Explorers Program

File:Pointing X-ray Eyes at our Resident Supermassive Black Hole.jpg

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

The Explorers Program<ref>{{#invoke:citation/CS1|citation |CitationClass=web }} Template:PD-notice</ref> 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

File:Ignition of Jupiter-C with Explorer 1.jpg

Launch of Explorer 1 on the Juno I launch vehicle.

File:Explorer1.jpg

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

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.<ref name=Boehm-NASA>Template:Citation-attribution</ref> 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.<ref>Template:Cite magazine</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }} Template:PD-notice</ref>

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).<ref name="Butrica">Template:Cite book Template:PD-notice</ref><ref name="nasa">Template:Cite book Template:PD-notice</ref> 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.<ref name="Butrica"/>

List of Interplanetary Monitoring Platform (IMP) missions

Mission	Photo	Satellite		Launch date	Decay date	Notes
		Explorer	IMP
IMP-1	File:Explorer-18 IMP-A.jpg	Explorer 18	IMP-A	citation	CitationClass=web }}</ref>	December 30, 1965	First use of integrated circuits in a spacecraft. First satellite in IMP-A/-B/-C design series.
IMP-2	File:Explorer-21 image.jpg	Explorer 21	IMP-B	4 October 1964, 03:45 UTC<ref name="jonathan" />	January 1, 1966	Second satellite in IMP-A/-B/-C design series.
IMP-3	File:Explorer 28.jpg	Explorer 28	IMP-C	29 May 1965, 12:00 UTC<ref name="jonathan" />	4 July 1968	Third satellite in IMP-A/-B/-C design series.
AIMP-1	File:IMP-D.jpg	Explorer 33	IMP-D	1 July 1966, 16:02 UTC<ref name="jonathan" />	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	File:Explorer 34.jpg	Explorer 34	IMP-F	24 May 1967, 14:05 UTC<ref name="jonathan" />	May 3, 1969	Similar design to IMP-G.
AIMP-2	File:IMP-E.jpg	Explorer 35	IMP-E	19 July 1967, 14:19 UTC<ref name="jonathan" />	After June 24, 1973	Similar design to IMP-D. Positioned in Selenocentric orbit.
IMP-5	File:Explorer-41 IMP-G.jpg	Explorer 41	IMP-G	21 June 1969, 08:47 UTC<ref name="jonathan" />	December 23, 1972	Similar design to IMP-F.
IMP-6	File:Explorer 43 IMP-I.jpg	Explorer 43	IMP-I	13 March 1971, 16:15 UTC<ref name="jonathan" />	October 2, 1974	First spacecraft in IMP-I/-H/-J series.
IMP-7	File:Explorer-47 IMP-I.jpg	Explorer 47	IMP-H	23 September 1972, 01:20 UTC<ref name="jonathan" />	In orbit	Second spacecraft in IMP-I/-H/-J series.
IMP-8	File:IMP 8.gif	Explorer 50	IMP-J	26 October 1973, 02:26 UTC<ref name="jonathan" />	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,<ref name= "nssdc_list"/> 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.Template:Citation needed

With decreases in NASA's budget, Explorer missions became infrequent in the early 1980s.Template:Citation needed

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.<ref name=NASAhistory>Template:Cite book Template:PD-notice</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web

}} Template:PD-notice</ref> The first three SMEX missions were chosen in April 1989 out of 51 candidates, and launched in 1992, 1996 and 1998<ref name=IEEE>Template:Cite conference</ref> The second set of two missions were announced in September 1994 and launched in 1998 and 1999.<ref name=NASAhistory/>

File:M101 combined low.jpg

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 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.<ref name="NASAhistory" /> This new launch vehicle was not developed and instead, these missions were flown on a modified Delta II rocket.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite magazine</ref> The first announcement opportunity for MIDEX was issued in March 1995, and the first launch under this new class was FUSE in 1999.<ref name="NASAhistory" />

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.<ref name=NASAhistory/> Later, NASA established the University-Class Explorer (UNEX) program for much cheaper missions, which is regarded as a successor to STEDI.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

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.<ref name="welcome">{{#invoke:citation/CS1|citation |CitationClass=web }} Template:PD-notice</ref> 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }} Template:PD-notice</ref>

Excluding the launches, the MIDEX class has a current mission cap cost of US$250 million in 2018,<ref name=Midex19>{{#invoke:citation/CS1|citation |CitationClass=web }} Template:PD-notice</ref> with future MIDEX missions being capped at US$350 million.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The cost cap for SMEX missions in 2017 was US$165 million.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }} Template:PD-notice</ref> UNEX missions are capped at US$15 million.<ref name=missions>{{#invoke:citation/CS1|citation |CitationClass=web }} Template:PD-notice</ref> 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.<ref name=Midex19/><ref name=missions/>

Classes

Medium-Class (MIDEX)

citation

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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	Template:Success
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	Template:Success
FAME	MIDEX-4	check|unknown=|preview=Page using Template:Center with unknown parameter "_VALUE_"|ignoreblank=y| 1 | style }}	Scheduled for 2004	Cancelled in 2002 (cost)
THEMIS A	MIDEX-5A	Explorer-85	17 February 2007	Template:Success
THEMIS B	MIDEX-5B	Explorer-86	17 February 2007	Template:Success
THEMIS C	MIDEX-5C	Explorer-87	17 February 2007	Template:Success
THEMIS D	MIDEX-5D	Explorer-88	17 February 2007	Template:Success
THEMIS E	MIDEX-5E	Explorer-89	17 February 2007	Template:Success
WISE / NEOWISE	MIDEX-6	Explorer-92	14 December 2009	citation	CitationClass=web }}</ref> Reentered on 2 November 2024
TESS	MIDEX-7	Explorer-95	18 April 2018	Template:Success
ICON	MIDEX-8	Explorer-96	11 October 2019	Template:Success
SPHEREx	MIDEX-9		12 March 2025	Template:Success
MUSE	MIDEX-10		2027	Template:Pending
HelioSwarm	MIDEX-11		2028	Template:Pending
UVEX	MIDEX-12		2030	Template:Pending

Small Explorers (SMEX)

The Small Explorers class was implemented in 1989 specifically to fund space exploration missions that cost no more than Template:US$.<ref name=missions/><ref name="IEEE" /> The missions are managed by the Explorers Project at the Goddard Space Flight Center (GSFC).<ref name="welcome" />

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.<ref name="welcome" />

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).<ref>{{#invoke:citation/CS1|citation |CitationClass=web }} Template:PD-notice</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }} Template:PD-notice</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }} Template:PD-notice</ref> In June 2019 NASA selected TRACERS and PUNCH for flight.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }} Template:PD-notice</ref>

citation

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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	Template:N/a	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	check|unknown=|preview=Page using Template:Center with unknown parameter "_VALUE_"|ignoreblank=y| 1 | style }}	Scheduled for 2005	Template:N/a	citation	CitationClass=web }}</ref>
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		Template:Success
NuSTAR	SMEX-11	Explorer-93	13 June 2012		Template:Success
IRIS	Template:Nowrap	Template:Nowrap	28 June 2013		Template:Success
GEMS	SMEX-13	check|unknown=|preview=Page using Template:Center with unknown parameter "_VALUE_"|ignoreblank=y| 1 | style }}	Scheduled for 2014	Template:N/a	citation	CitationClass=web }}</ref>
Imaging X-ray Polarimetry Explorer (IXPE)	SMEX-14	Template:Nowrap	9 December 2021		Template:Success
PUNCH	SMEX-15		citation	CitationClass=web }}</ref>		Template:Pending<ref name="nasa-20220803">{{#invoke:citation/CS1|citation	CitationClass=web }} Template:PD-notice</ref>
TRACERS	SMEX-16		citation	CitationClass=web }}</ref>		Template:Pending
COSI	SMEX-17		August 2027<ref>Template:Cite press release Template:PD-notice</ref>		Template:Pending

• SAMPEX 3.jpg

  SAMPEX

• RHESSI.jpg

  RHESSI

• IBEX spacecraft.jpg

  IBEX

• IXPE-artist-rendition.jpg

  IXPE

University-Class Explorers (UNEX)

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Name	UNEX number	Explorer number	Launch (UTC)	Status
SNOE	UNEX-1	Explorer-72	26 February 1998	Ended in 2000
IMEX	UNEX-2	check|unknown=|preview=Page using Template:Center with unknown parameter "_VALUE_"|ignoreblank=y| 1 | style }}	Scheduled for 2003	Cancelled before 2005 (cost)<ref name="nap">Template:Cite book</ref>
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.<ref>Explorers Missions: Missions of Opportunity (MO) NASA Goddard Space Flight Center (GSFC). Accessed on 18 August 2019. Template:PD-notice</ref>

List of MO<ref name=":1" />

Name	Launcher (mission)	Launch (UTC)	Status
HETE-2	NASA (Explorer-79)	9 October 2000	citation	CitationClass=web }} Template:PD-notice</ref>
INTEGRAL	ESA<ref>{{#invoke:citation/CS1|citation	CitationClass=web }} Template:PD-notice</ref>	17 October 2002	Template:Success
Suzaku (Astro-E2)	JAXA<ref>{{#invoke:citation/CS1|citation	CitationClass=web }} Template:PD-notice</ref>	10 July 2005	Ended in 2015
TWINS	NRO (USA-184;<ref>{{#invoke:citation/CS1|citation	CitationClass=web }} Template:PD-notice</ref> USA-200<ref>{{#invoke:citation/CS1|citation	CitationClass=web }} Template:PD-notice</ref>)	TWINS-1: 28 June 2006 TWINS-2: 13 March 2008	Template:Success
CINDI	DoD (C/NOFS)<ref>{{#invoke:citation/CS1|citation	CitationClass=web }} Template:PD-notice</ref>	16 April 2008	Ended in 2015
Hitomi (Astro-H)	JAXA<ref>{{#invoke:citation/CS1|citation	CitationClass=web }} Template:PD-notice</ref>	17 February 2016	Template:Failure
NICER	ISS (CRS-11)	3 June 2017	Template:Success
GOLD	SES (SES-14)	25 January 2018	Template:Success
XRISM	JAXA	6 September 2023	Template:Success
AWE	ISS (CRS-29)	December 2023	Template:Success
GUSTO	NASA (high-altitude balloon)	31 December 2023	citation	CitationClass=web }}</ref>
SunRISE	NASA (Maxar satellite)	2024	Template:Pending
EZIE	NASA, JHUAPL	2024	Template:Pending
Solar-C EUVST	JAXA	July 2028	Template:Pending
CASE	ESA (Cosmic Vision M4)	2029	Template:Pending

Beacon ExplorersTemplate:Anchor

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

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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

citation

CitationClass=web }} Template:PD-notice</ref><ref>{{#invoke:citation/CS1|citation	CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation	CitationClass=web }} Template:PD-notice</ref><ref name=skyrocket>{{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
No.	Name(s)	Launch date (UTC)	Rocket	Mass (kg)	Orbit regime	End of data	citation	CitationClass=web }}</ref>	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<ref>{{#invoke:citation/CS1|citation	CitationClass=web }} Template:PD-notice</ref>
2	Explorer 2	5 March 1958	Juno I	15	Failed	—	—	citation	CitationClass=web }} Template:PD-notice</ref>
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<ref>{{#invoke:citation/CS1|citation	CitationClass=web }} Template:PD-notice</ref>
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)<ref>{{#invoke:citation/CS1|citation	CitationClass=web }} Template:PD-notice</ref>
5	Explorer 5	24 August 1958	Juno I	17	failed	—	—	citation	CitationClass=web }} Template:PD-notice</ref>
—	Explorer S-1 (7X)	16 July 1959	Juno II	42	failed	—	—	citation	CitationClass=web }} Template:PD-notice</ref>
6	Explorer 6 (S-2, Able 3)	7 August 1959	Thor-Able	64	HEO	6 October 1959	12 July 1961	citation	CitationClass=web }} Template:PD-notice</ref><ref name="space-timeline">{{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
7	Explorer 7 (S-1A)	13 October 1959	Juno II	42	LEO	24 August 1961	In orbit	citation	CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation	CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation	CitationClass=web }} Template:PD-notice</ref>
–	S-46A (IE-B)	23 March 1960	Juno II	16	failed	—	—	citation	CitationClass=web }} Template:PD-notice</ref>
8	Explorer 8 (S-30)	3 November 1960	Juno II	41	LEO	27 December 1960	27 March 2012	citation	CitationClass=web }} Template:PD-notice</ref>
–	S-56	4 December 1960	Scout X-1	6	failed	—	—	citation	CitationClass=web }} Template:PD-notice</ref>
9	Explorer 9 (S-56A)	16 February 1961	Scout X-1	36	LEO	9 April 1964	9 April 1964	citation	CitationClass=web }} Template:PD-notice</ref>
–	S-45	24 February 1961	Juno II	34	failed	—	—	citation	CitationClass=web }} Template:PD-notice</ref>
10	Explorer 10 (P 14)	25 March 1961	Thor-Delta	79	HEO	25 March 1961	1 June 1968	citation	CitationClass=web }} Template:PD-notice</ref>
11	Explorer 11 (S 15)	27 April 1961	Juno II	37	LEO	17 November 1961	In orbit	citation	CitationClass=web }} Template:PD-notice</ref>
–	S-45A	25 May 1961	Juno II	34	failed	—	—	citation	CitationClass=web }} Template:PD-notice</ref>
–	S-55 (satellite) (Meteoroid Satellite-A, Micrometeorite Explorer)	30 June 1961	Scout X-1	85	failed	—	—	citation	CitationClass=web }} Template:PD-notice</ref>
12	EPE-A (S 3, Energetic Particle Explorer-A)	16 August 1961	Thor-Delta	38	HEO	6 December 1961	1 September 1963	citation	CitationClass=web }} Template:PD-notice</ref>
13	Explorer 13 (S-55A)	25 August 1961	Scout X-1	86	LEO	28 August 1961	28 August 1961	citation	CitationClass=web }} Template:PD-notice</ref>
14	EPE-B (Energetic Particle Explorer-B)	2 October 1962	Delta A	40	HEO	11 August 1963	1 July 1966	citation	CitationClass=web }} Template:PD-notice</ref>
15	EPE-C (S-3B, Energetic Particle Explorer-C)	27 October 1962	Delta A	44	HEO	30 January 1963	15 January 1978	citation	CitationClass=web }} Template:PD-notice</ref>
16	S-55B	16 December 1962	Scout X-3	101	LEO	22 July 1963	In orbit	citation	CitationClass=web }} Template:PD-notice</ref>
17	AE-A (S-6, Atmosphere Explorer-A)	3 April 1963	Delta B	184	LEO	10 July 1963	24 November 1966	citation	CitationClass=web }} Template:PD-notice</ref>
18	IMP-A (IMP 1, Interplanetary Monitoring Platform-A)	27 November 1963	Delta C	138	HEO	10 May 1965	30 December 1965	citation	CitationClass=web }} Template:PD-notice</ref>
19	AD-A (Atmospheric Density-A)	19 December 1963	Scout X-4	8	LEO	10 May 1981	10 May 1981	citation	CitationClass=web }} Template:PD-notice</ref>
–	BE-A (Beacon Explorer-A, S-66A)	19 March 1964	Delta B	114	failed	—	—	citation	CitationClass=web }} Template:PD-notice</ref>
20	IE-A (S 48, TOPSI, Ionosphere Explorer-A)	25 August 1964	Scout X-4	45	LEO	29 December 1965	In orbit	citation	CitationClass=web }} Template:PD-notice</ref>
21	IMP-B (IMP 2, Interplanetary Monitoring Platform-B)	4 October 1964	Delta C	135	HEO	13 October 1965	30 January 1966	citation	CitationClass=web }} Template:PD-notice</ref>
22	BE-B (Beacon Explorer-B, S-66B)	10 October 1964	Scout X-4	53	LEO	February 1970	In orbit	citation	CitationClass=web }} Template:PD-notice</ref>
23	S 55C	6 November 1964	Scout X-4	134	LEO	7 November 1965	29 June 1983	citation	CitationClass=web }} Template:PD-notice</ref>
24	AD-B (Atmospheric Density-B)	21 November 1964	Scout X-4	9	MEO	18 October 1968	18 October 1968	citation	CitationClass=web }} Template:PD-notice</ref>
25	Injun 4 (IE-B, Ionosphere Explorer-B)	21 November 1964	Scout X-4	40	LEO	December 1966	In orbit	citation	CitationClass=web }} Template:PD-notice</ref>
26	EPE-D (Energetic Particle Explorer-D)	21 December 1964	Delta C	46	MEO	27 December 1967	23 August 2021	citation	CitationClass=web }} Template:PD-notice</ref>
27	BE-C (Beacon Explorer-C, S-66C)	29 April 1965	Scout X-4	61	LEO	20 July 1973	In orbit	citation	CitationClass=web }} Template:PD-notice</ref>
28	IMP-C (IMP 3, Interplanetary Monitoring Platform-C)	29 May 1965	Delta C	128	HEO	12 May 1967	4 July 1968	citation	CitationClass=web }} Template:PD-notice</ref>
29	GEOS 1 (GEOS-A, Geodetic Earth Orbiting Satellite-1)	6 November 1965	Delta E	387	LEO	23 June 1978	In orbit	citation	CitationClass=web }} Template:PD-notice</ref>
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)<ref>{{#invoke:citation/CS1|citation	CitationClass=web }} Template:PD-notice</ref>
31	DME-A (Direct Measurements Explorer)	29 November 1965	Thor-Agena B	99	LEO	1 October 1969	In orbit	citation	CitationClass=web }}</ref>
32	AE-B (Atmosphere Explorer-B)	25 May 1966	Delta C1	225	LEO	March 1967	22 February 1985	citation	CitationClass=web }}</ref>
33	IMP-D (AIMP 1, Interplanetary Monitoring Platform-D)	1 July 1966	Delta E1	212	HEO	citation	CitationClass=web }}</ref>	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	citation	CitationClass=web }}</ref>
35	IMP-E (AIMP 2, Interplanetary Monitoring Platform-E)	19 July 1967	Delta E1	230	Lunar	24 June 1973	Lunar orbit	citation	CitationClass=web }}</ref>
36	GEOS 2 (GEOS-B, Geodetic Earth Orbiting Satellite-2)	11 January 1968	Delta E1	469	LEO	1 July 1982	In orbit	citation	CitationClass=web }}</ref>
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)<ref>{{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
38	RAE-A (RAE 1, Radio Astronomy Explorer-A)	4 July 1968	Delta J	602	MEO	(~1969)	In orbit	citation	CitationClass=web }}</ref>
39	AD-C (Atmospheric Density-C)	8 August 1968	Scout B	9	LEO	23 June 1971	22 June 1981	citation	CitationClass=web }}</ref>
40	Injun 5 (Injun C, IE-C, Ionosphere Explorer-C)	8 August 1968	Scout B	71	LEO	June 1971	In orbit	citation	CitationClass=web }}</ref>
41	IMP-G (IMP 5, Interplanetary Monitoring Platform-G)	21 June 1969	Delta E1	145	HEO	23 December 1972	23 December 1972	citation	CitationClass=web }}</ref>
42	Uhuru (SAS-A, SAS 1)	12 December 1970	Scout B	142	LEO	4 January 1975	5 April 1979	citation	CitationClass=web }}</ref>
43	IMP-H (IMP 7, Interplanetary Monitoring Platform-H)	13 March 1971	Delta M6	635	MEO	2 October 1974	2 October 1974	citation	CitationClass=web }}</ref>
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)<ref>{{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
45	SSS-A (S-Cubed A)	15 November 1971	Scout B	52	MEO	30 September 1974	10 January 1992	citation	CitationClass=web }}</ref>
46	MTS (Meteoroid Technology Satellite, METEC)<ref>{{#invoke:citation/CS1|citation	CitationClass=web }}</ref>	13 August 1972	Scout D-1	90	LEO	4 November 1974	2 November 1979	citation	CitationClass=web }}</ref>
47	IMP-I (IMP 6, Interplanetary Monitoring Platform-I)	23 September 1972	Delta 1604	635	HEO	31 October 1978	In orbit	citation	CitationClass=web }}</ref>
48	SAS-B (Small Astronomy Satellite-B, SAS 2)	15 November 1972	Scout D-1	166	LEO	8 June 1973	20 August 1980	citation	CitationClass=web }}</ref>
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<ref name="explorer49nssdc"/>	citation	CitationClass=web }}</ref>
50	IMP-J (IMP 8, Interplanetary Monitoring Platform-J)	26 October 1973	Delta 1604	371	HEO	7 October 2006	In orbit	citation	CitationClass=web }}</ref>
51	AE-C (Atmosphere Explorer-C)	16 December 1973	Delta 1900	658	LEO	(12 December 1978)	12 December 1978	citation	CitationClass=web }}</ref>
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	citation	CitationClass=web }}</ref>
53	SAS-C (Small Astronomy Satellite-C, SAS 3)	7 May 1975	Scout F-1	197	LEO	7 April 1979	9 April 1979	citation	CitationClass=web }}</ref>
54	AE-D (Atmosphere Explorer-D)	6 October 1975	Delta 2910	681	LEO	29 January 1976	12 March 1976	citation	CitationClass=web }}</ref>
55	AE-E (Atmosphere Explorer-E)	20 November 1975	Delta 2910	735	LEO	25 September 1980	10 June 1981	citation	CitationClass=web }}</ref>
—	DADE-A (Dual Air Density Explorer-A)	5 December 1975	Scout F-1	40	failed	—	—	citation	CitationClass=web }}</ref>
—	DADE-B (Dual Air Density Explorer-B)	5 December 1975	Scout F-1	43	failed	—	—	citation	CitationClass=web }}</ref>
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<ref>{{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
57	IUE	26 January 1978	Delta 2914	669	MEO	30 September 1996	In orbit	citation	CitationClass=web }}</ref>
58	HCMM (AEM-A)	26 April 1978	Scout F	117	LEO	30 September 1980	December 22, 1981	citation	CitationClass=web }}</ref>
59	ICE (ISEE 3, ISEE-C)	12 August 1978	Delta 2914	390	Sun–Earth L1	16 September 2014	Heliocentric orbit	citation	CitationClass=web }}</ref><ref name="space-timeline" />
60	SAGE (AEM-B)	18 February 1979	Scout D-1	149	LEO	7 January 1982	11 April 1989	citation	CitationClass=web }}</ref>
61	MAGSAT (AEM-C)	30 October 1979	Scout G-1	158	LEO	6 May 1980	11 June 1980	citation	CitationClass=web }}</ref>
62	Dynamics Explorer 1 (DE-1)	3 August 1981	Delta 3913	424	MEO	28 February 1991	In orbit	citation	CitationClass=web }}</ref>
63	Dynamics Explorer 2 (DE-2)	3 August 1981	Delta 3913	420	LEO	19 February 1983	19 February 1983	citation	CitationClass=web }}</ref>
64	SME	6 October 1981	Delta 2310	145	LEO	4 April 1989	5 March 1991	citation	CitationClass=web }}</ref>
65	AMPTE-CCE	16 August 1984	Delta 3924	242	MEO	12 July 1989	In orbit	Magnetosphere research<ref name="Display">Template:Citation-attribution</ref>
66	COBE	18 November 1989	Delta 5920	2,206	LEO	23 December 1993	In orbit	citation	CitationClass=web }}</ref>
67	EUVE (BERKSAT)	7 June 1992	Delta II 6920-X	3,275	LEO	31 January 2001	30 January 2002	citation	CitationClass=web }}</ref>
68	SAMPEX	3 July 1992	Scout G-1	158	LEO	citation	CitationClass=web }}</ref>	citation	CitationClass=web }}</ref>	citation	CitationClass=web }}</ref>
69	RXTE	30 December 1995	Delta II 7920	3,200	LEO	3 January 2012	citation	CitationClass=web }}</ref>	citation	CitationClass=web }}</ref>
70	FAST	21 August 1996	Pegasus XL	187	LEO	4 May 2009	In orbit	citation	CitationClass=web }}</ref>
—	HETE 1	4 November 1996	Pegasus XL	128	LEO	—	7 April 2002	citation	CitationClass=web }}</ref>
71	ACE	25 August 1997	Delta II 7920	596	Sun–Earth L1	Operational	In L1 orbit	citation	CitationClass=web }}</ref>
72	SNOE	26 February 1998	Pegasus XL	120	LEO	13 December 2003	13 December 2003	citation	CitationClass=web }}</ref>
73	TRACE	2 April 1998	Pegasus XL	250	LEO	citation	CitationClass=web }}</ref>	In orbit	citation	CitationClass=web }}</ref>
74	SWAS	6 December 1998	Pegasus XL	288	LEO	1 September 2005	In orbit	citation	CitationClass=web }}</ref>
75	WIRE	5 March 1999	Pegasus XL	250	SSO	citation	CitationClass=web }}</ref>	citation	CitationClass=web }}</ref>	citation	CitationClass=web }}</ref>
76	TERRIERS	18 May 1999	Pegasus XL	120	Polar LEO	18 May 1999	In orbit	citation	CitationClass=web }}</ref>
77	FUSE	23 June 1999	Delta II 7320	1,400	LEO	18 October 2007	In orbit	citation	CitationClass=web }}</ref>
78	IMAGE	25 March 2000	Delta II 7326	536	Polar MEO	citation	CitationClass=web }}</ref>	In orbit	citation	CitationClass=web }}</ref>
79	HETE-2	9 October 2000	Pegasus-H	124	LEO	citation	CitationClass=web }}</ref>	In orbit	citation	CitationClass=web }}</ref>
80	WMAP	30 June 2001	Delta II 7425-10	840	Sun–Earth L2	citation	CitationClass=web }}</ref>	Heliocentric orbit	citation	CitationClass=web }}</ref>
81	RHESSI	5 February 2002	Pegasus XL	230	LEO	16 August 2018	Deorbited	citation	CitationClass=web }}</ref>
—	INTEGRAL	17 October 2002	Proton-K Blok DM-2	4,000	HEO	Operational	In orbit	citation	CitationClass=web }}</ref>
82	CHIPSat	13 January 2003	Delta II 7320-10	60	LEO	citation	CitationClass=web }}</ref>	In orbit	citation	CitationClass=web }}</ref>
83	GALEX	28 April 2003	Pegasus XL	280	LEO	citation	CitationClass=web }}</ref>	In orbit	citation	CitationClass=web }}</ref>
84	Swift	20 November 2004	Delta II 7320-10C	1,470	LEO	Operational	In orbit	citation	CitationClass=web }}</ref>
—	Suzaku (Astro E2)	10 July 2005	M-V	1,706	LEO	2 September 2015	In orbit	citation	CitationClass=web }}</ref>
—	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	citation	CitationClass=web }}</ref>
86	THEMIS B (ARTEMIS P1)	17 February 2007	Delta II 7925	77	Lunar	Operational	Lunar orbit	citation	CitationClass=web }}</ref>
87	THEMIS C (ARTEMIS P2)	17 February 2007	Delta II 7925	77	Lunar	Operational	Lunar orbit	citation	CitationClass=web }}</ref>
88	THEMIS D	17 February 2007	Delta II 7925	77	HEO	Operational	In orbit	citation	CitationClass=web }}</ref>
89	THEMIS E	17 February 2007	Delta II 7925	77	HEO	Operational	In orbit	citation	CitationClass=web }}</ref>
90	AIM	25 April 2007	Pegasus XL	197	SSO	March 2023	19 August 2024	SMEX: noctilucent cloud observation<ref>{{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
—	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	citation	CitationClass=web }}</ref>
92	WISE	14 December 2009	Delta II 7320	661	LEO	August 2024	2 November 2024	MIDEX: infrared astronomy, NEOWISE extension. Discovered first Earth trojan.<ref>{{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
93	NuSTAR	13 June 2012	Pegasus XL	350	LEO	Operational	In orbit	citation	CitationClass=web }}</ref>
94	IRIS	27 June 2013	Pegasus XL	183	SSO	Operational	In orbit	citation	CitationClass=web }}</ref>
—	Hitomi (NeXT, ASTRO-H)	7 February 2016	H-2A-202	2,700	LEO	26 March 2016	In orbit	citation	CitationClass=web }}</ref>
—	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

File:WISE artist concept (PIA17254, crop).jpg

WISE was restarted after it was turned off

File:Thor-Able III Explorer 6.jpg

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

File:ISEE-C (ISEE 3) in dynamic test chamber.jpg

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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.Template:Citation needed

Examples of missions that were not developed or cancelled were:<ref name=skyrocket/>

• Owl 1 and 2 (cost, 1965)<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref>

• MSS A (Magnetic Storm Satellite, Explorer-A, 1970)<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

• CATSAT (STEDI 3) (cost)<ref name="nap" />
• IMEX (UNEX 2) (cost)<ref name="nap" />
• FAME (MIDEX 4)
• SPIDR (SMEX 8) (technical, 2003)<ref name="nap" />
• 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:<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Template:Bar graph

See also

• Cosmic Vision, a European Space Agency (ESA) programme
• Cosmic Vision S-class missions, the European Space Agency equivalent to the Small Explorers Program
• Template:Annotated link
• Template:Annotated link

External links

• {{#invoke:citation/CS1|citation

|CitationClass=web }}

• 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