Explorers Program: Difference between revisions

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[[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 programProgram'''<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 ExplorerExplorers programProgram 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]].

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.

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[[File: Explorer1.jpg|thumb|upright=1.0|right|Explorer 1, the first Earth satellite orbited by the United States]]

The ExplorerExplorers programProgram 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 ExplorerExplorers programProgram ===

With the establishment of NASA in 1958, the ExplorerExplorers programProgram 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"/>

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! rowspan="2" |Photo

! colspan="2" |Satellite

! rowspan="2" |Launch Datedate

! rowspan="2" |Decay Datedate

! rowspan="2" |Notes

|-

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|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>

|December 30, 1965

|First use of [[Integratedintegrated circuit|integrated circuits]]s in a spacecraft. First satellite in IMP-A/-B/-C design series.

|-

|[[Explorer 21|IMP-2]]

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|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]].{{factcitation needed|date=September 2024}}

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

=== SMEX, MIDEX, and Student ExplorerExplorers programsPrograms ===

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 ExplorerExplorers 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 ExplorerExplorers (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 ExplorerExplorers 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 ExplorerExplorers programProgram 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 ExplorerExplorers (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=ExplorerExplorers 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 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=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>

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| Explorer-69

| 30 December 1995

| Ended in 2012 / Reentered on 30 April 2018

|-

| [[Advanced Composition Explorer|ACE]]

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| 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

|-

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| [[SPHEREx]]

| MIDEX-9<!--The sequential number, but needs a reference-->

| Explorer-102

|

| 2712 FebruaryMarch 2025

| {{success|'''Operational'''}}

| {{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=Jet Propulsion Laboratory |publisher=NASA |access-date=7 July 2021}} {{PD-notice}}</ref>}}

|-

| [[Multi-slit Solar Explorer|MUSE]]

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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 ExplorerExplorers 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 ExplorerExplorers (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=ExplorerExplorers 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=ExplorerExplorers 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

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| 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>

|

|-

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| Explorer-90

| 25 April 2007

| 19 August 2024

| Reentered on 19 August 2024

|

|-

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| SMEX-15

|

| 2712 FebruaryMarch 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>

|

| {{pendingsuccess|In development'''Operational'''}}<ref name="nasa-20220803">{{cite web |last=InterranteOxford |first=AbbeyClarence |url=https://blogswww.nasaspacedaily.govcom/punch/2022/08/03/punch-announces-rideshare-with-spherex-and-new-launch-datereports/NASAs_PUNCH_Mission_Commences_Study_of_Solar_Wind_999.html |title=NASA's PUNCH AnnouncesMission RideshareCommences withStudy SPHERExof andSolar New Launch DateWind |date=313 AugustMarch 20222025 |access-date=319 AugustMarch 20222025 |work=NASASpace Daily}} {{PD-notice}}</ref>

|-

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| SMEX-16

|

| 23 July 2025

| 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|InSpacecraft developmentcommissioning}}

|-

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| Explorer-72

| 26 February 1998

| Ended in 2000; decayed from orbit in December 2003

|-

| IMEX

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== Launched spacecraft ==

ExplorerExplorers 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=ExplorerExplorers 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 ExplorerExplorers 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=ExplorerExplorers 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=ExplorerExplorers 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=ExplorerExplorers Program |website=space.skyrocket.de |access-date=2016-02-24}}</ref>

|-

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| [[Sun-synchronous orbit|SSO]]

| March 2023

| Reentered on 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>

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| LEO

| August 2024

| 2 November 2024

| In orbit

| 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>

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[[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.{{factcitation needed|date=September 2024}}

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

Line 1,866:

==Launch statistics==

Number of launches per decade:<ref>{{cite web|url=http://nssdc.gsfc.nasa.gov/multi/explorer.html |title=About NASA's ExplorerExplorers Program MissionsSatellites |website=Nssdc.gsfc.nasa.gov |access-date=2016-02-24}}</ref>

{{Bar graph

| title = Number of Explorer launches by decade

Line 1,896:

{{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 ExplorerExplorers Program program

* {{annotated link|Discovery program}}

* {{annotated link|New Frontiers program}}

Line 1,904:

==External links==

{{commons category|Explorer program|Small 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]