Editing Explorers Program (section)

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

The Explorers Program began as a [[United States Army|U.S. Army]] proposal ([[Project Orbiter]]) to place a "civilian" [[satellite|artificial satellite]] into orbit during the [[International Geophysical Year]] (IGY). Although that proposal was rejected in favor of the [[United States Navy|U.S. Navy]]'s [[Project Vanguard]], which made the first sub-orbital flight [[Vanguard TV0]] in December 1956, the [[Soviet Union]]'s launch of [[Sputnik 1]] on 4 October 1957 (and the resulting "[[Sputnik crisis]]") and the failure of the [[Vanguard 1]] launch attempt resulted in the Army program being funded to match the Soviet space achievements. [[Explorer 1]] was launched on the Juno I on 1 February 1958, becoming the first U.S. satellite, as well as discovering the [[Van Allen radiation belt]].

Four follow-up satellites of the Explorer series were launched by the Juno I launch vehicle in 1958, of which [[Explorer 3]] and [[Explorer 4]] were successful, while [[Explorer 2]] and [[Explorer 5]] failed to reach orbit.<ref name=Boehm-NASA>{{citation-attribution|1=J. Boehm, H.J. Fichtner and Otto A. Hoberg, [https://www.nasa.gov/sites/default/files/atoms/files/explorer_i_boehm_document.pdf EXPLORER SATELLITES LAUNCHED BY JUNO 1 AND JUNO 2 VEHICLES] NASA Report}}</ref> The Juno I vehicle was replaced by the [[Juno II]] in 1959.

=== Continuation of the Explorers Program ===
With the establishment of NASA in 1958, the Explorers Program was transferred to NASA from the [[United States Army|U.S. Army]]. NASA continued to use the name for an ongoing series of relatively small space missions, typically an artificial satellite with a specific science focus. [[Explorer 6]] in 1959 was the first scientific satellite under the project direction of NASA's [[Goddard Space Flight Center]] (GSFC) in [[Greenbelt, Maryland]].<ref>{{cite magazine |last=Portree |first=David S. F. |url=https://www.wired.com/2013/05/cometary-explorer-1973/ |title=Cometary Explorer (1973) |magazine=Wired |date=22 May 2013 |access-date=24 June 2019 |url-access=limited}}</ref><ref>{{cite web |url=https://www.nasa.gov/content/goddard-missions-past |title=Goddard Missions |work=Goddard Space Flight Center |publisher=NASA |date=5 June 2018 |access-date=24 June 2019}} {{PD-notice}}</ref>

The [[Interplanetary Monitoring Platform]] (IMP) was launched in 1963 and involved a network of eleven Explorer satellites designed to collect data on space radiation in support of the [[Apollo program]]. The IMP program was a major step forward in spacecraft [[electronics]] design, as it was the first space program to use [[integrated circuit]] (IC) chips and [[MOSFET]]s (MOS transistors).<ref name="Butrica">{{cite book |last1=Butrica |first1=Andrew J. |chapter=Chapter 3: NASA's Role in the Manufacture of Integrated Circuits |editor-last1=Dick |editor-first1=Steven J. |title=Historical Studies in the Societal Impact of Spaceflight |date=2015 |publisher=NASA |isbn=978-1-62683-027-1 |pages=149-250 (237-242) |chapter-url=https://www.nasa.gov/sites/default/files/atoms/files/historical-studies-societal-impact-spaceflight-ebook_tagged.pdf#page=237}} {{PD-notice}}</ref><ref name="nasa">{{cite book |title=Interplanetary Monitoring Platform |date=29 August 1989 |publisher=NASA |pages=1, 11, 134 |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19800012928.pdf |access-date=12 August 2019 |last1=Butler |first1=P. M.}} {{PD-notice}}</ref> The IMP-A ([[Explorer 18]]) in 1963 was the first spacecraft to use IC chips, and the IMP-D ([[Explorer 33]]) in 1966 was the first to use MOSFETs.<ref name="Butrica"/>
{| class="wikitable sortable"
|+List of [[Interplanetary Monitoring Platform]] (IMP) missions
! rowspan="2" |Mission
! rowspan="2" |Photo
! colspan="2" |Satellite
! rowspan="2" |Launch date
! rowspan="2" |Decay date
! rowspan="2" |Notes
|-
!Explorer
!IMP
|-
|[[Explorer 18|IMP-1]]
|[[File:Explorer-18_IMP-A.jpg|frameless|180x180px]]
|Explorer 18
|IMP-A
|27 November 1963, 02:30 UTC<ref name="jonathan">{{cite web |last=McDowell |first=Jonathan |title=Launch Log |url=http://www.planet4589.org/space/log/launchlog.txt |access-date=2018-06-24 |work=Jonathan's Space Page}}</ref>
|December 30, 1965
|First use of [[integrated circuit]]s in a spacecraft. First satellite in IMP-A/-B/-C design series.
|-
|[[Explorer 21|IMP-2]]
|[[File:Explorer-21 image.jpg|frameless|180x180px]]
|Explorer 21
|IMP-B
|4 October 1964, 03:45 UTC<ref name="jonathan" />
|January 1, 1966
|Second satellite in IMP-A/-B/-C design series.
|-
|[[Explorer 28|IMP-3]]
|[[File:Explorer 28.jpg|frameless|180x180px]]
|Explorer 28
|IMP-C
|29 May 1965, 12:00 UTC<ref name="jonathan" />
|4 July 1968
|Third satellite in IMP-A/-B/-C design series.
|-
|[[Explorer 33|AIMP-1]]
|[[File:IMP-D.jpg|frameless|180x180px]]
|Explorer 33
|IMP-D
|1 July 1966, 16:02 UTC<ref name="jonathan" />
|In orbit
|First use of [[MOSFET]] integrated circuits in a spacecraft, similar design to IMP-E. Originally intended to orbit the Moon, but placed in an elliptical high orbit instead.
|-
|[[Explorer 34|IMP-4]]
|[[File:Explorer 34.jpg|frameless|180x180px]]
|Explorer 34
|IMP-F
|24 May 1967, 14:05 UTC<ref name="jonathan" />
|May 3, 1969
|Similar design to IMP-G.
|-
|[[Explorer 35|AIMP-2]]
|[[File:IMP-E.jpg|frameless|180x180px]]
|Explorer 35
|IMP-E
|19 July 1967, 14:19 UTC<ref name="jonathan" />
|After June 24, 1973
|Similar design to IMP-D. Positioned in [[Moon|Selenocentric orbit]].
|-
|[[Explorer 41|IMP-5]]
|[[File:Explorer-41 IMP-G.jpg|frameless|180x180px]]
|Explorer 41
|IMP-G
|21 June 1969, 08:47 UTC<ref name="jonathan" />
|December 23, 1972
|Similar design to IMP-F.
|-
|[[Explorer 43|IMP-6]]
|[[File:Explorer 43 IMP-I.jpg|frameless|180x180px]]
|Explorer 43
|IMP-I
|13 March 1971, 16:15 UTC<ref name="jonathan" />
|October 2, 1974
|First spacecraft in IMP-I/-H/-J series.
|-
|[[Explorer 47|IMP-7]]
|[[File:Explorer-47 IMP-I.jpg|frameless|180x180px]]
|Explorer 47
|IMP-H
|23 September 1972, 01:20 UTC<ref name="jonathan" />
|In orbit
|Second spacecraft in IMP-I/-H/-J series.
|-
|[[Explorer 50|IMP-8]]
|[[File:IMP_8.gif|frameless|180x180px]]
|Explorer 50
|IMP-J
|26 October 1973, 02:26 UTC<ref name="jonathan" />
|In orbit
|Third spacecraft in IMP-I/-H/-J series, remained in service until 2006
|}
Over the following two decades, NASA has launched over 50 Explorer missions,<ref name= "nssdc_list"/> some in conjunction to military programs, usually of an exploratory or survey nature or had specific objectives not requiring the capabilities of a major space observatory. Explorer satellites have made many important discoveries on: Earth's [[magnetosphere]] and the shape of its [[Gravitational field|gravity field]]; the [[solar wind]]; properties of [[micrometeoroids]] raining down on the [[Earth]]; ultraviolet, cosmic and X-rays from the [[Solar System]] and beyond; [[Ionosphere|ionospheric physics]]; [[Solar flare|Solar plasma]]; [[solar energetic particles]]; and [[Atmospheric science|atmospheric physics]]. These missions have also investigated air density, radio astronomy, [[geodesy]], and [[gamma-ray astronomy]].{{citation needed|date=September 2024}}

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

=== SMEX, MIDEX, and Student Explorers Programs ===
In 1988, the '''Small Explorer (SMEX)''' class was established with a focus on frequent flight opportunities for highly focused and relatively inexpensive space science missions in the disciplines of astrophysics and space physics.<ref name=NASAhistory>{{cite book |last=Rumerman |first=Judy A. |url=https://history.nasa.gov/SP-4012v7ch4.pdf |title=NASA Historical Data Book, Vol. VII: NASA Launch Systems, Space Transportation, Human Spaceflight, and Space Science, 1989-1998 |publisher=NASA |date=2009 |access-date=24 June 2019}} {{PD-notice}}</ref><ref>{{cite web |url=https://www.nasa.gov/centers/goddard/pdf/106477main_smex.pdf |title=NASA's Small Explorers Program: Faster, Better, Cheaper |work=Goddard Space Flight Center |publisher=NASA |date=January 1998 |access-date=24 June 2019 |archive-date=16 February 2017 |archive-url=https://web.archive.org/web/20170216162632/https://www.nasa.gov/centers/goddard/pdf/106477main_smex.pdf |url-status=dead}} {{PD-notice}}</ref> The first three SMEX missions were chosen in April 1989 out of 51 candidates, and launched in 1992, 1996 and 1998<ref name=IEEE>{{cite conference |title=SAMPEX: NASA's First Small Explorer Satellite |conference=IEEE Aerospace Conference 21–28 March 1998 Aspen, Colorado |first1=G. M. |last1=Mason |first2=D. N. |last2=Baker |first3=J. B. |last3=Blake |first4=R. E. |last4=Boughner |first5=L. B. |last5=Callis |display-authors=et al. |volume=5 |pages=389–412 |date=1998 |doi=10.1109/AERO.1998.685848}}</ref> The second set of two missions were announced in September 1994 and launched in 1998 and 1999.<ref name=NASAhistory/>[[File:M101 combined low.jpg|thumb|upright=1.0|right|This artificially colored view of M101 maps ultraviolet light as blue while visible light is red since [[Ultraviolet|UV]] light does not have a "color" (the eye stopping at about violet). This view was taken by the MIDEX-3 [[Neil Gehrels Swift Observatory|Swift]], which can also detect X-rays, and has contributed to the study of [[gamma-ray burst]]s and other topics.]]

In the mid-1990s, NASA initiated the '''Medium-class Explorers (MIDEX)''' to enable more frequent flights. These are larger than SMEX missions and were to be launched aboard a new kind of medium-light class launch vehicle.<ref name="NASAhistory" /> This new launch vehicle was not developed and instead, these missions were flown on a modified [[Delta II]] rocket.<ref>{{cite web |author=Ed Kyle |url=http://www.spacelaunchreport.com/delta2.html |archive-url=https://web.archive.org/web/20100325164354/http://www.spacelaunchreport.com/delta2.html |url-status=usurped |archive-date=25 March 2010 |title=Delta II Data Sheet |publisher=Spacelaunchreport.com |access-date=2018-04-28}}</ref><ref>{{cite magazine |url=http://aviationweek.com/awin/nasa-taps-mcdonnell-med-lite-launches |title=NASA Taps Mcdonnell For Med-Lite Launches |magazine=Aviation Week |date=4 March 1996 |access-date=28 April 2018}}</ref> The first announcement opportunity for MIDEX was issued in March 1995, and the first launch under this new class was [[Far Ultraviolet Spectroscopic Explorer|FUSE]] in 1999.<ref name="NASAhistory" />

In May 1994, NASA started the '''Student Explorer Demonstration Initiative''' (STEDI) pilot program, to demonstrate that high-quality space science can be carried out with small, low-cost missions. Of the three selected missions, SNOE was launched in 1998 and TERRIERS in 1999, but the latter failed after launch. The STEDI program was terminated in 2001.<ref name=NASAhistory/> Later, NASA established the '''University-Class Explorer''' (UNEX) program for much cheaper missions, which is regarded as a successor to STEDI.<ref>{{cite web |url=https://web.stanford.edu/~sbuchman/publications-PDF/The%20Large%20Benefits%20of%20Small%20Satellite%20Missions.pdf |title=The Large Benefits of Small Satellite Missions |access-date=2018-04-28}}</ref>

The Explorer missions were at first managed by the Small Explorers Project Office at NASA's Goddard Space Flight Center (GSFC). In early 1999, that office was closed and with the announcement of opportunity for the third set of SMEX missions NASA converted the SMEX class so that each mission was managed by its [[principal investigator]], with oversight by the GSFC Explorer Project.<ref name="welcome">{{cite web |url=http://sunland.gsfc.nasa.gov/smex/ |title=Welcome to the Small Explorer's Web Site |publisher=NASA |date=18 February 2000 |url-status=dead |archive-url=https://web.archive.org/web/20000817054104/http://sunland.gsfc.nasa.gov/smex/ |archive-date=17 August 2000}} {{PD-notice}}</ref> The Explorers Program Office at Goddard Space Flight Center, provides management of the many operational scientific exploration missions that are characterized by relatively moderate costs and small to medium-sized missions that are capable of being built, tested, and launched in a short time interval compared to larger observatories like NASA's [[Great Observatories]].<ref>{{cite web |url=http://explorers.gsfc.nasa.gov/history.html |title=Explorers Program |website=explorers.gsfc.nasa.gov |date=1958-01-31 |access-date=2016-02-24 |archive-url=https://web.archive.org/web/20160304052517/http://explorers.gsfc.nasa.gov/history.html |archive-date=4 March 2016 |url-status=dead}} {{PD-notice}}</ref>

Excluding the launches, the MIDEX class has a current mission cap cost of US$250 million in 2018,<ref name=Midex19>{{cite web |url=https://www.nasa.gov/press-release/nasa-selects-proposals-to-study-galaxies-stars-planets |title=NASA Selects Proposals to Study Galaxies, Stars, Planets |date=9 August 2017 |publisher=NASA |access-date=28 April 2018}} {{PD-notice}}</ref> with future MIDEX missions being capped at US$350 million.<ref>{{cite web |author=Jeff Foust |url=http://spacenews.com/earth-science-decadal-report-recommends-mix-of-large-and-small-missions/ |title=Earth science decadal report recommends mix of large and small missions |date=5 January 2018 |publisher=SpaceNews |access-date=28 April 2018}}</ref> The cost cap for SMEX missions in 2017 was US$165 million.<ref>{{cite web |last1=Wu |first1=Chauncey |last2=Manuel |first2=Greg |last3=Salas |first3=Andrea |url=https://explorers.larc.nasa.gov/HPSMEX/pdf_files/05-2016_Helio_PPC_TMC_Wu_v2.pdf |title=2016 Heliophysics Small Explorers (SMEX) & Mission of Opportunity (MO) Solicitations Pre-Proposal Conference |publisher=NASA |date=15 August 2016 |access-date=24 June 2019}} {{PD-notice}}</ref> UNEX missions are capped at US$15 million.<ref name=missions>{{cite web |url=http://explorers.gsfc.nasa.gov/missions.html |title=Explorers Missions |publisher=NASA |access-date=28 August 2015 |url-status=dead |archive-url=https://web.archive.org/web/20100323182500/http://explorers.gsfc.nasa.gov/missions.html |archive-date=23 March 2010}} {{PD-notice}}</ref> A sub-project called '''Missions of Opportunity''' (MO) has funded science instruments or hardware components of onboard non-NASA space missions, and have a total NASA cost cap of US$70 million.<ref name=Midex19/><ref name=missions/>