View source for Explorers Program

{{Short description|Ongoing NASA space exploration program}}
{{Use American English|date=October 2021}}
{{Use dmy dates|date=October 2021}}

[[File:Pointing X-ray Eyes at our Resident Supermassive Black Hole.jpg|thumb|upright=1.0|right|An Explorer mission observes [[Sagittarius A*]], the [[Milky Way|Milky Way's]] central [[black hole]], flaring.]]

The '''Explorers Program'''<ref>{{cite web |title=Explorers Program |url=https://explorers.gsfc.nasa.gov/ |website=Explorers Program Home Page at NASA Goddard |publisher=NASA |access-date=3 May 2022}} {{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 (rocket family)|Thor]], [[Scout (rocket family)|Scout]], [[Delta (rocket family)|Delta]] and [[Northrop Grumman Pegasus|Pegasus]] launch vehicles, and [[Falcon 9]].{{fact|date=October 2025}}

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.{{fact|date=October 2025}}

== History ==
=== Early Explorer satellites ===
[[File:Ignition of Jupiter-C with Explorer 1.jpg|thumb|upright=1.0|right|Launch of Explorer 1 on the Juno I launch vehicle.]]
[[File: Explorer1.jpg|thumb|upright=1.0|right|Explorer 1, the first Earth satellite orbited by the United States]]

The Explorers Program began as a [[United States Army|U.S. Army]] proposal ([[Project Orbiter]]) to place a "civilian" [[satellite|artificial satellite]] into orbit during the [[International Geophysical Year]] (IGY). Although that proposal was rejected in favor of the [[United States Navy|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>{{citation-attribution|1=J. Boehm, H.J. Fichtner and Otto A. Hoberg, [https://www.nasa.gov/sites/default/files/atoms/files/explorer_i_boehm_document.pdf EXPLORER SATELLITES LAUNCHED BY JUNO 1 AND JUNO 2 VEHICLES] NASA Report}}</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 [[United States Army|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>{{cite magazine |last=Portree |first=David S. F. |url=https://www.wired.com/2013/05/cometary-explorer-1973/ |title=Cometary Explorer (1973) |magazine=Wired |date=22 May 2013 |access-date=24 June 2019 |url-access=limited}}</ref><ref>{{cite web |url=https://www.nasa.gov/content/goddard-missions-past |title=Goddard Missions |work=Goddard Space Flight Center |publisher=NASA |date=5 June 2018 |access-date=24 June 2019}} {{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 [[MOSFET]]s (MOS transistors).<ref name="Butrica">{{cite book |last1=Butrica |first1=Andrew J. |chapter=Chapter 3: NASA's Role in the Manufacture of Integrated Circuits |editor-last1=Dick |editor-first1=Steven J. |title=Historical Studies in the Societal Impact of Spaceflight |date=2015 |publisher=NASA |isbn=978-1-62683-027-1 |pages=149-250 (237-242) |chapter-url=https://www.nasa.gov/sites/default/files/atoms/files/historical-studies-societal-impact-spaceflight-ebook_tagged.pdf#page=237}} {{PD-notice}}</ref><ref name="nasa">{{cite book |title=Interplanetary Monitoring Platform |date=29 August 1989 |publisher=NASA |pages=1, 11, 134 |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19800012928.pdf |access-date=12 August 2019 |last1=Butler |first1=P. M.}} {{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"/>
{| class="wikitable sortable"
|+List of [[Interplanetary Monitoring Platform]] (IMP) missions
! rowspan="2" |Mission
! rowspan="2" |Photo
! colspan="2" |Satellite
! rowspan="2" |Launch date
! rowspan="2" |Decay date
! rowspan="2" |Notes
|-
!Explorer
!IMP
|-
|[[Explorer 18|IMP-1]]
|[[File:Explorer-18_IMP-A.jpg|frameless|180x180px]]
|Explorer 18
|IMP-A
|27 November 1963, 02:30 UTC<ref name="jonathan">{{cite web |last=McDowell |first=Jonathan |title=Launch Log |url=http://www.planet4589.org/space/log/launchlog.txt |access-date=2018-06-24 |work=Jonathan's Space Page}}</ref>
|30 December 1965
|First use of [[integrated circuit]]s in a spacecraft. First satellite in IMP-A/-B/-C design series.
|-
|[[Explorer 21|IMP-2]]
|[[File:Explorer-21 image.jpg|frameless|180x180px]]
|Explorer 21
|IMP-B
|4 October 1964, 03:45 UTC<ref name="jonathan" />
|1 January 1966
|Second satellite in IMP-A/-B/-C design series.
|-
|[[Explorer 28|IMP-3]]
|[[File:Explorer 28.jpg|frameless|180x180px]]
|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.
|-
|[[Explorer 33|AIMP-1]]
|[[File:IMP-D.jpg|frameless|180x180px]]
|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.
|-
|[[Explorer 34|IMP-4]]
|[[File:Explorer 34.jpg|frameless|180x180px]]
|Explorer 34
|IMP-F
|24 May 1967, 14:05 UTC<ref name="jonathan" />
|3 May 1969
|Similar design to IMP-G.
|-
|[[Explorer 35|AIMP-2]]
|[[File:IMP-E.jpg|frameless|180x180px]]
|Explorer 35
|IMP-E
|19 July 1967, 14:19 UTC<ref name="jonathan" />
|After 24 June 1973
|Similar design to IMP-D. Positioned in [[Moon|Selenocentric orbit]].
|-
|[[Explorer 41|IMP-5]]
|[[File:Explorer-41 IMP-G.jpg|frameless|180x180px]]
|Explorer 41
|IMP-G
|21 June 1969, 08:47 UTC<ref name="jonathan" />
|23 December 1972
|Similar design to IMP-F.
|-
|[[Explorer 43|IMP-6]]
|[[File:Explorer 43 IMP-I.jpg|frameless|180x180px]]
|Explorer 43
|IMP-I
|13 March 1971, 16:15 UTC<ref name="jonathan" />
|2 October 1974
|First spacecraft in IMP-I/-H/-J series.
|-
|[[Explorer 47|IMP-7]]
|[[File:Explorer-47 IMP-I.jpg|frameless|180x180px]]
|Explorer 47
|IMP-H
|23 September 1972, 01:20 UTC<ref name="jonathan" />
|In orbit
|Second spacecraft in IMP-I/-H/-J series.
|-
|[[Explorer 50|IMP-8]]
|[[File:IMP_8.gif|frameless|180x180px]]
|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 [[Gravitational field|gravity field]]; the [[solar wind]]; properties of [[micrometeoroids]] raining down on the [[Earth]]; ultraviolet, cosmic and X-rays from the [[Solar System]] and beyond; [[Ionosphere|ionospheric physics]]; [[Solar flare|Solar plasma]]; [[solar energetic particles]]; and [[Atmospheric science|atmospheric physics]]. These missions have also investigated air density, radio astronomy, [[geodesy]], and [[gamma-ray astronomy]].{{citation needed|date=September 2024}}

With decreases in NASA's budget, Explorer missions became infrequent in the early 1980s.{{citation needed|date=September 2024}}

=== 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>{{cite book |last=Rumerman |first=Judy A. |url=https://history.nasa.gov/SP-4012v7ch4.pdf |title=NASA Historical Data Book, Vol. VII: NASA Launch Systems, Space Transportation, Human Spaceflight, and Space Science, 1989-1998 |publisher=NASA |date=2009 |access-date=24 June 2019}} {{PD-notice}}</ref><ref>{{cite web |url=https://www.nasa.gov/centers/goddard/pdf/106477main_smex.pdf |title=NASA's Small Explorers Program: Faster, Better, Cheaper |work=Goddard Space Flight Center |publisher=NASA |date=January 1998 |access-date=24 June 2019 |archive-date=16 February 2017 |archive-url=https://web.archive.org/web/20170216162632/https://www.nasa.gov/centers/goddard/pdf/106477main_smex.pdf |url-status=dead}} {{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>{{cite conference |title=SAMPEX: NASA's First Small Explorer Satellite |conference=IEEE Aerospace Conference 21–28 March 1998 Aspen, Colorado |first1=G. M. |last1=Mason |first2=D. N. |last2=Baker |first3=J. B. |last3=Blake |first4=R. E. |last4=Boughner |first5=L. B. |last5=Callis |display-authors=et al. |volume=5 |pages=389–412 |date=1998 |doi=10.1109/AERO.1998.685848}}</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|thumb|upright=1.0|right|This artificially colored view of M101 maps ultraviolet light as blue while visible light is red since [[Ultraviolet|UV]] light does not have a "color" (the eye stopping at about violet). This view was taken by the MIDEX-3 [[Neil Gehrels Swift Observatory|Swift]], which can also detect X-rays, and has contributed to the study of [[gamma-ray burst]]s 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>{{cite web |author=Ed Kyle |url=http://www.spacelaunchreport.com/delta2.html |archive-url=https://web.archive.org/web/20100325164354/http://www.spacelaunchreport.com/delta2.html |url-status=usurped |archive-date=25 March 2010 |title=Delta II Data Sheet |publisher=Spacelaunchreport.com |access-date=2018-04-28}}</ref><ref>{{cite magazine |url=http://aviationweek.com/awin/nasa-taps-mcdonnell-med-lite-launches |title=NASA Taps Mcdonnell For Med-Lite Launches |magazine=Aviation Week |date=4 March 1996 |access-date=28 April 2018}}</ref> The first announcement opportunity for MIDEX was issued in March 1995, and the first launch under this new class was [[Far Ultraviolet Spectroscopic Explorer|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>{{cite web |url=https://web.stanford.edu/~sbuchman/publications-PDF/The%20Large%20Benefits%20of%20Small%20Satellite%20Missions.pdf |title=The Large Benefits of Small Satellite Missions |access-date=2018-04-28}}</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">{{cite web |url=http://sunland.gsfc.nasa.gov/smex/ |title=Welcome to the Small Explorer's Web Site |publisher=NASA |date=18 February 2000 |url-status=dead |archive-url=https://web.archive.org/web/20000817054104/http://sunland.gsfc.nasa.gov/smex/ |archive-date=17 August 2000}} {{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>{{cite web |url=http://explorers.gsfc.nasa.gov/history.html |title=Explorers Program |website=explorers.gsfc.nasa.gov |date=1958-01-31 |access-date=2016-02-24 |archive-url=https://web.archive.org/web/20160304052517/http://explorers.gsfc.nasa.gov/history.html |archive-date=4 March 2016 |url-status=dead}} {{PD-notice}}</ref>

Excluding the launches, the MIDEX class has a current mission cap cost of US$250 million in 2018,<ref name=Midex19>{{cite web |url=https://www.nasa.gov/press-release/nasa-selects-proposals-to-study-galaxies-stars-planets |title=NASA Selects Proposals to Study Galaxies, Stars, Planets |date=9 August 2017 |publisher=NASA |access-date=28 April 2018}} {{PD-notice}}</ref> with future MIDEX missions being capped at US$350 million.<ref>{{cite web |author=Jeff Foust |url=http://spacenews.com/earth-science-decadal-report-recommends-mix-of-large-and-small-missions/ |title=Earth science decadal report recommends mix of large and small missions |date=5 January 2018 |publisher=SpaceNews |access-date=28 April 2018}}</ref> The cost cap for SMEX missions in 2017 was US$165 million.<ref>{{cite web |last1=Wu |first1=Chauncey |last2=Manuel |first2=Greg |last3=Salas |first3=Andrea |url=https://explorers.larc.nasa.gov/HPSMEX/pdf_files/05-2016_Helio_PPC_TMC_Wu_v2.pdf |title=2016 Heliophysics Small Explorers (SMEX) & Mission of Opportunity (MO) Solicitations Pre-Proposal Conference |publisher=NASA |date=15 August 2016 |access-date=24 June 2019}} {{PD-notice}}</ref> UNEX missions are capped at US$15 million.<ref name=missions>{{cite web |url=http://explorers.gsfc.nasa.gov/missions.html |title=Explorers Missions |publisher=NASA |access-date=28 August 2015 |url-status=dead |archive-url=https://web.archive.org/web/20100323182500/http://explorers.gsfc.nasa.gov/missions.html |archive-date=23 March 2010}} {{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) ===
{| class="wikitable"
|+List of MIDEX missions<ref>{{cite web |url=http://explorers.gsfc.nasa.gov/midex.html |title=Explorers Program |website=explorers.gsfc.nasa.gov |access-date=8 April 2016 |archive-url=https://web.archive.org/web/20160323014953/http://explorers.gsfc.nasa.gov/midex.html |archive-date=23 March 2016 |url-status=dead}} {{PD-notice}}</ref><ref name=":0" /><ref>{{cite web |url=https://explorers.larc.nasa.gov/MIDEX/MIDEX.html |title=Medium-class Explorers (MIDEX) |publisher=Explorers.larc.nasa.gov |access-date=28 April 2018}} {{PD-notice}}</ref> 
|-
! Name
! MIDEX<br/>number
! Explorer<br/>number
! Launch (UTC)
! Status
|-
| [[Rossi X-ray Timing Explorer|RXTE]]
| 
| Explorer-69
| 30 December 1995
| Ended in 2012 / Reentered on 30 April 2018 
|-
| [[Advanced Composition Explorer|ACE]]
| 
| Explorer-71
| 25 August 1997
| {{success|'''Operational'''}}
|-
| [[Far Ultraviolet Spectroscopic Explorer|FUSE]]
| MIDEX-0
| Explorer-77
| 23 June 1999
| Ended in 2007
|-
| [[IMAGE (spacecraft)|IMAGE]]
| MIDEX-1
| Explorer-78
| 25 March 2000
| Lost contact in 2005. Partial contact reestablished in January 2018
|-
| [[Wilkinson Microwave Anisotropy Probe|WMAP]]
| MIDEX-2
| Explorer-80
| 30 June 2001
| Ended in 2010
|-
| [[Neil Gehrels Swift Observatory|Swift]]
| MIDEX-3
| Explorer-84
| 20 November 2004
| {{success|'''Operational'''}}
|-
| [[Full-sky Astrometric Mapping Explorer|FAME]]
| MIDEX-4
| {{center|—}}
| ''Scheduled for 2004''
| Cancelled in 2002 (cost)
|-
| [[THEMIS (satellite)|THEMIS]] A
| MIDEX-5A
| Explorer-85
| 17 February 2007
| {{success|'''Operational'''}}
|-
| [[THEMIS (satellite)|THEMIS]] B
| MIDEX-5B
| Explorer-86
| 17 February 2007
| {{success|'''Operational'''}}
|-
| [[THEMIS (satellite)|THEMIS]] C
| MIDEX-5C
| Explorer-87
| 17 February 2007
| {{success|'''Operational'''}}
|-
| [[THEMIS (satellite)|THEMIS]] D
| MIDEX-5D
| Explorer-88
| 17 February 2007
| {{success|'''Operational'''}}
|-
| [[THEMIS (satellite)|THEMIS]] E
| MIDEX-5E
| Explorer-89
| 17 February 2007
| {{success|'''Operational'''}}
|-
| [[Wide-field Infrared Survey Explorer|WISE / NEOWISE]] 
| MIDEX-6
| Explorer-92
| 14 December 2009
| Mission operations Completed on 31 July 2024.<ref>{{Cite web|url=https://www.nasa.gov/missions/neowise/nasas-neowise-infrared-heritage-will-live-on/|title=NASA’s NEOWISE Infrared Heritage Will Live On - NASA|date=1 July 2024}}</ref> Reentered on 2 November 2024

|-
| [[Transiting Exoplanet Survey Satellite|TESS]]
| MIDEX-7
| Explorer-95
| 18 April 2018
| {{success|'''Operational'''}}
|-
| [[Ionospheric Connection Explorer|ICON]]
| MIDEX-8
| Explorer-96
| 11 October 2019 
| {{success|'''Operational'''}}
|-
| [[SPHEREx]]
| MIDEX-9<!--The sequential number, but needs a reference-->
| Explorer-102
| 12 March 2025 
|  {{success|'''Operational'''}}
|-
| [[Multi-slit Solar Explorer|MUSE]]
| MIDEX-10<!--The sequential number, but needs a reference-->
| 
| 2027 
| {{pending|In development}}
|-
| [[HelioSwarm]]
| MIDEX-11<!--The sequential number, but needs a reference-->
| 
| 2028 
| {{pending|In development}}
|-
| [[UVEX]]
| MIDEX-12<!--The sequential number, but needs a reference-->
| 
| 2030 
| {{pending|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}}.<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|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>{{cite web |url=https://www.nasa.gov/press-release/nasa-selects-proposals-to-study-sun-space-environment |title=NASA Selects Proposals to Study Sun, Space Environment |publisher=NASA |first=Dwayne |last=Brown |date=28 July 2017 |access-date=7 December 2017}} {{PD-notice}}</ref><ref>{{cite web |url=https://nspires.nasaprs.com/external/viewrepositorydocument/cmdocumentid=524225/solicitationId=%7BA0C496AC-9B9D-8F7D-A506-B1695BF9BDE8%7D/viewSolicitationDocument=1/2016%20Helio%20SMEX%20AO_amend1_clarify.pdf |title=Announcement of Opportunity: Heliophysics Explorers Program, 2016 Small Explorers (SMEX) |publisher=NASA |date=13 July 2016 |id=NNH16ZDA005O}} {{PD-notice}}</ref><ref>{{cite web |url=https://explorers.larc.nasa.gov/HPSMEX/pdf_files/8_SMEX-AO-2016Helio-CSR-Kickoff-LSP-MENDOZA-HILL.pdf |title=Heliophysics Small Explorers 2016 Announcement of Opportunity: Concept Study Report Kickoff |publisher=NASA{{\}}Launch Services Program |first=Alicia |last=Mendoza-Hill |date=25 August 2017}} {{PD-notice}}</ref> In June 2019 NASA selected TRACERS and PUNCH for flight.<ref>{{cite web |url=https://www.nasa.gov/press-release/nasa-selects-missions-to-study-our-sun-its-effects-on-space-weather |title=NASA Selects Missions to Study Our Sun, Its Effects on Space Weather |publisher=NASA |date=20 June 2019 |url-status=live |archive-url=https://web.archive.org/web/20231031190231/https://www.nasa.gov/news-release/nasa-selects-missions-to-study-our-sun-its-effects-on-space-weather/ |archive-date=31 October 2023}} {{PD-notice}}</ref>

{| class="wikitable"
|+List of SMEX missions <ref name=":0">{{cite web |url=http://space.skyrocket.de/doc_sat/explorer.htm |title=Explorers Program |website=space.skyrocket.de |access-date=8 April 2016}}</ref><ref>{{cite web |url=http://explorers.gsfc.nasa.gov/smex.html |title=Operational Small Explorers (SMEX) Missions |website=Explorers Program |access-date=8 April 2016 |archive-url=https://web.archive.org/web/20160325193350/http://explorers.gsfc.nasa.gov/smex.html |archive-date=25 March 2016 |url-status=dead}} {{PD-notice}}</ref>
|-
! Name
! SMEX<br/>number
! Explorer<br/>number
! Launch (UTC)
! End of<br/>mission
! Status
|-

| [[Solar Anomalous and Magnetospheric Particle Explorer|SAMPEX]]
| SMEX-1
| Explorer-68
| 3 July 1992
| 30 June 2004
| Reentered on 13 November 2012
|-

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| [[TOMS]]-EP
| 
| 
| 2 July 1996
| <ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1996-037A |title=NASA - NSSDCA - Spacecraft |publisher=NASA |date=21 March 2017 |access-date=20 April 2018}} {{PD-notice}}</ref>
-->
|-

| [[Fast Auroral Snapshot Explorer|FAST]]
| SMEX-2
| Explorer-70
| 21 August 1996
| 4 May 2009
| 
|-

| [[Submillimeter Wave Astronomy Satellite|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<ref>{{Cite web |last=McDowell |first=Jonathan |date=18 July 2025 |title=NASA's TRACE solar observatory, which operated from 1998 to 2010,  reentered over the ocean 600 km south of Perth, W Australia at 1137 UTC  Jul 18. |url=https://x.com/planet4589/status/1946352736775135363}}</ref>
|-

| [[Wide Field Infrared Explorer|WIRE]]
| SMEX-5
| Explorer-75
| 5 March 1999
| {{n/a}}
| Spacecraft equipment failure; reentered on 10 May 2011
|-

| [[Reuven Ramaty High Energy Solar Spectroscopic Imager|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
| {{center|—}}
| ''Scheduled for 2005''
| {{n/a}}
| Cancelled in 2003 due to poor instrument sensitivity <ref>{{cite web |url=http://www.spacetoday.net/Summary/1695 |title=NASA cancels space science mission |website=spacetoday.net |date=4 June 2003 |access-date=28 April 2018 |url-status=live |archive-url=https://web.archive.org/web/20190427111301/http://www.spacetoday.net/Summary/1695 |archive-date=27 April 2019}}</ref>
|-

| [[Aeronomy of Ice in the Mesosphere]] (AIM)
| SMEX-9
| Explorer-90
| 25 April 2007
| 19 August 2024
| Reentered on 19 August 2024
|-

| [[Interstellar Boundary Explorer|IBEX]]
| SMEX-10
| Explorer-91
| 19 October 2008
| 
| {{success|'''Operational'''}}
|-

| [[NuSTAR]]
| SMEX-11
| Explorer-93
| 13 June 2012
| 
| {{success|'''Operational'''}}
|-

| [[Interface Region Imaging Spectrograph|IRIS]]
| {{nowrap|SMEX-12}}
| {{nowrap|Explorer-94}}
| 28 June 2013
| 
| {{success|'''Operational'''}}
|-

| [[Gravity and Extreme Magnetism|GEMS]]
| SMEX-13
| {{center|—}}
| ''Scheduled for 2014''
| {{n/a}}
| Cancelled in 2012 due to expected cost overruns<ref>{{cite web |last1=Berger |first1=Brian |first2=Dan |last2=Leone |agency=Space News |url=https://www.space.com/16041-gems-spacecraft-nasa-cancellation.html |title=GEMS Spacecraft Team Appeals NASA Cancellation Decision |publisher=Space.com |date=7 June 2012 |access-date=28 April 2018}}</ref>
|-

| [[Imaging X-ray Polarimetry Explorer]] (IXPE)
| SMEX-14
| {{nowrap|Explorer-97}}
| 9 December 2021
| 
| {{success|'''Operational'''}}
|-

| [[Polarimeter to Unify the Corona and Heliosphere|PUNCH]]
| SMEX-15
| 
| 12 March 2025<ref name="nasa-lspedu">{{cite web |url=https://public.ksc.nasa.gov/lspeducation/upcoming-missions/ |title=Upcoming Missions |work=[[NASA Launch Services Program]] |date=1 July 2024 |access-date=3 August 2024}}</ref>
| 
| {{success|'''Operational'''}}<ref name="nasa-20220803">{{cite web |last=Oxford |first=Clarence |url=https://www.spacedaily.com/reports/NASAs_PUNCH_Mission_Commences_Study_of_Solar_Wind_999.html |title=NASA's PUNCH Mission Commences Study of Solar Wind |date=13 March 2025 |access-date=19 March 2025 |work=Space Daily}} {{PD-notice}}</ref>
|-

| [[Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites|TRACERS]]
| SMEX-16
| 
| 23 July 2025
| 
| {{pending|Spacecraft commissioning}}
|-

| [[Compton Spectrometer and Imager|COSI]]
| SMEX-17
| 
| August 2027<ref>{{cite press release |url=https://www.nasa.gov/news-release/nasa-awards-launch-services-contract-for-space-telescope-mission/ |title=NASA Awards Launch Services Contract for Space Telescope Mission |work=NASA |date=2 July 2024 |access-date=3 July 2024}} {{PD-notice}}</ref>
| 
| {{pending|In development}}
|}

<gallery>
File:SAMPEX 3.jpg|SAMPEX
File:RHESSI.jpg|RHESSI
File:IBEX spacecraft.jpg|IBEX
File:IXPE-artist-rendition.jpg|IXPE
</gallery>

=== University-Class Explorers (UNEX) ===
{| class="wikitable"
|+List of UNEX missions<ref name=":1">{{cite web |url=http://explorers.gsfc.nasa.gov/unex_mo_intern.html |title=Explorers Program |website=explorers.gsfc.nasa.gov |access-date=2016-04-08 |archive-url=https://web.archive.org/web/20160322091118/http://explorers.gsfc.nasa.gov/unex_mo_intern.html |archive-date=22 March 2016 |url-status=dead}} {{PD-notice}}</ref>
|-
! Name
! UNEX number
! Explorer<br/>number
! Launch (UTC)
! Status
|-
| [[Student Nitric Oxide Explorer|SNOE]]
| UNEX-1
| Explorer-72
| 26 February 1998
| Ended in 2000; decayed from orbit in December 2003
|-
| IMEX
| UNEX-2
| {{center|—}}
| ''Scheduled for 2003''
| Cancelled before 2005 (cost)<ref name="nap">{{cite book |url=https://www.nap.edu/read/11530/chapter/8#54 |title=6 Lessons Learned from PI-Led Mission Experiences &#124; Principal-Investigator-Led Missions in the Space Sciences &#124; The National Academies Press |doi=10.17226/11530 |publisher=Nap.edu |date=2006 |isbn=978-0-309-10070-0 |access-date=28 April 2018}}</ref>

|-
| [[Cosmic Hot Interstellar Plasma Spectrometer|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>[https://explorers.gsfc.nasa.gov/missions.html Explorers Missions: Missions of Opportunity (MO)] NASA [[Goddard Space Flight Center]] (GSFC). Accessed on 18 August 2019. {{PD-notice}}</ref>

{| class="wikitable"
|+List of MO<ref name=":1" />
|-
! Name
! Launcher (mission)
! Launch (UTC)
! Status
|-
| [[HETE-2]]
| NASA (Explorer-79)
| 9 October 2000
| Ended in 2008<ref>{{cite web |url=https://heasarc.gsfc.nasa.gov/docs/hete2/hete2.html |title=The HETE-2 Satellite |publisher=NASA |access-date=28 April 2018}} {{PD-notice}}</ref>
|-
| [[INTEGRAL]]
| [[ESA]]<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2002-048A |title=NASA - NSSDCA - Spacecraft |publisher=NASA |access-date=20 April 2018}} {{PD-notice}}</ref>
| 17 October 2002
| {{success|'''Operational'''}}
|-
| [[Suzaku (satellite)|Suzaku]] (Astro-E2)
| [[JAXA]]<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2005-025A |title=NASA - NSSDCA - Spacecraft |publisher=NASA |date=21 March 2017 |access-date=20 April 2018}} {{PD-notice}}</ref>
| 10 July 2005
| Ended in 2015
|-
| [[TWINS]]
| [[National Reconnaissance Office|NRO]] ([[USA-184]];<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2006-027A |title=NASA - NSSDCA - Spacecraft |publisher=NASA |access-date=2018-04-20}} {{PD-notice}}</ref><br/>[[USA-200]]<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2008-010A |title=NASA - NSSDCA - Spacecraft |publisher=NASA |access-date=2018-04-20}} {{PD-notice}}</ref>)
| TWINS-1: 28 June 2006<br/>TWINS-2: 13 March 2008
| {{success|'''Operational'''}}
|-
| [[CINDI]]
| [[DoD]] ([[C/NOFS]])<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2008-017A |title=NASA - NSSDCA - Spacecraft |publisher=NASA |date=2017-03-21 |access-date=2018-04-20}} {{PD-notice}}</ref>
| 16 April 2008
| Ended in 2015
|-
| [[Hitomi (satellite)|Hitomi]] (Astro-H)
| [[JAXA]]<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2016-012A |title=NASA - NSSDCA - Spacecraft |publisher=NASA |date=2017-03-21 |access-date=2018-04-20}} {{PD-notice}}</ref>
| 17 February 2016 
| {{failure|Failed}}
|-
| [[Neutron Star Interior Composition Explorer|NICER]]
| [[ISS]] ([[SpaceX CRS-11|CRS-11]])
| 3 June 2017
| {{success|'''Operational'''}}
|-
| [[Global-scale Observations of the Limb and Disk|GOLD]]
| [[SES (company)|SES]] ([[SES-14]])
| 25 January 2018
| {{success|'''Operational'''}}
|-
| [[X-Ray Imaging and Spectroscopy Mission|XRISM]]
| [[JAXA]]
| 6 September 2023
| {{success|'''Operational'''}}
|-
| [[Atmospheric Waves Experiment|AWE]]
| [[ISS]] ([[SpaceX CRS-29|CRS-29]])
| December 2023
| {{success|'''Operational'''}}
|-
| [[GUSTO (telescope)|GUSTO]]
| [[NASA]] ([[high-altitude balloon]])
| 31 December 2023
| Ended on 26 February 2024<ref>{{cite web |last=Littleton |first=Olivia F. |url=https://www.nasa.gov/missions/scientific-balloons/nasa-scientific-balloons-ready-for-flights-over-antarctica/ |title=NASA Scientific Balloons Ready for Flights Over Antarctica |work=[[NASA]] |date=27 February 2024 |access-date=3 August 2024}}</ref>
|-
| [[Sun Radio Interferometer Space Experiment|SunRISE]]
| [[NASA]] ([[Maxar]] satellite)
| 2024
| {{pending|In development<ref>{{cite web |last=Ng |first=Joy |url=https://blogs.nasa.gov/sunspot/2021/09/09/nasas-sunrise-mission-studying-solar-particle-storms-moves-toward-launch/ |title=NASA's SunRISE Mission Studying Solar Particle Storms Moves Toward Launch |work=NASA |date=9 September 2021 |access-date=9 September 2021}} {{PD-notice}}</ref><ref>{{cite press release |last1=Hautaluoma |first1=Grey |last2=Fox |first2=Karen |url=https://www.nasa.gov/press-release/nasa-selects-mission-to-study-causes-of-giant-solar-particle-storms |title=NASA Selects Mission to Study Causes of Giant Solar Particle Storms |publisher=NASA |date=30 March 2020 |access-date=31 March 2020}} {{PD-notice}}</ref>}}
|-
| [[Electrojet Zeeman Imaging Explorer|EZIE]]
| [[NASA]], [[JHUAPL]]
| 2024
| {{success|'''Operational'''}}
|-
| [[Solar-C EUVST]]
| [[JAXA]]
| July 2028
| {{pending|In development<ref>{{cite press release |last1=Hautaluoma |first1=Grey |last2=Frazier |first2=Sarah |url=https://www.nasa.gov/press-release/nasa-approves-heliophysics-missions-to-explore-sun-earth-s-aurora |title=NASA Approves Heliophysics Missions to Explore Sun, Earth's Aurora |publisher=NASA |date=29 December 2020 |access-date=30 December 2020}} {{PD-notice}}</ref><ref>{{cite conference |last=Shimizu |first=Toshifumi |url=https://agu.confex.com/agu/fm23/meetingapp.cgi/Paper/1295860 |title=SH54A-03 The SOLAR-C EUVST mission: Coronal physics advanced by novel EUV spectroscopy |conference=American Geophysical Union (AGU23) |date=15 December 2023 |access-date=26 December 2023}}</ref>}}
|-
| [[Contribution to ARIEL Spectroscopy of Exoplanets|CASE]]
| [[ESA]] ([[Cosmic Vision]] [[ARIEL|M4]])
| 2029
| {{pending|In development<ref>{{cite web |last=Landau |first=Elizabeth |url=https://www.nasa.gov/feature/jpl/nasa-instrument-to-probe-planet-clouds-on-european-mission |title=NASA Instrument to Probe Planet Clouds on European Mission |publisher=NASA |date=8 November 2019 |access-date=12 November 2019}} {{PD-notice}}</ref><ref>{{Cite journal |arxiv=1906.02820 |doi=10.1088/1538-3873/ab2d54 |title=Constraining Exoplanet Metallicities and Aerosols with the Contribution to ARIEL Spectroscopy of Exoplanets (CASE) |journal=Publications of the Astronomical Society of the Pacific |volume=131 |issue=1003 |pages=094401 |year=2019 |last1=Zellem |first1=Robert T. |display-authors=et al. |bibcode=2019PASP..131i4401Z |s2cid=174801052}} {{PD-notice}}</ref>}}
|}

=== Beacon Explorers{{anchor|Beacon}} ===
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 [[NASA Space Science Data Coordinated Archive|NSSDC master catalog]], typically assigned to each spacecraft in a mission. These numbers were not officially assigned until after 1975.<ref>{{cite web |url=http://www.planet4589.org/space/misc/explorer.html |title=Explorers Program |website=planet4589.org |access-date=2016-02-24 |archive-url=https://web.archive.org/web/20160303194728/http://www.planet4589.org/space/misc/explorer.html |archive-date=2016-03-03 |url-status=dead}}</ref>

{| class="wikitable sortable" style="margin:1em auto;"
|+ '''Explorers Program satellites'''<ref name= "nssdc_list">{{cite web |url=https://nssdc.gsfc.nasa.gov/multi/explorer.html |title=NASA's Explorers Program Satellites |publisher=National Space Science Data Center |access-date=2018-04-20}} {{PD-notice}}</ref><ref>{{cite web |url=http://www.planet4589.org/space/misc/explorer.html |title=Explorers Program |publisher=Planet4589.org |access-date=2018-04-20 |archive-url=https://web.archive.org/web/20160303194728/http://www.planet4589.org/space/misc/explorer.html |archive-date=2016-03-03 |url-status=dead}}</ref><ref>{{cite web |url=https://history.nasa.gov/explorer.html |title=Explorers Spacecraft Series |publisher=History.nasa.gov |access-date=2018-04-20}} {{PD-notice}}</ref><ref name=skyrocket>{{cite web |url=http://space.skyrocket.de/doc_sat/explorer.htm |title=Explorers Program |website=space.skyrocket.de |access-date=2016-02-24}}</ref> 
|-

! No.
! Name(s)
! Launch date (UTC)
! Rocket
! Mass (kg)
! Orbit regime
! End of data
! Re-entry<br><ref>{{cite web |url=http://www.unoosa.org/oosa/osoindex/search-ng.jspx?match=explorer |title=Search OSOidx |publisher=Unoosa.org |date=2018-04-23 |access-date=2018-04-28}}</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 [[United States Army|U.S. Army]]<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1958-001A |title=Explorer 1 (1958-001A) |publisher=NASA |date=7 January 2022 |access-date=12 February 2022}} {{PD-notice}}</ref>
|- style="background:#fbb;"

! style="background:#fbb;"| 2
| [[Explorer 2]]
| 5 March 1958
| [[Juno I]]
| 15
| ''Failed''
| —
| —
| Failed to achieve orbit.<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=EXPLR2 |title=Explorer 1 (EXPLR2) |publisher=NASA |date=7 January 2022 |access-date=12 February 2022}} {{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>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1958-003A |title=Explorer 3 (1958-003A) |publisher=NASA |date=7 January 2022 |access-date=12 February 2022}} {{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>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1958-005A |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |access-date=2018-04-20}} {{PD-notice}}</ref>
|- style="background:#fbb;"

! style="background:#fbb;"| 5
| [[Explorer 5]]
| 24 August 1958
| [[Juno I]]
| 17
| ''failed''
| —
| —
| Planned in conjunction with Explorer 4, but launch failed<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=EXPLR5 |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |access-date=2018-04-20}} {{PD-notice}}</ref>
|- style="background:#fbb;"

! style="background:#fbb;"|  —
| [[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<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=EXP-7X |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |access-date=2018-04-20}} {{PD-notice}}</ref>
|-

! 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<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1959-004A |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |access-date=2018-04-20}} {{PD-notice}}</ref><ref name="space-timeline">{{cite web |url=https://www.space.com/4422-timeline-50-years-spaceflight.html |title=Timeline: 50 Years of Spaceflight |website=space.com |date=28 September 2012 |access-date=24 June 2019}}</ref>
|-

! 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<ref>{{cite web |url=https://www.ssec.wisc.edu/media/spotlight/explorer7.html |title=50 Year Anniversary of Explorer 7 Launch |publisher=ssec.wisc.edu |access-date=2018-04-20 |archive-date=10 June 2017 |archive-url=https://web.archive.org/web/20170610094052/http://www.ssec.wisc.edu/media/spotlight/explorer7.html |url-status=dead}}</ref><ref>{{cite web |last=Evans |first=Gareth |url=https://www.aerospace-technology.com/features/feature78185/ |title=Nasa's High-Tech Climate Monitoring |publisher=Aerospace Technology |date=2010-04-05 |access-date=2018-04-20}}</ref><ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1959-009A |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |access-date=2018-04-20 }} {{PD-notice}}</ref>
|- style="background:#fbb;"

! style="background:#fbb;"| –
| [[Explorer S-46 (satellite)|S-46A]] (IE-B)
| 23 March 1960
| [[Juno II]]
| 16
| ''failed''
| —
| —
| Analyze electron and proton radiation energies, failed to achieve orbit<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=EXS-46 |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}} {{PD-notice}}</ref>
|-

! 8
| [[Explorer 8]] (S-30)
| 3 November 1960
| [[Juno II]]
| 41
| LEO
| 27 December 1960
| 27 March 2012
| Measured atmospheric composition of the ionosphere<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1960-014A |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |access-date=2018-04-20}} {{PD-notice}}</ref>
|- style="background:#fbb;"

! style="background:#fbb;"| –
| [[S-56 (satellite)|S-56]]
| 4 December 1960
| [[Scout X-1]]
| 6
| ''failed''
| —
| —
| Atmosphere density measurement, but failed to achieve orbit<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=EXS-56 |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}} {{PD-notice}}</ref>
|-

! 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<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1961-004A |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |access-date=2018-04-20}} {{PD-notice}}</ref>
|- style="background:#fbb;"

! style="background:#fbb;"| –
| [[S-45 (satellite)|S-45]]
| 24 February 1961
| [[Juno II]]
| 34
| ''failed''
| —
| —
| Ionosphere research, but failed to achieve orbit<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=EXS-451 |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}} {{PD-notice}}</ref>
|-

! 10
| [[Explorer 10]] (P 14)
| 25 March 1961
| [[Delta (rocket family)|Thor-Delta]]
| 79
| HEO
| 25 March 1961
| 1 June 1968
| Investigated the magnetic field between the Earth and Moon<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1961-010A |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}} {{PD-notice}}</ref>
|-

! 11
| [[Explorer 11]] (S 15)
| 27 April 1961
| [[Juno II]]
| 37
| LEO
| 17 November 1961
| In orbit
| Gamma ray astronomy<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1961-013A |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}} {{PD-notice}}</ref>
|- style="background:#fbb;"

! style="background:#fbb;"| –
| [[S-45A]]
| 25 May 1961
| [[Juno II]]
| 34
| ''failed''
| —
| —
| Ionosphere research, failed to achieve orbit. Last Juno II launch.<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=EXS-452 |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}} {{PD-notice}}</ref>
|- style="background:#fbb;"

! style="background:#fbb;"| –
| [[S-55 (satellite)]] (Meteoroid Satellite-A, Micrometeorite Explorer)
| 30 June 1961
| [[Scout (rocket)|Scout X-1]]
| 85
| ''failed''
| —
| —
| Micrometeoroid research, failed to achieve orbit<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=EXS-55 |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}} {{PD-notice}}</ref>
|-

! 12
| [[Explorer 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<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1961-020A |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |access-date=2018-04-20}} {{PD-notice}}</ref>
|- style="background: #ffb;"

! style="background: #FFB;" | 13
| [[Explorer 13]] (S-55A)
| 25 August 1961
| [[Scout X-1]]
| 86
| LEO
| 28 August 1961
| 28 August 1961
| Micrometeoroid research; partial failure<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1961-022A |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |access-date=2018-04-20}} {{PD-notice}}</ref>
|-

! 14
| [[Explorer 14|EPE-B]] (Energetic Particle Explorer-B)
| 2 October 1962
| [[Delta A]]
| 40
| HEO
| 11 August 1963
| 1 July 1966
| Energetic particle research<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1962-051A |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |access-date=2018-04-20}} {{PD-notice}}</ref>
|-

! 15
| [[Explorer 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<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1962-059A |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}} {{PD-notice}}</ref>
|-

! 16
| [[Explorer 16|S-55B]]
| 16 December 1962
| [[Scout (rocket)|Scout X-3]]
| 101
| LEO
| 22 July 1963
| In orbit
| Micrometeoroid research<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1962-070A |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |access-date=2018-04-20}} {{PD-notice}}</ref>
|-

! 17
| [[Explorer 17|AE-A (S-6, Atmosphere Explorer-A)]]
| 3 April 1963
| [[Delta B]]
| 184
| LEO
| 10 July 1963
| 24 November 1966
| Atmospheric research<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1963-009A |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |access-date=2018-04-20}} {{PD-notice}}</ref>
|-

! 18
| [[Explorer 18|IMP-A]] (IMP 1, Interplanetary Monitoring Platform-A)
| 27 November 1963
| [[Delta C]]
| 138
| HEO
| 10 May 1965
| 30 December 1965
| Magnetospheric research<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1963-046A |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |access-date=2018-04-20}} {{PD-notice}}</ref>
|-

! 19
| [[Explorer 19|AD-A (Atmospheric Density-A)]]
| 19 December 1963
| [[Scout (rocket)|Scout X-4]]
| 8
| LEO
| 10 May 1981
| 10 May 1981
| Atmospheric density measurements<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1963-053A |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}} {{PD-notice}}</ref>
|- style="background:#fbb;"

! style="background:#fbb;"| –
| BE-A ([[Beacon Explorer-A]], S-66A)
| 19 March 1964
| [[Delta B]]
| 114
| ''failed''
| —
| —
| Launch failure<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=EXS-66A |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}} {{PD-notice}}</ref>
|-

! 20
| [[Explorer 20|IE-A]] (S 48, TOPSI, Ionosphere Explorer-A)
| 25 August 1964
| [[Scout (rocket)|Scout X-4]]
| 45
| LEO
| 29 December 1965
| In orbit
| Ionosphere research<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1964-051A |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |access-date=2018-04-20}} {{PD-notice}}</ref>
|-

! 21
| [[Explorer 21|IMP-B]] (IMP 2, Interplanetary Monitoring Platform-B)
| 4 October 1964
| [[Delta C]]
| 135
| HEO
| 13 October 1965
| 30 January 1966
| Magnetospheric research<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1964-060A |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |access-date=2018-04-20}} {{PD-notice}}</ref>
|-

! 22
| [[Explorer 22|BE-B]] (Beacon Explorer-B, S-66B)
| 10 October 1964
| [[Scout (rocket)|Scout X-4]]
| 53
| LEO
| February 1970
| In orbit
| Ionospheric and geodetic research<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1964-064A |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}} {{PD-notice}}</ref>
|-

! 23
| [[Explorer 23|S 55C]]
| 6 November 1964
| [[Scout (rocket)|Scout X-4]]
| 134
| LEO
| 7 November 1965
| 29 June 1983
| Micrometeoric research<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1964-074A |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |access-date=2018-04-20}} {{PD-notice}}</ref>
|-

! 24
| [[Explorer 24|AD-B]] (Atmospheric Density-B)
| 21 November 1964
| [[Scout (rocket)|Scout X-4]]
| 9
| MEO
| 18 October 1968
| 18 October 1968
| Atmospheric density measurements<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1964-076A |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |access-date=2018-04-20}} {{PD-notice}}</ref>
|-

! 25
| [[Injun (satellite)|Injun 4]] (IE-B, Ionosphere Explorer-B)
| 21 November 1964
| [[Scout (rocket)|Scout X-4]]
| 40
| LEO
| December 1966
| In orbit
| Ionospheric research<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1964-076B |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}} {{PD-notice}}</ref>
|-

! 26
| [[Explorer 26|EPE-D (Energetic Particle Explorer-D)]]
| 21 December 1964
| [[Delta C]]
| 46
| MEO
| 27 December 1967
| 23 August 2021
| High energy particle observations<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1964-086A |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |access-date=2018-04-20}} {{PD-notice}}</ref>
|-

! 27
| [[Explorer 27|BE-C (Beacon Explorer-C, S-66C)]]
| 29 April 1965
| [[Scout (rocket)|Scout X-4]]
| 61
| LEO
| 20 July 1973
| In orbit
| Magnetospheric research<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1965-032A |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |access-date=2018-04-20}} {{PD-notice}}</ref>
|-

! 28
| [[Explorer 28|IMP-C]] (IMP 3, Interplanetary Monitoring Platform-C)
| 29 May 1965
| [[Delta C]]
| 128
| HEO
| 12 May 1967
| 4 July 1968
| Magnetospheric research<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1965-042A |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |access-date=2018-04-20}} {{PD-notice}}</ref>
|-

! 29
| [[Explorer 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<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1965-089A |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}} {{PD-notice}}</ref>
|-

! 30
| [[Solrad 8|SOLRAD 8]] (SE-A)
| 19 November 1965
| [[Scout (rocket family)|Scout X-4]]
| 57
| LEO
| 5 November 1967
| In orbit
| Solar radiation monitoring (Cover for covert [[Signals intelligence|ELINT]] mission)<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1965-093A |title=NASA - NSSDCA - Spacecraft |publisher=National Space Science Data Center |access-date=2018-04-20}} {{PD-notice}}</ref>
|-

! 31
| [[Explorer 31|DME-A]] (Direct Measurements Explorer)
| 29 November 1965
| [[Thor-Agena]] B
| 99
| LEO
| 1 October 1969
| In orbit
| Ionospheric research<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1965-098B |title=NASA - NSSDCA - Spacecraft - Details |publisher=National Space Science Data Center |access-date=2018-04-20}}</ref>
|-

! 32
| [[Explorer 32|AE-B (Atmosphere Explorer-B)]]
| 25 May 1966
| [[Delta C]]1
| 225
| LEO
| March 1967
| 22 February 1985
| Atmospheric research<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1966-044A |title=NASA - NSSDCA - Spacecraft - Details |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}}</ref>
|-

! 33
| [[Explorer 33|IMP-D (AIMP 1, Interplanetary Monitoring Platform-D)]]
| 1 July 1966
| [[Delta E]]1
| 212
| HEO
| 21 September 1971<ref>{{cite web| url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1966-058A| title=Explorer 33 (NSSDC ID: 1966-058A)| publisher=NASA / National Space Science Data Center| date=2008-04-02| access-date=2008-07-04}}</ref>
| In orbit
| Magnetospheric research
|-

! 34
| [[Explorer 34|IMP-F]] (IMP 4, Interplanetary Monitoring Platform-F)
| 24 May 1967
| [[Delta E]]1
| 163
| MEO
| 3 May 1969
| 3 May 1969
| Magnetospheric research<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1967-051A |title=NASA - NSSDCA - Spacecraft - Details |publisher=National Space Science Data Center|access-date=2018-04-20}}</ref>
|-

! 35
| [[Explorer 35|IMP-E (AIMP 2, Interplanetary Monitoring Platform-E)]]
| 19 July 1967
| [[Delta E]]1
| 230
| [[Lunar orbit|Lunar]]
| 24 June 1973
| Lunar orbit
| Magnetospheric research<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1967-070A |title=NASA - NSSDCA - Spacecraft - Details |publisher=National Space Science Data Center |access-date=2018-04-20}}</ref>
|-

! 36
| [[GEOS 2]] (GEOS-B, Geodetic Earth Orbiting Satellite-2)
| 11 January 1968
| [[Delta E]]1
| 469
| LEO
| 1 July 1982
| In orbit
| Geodetic Earth monitoring<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1968-002A |title=NASA - NSSDCA - Spacecraft - Details |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}}</ref>
|-

! 37
| [[SOLRAD 9]] (SE B)
| 5 March 1968
| [[Scout B]]
| 198
| LEO
| 30 April 1974
| 16 November 1990
| Solar radiation monitoring<br>(Cover for covert [[ELINT]] mission)<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1968-017A |title=NASA - NSSDCA - Spacecraft - Details |publisher=National Space Science Data Center|access-date=2018-04-20}}</ref>
|-

! 38
| [[Explorer 38|RAE-A (RAE 1, Radio Astronomy Explorer-A)]]
| 4 July 1968
| [[Delta J]]
| 602
| MEO
| (~1969)
| In orbit
| Radio astronomy<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1968-055A |title=NASA - NSSDCA - Spacecraft - Details |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}}</ref>
|-

! 39
| [[Explorer 39|AD-C]] (Atmospheric Density-C)
| 8 August 1968
| [[Scout B]]
| 9
| LEO
| 23 June 1971
| 22 June 1981
| Atmospheric density measurements<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1968-066A |title=NASA - NSSDCA - Spacecraft - Details |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}}</ref>
|-

! 40
| [[Injun (satellite)|Injun 5]] (Injun C, IE-C, Ionosphere Explorer-C)
| 8 August 1968
| [[Scout B]]
| 71
| LEO
| June 1971
| In orbit
| Magnetospheric Research<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1968-066B |title=NASA - NSSDCA - Spacecraft - Details |publisher=National Space Science Data Center |access-date=2018-04-20}}</ref>
|-

! 41
| [[Explorer 41|IMP-G]] (IMP 5, Interplanetary Monitoring Platform-G)
| 21 June 1969
| [[Delta E]]1
| 145
| HEO
| 23 December 1972
| 23 December 1972
| Magnetospheric research<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1969-053A |title=NASA - NSSDCA - Spacecraft - Details |publisher=National Space Science Data Center |access-date=2018-04-20}}</ref>
|-

! 42
| [[Uhuru (satellite)|Uhuru]] (SAS-A, SAS 1)
| 12 December 1970
| [[Scout B]]
| 142
| LEO
| 4 January 1975
| 5 April 1979
| X-ray astronomy<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1970-107A |title=NASA - NSSDCA - Spacecraft - Details |publisher=National Space Science Data Center |access-date=2018-04-20}}</ref>
|-

! 43
| [[Explorer 43|IMP-H]] (IMP 7, Interplanetary Monitoring Platform-H)
| 13 March 1971
| [[Delta M]]6
| 635
| MEO
| 2 October 1974
| 2 October 1974
| Magnetospheric research<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1971-019A |title=NASA - NSSDCA - Spacecraft - Details |publisher=National Space Science Data Center |access-date=2018-04-20}}</ref>
|-

! 44
| [[SOLRAD 10]] (SE-C, SOLRAD-C)
| 8 July 1971
| [[Scout B]]
| 260
| LEO
| 30 June 1973
| 15 December 1979
| Solar radiation monitoring<br>(Cover for covert [[ELINT]] mission)<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1971-058A |title=NASA - NSSDCA - Spacecraft - Details |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}}</ref>
|-

! 45
| [[Explorer 45|SSS-A]] (S-Cubed A)
| 15 November 1971
| [[Scout B]]
| 52
| MEO
| 30 September 1974
| 10 January 1992
| Magnetospheric research<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1971-096A |title=NASA - NSSDCA - Spacecraft - Details |publisher=National Space Science Data Center |access-date=2018-04-20 }}</ref>
|-

! 46
| MTS ([[Meteoroid Technology Satellite]], METEC)<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1972-061A |title=NASA - NSSDCA - Spacecraft - Details |website=National Space Science Data Center |date=2016-02-12 |access-date=2016-02-24}}</ref>
| 13 August 1972
| [[Scout (rocket)|Scout D-1]]
| 90
| LEO
| 4 November 1974
| 2 November 1979
| Micrometeoroids research<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1972-061A |title=NASA - NSSDCA - Spacecraft - Details |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}}</ref>
|-

! 47
| [[Explorer 47|IMP-I]] (IMP 6, Interplanetary Monitoring Platform-I)
| 23 September 1972
| [[Delta 1000|Delta 1604]]
| 635
| HEO
| 31 October 1978
| In orbit
| Magnetospheric research<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1972-073A |title=NASA - NSSDCA - Spacecraft - Details |publisher=National Space Science Data Center |access-date=2018-04-20}}</ref>
|-

! 48
| [[Second Small Astronomy Satellite|SAS-B (Small Astronomy Satellite-B, SAS 2)]]
| 15 November 1972
| [[Scout (rocket)|Scout D-1]]
| 166
| LEO
| 8 June 1973
| 20 August 1980
| X-ray astronomy<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1972-091A |title=NASA - NSSDCA - Spacecraft - Details |publisher=National Space Science Data Center |access-date=2018-04-20}}</ref>
|-

! 49
| [[Explorer 49|RAE-B (RAE 2, Radio Astronomy Explorer-B)]]
| 10 June 1973
| [[Delta 1000|Delta 1913]]
| 328
| [[Lunar orbit|Lunar]]
| 26 April 1977
| Presumed crashed into Moon sometime after August 1977<ref name="explorer49nssdc"/>
| Radio astronomy<ref name="explorer49nssdc">{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1973-039A|title=NASA - NSSDCA - Spacecraft - Details |publisher=National Space Science Data Center |access-date=2018-04-20}}</ref>
|-

! 50
| [[Explorer 50|IMP-J]] (IMP 8, Interplanetary Monitoring Platform-J)
| 26 October 1973
| [[Delta 1000|Delta 1604]]
| 371
| HEO
| 7 October 2006
| In orbit
| Magnetospheric research<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1973-078A |title=NASA - NSSDCA - Spacecraft - Details |publisher=National Space Science Data Center |access-date=2018-04-20}}</ref>
|-

! 51
| [[Explorer 51|AE-C]] (Atmosphere Explorer-C)
| 16 December 1973
| [[Delta 1000|Delta 1900]]
| 658
| LEO
| (12 December 1978)
| 12 December 1978
| Atmospheric research<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1973-101A |title=NASA - NSSDCA - Spacecraft - Details |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}}</ref>
|-

! 52
| [[Explorer 52|Hawkeye 1]] (Injun-F, Injun 6, IE-D, Ionosphere Explorer-D)
| 3 June 1974
| [[Scout (rocket)|Scout E-1]]
| 23
| HEO
| 28 April 1978
| 28 April 1978
| Magnetospheric research<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1974-040A |title=NASA - NSSDCA - Spacecraft - Details |publisher=National Space Science Data Center |access-date=2018-04-20}}</ref>
|-

! 53
| [[Third Small Astronomy Satellite|SAS-C (Small Astronomy Satellite-C, SAS 3)]]
| 7 May 1975
| [[Scout (rocket)|Scout F-1]]
| 197
| LEO
| 7 April 1979
| 9 April 1979
| X-ray astronomy<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1975-037A |title=NASA - NSSDCA - Spacecraft - Details |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}}</ref>
|-

! 54
| [[Explorer 54|AE-D]] (Atmosphere Explorer-D)
| 6 October 1975
| [[Delta 2000|Delta 2910]]
| 681
| LEO
| 29 January 1976
| 12 March 1976
| Atmospheric research<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1975-096A |title=AE-D |publisher=National Space Science Data Center |access-date=2018-04-20}}</ref>
|-

! 55
| [[Explorer 55|AE-E]] (Atmosphere Explorer-E)
| 20 November 1975
| [[Delta 2000|Delta 2910]]
| 735
| LEO
| 25 September 1980
| 10 June 1981
| Atmospheric research<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1975-107A |title=AE-E |publisher=National Space Science Data Center |access-date=2018-04-20}}</ref>
|- style="background:#fbb;"

! style="background:#fbb;"| —
| [[Dual Air Density Explorer|DADE-A]] (Dual Air Density Explorer-A)
| 5 December 1975
| [[Scout (rocket)|Scout F-1]]
| 40
| ''failed''
| —
| —
| Atmospheric research; failed during launch<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=DADE-A |title=DADE-A |publisher=National Space Science Data Center |access-date=2018-04-20}}</ref>
|- style="background:#fbb;"

! style="background:#fbb;"| —
| [[Dual Air Density Explorer|DADE-B]] (Dual Air Density Explorer-B)
| 5 December 1975
| [[Scout (rocket)|Scout F-1]]
| 43
| ''failed''
| —
| —
| Atmospheric research; failed during launch<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=DADE-B |title=DADE-B |publisher=National Space Science Data Center |access-date=2018-04-20}}</ref>
|-

! 56
| [[ISEE-1]]  (ISEE-A)
| 22 October 1977
| [[Delta 2000|Delta 2914]]
| 340
| HEO
| 26 September 1987
| 26 September 1987
| Magnetospheric research; launched with ESA's [[ISEE-2]]; co-mission with ISEE 3<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1977-102A |title=ISEE 1 |publisher=NASA - NSSDCA |access-date=2018-04-20}}</ref>
|-

! 57
| [[International Ultraviolet Explorer|IUE]]
| 26 January 1978
| [[Delta 2000|Delta 2914]]
| 669
| MEO
| 30 September 1996
| In orbit
| Ultraviolet astronomy<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1978-012A |title=IUE |publisher=NASA - NSSDCA |access-date=2018-04-20}}</ref>
|-

! 58
| [[Heat Capacity Mapping Mission|HCMM]] (AEM-A)
| 26 April 1978
| [[Scout (rocket)|Scout F]]
| 117
| LEO
| 30 September 1980
| December&nbsp;22, 1981
| Thermal mapping of the Earth<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1978-041A |title= HCMM |publisher=NASA - NSSDCA |access-date=2018-04-20}}</ref>
|-

! 59
| [[International Cometary Explorer|ICE]] (ISEE 3, ISEE-C)
| 12 August 1978
| [[Delta 2000|Delta 2914]]
| 390
| [[Lissajous orbit|Sun–Earth L<sub>1</sub>]]
| 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.<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1978-079A |title= ISEE 3 |publisher=NASA - NSSDCA |access-date=2018-04-20}}</ref><ref name="space-timeline" />
|-

! 60
| [[Stratospheric Aerosol and Gas Experiment|SAGE]] (AEM-B)
| 18 February 1979
| [[Scout (rocket)|Scout D-1]]
| 149
| LEO
| 7 January 1982
| 11 April 1989
| Stratospheric aerosol and ozone data<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1979-013A |title= SAGE |publisher=NASA - NSSDCA |access-date=2018-04-20 }}</ref>
|-

! 61
| [[Magsat|MAGSAT]] (AEM-C)
| 30 October 1979
| [[Scout (rocket)|Scout G-1]]
| 158
| LEO
| 6 May 1980
| 11 June 1980
| Mapped the near surface magnetic field of the Earth<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1979-094A |title=Magsat |publisher=NASA - NSSDCA |access-date=2018-04-20}}</ref>
|-

! 62
| [[Dynamics Explorer 1]] (DE-1)
| 3 August 1981
| [[Delta (rocket)|Delta 3913]]
| 424
| MEO
| 28 February 1991
| In orbit
| Magnetospheric research<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1981-070A |title=Dynamics Explorer 1 |publisher=NASA - NSSDCA |access-date=2018-04-20}}</ref>
|-

! 63
| [[Dynamics Explorer 2]] (DE-2)
| 3 August 1981
| [[Delta (rocket)|Delta 3913]]
| 420
| LEO
| 19 February 1983
| 19 February 1983
| Magnetospheric research<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1981-070B |title= Dynamics Explorer 2 |publisher=NASA - NSSDCA |access-date=2018-04-20}}</ref>
|-

! 64
| [[Solar Mesosphere Explorer|SME]]
| 6 October 1981
| [[Delta (rocket)|Delta 2310]]
| 145
| LEO
| 4 April 1989
| 5 March 1991
| Atmospheric research<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1981-100A |title= SME |publisher=NASA - NSSDCA |access-date=2018-04-20 |url-status=live |archive-url=https://web.archive.org/web/20200404213307/https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1981-100A |archive-date= 4 April 2020 }}</ref>
|-

! 65
| [[AMPTE-CCE]]
| 16 August 1984
| [[Delta (rocket)|Delta 3924]]
| 242
| MEO
| 12 July 1989
| In orbit
| [[Magnetosphere|Magnetosphere research]]<ref name="Display">{{citation-attribution|1={{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1984-088A|title=AMPTE-CEE (1984-088A)|publisher=NSSDCA |access-date=26 November 2021}} }}</ref>
|-

! 66
| [[Cosmic Background Explorer|COBE]]
| 18 November 1989
| [[Delta (rocket)|Delta 5920]]
| 2,206
| LEO
| 23 December 1993
| In orbit
| Microwave astronomy<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1989-089A |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |access-date=2018-04-20}}</ref>
|-

! 67
| [[Extreme Ultraviolet Explorer|EUVE]] (BERKSAT)
| 7 June 1992
| [[Delta II]] 6920-X
| 3,275
| LEO
| 31 January 2001
| 30 January 2002
| Ultraviolet astronomy<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1992-031A |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |access-date=2018-04-20}}</ref>
|-

! 68
| [[Solar Anomalous and Magnetospheric Particle Explorer|SAMPEX]]
| 3 July 1992
| [[Scout (rocket)|Scout G-1]]
| 158
| LEO
| 30 June 2004<ref>{{cite web|url=http://www.srl.caltech.edu/sampex/DataCenter/ |title=The SAMPEX Data Center |website=Srl.caltech.edu |access-date=2016-02-24}}</ref>
| 13 November 2012.<ref>{{cite web |url=http://www.aerospace.org/cords/reentry-predictions/upcoming-reentries/1992-038a/ |archive-url=https://archive.today/20130224130248/http://www.aerospace.org/cords/reentry-predictions/upcoming-reentries/1992-038a/ |url-status=dead |archive-date=2013-02-24 |title=SAMPEX &#124; The Aerospace Corporation |website=Aerospace.org |access-date=2016-02-24 }}</ref>
| SMEX: magnetospheric research<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1992-038A |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |access-date=2018-04-20}}</ref>
|-

! 69
| [[Rossi X-ray Timing Explorer|RXTE]]
| 30 December 1995
| [[Delta II]] 7920
| 3,200
| LEO
| 3 January 2012 
| 30 April 2018<ref>{{cite web |last=Reddy |first=Francis |url=https://phys.org/news/2018-05-nasa-rossi-x-ray-explorer-scientific.html |title=Nasa's Rossi X-ray Timing Explorer leaves scientific legacy |website=[[Phys.org]] |date=2018-05-04 |access-date=2019-02-18}}</ref>
| MIDEX: X-ray astronomy<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1995-074A |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |date=2017-03-21 |access-date=2018-04-20 }}</ref>
|-

! 70
| [[Fast Auroral Snapshot Explorer|FAST]]
| 21 August 1996
| [[Pegasus XL]]
| 187
| LEO
| 4 May 2009
| In orbit
| SMEX: auroral phenomena<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1996-049A |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |access-date=2018-04-20}}</ref>
|- style="background:#fbb;"

! style="background:#fbb;"| —
| [[High Energy Transient Explorer|HETE 1]]
| 4 November 1996
| [[Pegasus XL]]
| 128
| LEO
| —
| 7 April 2002
| Separation failure, mission relaunched as HETE 2<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1996-061A |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |date=2017-03-21 |access-date=2018-04-20}}</ref>
|- style="background:#bfb;"

! style="background:#bfb;"| 71
| [[Advanced Composition Explorer|ACE]]
| 25 August 1997
| [[Delta II]] 7920
| 596
| [[Lissajous orbit|Sun–Earth L<sub>1</sub>]]
| '''Operational'''
| In [[Lissajous orbit|L<sub>1</sub> orbit]]
| MIDEX: solar/interplanetary/interstellar particle research<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1997-045A |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |date=2017-03-21 |access-date=2018-04-20}}</ref>
|-

! 72
| [[Student Nitric Oxide Explorer|SNOE]]
| 26 February 1998
| [[Pegasus XL]]
| 120
| LEO
| 13 December 2003
| 13 December 2003
| STEDI, UNEX: atmospheric research<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1998-012A |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |access-date=2018-04-20 }}</ref>
|-

! 73
| [[TRACE]]
| 2 April 1998
| [[Pegasus XL]]
| 250
| LEO
| 21 June 2010<ref>{{cite web|url=http://trace.lmsal.com/ |title=< Welcome to T R A C E on-line > |website=Trace.lmsal.com |date=2010-06-21 |access-date=2016-02-24}}</ref> <!-- (Hibernation) ({{For year month day| year=1998 | month=04 | day=02}}) -->
| In orbit
| SMEX: solar observatory<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1998-020A |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |date=2017-03-21 |access-date=2018-04-20 }}</ref>
|-

! 74
| [[Submillimeter Wave Astronomy Satellite|SWAS]]
| 6 December 1998
| [[Pegasus XL]]
| 288
| LEO
| 1 September 2005 <!-- (hiberation) -->
| In orbit
| SMEX: submillimeter astronomy<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1998-071A |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |access-date=2018-04-20}}</ref>
|- style="background: #ffb;"

! style="background: #FFB;" | 75
| [[Wide Field Infrared Explorer|WIRE]]
| 5 March 1999
| [[Pegasus XL]]
| 250
| [[Sun-synchronous orbit|SSO]]
| 30 September 2000<ref>{{cite web |url=http://spider.ipac.caltech.edu/staff/laher/asteroseismology/wireobs2.html |title=Complete WIRE Star-Camera Observation History |publisher=Spider.ipac.caltech.edu |access-date=2018-04-20 |archive-date=6 August 2020 |archive-url=https://web.archive.org/web/20200806214243/http://spider.ipac.caltech.edu/staff/laher/asteroseismology/wireobs2.html |url-status=dead }}</ref>
| 10 May 2011<ref>{{cite web|author=WIRE Web Team |url=http://www.ipac.caltech.edu/wire/ |title=WIRE: The Wide Field Infrared Explorer |website=Ipac.caltech.edu |access-date=2016-02-24}}</ref>
| SMEX, Infrared astronomy, primary mission failed due to loss of coolant<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1999-011A |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |date=2017-03-21 |access-date=2018-04-20 }}</ref>
|- style="background:#fbb;"

! style="background:#fbb;"| 76
| TERRIERS
| 18 May 1999
| [[Pegasus XL]]
| 120
| Polar LEO
| 18 May 1999
| In orbit
| STEDI: atmospheric research, satellite failed shortly after achieving orbit<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1999-026A |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |access-date=2018-04-20}}</ref>
|-

! 77
| [[Far Ultraviolet Spectroscopic Explorer|FUSE]]
| 23 June 1999
| [[Delta II]] 7320
| 1,400
| LEO
| 18 October 2007
| In orbit
| MIDEX: ultraviolet astronomy<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1999-035A |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |access-date=2018-04-20}}</ref>
|-

! 78
| [[IMAGE (spacecraft)|IMAGE]]
| 25 March 2000
| [[Delta II]] 7326
| 536
| Polar MEO
| 18 December 2005<ref>{{cite web|url=http://image.gsfc.nasa.gov/ |title=NASA - IMAGE Science Center |website=Image.gsfc.nasa.gov |access-date=2016-02-24}}</ref>
| In orbit
| MIDEX: magnetospheric research<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2000-017A |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |access-date=2018-04-20}}</ref>
|-

! 79
| [[High Energy Transient Explorer|HETE-2]]
| 9 October 2000
| [[Pegasus (rocket)|Pegasus-H]]
| 124
| LEO
| 28 March 2007<ref>{{cite web |url=https://science.nasa.gov/missions/hete-2/ |title=Missions - HETE-2 - NASA Science |website=Science.nasa.gov |access-date=2016-02-24 |archive-date=22 July 2016 |archive-url=https://web.archive.org/web/20160722132241/http://science.nasa.gov/missions/hete-2/ |url-status=dead }}</ref>
| In orbit
| MO: UV, X-ray, and gamma ray astronomy<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2000-061A |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |date=2017-03-21 |access-date=2018-04-20}}</ref>
|-

! 80
| [[WMAP]]
| 30 June 2001
| [[Delta II]] 7425-10
| 840
| [[Lissajous orbit|Sun–Earth L<sub>2</sub>]]
| October 2010<ref name="graveyard">{{cite web |title=Mission Complete! WMAP fires its thrusters for the last time |url=http://news.discovery.com/space/mission-complete-wmap-fires-its-thrusters-for-the-last-time.html |access-date=8 November 2010 |archive-date=14 July 2012 |archive-url=https://web.archive.org/web/20120714021615/http://news.discovery.com/space/mission-complete-wmap-fires-its-thrusters-for-the-last-time.html |url-status=dead }}</ref>
| Heliocentric orbit
| MIDEX: microwave astronomy<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2001-027A |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |date=2017-03-21 |access-date=2018-04-20}}</ref>
|-

! 81
| [[RHESSI]]
| 5 February 2002
| [[Pegasus XL]]
| 230
| LEO
| 16 August 2018
| Deorbited
| SMEX: X-ray and gamma ray solar flare imaging<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2002-004A |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |access-date=2018-04-20}}</ref>
|-

| —
| [[INTEGRAL]]
| 17 October 2002
| [[Proton-K]] [[Blok D|Blok DM-2]]
| 4,000
| HEO
| 28 February 2025
| In orbit
| International: space telescope for observing gamma rays<ref>{{cite web|url=https://heasarc.gsfc.nasa.gov/docs/integral/integral.html |title=The International Gamma-Ray Astrophysics Laboratory (INTEGRAL) |publisher=Heasarc.gsfc.nasa.gov |date=2002-10-17 |access-date=2018-04-28}}</ref>

|- 
! 82
| [[CHIPSat]]
| 13 January 2003
| [[Delta II]] 7320-10
| 60
| LEO
| 11 April 2008<ref>{{cite web|last=Savage |first=Sam |url=http://www.redorbit.com/news/space/1417075/chipsat_quietly_shut_down/ |title=CHIPSat Quietly Shut Down |website=Redorbit.com |date=2008-06-04 |access-date=2016-02-24}}</ref>
| In orbit
| UNEX: ultraviolet spectroscopy and astronomy<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2003-002B |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |access-date=2018-04-20}}</ref>

|- 
! 83
| [[GALEX]]
| 28 April 2003
| [[Pegasus XL]]
| 280
| LEO
| 28 June 2013<ref>{{cite web|url=http://www.galex.caltech.edu/newsroom/glx2013-03r.html |title=Press Release: NASA Decommissions Its Galaxy Hunter Spacecraft |website=Galex.caltech.edu |date=2013-06-28 |access-date=2016-02-24}}</ref>
| In orbit
| SMEX: ultraviolet astronomy<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2003-017A |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |date=2017-03-21 |access-date=2018-04-20}}</ref>

|- style="background:#bfb;"
! style="background:#bfb;"| 84
| [[Neil Gehrels Swift Observatory|Swift]]
| 20 November 2004
| [[Delta II]] 7320-10C
| 1,470
| LEO
| '''Operational'''
| In orbit
| MIDEX: gamma ray astronomy<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2004-047A |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |date=2017-03-21 |access-date=2018-04-20 }}</ref>

|- 
! —
| [[Suzaku (satellite)|Suzaku]] (Astro E2)
| 10 July 2005
| [[M-V]]
| 1,706
| LEO
| 2 September 2015
| In orbit
| MO: instrument on JAXA's Suzaku mission<ref>{{cite web|url=https://heasarc.gsfc.nasa.gov/docs/astroe/astroegof.html |title=HEASARC: Suzaku Guest Observer Facility |publisher=Heasarc.gsfc.nasa.gov |access-date=2018-04-28}}</ref>

|- style="background:#bfb;"
! style="background:#bfb;"| —
| [[TWINS]] A
| 28 June 2006
| [[Delta IV]] M+(4,2)
| ''classified''
| [[Molniya orbit|Molniya]]
| '''Operational'''
| In orbit
| MO: payload on Trumpet-F/O-1 1 ([[USA-184]])

|- style="background:#bfb;"
! style="background:#bfb;"| 85
| [[THEMIS (satellite)|THEMIS]] A
| 17 February 2007
|  [[Delta II]] 7925
| 77
| HEO
| '''Operational'''
| In orbit
| MIDEX: magnetospheric research<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2007-004A |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |date=2017-03-21 |access-date=2018-04-20}}</ref>
|- style="background:#bfb;"
! style="background:#bfb;"| 86
| [[THEMIS (satellite)|THEMIS]] B (ARTEMIS P1)
| 17 February 2007
| [[Delta II]] 7925
| 77
| [[Lunar orbit|Lunar]]
| '''Operational'''
| Lunar orbit
| MIDEX; Magnetospheric research<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2007-004B |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |access-date=2018-04-20}}</ref>
|- style="background:#bfb;"
! style="background:#bfb;"| 87
| [[THEMIS (satellite)|THEMIS]] C (ARTEMIS P2)
| 17 February 2007
|  [[Delta II]] 7925
| 77
| [[Lunar orbit|Lunar]]
| '''Operational'''
| Lunar orbit 
| MIDEX: magnetospheric research<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2007-004C |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |access-date=2018-04-20}}</ref>
|- style="background:#bfb;"
! style="background:#bfb;"| 88
| [[THEMIS (satellite)|THEMIS]] D
| 17 February 2007
|  [[Delta II]] 7925
| 77
| HEO
| '''Operational'''
| In orbit
| MIDEX: magnetospheric research<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2007-004D |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |date=2017-03-21 |access-date=2018-04-20}}</ref>
|- style="background:#bfb;"
! style="background:#bfb;"| 89
| [[THEMIS (satellite)|THEMIS]] E
| 17 February 2007
| [[Delta II]] 7925
| 77
| HEO
| '''Operational'''
| In orbit
| MIDEX: magnetospheric research<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2007-004E |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |date=2017-03-21 |access-date=2018-04-20}}</ref>

|-
! 90
| [[Aeronomy of Ice in the Mesosphere|AIM]]
| 25 April 2007
| [[Pegasus XL]]
| 197
| [[Sun-synchronous orbit|SSO]]
| March 2023
| 19 August 2024 
| SMEX: [[noctilucent cloud]] observation<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2007-015A |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |date=2017-03-21 |access-date=2018-04-20}}</ref>

|- style="background:#bfb;"
! style="background:#bfb;"| —
| [[TWINS]] B
| 13 March 2008
| [[Atlas V]] 411
| ''classified''
| [[Molniya orbit|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]]

|- style="background:#bfb;"
! style="background:#bfb;"| 91
| [[Interstellar Boundary Explorer|IBEX]]
| 19 October 2008
| [[Pegasus XL]]
| 107
| MEO
| '''Operational'''
| In orbit
| SMEX: mapping the boundary between the Solar System and interstellar space.<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2008-051A |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |access-date=2018-04-20}}</ref>

|-
! 92
| [[Wide-field Infrared Survey Explorer|WISE]]
| 14 December 2009
| [[Delta II]] 7320
| 661
| LEO 
| August 2024
| 2 November 2024
| MIDEX: infrared astronomy, NEOWISE extension. Discovered first [[Earth trojan]].<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2009-071A |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |access-date=2018-04-20}}</ref>

|- style="background:#bfb;"
! style="background:#bfb;"| 93
| [[Nuclear Spectroscopic Telescope Array|NuSTAR]]
| 13 June 2012
| [[Pegasus XL]]
| 350
| LEO
| '''Operational'''
| In orbit
| SMEX: high-energy X-ray astronomy<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2012-031A |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |date=2017-03-21 |access-date=2018-04-20}}</ref>

|- style="background:#bfb;"
! style="background:#bfb;"| 94
| [[Interface Region Imaging Spectrograph|IRIS]]
| 27 June 2013
| [[Pegasus XL]]
| 183
| [[Sun-synchronous orbit|SSO]]
| '''Operational'''
| In orbit
| SMEX: solar UV astronomy<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2013-033A |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |access-date=2018-04-20}}</ref>

|- 
! —
| [[Hitomi (satellite)|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<ref>{{cite web|url=http://global.jaxa.jp/press/2016/03/20160327_hitomi.html |title=JAXA &#124; Communication anomaly of X-ray Astronomy Satellite "Hitomi" (ASTRO-H) |publisher=Global.jaxa.jp |access-date=2018-04-28}}</ref>

|- style="background:#bfb;"
! style="background:#bfb;"| —
| [[NICER]] 
| 3 May 2017
| [[Falcon 9]] FT
| 372
| [[ISS]]
| '''Operational'''
| ISS
| MO: instrument on ISS for [[neutron star]] observations

|- style="background:#bfb;"
! style="background:#bfb;"| —
| [[Global-scale Observations of the Limb and Disk|GOLD]] 
| 25 January 2018
| [[Ariane 5]] ECA
| 37
| [[Geostationary orbit|GEO]]
| '''Operational'''
| In orbit
| MO: instrument on [[SES-14]] comsat for studying Earth–space boundary

|- style="background:#bfb;"
! style="background:#bfb;"| 95
| [[Transiting Exoplanet Survey Satellite|TESS]]
| 18 April 2018
| [[Falcon 9]] FT
| 362
| HEO
| '''Operational'''
| In orbit
| MIDEX: survey for transiting exoplanets

|- style="background:#bfb;"
! style="background:#bfb;"| 96
| [[Ionospheric Connection Explorer|ICON]]
| 11 October 2019
| [[Pegasus (rocket)|Pegasus]] XL
| 287
| LEO
| '''Operational'''
| In orbit
| MIDEX: ionospheric studies

|- style="background:#bfb;"
! style="background:#bfb;"| 97
| [[IXPE]]
| 9 December 2021
| [[Falcon 9]] Block 5
| 330
| LEO
| '''Operational'''
| In orbit
| SMEX: X-ray studies

|- style="background:#bfb;"
! style="background:#bfb;"| —
| [[X-Ray Imaging and Spectroscopy Mission|XRISM]]
| 6 September 2023
| [[H-IIA]] 202
| 2,300
| LEO
| '''Operational'''
| In orbit
| MO: instruments on [[JAXA]]'s XRISM x-ray space telescope

|- style="background:#bfb;"
! style="background:#bfb;"| 98
| [[Polarimeter to Unify the Corona and Heliosphere|PUNCH-NFI]]
| 12 March 2025
| [[Falcon 9]] Block 5
| 40
| LEO
| '''Operational'''
| In orbit
| SMEX: Heliosphere studies

|- style="background:#bfb;"
! style="background:#bfb;"| 99
| [[Polarimeter to Unify the Corona and Heliosphere|PUNCH-WFI 1]]
| 12 March 2025
| [[Falcon 9]] Block 5
| 40
| LEO
| '''Operational'''
| In orbit
| SMEX: Heliosphere studies

|- style="background:#bfb;"
! style="background:#bfb;"| 100
| [[Polarimeter to Unify the Corona and Heliosphere|PUNCH-WFI 3]]
| 12 March 2025
| [[Falcon 9]] Block 5
| 40
| LEO
| '''Operational'''
| In orbit
| SMEX: Heliosphere studies

|- style="background:#bfb;"
! style="background:#bfb;"| 101
| [[Polarimeter to Unify the Corona and Heliosphere|PUNCH-WFI 2]]
| 12 March 2025
| [[Falcon 9]] Block 5
| 40
| LEO
| '''Operational'''
| In orbit
| SMEX: Heliosphere studies

|- style="background:#bfb;"
! style="background:#bfb;"| 102
| [[SPHEREx]]
| 12 March 2025
| [[Falcon 9]] Block 5
| 178 
| LEO
| '''Operational'''
| In orbit
| MIDEX: Infrared studies

|-
|}

==Cancelled missions==
[[File:WISE artist concept (PIA17254, crop).jpg|thumb|200px|WISE was restarted after it was turned off]]
[[File:Thor-Able III Explorer 6.jpg|thumb|right|200px|[[Explorer 6]] on a [[Thor-Able|Thor-Able III]] launch in August 1959]]
[[File:ISEE-C (ISEE 3) in dynamic test chamber.jpg|thumb|200px|right|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>{{cite web|url=http://explorers.larc.nasa.gov/EX/ |title=Science Office for Mission Assessments: Explorer 2011 |website=Explorers.larc.nasa.gov |access-date=2016-02-24}}</ref> <!-- Missions of Opportunity (MO) are small collaborative missions with spacecraft not operated by NASA, such as an additional instrument. Examples of this include [[Astro-H]], [[CINDI]], [[TWINS]], and [[HETE-2]].  -->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|date=September 2024}}

Examples of missions that were not developed or cancelled were:<ref name=skyrocket/>
*Owl 1 and 2 (cost, 1965)<ref>{{cite web|url=http://space.skyrocket.de/doc_sdat/explorer_owl.htm |title=Explorer: Owl 1, 2 |publisher=Space.skyrocket.de |access-date=2018-04-28}}</ref>
*MSS A (Magnetic Storm Satellite, Explorer-A, 1970)<ref>{{cite web|url=http://space.skyrocket.de/doc_sdat/explorer_mss-a.htm |title=Explorer: MSS A |publisher=Space.skyrocket.de |access-date=2018-04-28}}</ref><ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=MSS-A |title=NASA - NSSDCA - Spacecraft - Details |publisher=Nssdc.gsfc.nasa.gov |date=2017-03-21 |access-date=2018-04-20}}</ref>
*CATSAT (STEDI 3) (cost)<ref name="nap" />
*IMEX (UNEX 2) (cost)<ref name="nap" />
*[[Full-sky Astrometric Mapping Explorer|FAME]] (MIDEX 4)
*SPIDR (SMEX 8) (technical, 2003)<ref name="nap" />
*[[Gravity and Extreme Magnetism|GEMS]] (SMEX 13)

Recent examples of conclusions of launched missions, cancelled due to budgetary constraints:
*[[Fast Auroral Snapshot Explorer|FAST]] - 2009
*[[TRACE]] - 2010 (Solar observatory, see [[Solar Dynamics Observatory]])
*[[Wilkinson Microwave Anisotropy Probe|Wilkinson MAP]] - 2010
*[[Wide-field Infrared Survey Explorer|WISE]] - 2011 (extended in 2013 as NEOWISE mission)
*[[Rossi X-ray Timing Explorer|RXTE]] - 2012
*[[GALEX|Galaxy Evolution Explorer]] - 2013

==Launch statistics==
Number of launches per decade:<ref>{{cite web|url=http://nssdc.gsfc.nasa.gov/multi/explorer.html |title=NASA's Explorers Program Satellites |website=Nssdc.gsfc.nasa.gov |access-date=2016-02-24}}</ref> 
{{Bar graph
| title = Number of Explorer launches by decade
| float = left
| bar_width = 20
| width_units = em
| data_max = 35
| label_type = Decade
| data_type = #
| label1 = 1950s
| data1 = 8
| label2 = 1960s
| data2 = 34
| label3 = 1970s
| data3 = 20
| label4 = 1980s
| data4 = 5
| label5 = 1990s
| data5 = 11 
| label6 = 2000s
| data6 = 15
| label7 = 2010s
| data7 = 7
| label8 = 2020s
| data8 = 2}}
{{clear}}

== See also ==
{{Portal|Spaceflight}}
* [[Cosmic Vision]], a [[European Space Agency]] (ESA) programme 
* [[Cosmic Vision#Small class|Cosmic Vision S-class missions]], the European Space Agency equivalent to the Small Explorers Program 
* {{annotated link|Discovery program}}
* {{annotated link|New Frontiers program}}

==References==
{{reflist}}

==External links==
{{commons category|Explorer program}}
*{{cite web |url=http://explorers.gsfc.nasa.gov/index.html |title=Explorers Program |access-date=2009-12-05 |year=2009 |work=Goddard Space Flight Center |publisher=[[NASA]] |archive-url=https://web.archive.org/web/20091031051247/http://explorers.gsfc.nasa.gov/index.html |archive-date=2009-10-31 |url-status=dead }}
* [https://web.archive.org/web/20100323182500/http://explorers.gsfc.nasa.gov/missions.html NASA Explorers Program missions page]
* [http://nssdc.gsfc.nasa.gov/multi/explorer.html NSSDC updated list of Explorers missions]
* [https://web.archive.org/web/20151220110322/http://solarsystem.nasa.gov/missions/profile.cfm?MCode=Explorer_Moon Explorers Program Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
* [http://explorers.gsfc.nasa.gov/smex.html Small Explorers Missions] by [[Goddard Space Flight Center]]
* [http://rammb.cira.colostate.edu/dev/hillger/SMEX.htm SMEX-series satellites] by Colorado State University

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{{NASA planetary exploration programs}}
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