	225	LEO	March 1967	22 February 1985	Atmospheric research[101]
33	IMP-D (AIMP 1, Interplanetary Monitoring Platform-D)	1 July 1966	Delta E1	212	HEO	21 September 1971[102]	In orbit	Magnetospheric research
34	IMP-F (IMP 4, Interplanetary Monitoring Platform-F)	24 May 1967	Delta E1	163	MEO	3 May 1969	3 May 1969	Magnetospheric research[103]
35	IMP-E (AIMP 2, Interplanetary Monitoring Platform-E)	19 July 1967	Delta E1	230	Lunar	24 June 1973	Lunar orbit	Magnetospheric research[104]
36	GEOS 2 (GEOS-B, Geodetic Earth Orbiting Satellite-2)	11 January 1968	Delta E1	469	LEO	1 July 1982	In orbit	Geodetic Earth monitoring[105]
37	SOLRAD 9 (SE B)	5 March 1968	Scout B	198	LEO	30 April 1974	16 November 1990	Solar radiation monitoring (Cover for covert ELINT mission)[106]
38	RAE-A (RAE 1, Radio Astronomy Explorer-A)	4 July 1968	Delta J	602	MEO	(~1969)	In orbit	Radio astronomy[107]
39	AD-C (Atmospheric Density-C)	8 August 1968	Scout B	9	LEO	23 June 1971	22 June 1981	Atmospheric density measurements[108]
40	Injun 5 (Injun C, IE-C, Ionosphere Explorer-C)	8 August 1968	Scout B	71	LEO	June 1971	In orbit	Magnetospheric Research[109]
41	IMP-G (IMP 5, Interplanetary Monitoring Platform-G)	21 June 1969	Delta E1	145	HEO	23 December 1972	23 December 1972	Magnetospheric research[110]
42	Uhuru (SAS-A, SAS 1)	12 December 1970	Scout B	142	LEO	4 January 1975	5 April 1979	X-ray astronomy[111]
43	IMP-H (IMP 7, Interplanetary Monitoring Platform-H)	13 March 1971	Delta M6	635	MEO	2 October 1974	2 October 1974	Magnetospheric research[112]
44	SOLRAD 10 (SE-C, SOLRAD-C)	8 July 1971	Scout B	260	LEO	30 June 1973	15 December 1979	Solar radiation monitoring (Cover for covert ELINT mission)[113]
45	SSS-A (S-Cubed A)	15 November 1971	Scout B	52	MEO	30 September 1974	10 January 1992	Magnetospheric research[114]
46	MTS (Meteoroid Technology Satellite, METEC)[115]	13 August 1972	Scout D-1	90	LEO	4 November 1974	2 November 1979	Micrometeoroids research[116]
47	IMP-I (IMP 6, Interplanetary Monitoring Platform-I)	23 September 1972	Delta 1604	635	HEO	31 October 1978	In orbit	Magnetospheric research[117]
48	SAS-B (Small Astronomy Satellite-B, SAS 2)	15 November 1972	Scout D-1	166	LEO	8 June 1973	20 August 1980	X-ray astronomy[118]
49	RAE-B (RAE 2, Radio Astronomy Explorer-B)	10 June 1973	Delta 1913	328	Lunar	26 April 1977	Presumed crashed into Moon sometime after August 1977[119]	Radio astronomy[119]
50	IMP-J (IMP 8, Interplanetary Monitoring Platform-J)	26 October 1973	Delta 1604	371	HEO	7 October 2006	In orbit	Magnetospheric research[120]
51	AE-C (Atmosphere Explorer-C)	16 December 1973	Delta 1900	658	LEO	(12 December 1978)	12 December 1978	Atmospheric research[121]
52	Hawkeye 1 (Injun-F, Injun 6, IE-D, Ionosphere Explorer-D)	3 June 1974	Scout E-1	23	HEO	28 April 1978	28 April 1978	Magnetospheric research[122]
53	SAS-C (Small Astronomy Satellite-C, SAS 3)	7 May 1975	Scout F-1	197	LEO	7 April 1979	9 April 1979	X-ray astronomy[123]
54	AE-D (Atmosphere Explorer-D)	6 October 1975	Delta 2910	681	LEO	29 January 1976	12 March 1976	Atmospheric research[124]
55	AE-E (Atmosphere Explorer-E)	20 November 1975	Delta 2910	735	LEO	25 September 1980	10 June 1981	Atmospheric research[125]
—	DADE-A (Dual Air Density Explorer-A)	5 December 1975	Scout F-1	40	failed	—	—	Atmospheric research; failed during launch[126]
—	DADE-B (Dual Air Density Explorer-B)	5 December 1975	Scout F-1	43	failed	—	—	Atmospheric research; failed during launch[127]
56	ISEE-1 (ISEE-A)	22 October 1977	Delta 2914	340	HEO	26 September 1987	26 September 1987	Magnetospheric research; launched with ESA's ISEE-2; co-mission with ISEE 3[128]
57	IUE	26 January 1978	Delta 2914	669	MEO	30 September 1996	In orbit	Ultraviolet astronomy[129]
58	HCMM (AEM-A)	26 April 1978	Scout F	117	LEO	30 September 1980	December 22, 1981	Thermal mapping of the Earth[130]
59	ICE (ISEE 3, ISEE-C)	12 August 1978	Delta 2914	390	Sun–Earth L1	16 September 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.[131][67]
60	SAGE (AEM-B)	18 February 1979	Scout D-1	149	LEO	7 January 1982	11 April 1989	Stratospheric aerosol and ozone data[132]
61	MAGSAT (AEM-C)	30 October 1979	Scout G-1	158	LEO	6 May 1980	11 June 1980	Mapped the near surface magnetic field of the Earth[133]
62	Dynamics Explorer 1 (DE-1)	3 August 1981	Delta 3913	424	MEO	28 February 1991	In orbit	Magnetospheric research[134]
63	Dynamics Explorer 2 (DE-2)	3 August 1981	Delta 3913	420	LEO	19 February 1983	19 February 1983	Magnetospheric research[135]
64	SME	6 October 1981	Delta 2310	145	LEO	4 April 1989	5 March 1991	Atmospheric research[136]
65	AMPTE-CCE	16 August 1984	Delta 3924	242	MEO	12 July 1989	In orbit	Magnetosphere research[137]
66	COBE	18 November 1989	Delta 5920	2,206	LEO	23 December 1993	In orbit	Microwave astronomy[138]
67	EUVE (BERKSAT)	7 June 1992	Delta II 6920-X	3,275	LEO	31 January 2001	30 January 2002	Ultraviolet astronomy[139]
68	SAMPEX	3 July 1992	Scout G-1	158	LEO	30 June 2004[140]	13 November 2012.[141]	SMEX: magnetospheric research[142]
69	RXTE	30 December 1995	Delta II 7920	3,200	LEO	3 January 2012	30 April 2018[143]	MIDEX: X-ray astronomy[144]
70	FAST	21 August 1996	Pegasus XL	187	LEO	4 May 2009	In orbit	SMEX: auroral phenomena[145]
—	HETE 1	4 November 1996	Pegasus XL	128	LEO	—	7 April 2002	Separation failure, mission relaunched as HETE 2[146]
71	ACE	25 August 1997	Delta II 7920	596	Sun–Earth L1	Operational	In L1 orbit	MIDEX: solar/interplanetary/interstellar particle research[147]
72	SNOE	26 February 1998	Pegasus XL	120	LEO	13 December 2003	13 December 2003	STEDI, UNEX: atmospheric research[148]
73	TRACE	2 April 1998	Pegasus XL	250	LEO	21 June 2010[149]	In orbit	SMEX: solar observatory[150]
74	SWAS	6 December 1998	Pegasus XL	288	LEO	1 September 2005	In orbit	SMEX: submillimeter astronomy[151]
75	WIRE	5 March 1999	Pegasus XL	250	SSO	30 September 2000[152]	10 May 2011[153]	SMEX, Infrared astronomy, primary mission failed due to loss of coolant[154]
76	TERRIERS	18 May 1999	Pegasus XL	120	Polar LEO	18 May 1999	In orbit	STEDI: atmospheric research, satellite failed shortly after achieving orbit[155]
77	FUSE	23 June 1999	Delta II 7320	1,400	LEO	18 October 2007	In orbit	MIDEX: ultraviolet astronomy[156]
78	IMAGE	25 March 2000	Delta II 7326	536	Polar MEO	18 December 2005[157]	In orbit	MIDEX: magnetospheric research[158]
79	HETE-2	9 October 2000	Pegasus-H	124	LEO	28 March 2007[159]	In orbit	MO: UV, X-ray, and gamma ray astronomy[160]
80	WMAP	30 June 2001	Delta II 7425-10	840	Sun–Earth L2	October 2010[161]	Heliocentric orbit	MIDEX: microwave astronomy[162]
81	RHESSI	5 February 2002	Pegasus XL	230	LEO	16 August 2018	Deorbited	SMEX: X-ray and gamma ray solar flare imaging[163]
—	INTEGRAL	17 October 2002	Proton-K Blok DM-2	4,000	HEO	Operational	In orbit	International: space telescope for observing gamma rays[164]
82	CHIPSat	13 January 2003	Delta II 7320-10	60	LEO	11 April 2008[165]	In orbit	UNEX: ultraviolet spectroscopy and astronomy[166]
83	GALEX	28 April 2003	Pegasus XL	280	LEO	28 June 2013[167]	In orbit	SMEX: ultraviolet astronomy[168]
84	Swift	20 November 2004	Delta II 7320-10C	1,470	LEO	Operational	In orbit	MIDEX: gamma ray astronomy[169]
—	Suzaku (Astro E2)	10 July 2005	M-V	1,706	LEO	2 September 2015	In orbit	MO: instrument on JAXA's Suzaku mission[170]
—	TWINS A	28 June 2006	Delta IV M+(4,2)	classified	Molniya	Operational	In orbit	MO: payload on Trumpet-F/O-1 1 (USA-184)
85	THEMIS A	17 February 2007	Delta II 7925	77	HEO	Operational	In orbit	MIDEX: magnetospheric research[171]
86	THEMIS B (ARTEMIS P1)	17 February 2007	Delta II 7925	77	Lunar	Operational	Lunar orbit	MIDEX; Magnetospheric research[172]
87	THEMIS C (ARTEMIS P2)	17 February 2007	Delta II 7925	77	Lunar	Operational	Lunar orbit	MIDEX: magnetospheric research[173]
88	THEMIS D	17 February 2007	Delta II 7925	77	HEO	Operational	In orbit	MIDEX: magnetospheric research[174]
89	THEMIS E	17 February 2007	Delta II 7925	77	HEO	Operational	In orbit	MIDEX: magnetospheric research[175]
90	AIM	25 April 2007	Pegasus XL	197	SSO	March 2023	19 August 2024	SMEX: noctilucent cloud observation[176]
—	TWINS B	13 March 2008	Atlas V 411	classified	Molniya	Operational	In orbit	MO: payload on Trumpet-F/O-1 2 (USA-200)
—	CINDI	16 April 2008	Pegasus XL	395	LEO	28 November 2015	28 November 2015	MO: instruments on C/NOFS
91	IBEX	19 October 2008	Pegasus XL	107	MEO	Operational	In orbit	SMEX: mapping the boundary between the Solar System and interstellar space.[177]
92	WISE	14 December 2009	Delta II 7320	661	LEO	August 2024	02 November 2024	MIDEX: infrared astronomy, NEOWISE extension. Discovered first Earth trojan.[178]
93	NuSTAR	13 June 2012	Pegasus XL	350	LEO	Operational	In orbit	SMEX: high-energy X-ray astronomy[179]
94	IRIS	27 June 2013	Pegasus XL	183	SSO	Operational	In orbit	SMEX: solar UV astronomy[180]
—	Hitomi (NeXT, ASTRO-H)	7 February 2016	H-2A-202	2,700	LEO	26 March 2016	In orbit	MO: X-ray instrument on JAXA's Hitomi, but spacecraft failed after initial checkouts[181]
—	NICER	3 May 2017	Falcon 9 FT	372	ISS	Operational	ISS	MO: instrument on ISS for neutron star observations
—	GOLD	25 January 2018	Ariane 5 ECA	37	GEO	Operational	In orbit	MO: instrument on SES-14 comsat for studying Earth–space boundary
95	TESS	18 April 2018	Falcon 9 FT	362	HEO	Operational	In orbit	MIDEX: survey for transiting exoplanets
96	ICON	11 October 2019	Pegasus XL	287	LEO	Operational	In orbit	MIDEX: ionospheric studies
97	IXPE	9 December 2021	Falcon 9 Block 5	330	LEO	Operational	In orbit	SMEX: X-ray studies
—	XRISM	6 September 2023	H-IIA 202	2,300	LEO	Operational	In orbit	MO: instruments on JAXA's XRISM x-ray space telescope

Cancelled missions

WISE was restarted after it was turned off

Explorer 6 on a Thor-Able III launch in August 1959

ISEE-C in a dynamic test chamber, 1978

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.^[182] 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.^{[citation needed]}

Examples of missions that were not developed or cancelled were:^[58]

• Owl 1 and 2 (cost, 1965)^[183]
• MSS A (Magnetic Storm Satellite, Explorer-A, 1970)^[184][185]
• CATSAT (STEDI 3) (cost)^[38]
• IMEX (UNEX 2) (cost)^[38]
• FAME (MIDEX 4)
• SPIDR (SMEX 8) (technical, 2003)^[38]
• 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

Launch statistics

Number of launches per decade:^[186]

Number of Explorer launches by decade

Decade
1950s	8
1960s	34
1970s	20
1980s	5
1990s	11
2000s	15
2010s	7
2020s	2

See also

• Spaceflight portal

• Cosmic Vision, a European Space Agency (ESA) programme
• Cosmic Vision S-class missions, the European Space Agency equivalent to the Small Explorer program
• Discovery program – Ongoing solar system exploration program by NASAPages displaying short descriptions of redirect targets
• New Frontiers program – Series of space exploration missions being conducted by NASA

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External links

• "Explorers Program". Goddard Space Flight Center. NASA. 2009. Archived from the original on 31 October 2009. Retrieved 5 December 2009.
• NASA Explorers Program missions page
• NSSDC updated list of Explorers missions
• Explorers Program Profile by NASA's Solar System Exploration
• Small Explorers Missions by Goddard Space Flight Center
• SMEX-series satellites by Colorado State University