View source for Astronomy:Explorers Program

{{Short description|Ongoing United States space exploration program}}

[[File:Pointing X-ray Eyes at our Resident Supermassive Black Hole.jpg|thumb|upright=1.0|right|An Explorer mission observes [[Astronomy:Sagittarius A*|Sagittarius A*]], the 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}}</ref> is a [[Organization:NASA|NASA]] exploration program that provides flight opportunities for physics, [[Physics:Geophysics|geophysics]], [[Physics:Heliophysics|heliophysics]], and [[Physics:Astrophysics|astrophysics]] investigations from space. Launched in 1958, [[Astronomy:Explorer 1|Explorer 1]] was the first spacecraft of the United States to achieve orbit. Over 90 space missions have been launched since. Starting with [[Astronomy:Explorer 6|Explorer 6]], it has been operated by NASA, with regular collaboration with a variety of other institutions, including many international partners.

Launchers for the Explorer program have included Juno I, [[Engineering:Juno II|Juno II]], various [[Engineering:Thor (rocket family)|Thor]], [[Engineering:Scout (rocket family)|Scout]], [[Engineering:Delta (rocket family)|Delta]] and [[Engineering:Pegasus (rocket)|Pegasus]] launch vehicles, and [[Engineering:Falcon 9|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|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 Explorer program began as a U.S. Army proposal ([[Engineering:Project Orbiter|Project Orbiter]]) to place a "civilian" [[Astronomy:Satellite|artificial satellite]] into orbit during the [[Earth:International Geophysical Year|International Geophysical Year]] (IGY). Although that proposal was rejected in favor of the U.S. Navy's [[Astronomy:Project Vanguard|Project Vanguard]], which made the first sub-orbital flight Vanguard TV0 in December 1956, the [[Place:Soviet Union|Soviet Union]]'s launch of [[Engineering:Sputnik 1|Sputnik 1]] on 4 October 1957 (and the resulting "[[Engineering:Sputnik crisis|Sputnik crisis]]") and the failure of the [[Engineering:Vanguard 1|Vanguard 1]] launch attempt resulted in the Army program being funded to match the Soviet space achievements. [[Astronomy:Explorer 1|Explorer 1]] was launched on the Juno I on 1 February 1958, becoming the first U.S. satellite, as well as discovering the [[Astronomy:Van Allen radiation belt|Van Allen radiation belt]].

Four follow-up satellites of the Explorer series were launched by the Juno I launch vehicle in 1958, of which [[Astronomy:Explorer 3|Explorer 3]] and [[Astronomy:Explorer 4|Explorer 4]] were successful, while [[Astronomy:Explorer 2|Explorer 2]] and [[Astronomy:Explorer 5|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 [[Engineering:Juno II|Juno II]] in 1959.

=== Continuation of the Explorer program ===
[[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 [[Physics:Ultraviolet|UV]] light does not have a "color" (the eye stopping at about violet). This view was taken by the Explorer Swift, which can also detect X-rays, and has contributed to the study of gamma-ray bursts and other topics.]]

With the establishment of NASA in 1958, the Explorer program was transferred to NASA from the U.S. Army. NASA continued to use the name for an ongoing series of relatively small space missions, typically an artificial satellite with a specific science focus. [[Astronomy:Explorer 6|Explorer 6]] in 1959 was the first scientific satellite under the project direction of NASA's [[Organization:Goddard Space Flight Center|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>{{citation-attribution|1={{cite web|url=https://www.nasa.gov/content/goddard-missions-past|title=Goddard Missions - Past|work=Goddard Space Flight Center|publisher=NASA|date=5 June 2018|access-date=24 June 2019}} }}</ref>

The [[Astronomy:Interplanetary Monitoring Platform|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 [[Astronomy:Apollo program|Apollo program]]. The IMP program was a major step forward in spacecraft [[Engineering:Electronics|electronics]] design, as it was the first space program to use [[Engineering:Integrated circuit|integrated circuit]] (IC) chips and [[Engineering:MOSFET|MOSFET]]s (MOS transistors).<ref name="Butrica">{{citation-attribution|1={{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 (237-42)|chapter-url=https://www.nasa.gov/sites/default/files/atoms/files/historical-studies-societal-impact-spaceflight-ebook_tagged.pdf#page=237}} }}</ref><ref name="nasa">{{citation-attribution|1={{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.}} }}</ref> The IMP-A ([[Astronomy:Explorer 18|Explorer 18]]) in 1963 was the first spacecraft to use IC chips, and the IMP-D ([[Astronomy:Explorer 33|Explorer 33]]) in 1966 was the first to use MOSFETs.<ref name="Butrica"/>

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 [[Astronomy:Magnetosphere|magnetosphere]] and the shape of its [[Physics:Gravitational field|gravity field]]; the [[Astronomy:Solar wind|solar wind]]; properties of micrometeoroids raining down on the [[Astronomy:Earth|Earth]]; ultraviolet, cosmic and X-rays from the [[Astronomy:Solar System|Solar System]] and beyond; [[Astronomy:Ionosphere|ionospheric physics]]; [[Astronomy:Solar flare|Solar plasma]]; [[Astronomy:Solar energetic particles|solar energetic particles]]; and [[Physics:Atmospheric science|atmospheric physics]]. These missions have also investigated air density, radio astronomy, [[Geodesy|geodesy]], and [[Astronomy:Gamma-ray astronomy|gamma-ray astronomy]].

With drops in NASA's budget, Explorer missions became infrequent in the early 1980s.

=== SMEX, MIDEX, and Student Explorer programs ===
In 1988, the '''Small Explorer (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}}</ref><ref>{{cite web|url=https://www.nasa.gov/centers/goddard/pdf/106477main_smex.pdf |title=NASA's Small Explorer Program: Faster, Better, Cheaper|work=Goddard Space Flight Center|publisher=NASA|date=January 1998|access-date=24 June 2019}}</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/>

In the mid 1990s, NASA initiated the '''Medium-class Explorer (MIDEX)''' to enable more frequent flights. These are larger than SMEX missions and were to be launched aboard a new kind of medium-light class launch vehicle.<ref name=NASAhistory/> This new launch vehicle was not developed and instead, these missions were flown on a modified [[Engineering:Delta II|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=1996-03-04|access-date=2018-04-28}}</ref> The first announcement opportunity for MIDEX was issued in March 1995, and the first launch under this new class was [[Astronomy: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 Explorer Project Office at NASA's Goddard Space Flight Center (GSFC). In early 1999, that office was closed and with the announcement of opportunity for the third set of SMEX missions NASA converted the SMEX class so that each mission was managed by its [[Principal investigator|principal investigator]], with oversight by the GSFC Explorer Project.<ref name="welcome">{{citation-attribution|1={{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}} }}</ref> The Explorer program Office at Goddard Space Flight Center, provides management of the many operational scientific exploration missions that are characterized by relatively moderate costs and small to medium-sized missions that are capable of being built, tested, and launched in a short time interval compared to larger observatories like NASA's Great Observatories.<ref>{{citation-attribution|1={{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=2016-03-04|url-status=dead }} }}</ref>

Excluding the launches, the MIDEX class has a current mission cap cost of US$250 million in 2018,<ref name=Midex19>{{citation-attribution|1={{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=2018-04-28}} }}</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=2018-04-28}}</ref> The cost cap for SMEX missions in 2017 was US$165 million.<ref>{{citation-attribution|1={{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 Explorer (SMEX) & Mission of Opportunity (MO) Solicitations Pre-Proposal Conference|publisher=NASA|date=15 August 2016|access-date=24 June 2019}} }}</ref> UNEX missions are capped at US$15 million.<ref name=missions>{{citation-attribution|1={{cite web |url=http://explorers.gsfc.nasa.gov/missions.html|title=Explorer 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}} }}</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 Explorers (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=2016-04-08|archive-url=https://web.archive.org/web/20160323014953/http://explorers.gsfc.nasa.gov/midex.html|archive-date=2016-03-23|url-status=dead}}</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=2018-04-28}}</ref> 
|-
! Name
! MIDEX<br/>number
! Explorer<br/>number
! Launch (UTC)
! Status
|-
| [[Astronomy:Rossi X-ray Timing Explorer|RXTE]]
| 
| Explorer-69
| 30 December 1995
| Ended in 2012
|-
| [[Astronomy:Advanced Composition Explorer|ACE]]
| 
| Explorer-71
| 25 August 1997
| {{success|'''Operational'''}}
|-
| [[Astronomy:Far Ultraviolet Spectroscopic Explorer|FUSE]]
| MIDEX-0
| Explorer-77
| 23 June 1999
| Ended in 2007
|-
| [[Engineering:IMAGE (spacecraft)|IMAGE]]
| MIDEX-1
| Explorer-78
| 25 March 2000
| Lost contact in 2005. Partial contact reestablished in January 2018
|-
| [[Astronomy:Wilkinson Microwave Anisotropy Probe|WMAP]]
| MIDEX-2
| Explorer-80
| June 30, 2001
| Ended in 2010
|-
| [[Astronomy:Neil Gehrels Swift Observatory|Swift]]
| MIDEX-3
| Explorer-84
| November 20, 2004
| {{success|'''Operational'''}}
|-
| [[Astronomy:Full-sky Astrometric Mapping Explorer|FAME]]
| MIDEX-4
| {{center|—}}
| ''Scheduled for 2004''
| Cancelled in 2002 (cost)
|-
| THEMIS A
| MIDEX-5A
| Explorer-85
| February 17, 2007
| {{success|'''Operational'''}}
|-
| THEMIS B
| MIDEX-5B
| Explorer-86
| February 17, 2007
| {{success|'''Operational'''}}
|-
| THEMIS C
| MIDEX-5C
| Explorer-87
| February 17, 2007
| {{success|'''Operational'''}}
|-
| THEMIS D
| MIDEX-5D
| Explorer-88
| February 17, 2007
| {{success|'''Operational'''}}
|-
| THEMIS E
| MIDEX-5E
| Explorer-89
| February 17, 2007
| {{success|'''Operational'''}}
|-
| [[Astronomy:Wide-field Infrared Survey Explorer|WISE / NEOWISE]] 
| MIDEX-6
| Explorer-92
| December 14, 2009
| {{success|'''Operational'''}}<ref>[https://www.nasa.gov/feature/jpl/neowise-celebrates-five-years-of-asteroid-data/ NEOWISE Celebrates Five Years of Asteroid Data]. NASA. 15 April 2019.</ref>

|-
| [[Astronomy:Transiting Exoplanet Survey Satellite|TESS]]
| MIDEX-7
| Explorer-95
| April 18, 2018
| {{success|'''Operational'''}}
|-
| [[Astronomy:Ionospheric Connection Explorer|ICON]]
| MIDEX-8
| Explorer-96
| 11 October 2019 
| {{success|'''Operational'''}}
|-
| [[Astronomy:SPHEREx|SPHEREx]]
| MIDEX-9<!--The sequential number, but needs a reference-->
| 
| April 2025 
| {{pending|In development<ref>{{cite web |url=https://www.jpl.nasa.gov/missions/spherex |title=Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer |work=[[Organization:Jet Propulsion Laboratory|JPL]] |publisher=[[Organization:NASA|NASA]] |access-date=7 July 2021}}</ref>}}
|}

===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 [[Organization:Goddard Space Flight Center|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 Explorer Project Office at Goddard Space Flight Center. In early 1999, that office was closed and with the announcement of opportunity for the third set of SMEX missions NASA converted the program so that each mission was managed by its Principal Investigator, with oversight by the GSFC Explorers Project.<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}}</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 Explorer (SMEX)|publisher=NASA |date=13 July 2016 |id=NNH16ZDA005O}}</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}}</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= Oct 31, 2023 }}</ref>

{| class="wikitable"
|+List of SMEX missions <ref name=":0">{{cite web|url=http://space.skyrocket.de/doc_sat/explorer.htm|title=Explorer Program|website=space.skyrocket.de|access-date=8 April 2016}}</ref><ref>{{citation-attribution|1={{cite web |url=http://explorers.gsfc.nasa.gov/smex.html|title=Operational Small Explorers (SMEX) Missions |website=Explorer 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}} }}</ref>
|-
! Name
! SMEX<br/>number
! Explorer<br/>number
! Launch (UTC)
! End of<br/>mission
! Status
|-

| [[Astronomy: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 - Details|publisher=NASA|date=21 March 2017|access-date=20 April 2018}}</ref>
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|-

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

| [[Astronomy:Submillimeter Wave Astronomy Satellite|SWAS]]
| SMEX-3
| Explorer-74
| 6 December 1998
| 21 July 2004
| 
|-

| [[Astronomy:TRACE|TRACE]]
| SMEX-4
| Explorer-73
| 2 April 1998
| 21 June 2010
| 
|-

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

| [[Astronomy:Reuven Ramaty High Energy Solar Spectroscopic Imager|RHESSI]]
| SMEX-6
| Explorer-81
| 5 February 2002
| April 2018
| Deorbited on April 20, 2023
|-

| [[Astronomy:GALEX|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= Apr 27, 2019 }}</ref>
|-

| [[Astronomy:Aeronomy of Ice in the Mesosphere|Aeronomy of Ice in the Mesosphere]] (AIM)
| SMEX-9
| Explorer-90
| 25 April 2007
| 
| {{success|'''Operational'''}}
|-

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

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

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

| [[Astronomy: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>
|-

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

| [[Astronomy:Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites|TRACERS]]
| 
| 
| April 2025<ref>{{cite web |last=Foust |first=Jeff |url=https://spacenews.com/nasa-selects-spacex-for-rideshare-launch-of-smallsat-mission/ |title=NASA selects SpaceX for rideshare launch of smallsat mission |work=SpaceNews |date=30 September 2023 |access-date=1 October 2023 |url-status=live |archive-url=https://archive.today/20231219234720/https://spacenews.com/nasa-selects-spacex-for-rideshare-launch-of-smallsat-mission/ |archive-date= 19 December 2023 }}</ref>
| 
| {{pending|In development}}
|-

| [[Astronomy:Polarimeter to Unify the Corona and Heliosphere|PUNCH]]
| 
| 
| April 2025<ref name="nasa-20220803">{{cite web |last=Interrante |first=Abbey |url=https://blogs.nasa.gov/punch/2022/08/03/punch-announces-rideshare-with-spherex-and-new-launch-date/ |title=PUNCH Announces Rideshare with SPHEREx and New Launch Date |date=3 August 2022 |access-date=3 August 2022 |work=[[Organization:NASA|NASA]]}}</ref>
| 
| {{pending|In development}}
|-

| [[Astronomy:Compton Spectrometer and Imager|COSI]]
| 
| 
| 2027<ref>{{cite web |url=https://science.nasa.gov/mission/cosi |title=COSI |work=[[Organization:NASA|NASA]] |date=16 June 2023 |access-date=22 December 2023}}</ref><ref>{{citation-attribution|1={{cite press release|url=https://www.nasa.gov/press-release/nasa-selects-gamma-ray-telescope-to-chart-milky-way-evolution|title=NASA Selects Gamma-ray Telescope to Chart Milky Way Evolution |publisher=NASA|date=18 October 2021|access-date=18 October 2021}} }}</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=2016-03-22|url-status=dead}}</ref>
|-
! Name
! UNEX number
! Explorer<br/>number
! Launch (UTC)
! Status
|-
| [[Astronomy:Student Nitric Oxide Explorer|SNOE]]
| UNEX-1
| Explorer-72
| 26 February 1998
| Ended in 2000
|-
| 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=2018-04-28}}</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>[https://explorers.gsfc.nasa.gov/missions.html Explorers Missions: Missions of Opportunity (MO)] NASA [[Organization:Goddard Space Flight Center|Goddard Space Flight Center]] (GSFC). Accessed on 18 August 2019.</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>{{citation-attribution|1={{cite web|url=https://heasarc.gsfc.nasa.gov/docs/hete2/hete2.html|title=The HETE-2 Satellite|publisher=NASA|access-date=2018-04-28}} }}</ref>
|-
| [[Astronomy:INTEGRAL|INTEGRAL]]
| ESA<ref>{{citation-attribution|1={{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2002-048A|title=NASA - NSSDCA - Spacecraft|publisher=NASA|access-date=2018-04-20}} }}</ref>
| 17 October 2002
| {{success|'''Operational'''}}
|-
| [[Astronomy:Suzaku (satellite)|Suzaku]] (Astro-E2)
| [[Organization:JAXA|JAXA]]<ref>{{citation-attribution|1={{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2005-025A|title=NASA - NSSDCA - Spacecraft|publisher=NASA|date=2017-03-21|access-date=2018-04-20}} }}</ref>
| 10 July 2005
| Ended in 2015
|-
| [[Engineering:TWINS|TWINS]]
| NRO ([[Engineering:USA-184|USA-184]];<ref>{{citation-attribution|1={{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}} }}</ref><br/>[[Engineering:USA-200|USA-200]]<ref>{{citation-attribution|1={{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}} }}</ref>)
| TWINS-1: 28 June 2006<br/>TWINS-2: 13 March 2008
| {{success|'''Operational'''}}
|-
| [[Astronomy:CINDI|CINDI]]
| DoD (C/NOFS)<ref>{{citation-attribution|1={{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}} }}</ref>
| 16 April 2008
| Ended in 2015
|-
| [[Astronomy:Hitomi (satellite)|Hitomi]] (Astro-H)
| [[Organization:JAXA|JAXA]]<ref>{{citation-attribution|1={{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}} }}</ref>
| 17 February 2016 
| {{failure|Failed}}
|-
| [[Astronomy:Neutron Star Interior Composition Explorer|NICER]]
| ISS ([[Engineering:SpaceX CRS-11|CRS-11]])
| 3 June 2017
| {{success|'''Operational'''}}
|-
| [[Physics:Global-scale Observations of the Limb and Disk|GOLD]]
| SES ([[Engineering:SES-14|SES-14]])
| 25 January 2018
| {{success|'''Operational'''}}
|-
| [[Astronomy:X-Ray Imaging and Spectroscopy Mission|XRISM]]
| [[Organization:JAXA|JAXA]]
| 6 September 2023
| {{success|'''Operational'''}}
|-
| [[Engineering:Atmospheric Waves Experiment|AWE]]
| ISS ([[Engineering:SpaceX CRS-29|CRS-29]])
| December 2023
| {{pending|In development<ref>{{cite web |last=Hatfield |first=Miles |url=https://blogs.nasa.gov/awe/2023/03/10/awe-set-to-launch-in-december/ |title=AWE Set to Launch in December |work=[[Organization:NASA|NASA]] |date=10 March 2023 |access-date=11 March 2023}}</ref>}}
|-
| [[Astronomy:GUSTO (telescope)|GUSTO]]
| NASA, high-altitude balloon
| December 2023
| {{pending|In development<ref>{{cite web |url=https://www.nasa.gov/content/galacticextragalactic-uldb-spectroscopic-terahertz-observatory-gusto/ |title=Galactic/Extragalactic ULDB Spectroscopic Terahertz Observatory (GUSTO) |work=[[Organization:NASA|NASA]] |access-date=11 March 2023}}</ref><ref>{{cite web|url=https://www.nasa.gov/press-release/nasa-selects-mission-to-study-churning-chaos-in-our-milky-way-and-beyond |title=NASA Selects Mission to Study Churning Chaos of Nearby Cosmos &#124; NASA |publisher=Nasa.gov |date= 2017-03-24|access-date=2018-04-20}}</ref>}}
|-
| [[Astronomy:Sun Radio Interferometer Space Experiment|SunRISE]]
| [[Organization:NASA|NASA]] (Maxar satellite)
| April 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=[[Organization:NASA|NASA]] |date=9 September 2021 |access-date=9 September 2021}}</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=[[Organization:NASA|NASA]] |date=30 March 2020 |access-date=31 March 2020}}</ref>}}
|-
| EZIE
| [[Organization:NASA|NASA]], JHUAPL
| June 2024
| {{pending|In development<ref>{{cite press release |url=https://www.jhuapl.edu/PressRelease/201229-NASA-selects-EZIE-heliophysics |title=Johns Hopkins APL Space Weather Mission Selected by NASA |publisher=JHUAPL |date=29 December 2020 |access-date=30 December 2020}}</ref>}}
|-
| Solar-C EUVST
| [[Organization:JAXA|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=[[Organization:NASA|NASA]] |date=29 December 2020 |access-date=30 December 2020}}</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=[[Organization:American Geophysical Union|AGU23]] |date=15 December 2023 |access-date=26 December 2023}}</ref>}}
|-
| [[Astronomy:Contribution to ARIEL Spectroscopy of Exoplanets|CASE]]
| ESA (Cosmic Vision [[Astronomy: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=[[Organization:NASA|NASA]] |date=8 November 2019 |access-date=12 November 2019}}</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=0|bibcode = 2019PASP..131i4401Z|s2cid = 174801052}}</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 ==
Explorer name numbers can be found in the [[Engineering: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=Explorer 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 Explorer Program Satellites|publisher=National Space Science Data Center |access-date=2018-04-20}}</ref><ref>{{cite web|url=http://www.planet4589.org/space/misc/explorer.html|title=Explorer 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=Explorer Spacecraft Series|publisher=History.nasa.gov|access-date=2018-04-20}}</ref><ref name=skyrocket>{{cite web|url=http://space.skyrocket.de/doc_sat/explorer.htm|title=Explorer 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
| [[Astronomy:Explorer 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 [[Astronomy:Van Allen radiation belt|Van Allen radiation belt]]; launched by the U.S. Army<ref>{{citation-attribution|1={{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}} }}</ref>
|- style="background:#fbb;"

! style="background:#fbb;"| 2
| [[Astronomy:Explorer 2|Explorer 2]]
| 5 March 1958
| Juno I
| 15
| ''Failed''
| —
| —
| Failed to achieve orbit.<ref>{{citation-attribution|1={{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}} }}</ref>
|-

! 3
| [[Astronomy:Explorer 3|Explorer 3]]
| 26 March 1958
| Juno I
| 14
| MEO
| 27 June 1958
| 27 June 1958
| Energetic particle studies helped confirm the presence of [[Astronomy:Van Allen radiation belt|Van Allen radiation belt]]<ref>{{citation-attribution|1={{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}} }}</ref>
|-

! 4
| [[Astronomy:Explorer 4|Explorer 4]]
| July 26, 1958
| Juno I
| 26
| MEO
| October 5, 1958
| October 23, 1959
| Monitor charged particles inside Van Allen belts from nuclear detonations (during [[Engineering:Operation Argus|Operation Argus]])<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1958-005A |title=NASA - NSSDCA - Spacecraft - Details |publisher=National Space Science Data Center |access-date=2018-04-20}}</ref>
|- style="background:#fbb;"

! style="background:#fbb;"| 5
| [[Astronomy:Explorer 5|Explorer 5]]
| August 24, 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 - Details |publisher=National Space Science Data Center |access-date=2018-04-20}}</ref>
|- style="background:#fbb;"

! style="background:#fbb;"|  —
| Explorer S-1 (7X)
| July 16, 1959
| [[Engineering:Juno II|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 - Details |publisher=National Space Science Data Center |access-date=2018-04-20}}</ref>
|-

! 6
| [[Astronomy:Explorer 6|Explorer 6]] (S-2, Able 3)
| August 7, 1959
| [[Engineering:Thor-Able|Thor-Able]]
| 64
| HEO
| October 6, 1959
| July 1, 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 - Details |publisher=National Space Science Data Center |access-date=2018-04-20 }}</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
| [[Astronomy:Explorer 7|Explorer 7]] (S-1A)
| October 13, 1959
| [[Engineering:Juno II|Juno II]]
| 42
| LEO
| August 24, 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}}</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 - Details |publisher=National Space Science Data Center |access-date=2018-04-20 }}</ref>
|- style="background:#fbb;"

! style="background:#fbb;"| –
| S-46A (IE-B)
| March 23, 1960
| [[Engineering:Juno II|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 - Details |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}}</ref>
|-

! 8
| [[Astronomy:Explorer 8|Explorer 8]] (S-30)
| 3 November 1960
| [[Engineering:Juno II|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 - Details |publisher=National Space Science Data Center|access-date=2018-04-20}}</ref>
|- style="background:#fbb;"

! style="background:#fbb;"| –
| [[Astronomy:S-56 (satellite)|S-56]]
| December 4, 1960
| [[Engineering:Scout X-1|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 - Details |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}}</ref>
|-

! 9
| [[Astronomy:Explorer 9|Explorer 9]] (S-56A)
| February 16, 1961
| [[Engineering:Scout X-1|Scout X-1]]
| 36
| LEO
| April 9, 1964
| April 9, 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 - Details |publisher=National Space Science Data Center |access-date=2018-04-20}}</ref>
|- style="background:#fbb;"

! style="background:#fbb;"| –
| [[Astronomy:S-45 (satellite)|S-45]]
| February 24, 1961
| [[Engineering:Juno II|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 - Details |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}}</ref>
|-

! 10
| [[Astronomy:Explorer 10|Explorer 10]] (P 14)
| March 25, 1961
| [[Engineering:Thor-Delta|Thor-Delta]]
| 79
| HEO
| March 25, 1961
| June 1, 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 - Details |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}}</ref>
|-

! 11
| [[Astronomy:Explorer 11|Explorer 11]] (S 15)
| April 27, 1961
| [[Engineering:Juno II|Juno II]]
| 37
| LEO
| November 17, 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 - Details |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}}</ref>
|- style="background:#fbb;"

! style="background:#fbb;"| –
| [[Astronomy:S-45A|S-45A]]
| May 25, 1961
| [[Engineering:Juno II|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 - Details |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}}</ref>
|- style="background:#fbb;"

! style="background:#fbb;"| –
| [[Astronomy:S-55 (satellite)|S-55 (satellite)]] (Meteoroid Satellite-A, Micrometeorite Explorer)
| June 30, 1961
| 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 - Details |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}}</ref>
|-

! 12
| [[Astronomy:Explorer 12|EPE-A]] (S 3, Energetic Particle Explorer-A)
| August 16, 1961
| [[Engineering:Thor-Delta|Thor-Delta]]
| 38
| HEO
| December 6, 1961
| September 1, 1963
| Energetic particle research<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1961-020A |title=NASA - NSSDCA - Spacecraft - Details |publisher=National Space Science Data Center |access-date=2018-04-20}}</ref>
|- style="background: #ffb;"

! style="background: #FFB;" | 13
| S-55A
| August 25, 1961
| [[Engineering:Scout X-1|Scout X-1]]
| 86
| LEO
| August 28, 1961
| August 28, 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 - Details |publisher=National Space Science Data Center |access-date=2018-04-20}}</ref>
|-

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

! 15
| EPE-C (S-3B, Energetic Particle Explorer-C)
| October 27, 1962
| [[Engineering:Delta A|Delta A]]
| 44
| HEO
| January 30, 1963
| January 15, 1978
| Energetic particle research<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1962-059A |title=NASA - NSSDCA - Spacecraft - Details |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}}</ref>
|-

! 16
| S-55B
| December&nbsp;16, 1962
| Scout X-3
| 101
| LEO
| July 22, 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 - Details |publisher=National Space Science Data Center |access-date=2018-04-20}}</ref>
|-

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

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

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

! style="background:#fbb;"| –
| BE-A (Beacon Explorer-A, S-66A)
| March 19, 1964
| [[Engineering:Delta B|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 - Details |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}}</ref>
|-

! 20
| IE-A (S 48, TOPSI, Ionosphere Explorer-A)
| August 25, 1964
| Scout X-4
| 45
| LEO
| December 29, 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 - Details |publisher=National Space Science Data Center |access-date=2018-04-20}}</ref>
|-

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

! 22
| BE-B (Beacon Explorer-B, S-66B)
| October 10, 1964
| 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 - Details |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}}</ref>
|-

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

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

! 25
| [[Astronomy:Injun (satellite)|Injun 4]] (IE-B, Ionosphere Explorer-B)
| November 21, 1964
| 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 - Details |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}}</ref>
|-

! 26
| [[Astronomy:Explorer 26|EPE-D (Energetic Particle Explorer-D)]]
| December 21, 1964
| [[Engineering:Delta C|Delta C]]
| 46
| MEO
| December 27, 1967
| August 23, 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 - Details |publisher=National Space Science Data Center |access-date=2018-04-20}}</ref>
|-

! 27
| [[Astronomy:Explorer 27|BE-C (Beacon Explorer-C,S-66C)]]
| April 29, 1965
| Scout X-4
| 61
| LEO
| July 20, 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 - Details |publisher=National Space Science Data Center |access-date=2018-04-20}}</ref>
|-

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

! 29
| [[Astronomy:Explorer 29|GEOS 1]] (GEOS-A, Geodetic Earth Orbiting Satellite-1)
| November 6, 1965
| [[Engineering:Delta E|Delta E]]
| 387
| LEO
| June 23, 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 - Details |publisher=National Space Science Data Center |date=2017-03-21 |access-date=2018-04-20}}</ref>
|-

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

! 31
| DME-A (Direct Measurements Explorer)
| November 29, 1965
| [[Engineering:Thor-Agena|Thor-Agena]] B
| 99
| LEO
| October 1, 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
| [[Astronomy:Explorer 32|AE-B (Atmosphere Explorer-B)]]
| May 25, 1966
| [[Engineering:Delta C|Delta C]]1
| 225
| LEO
| March 1967
| February 22, 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
| [[Astronomy:Explorer 33|IMP-D (AIMP 1, Interplanetary Monitoring Platform-D)]]
| July 1, 1966
| [[Engineering:Delta E|Delta E]]1
| 212
| HEO
| September 21, 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
| IMP-F (IMP 4, Interplanetary Monitoring Platform-F)
| May 24, 1967
| [[Engineering:Delta E|Delta E]]1
| 163
| MEO
| May 3, 1969
| May 3, 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
| [[Astronomy:Explorer 35|IMP-E (AIMP 2, Interplanetary Monitoring Platform-E)]]
| July 19, 1967
| [[Engineering:Delta E|Delta E]]1
| 230
| [[Astronomy:Lunar orbit|Lunar]]
| June 24, 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)
| January 11, 1968
| [[Engineering:Delta E|Delta E]]1
| 469
| LEO
| July 1, 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
| [[Astronomy:SOLRAD 9|SOLRAD 9]] (SE B)
| March 5, 1968
| Scout B
| 198
| LEO
| April 30, 1974
| November 16, 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
| [[Astronomy:Explorer 38|RAE-A (RAE 1, Radio Astronomy Explorer-A)]]
| July 4, 1968
| [[Engineering:Delta J|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
| AD-C (Atmospheric Density-C)
| August 8, 1968
| Scout B
| 9
| LEO
| June 23, 1971
| June 22, 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
| [[Astronomy:Injun (satellite)|Injun 5]] (Injun C, IE-C, Ionosphere Explorer-C)
| August 8, 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
| IMP-G (IMP 5, Interplanetary Monitoring Platform-G)
| June 21, 1969
| [[Engineering:Delta E|Delta E]]1
| 145
| HEO
| December 23, 1972
| December 23, 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
| [[Astronomy:Uhuru (satellite)|Uhuru]] (SAS-A, SAS 1)
| December 12, 1970
| Scout B
| 142
| LEO
| January 4, 1975
| April 5, 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
| IMP-H (IMP 7, Interplanetary Monitoring Platform-H)
| March 13, 1971
| [[Engineering:Delta M|Delta M]]6
| 635
| MEO
| October 2, 1974
| October 2, 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
| [[Astronomy:SOLRAD 10|SOLRAD 10]] (SE-C, SOLRAD-C)
| July 8, 1971
| Scout B
| 260
| LEO
| June 30, 1973
| December 15, 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
| SSS-A (S-Cubed A)
| November 15, 1971
| Scout B
| 52
| MEO
| September 30, 1974
| January 10, 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 ([[Engineering:Meteoroid Technology Satellite|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>
| August 13, 1972
| Scout D-1
| 90
| LEO
| November 4, 1974
| November 2, 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
| IMP-I (IMP 6, Interplanetary Monitoring Platform-I)
| September 23, 1972
| [[Engineering:Delta 1000|Delta 1604]]
| 635
| HEO
| October 31, 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
| SAS-B (Small Astronomy Satellite-B, SAS 2)
| November 15, 1972
| Scout D-1
| 166
| LEO
| June 8, 1973
| August 20, 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
| [[Astronomy:Explorer 49|RAE-B (RAE 2, Radio Astronomy Explorer-B)]]
| June 10, 1973
| [[Engineering:Delta 1000|Delta 1913]]
| 328
| [[Astronomy:Lunar orbit|Lunar]]
| April 26, 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
| IMP-J (IMP 8, Interplanetary Monitoring Platform-J)
| October 26, 1973
| [[Engineering:Delta 1000|Delta 1604]]
| 371
| HEO
| October 7, 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
| AE-C (Atmosphere Explorer-C)
| December 16, 1973
| [[Engineering:Delta 1000|Delta 1900]]
| 658
| LEO
| (December 12, 1978)
| December 12, 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
| [[Astronomy:Explorer 52|Hawkeye 1]] (Injun-F, Injun 6, IE-D, Ionosphere Explorer-D)
| June 3, 1974
| Scout E-1
| 23
| HEO
| April 28, 1978
| April 28, 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
| SAS-C (Small Astronomy Satellite-C, SAS 3)
| May 7, 1975
| Scout F-1
| 197
| LEO
| April 7, 1979
| April 9, 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
| AE-D (Atmosphere Explorer-D)
| October 6, 1975
| [[Engineering:Delta 2000|Delta 2910]]
| 681
| LEO
| January 29, 1976
| March 12, 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
| AE-E (Atmosphere Explorer-E)
| November 20, 1975
| [[Engineering:Delta 2000|Delta 2910]]
| 735
| LEO
| September 25, 1980
| June 10, 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;"| —
| [[Astronomy:Dual Air Density Explorer|DADE-A]] (Dual Air Density Explorer-A)
| December 5, 1975
| 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;"| —
| [[Astronomy:Dual Air Density Explorer|DADE-B]] (Dual Air Density Explorer-B)
| December 5, 1975
| 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
| [[Astronomy:ISEE-1|ISEE-1]]  (ISEE-A)
| October 22, 1977
| [[Engineering:Delta 2000|Delta 2914]]
| 340
| HEO
| September 26, 1987
| September 26, 1987
| Magnetospheric research; launched with ESA's [[Engineering:ISEE-2|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
| [[Astronomy:International Ultraviolet Explorer|IUE]]
| January 26, 1978
| [[Engineering:Delta 2000|Delta 2914]]
| 669
| MEO
| September 30, 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
| [[Astronomy:Heat Capacity Mapping Mission|HCMM]] (AEM-A)
| April 26, 1978
| Scout F
| 117
| LEO
| September 30, 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
| [[Astronomy:International Cometary Explorer|ICE]] (ISEE 3, ISEE-C)
| August 12, 1978
| [[Engineering:Delta 2000|Delta 2914]]
| 390
| [[Lissajous orbit|Sun–Earth L<sub>1</sub>]]
| September 16, 2014
| [[Astronomy:Heliocentric orbit|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
| [[Astronomy:Stratospheric Aerosol and Gas Experiment|SAGE]] (AEM-B)
| February 18, 1979
| Scout D-1
| 149
| LEO
| January 7, 1982
| April 11, 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
| [[Astronomy:Magsat|MAGSAT]] (AEM-C)
| October 30, 1979
| Scout G-1
| 158
| LEO
| May 6, 1980
| June 11, 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
| [[Engineering:Dynamics Explorer 1|Dynamics Explorer 1]] (DE-1)
| 3 August 1981
| 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
| [[Engineering:Dynamics Explorer 2|Dynamics Explorer 2]] (DE-2)
| 3 August 1981
| 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
| [[Astronomy:Solar Mesosphere Explorer|SME]]
| October 6, 1981
| Delta 2310
| 145
| LEO
| April 4, 1989
| March 5, 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
| [[Engineering:AMPTE-CCE|AMPTE-CCE]]
| 16 August 1984
| Delta 3924
| 242
| MEO
| 12 July 1989
| In orbit
| [[Astronomy: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
| [[Astronomy:Cosmic Background Explorer|COBE]]
| November 18, 1989
| Delta 5920
| 2,206
| LEO
| December 23, 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
| [[Astronomy:Extreme Ultraviolet Explorer|EUVE]] (BERKSAT)
| June 7, 1992
| [[Engineering:Delta II|Delta II]] 6920-X
| 3,275
| LEO
| January 31, 2001
| January 30, 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
| [[Astronomy:Solar Anomalous and Magnetospheric Particle Explorer|SAMPEX]]
| July 3, 1992
| Scout G-1
| 158
| LEO
| June 30, 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>
| November 13, 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
| [[Astronomy:Rossi X-ray Timing Explorer|RXTE]]
| December 30, 1995
| [[Engineering:Delta II|Delta II]] 7920
| 3,200
| LEO
| January 3, 2012 
| April 30, 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
| [[Astronomy:Fast Auroral Snapshot Explorer|FAST]]
| August 21, 1996
| [[Astronomy:Pegasus XL|Pegasus XL]]
| 187
| LEO
| May 4, 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;"| —
| [[Astronomy:High Energy Transient Explorer|HETE 1]]
| November 4, 1996
| [[Astronomy:Pegasus XL|Pegasus XL]]
| 128
| LEO
| —
| April 7, 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
| [[Astronomy:Advanced Composition Explorer|ACE]]
| August 25, 1997
| [[Engineering:Delta II|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
| [[Astronomy:Student Nitric Oxide Explorer|SNOE]]
| February 26, 1998
| [[Astronomy:Pegasus XL|Pegasus XL]]
| 120
| LEO
| December 13, 2003
| December 13, 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
| [[Astronomy:TRACE|TRACE]]
| April 2, 1998
| [[Astronomy:Pegasus XL|Pegasus XL]]
| 250
| LEO
| June 21, 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
| [[Astronomy:Submillimeter Wave Astronomy Satellite|SWAS]]
| December 6, 1998
| [[Astronomy:Pegasus XL|Pegasus XL]]
| 288
| LEO
| September 1, 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
| [[Astronomy:Wide Field Infrared Explorer|WIRE]]
| March 5, 1999
| [[Astronomy:Pegasus XL|Pegasus XL]]
| 250
| [[Astronomy:Sun-synchronous orbit|SSO]]
| September 30, 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>
| May 10, 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
| May 18, 1999
| [[Astronomy:Pegasus XL|Pegasus XL]]
| 120
| Polar LEO
| May 18, 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
| [[Astronomy:Far Ultraviolet Spectroscopic Explorer|FUSE]]
| June 23, 1999
| [[Engineering:Delta II|Delta II]] 7320
| 1,400
| LEO
| October 18, 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
| [[Engineering:IMAGE (spacecraft)|IMAGE]]
| March 25, 2000
| [[Engineering:Delta II|Delta II]] 7326
| 536
| Polar MEO
| December 18, 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
| [[Astronomy:High Energy Transient Explorer|HETE-2]]
| October 9, 2000
| [[Engineering:Pegasus (rocket)|Pegasus-H]]
| 124
| LEO
| March 28, 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}}</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
| June 30, 2001
| [[Engineering:Delta II|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
| February 5, 2002
| [[Astronomy:Pegasus XL|Pegasus XL]]
| 230
| LEO
| August 16, 2018
| In orbit
| 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>
|-

|- style="background:#bfb;"
! style="background:#bfb;"| —
| [[Astronomy:INTEGRAL|INTEGRAL]]
| October 17, 2002
| [[Engineering:Proton-K|Proton-K]] [[Engineering:Blok D|Blok DM-2]]
| 4,000
| HEO
| '''Operational'''
| 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
| [[Astronomy:CHIPSat|CHIPSat]]
| January 13, 2003
| [[Engineering:Delta II|Delta II]] 7320-10
| 60
| LEO
| April 11, 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
| [[Astronomy:GALEX|GALEX]]
| April 28, 2003
| [[Astronomy:Pegasus XL|Pegasus XL]]
| 280
| LEO
| June 28, 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
| [[Astronomy:Neil Gehrels Swift Observatory|Swift]]
| November 20, 2004
| [[Engineering:Delta II|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>

|- 
! —
| [[Astronomy:Suzaku (satellite)|Suzaku]] (Astro E2)
| July 10, 2005
| [[Engineering:M-V|M-V]]
| 1,706
| LEO
| September 2, 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;"| —
| [[Engineering:TWINS|TWINS]] A
| June 28, 2006
| [[Engineering:Delta IV|Delta IV]] M+(4,2)
| ''classified''
| [[Astronomy:Molniya orbit|Molniya]]
| '''Operational'''
| In orbit
| MO: payload on Trumpet-F/O-1 1 ([[Engineering:USA-184|USA-184]])

|- style="background:#bfb;"
! style="background:#bfb;"| 85
| THEMIS A
| February 17, 2007
|  [[Engineering:Delta II|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 B (ARTEMIS P1)
| February 17, 2007
| [[Engineering:Delta II|Delta II]] 7925
| 77
| [[Astronomy: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 C (ARTEMIS P2)
| February 17, 2007
|  [[Engineering:Delta II|Delta II]] 7925
| 77
| [[Astronomy: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 D
| February 17, 2007
|  [[Engineering:Delta II|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 E
| February 17, 2007
| [[Engineering:Delta II|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>

|- style="background:#bfb;"
! style="background:#bfb;"| 90
| [[Astronomy:Aeronomy of Ice in the Mesosphere|AIM]]
| April 25, 2007
| [[Astronomy:Pegasus XL|Pegasus XL]]
| 197
| [[Astronomy:Sun-synchronous orbit|SSO]]
| '''Operational'''
| In orbit
| SMEX: [[Earth:Noctilucent cloud|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;"| —
| [[Engineering:TWINS|TWINS]] B
| March 13, 2008
| [[Engineering:Atlas V|Atlas V]] 411
| ''classified''
| [[Astronomy:Molniya orbit|Molniya]]
| '''Operational'''
| In orbit
| MO: payload on Trumpet-F/O-1 2 ([[Engineering:USA-200|USA-200]])

|- 
! —
| [[Astronomy:CINDI|CINDI]] 
| April 16, 2008
| [[Astronomy:Pegasus XL|Pegasus XL]]
| 395
| LEO
| November 28, 2015
| November 28, 2015
| MO: instruments on C/NOFS

|- style="background:#bfb;"
! style="background:#bfb;"| 91
| [[Astronomy:Interstellar Boundary Explorer|IBEX]]
| October 19, 2008
| [[Astronomy:Pegasus XL|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>

|- style="background:#bfb;"
! style="background:#bfb;"| 92
| [[Astronomy:Wide-field Infrared Survey Explorer|WISE]]
| December 14, 2009
| [[Engineering:Delta II|Delta II]] 7320
| 661
| LEO 
|  '''Operational'''
| In orbit
| MIDEX: infrared astronomy, NEOWISE extension. Discovered first [[Astronomy:Earth trojan|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
| NuSTAR
| June 13, 2012
| [[Astronomy:Pegasus XL|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
| [[Astronomy:Interface Region Imaging Spectrograph|IRIS]]
| June 27, 2013
| [[Astronomy:Pegasus XL|Pegasus XL]]
| 183
| [[Astronomy: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>

|- 
! —
| [[Astronomy:Hitomi (satellite)|Hitomi]] (NeXT,  ASTRO-H)
| February 7, 2016
| H-2A-202
| 2,700
| LEO
| March 26, 2016
| In orbit
| MO: X-ray instrument on JAXA's Hitomi, but spacecraft failed after initial checkouts<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 
| May 3, 2017
| [[Engineering:Falcon 9|Falcon 9]] FT
| 372
| ISS
| '''Operational'''
| ISS
| MO: instrument on ISS for [[Astronomy:Neutron star|neutron star]] observations

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

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

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

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

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

==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 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, [[Astronomy:CINDI|CINDI]], [[Engineering:TWINS|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.

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" />
*[[Astronomy:Full-sky Astrometric Mapping Explorer|FAME]] (MIDEX 4)
*SPIDR (SMEX 8) (technical, 2003)<ref name="nap" />
*[[Astronomy:Gravity and Extreme Magnetism|GEMS]] (SMEX 13)

Recent examples of conclusions of launched missions, cancelled due to budgetary constraints:
*[[Astronomy:Fast Auroral Snapshot Explorer|FAST]] - 2009
*[[Astronomy:TRACE|TRACE]] - 2010 (Solar observatory, see [[Astronomy:Solar Dynamics Observatory|Solar Dynamics Observatory]])
*[[Astronomy:Wilkinson Microwave Anisotropy Probe|Wilkinson MAP]] - 2010
*[[Astronomy:Wide-field Infrared Survey Explorer|WISE]] - 2011 (extended in 2013 as NEOWISE mission)
*[[Astronomy:Rossi X-ray Timing Explorer|RXTE]] - 2012
*[[Astronomy: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=About NASA's Explorer Missions |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 ==
* Cosmic Vision, a [[Organization:European Space Agency|European Space Agency]] (ESA) programme 
* Cosmic Vision S-class missions, the European Space Agency equivalent to the Small Explorer program
* Discovery program
* [[Astronomy:New Frontiers program|New Frontiers program]]

==References==
{{reflist|colwidth=30em}}

==External links==
*{{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=[[Organization:NASA|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 [[Organization:Goddard Space Flight Center|Goddard Space Flight Center]]
* [http://rammb.cira.colostate.edu/dev/hillger/SMEX.htm SMEX-series satellites] by Colorado State University

{{Explorers program}}
{{NASA planetary exploration programs}}
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