Space Shuttle Explorer (disambiguation)

Pointing X-ray Eyes at our Resident Supermassive Black Hole

An explorer mission observes the Milky Way's black hole flaring

The Explorers program is a United States space exploration program that provides flight opportunities for physics, geophysics, heliophysics, and astrophysics investigations from space.[1] Over 90 space missions have been launched since 1958, and it is still active. Starting with Explorer 6, it has been operated by NASA, with regular collaboration with a variety of other institutions, including many international partners.


Ignition of Jupiter-C with Explorer 1

Explorer 1's Jupiter rocket ignites

Sputnik 1

Sputnik caused an uproar in the West

The Explorers program was the United States's first successful attempt to launch an artificial satellite. It began as a U.S. Army proposal (Project Orbiter) to place a scientific satellite into orbit during the International Geophysical Year; however, that proposal was rejected in favor of the U.S. Navy's Project Vanguard. The Explorers program was later reestablished to catch up with the Soviet Union after that nation's launch of Sputnik 1 on October 4, 1957. (See: Sputnik crisis) Explorer 1 was launched on January 31, 1958. Besides being the first U.S. satellite, it is known for discovering the Van Allen radiation belt.

The Explorers program was transferred to NASA, which continued to use the name for an ongoing series of relatively small space missions, typically an artificial satellite with a specific science focus. Over the years, NASA has launched a series of Explorers spacecraft carrying a wide variety of scientific investigations.

M101 combined low

This artificially colored view of M101 maps ultraviolet light as blue while visible light is red since UV light does not have a "color" (the eye stopping at about violet). This view was taken by the Explorer SWIFT, which can also detect X-Rays, and has contributed to the study of Gamma-ray bursts and other topics

Explorers satellites have made many important discoveries on: Earth's magnetosphere and the shape of its gravity field; the solar wind; properties of micrometeoroids raining down on the Earth; ultraviolet, cosmic, and X-rays from the Solar System and universe beyond; ionospheric physics; Solar plasma; solar energetic particles; and atmospheric physics. These missions have also investigated air density, radio astronomy, geodesy, and gamma ray astronomy. Various space telescopes have made a variety of discoveries, including the first known Earth Trojan asteroid.

The Explorers Program Office at Goddard Space Flight Center in Greenbelt, Maryland, provides management of the many operational scientific exploration missions in the program. The missions 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.[2]

Explorers categories have included MIDEX (Medium Explorer), SMEX (Small Explorer), UNEX (University-Class Explorer), and others.[3] A subprogram called Missions of Opportunity (MO) has funded instruments on non-NASA missions.

Launchers used for these missions include Jupiter C (Juno I), Juno II, various Thor rockets such as the Thor-Able, Scout, various Delta and Delta II rockets of the Delta rocket family, and Pegasus.

Mission typesEdit

Medium-Class Explorers (MIDEX)Edit

The Medium-Class Explorer (MIDEX) program is an effort within NASA to fund space exploration missions that cost no more than US$180 million.[4]

List of MIDEX missions[5][6]
Launch (UTC) Status
Interplanetary Monitoring Platform-8 (IMP-8) Explorer-50 October 26, 1973 Completed: October 28, 2001
RXTE Explorer-69 December 30, 1995 Completed: January 5, 2012
ACE Explorer-71 August 25, 1997 Operational
FUSE MIDEX 0 Explorer-77 June 23, 1999 Completed: October 18, 2007
IMAGE MIDEX 1 Explorer-78 March 25, 2000 Completed
WMAP MIDEX 2 Explorer-80 June 30, 2001 Completed: October 2012
SWIFT MIDEX 3 Explorer-84 November 20, 2004 Operational
FAME MIDEX 4 Cancelled
THEMIS A MIDEX 5A Explorer-85 February 17, 2007 Operational
THEMIS B MIDEX 5B Explorer-86 February 17, 2007 Operational
THEMIS C MIDEX 5C Explorer-87 February 17, 2007 Operational
THEMIS D MIDEX 5D Explorer-88 February 17, 2007 Operational
THEMIS E MIDEX 5E Explorer-89 February 17, 2007 Operational
WISE MIDEX 6 Explorer-92 December 14, 2009 Operational
ICON Summer 2017 In development
TESS Summer 2017 In development

Small Explorers (SMEX)Edit

The Small Explorer (SMEX) program is an effort within NASA to fund space exploration missions that cost no more than US$120 million.[4] started in 1989.[7] 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 missions were 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 program so that each mission was managed by its Principal Investigator, with oversight by the GSFC Explorers Project.[8]

List of SMEX missions[6][9]
Launch (UTC) Status
SAMPEX SMEX-1 Explorer-68 3 July 1992 Ended: 30 June 2004
Reentered: 13 November 2012
FAST SMEX-2 Explorer-70 21 August 1996 Ended: 4 May 2009
SWAS SMEX-3 Explorer-74 6 December 1998 Ended: 21 July 2004
TRACE SMEX-4 Explorer-73 2 April 1998 Ended: 21 June 2010
WIRE SMEX-5 Explorer-75 5 March 1999 Spacecraft equipment failure
Reentered: 10 May 2011
RHESSI SMEX-6 Explorer-81 5 February 2002 Operational
GALEX SMEX-7 Explorer-83 28 April 2003 Ended: May 2012
Decommissioned: 28 June 2013
SPIDR SMEX-8 Cancelled, instrument not sensitive as expected
AIM SMEX-9 Explorer-90 25 April 2007 Operational
IBEX SMEX-10 Explorer-91 19 October 2008 Operational
NuSTAR SMEX-11 Explorer-93 13 June 2012 Operational
IRIS SMEX-12 Explorer-94 28 June 2013 Operational
GEMS SMEX-13 Cancelled, expected cost overrun

University-Class Explorers (UNEX)Edit

Investigations characterized by definition, development, mission operations, and data analysis costs not to exceed $15.0M (real year dollars) total cost to NASA. UNEX missions will be launched by a variety of low cost methods.

List of UNEX missions[10]
Launch (UTC) Status
SNOE Explorer-72 26 February 1998 Completed: 30 September 2000
CHIPS Explorer-82 12 January 2003 Completed

Missions of Opportunity (MO)Edit

The Missions of Opportunity (MO) program provides funding for science instruments or hardware components of onboard non-NASA space missions. MO missions have a total NASA cost of under $55 million.[4]

List of MO missions[10]
Name Explorer
Launch (UTC) Status
HETE-2 Explorer-79 9 October 2000 Ended
CINDI - 16 April 2008 Operational
Suzaku - 10 July 2005 Completed: 2 September 2015
TWINS - TWINS-1: 28 June 2006
TWINS-2: 13 March 2008
Astro-H - 17 February 2016 Failed
GOLD - 2017 In development
NICER - Early 2017 In development

International MissionsEdit

Main article: INTEGRAL

Spacecraft by yearEdit

Explorers name numbers can be found in the NSSDC master catalog, typically assigned to each spacecraft in a mission. These numbers were not officially assigned until after 1975.[11]


WISE artist concept (PIA17254, crop)

WISE was restarted after it was turned off

Thor-Able III Explorer 6

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

ISEE-C (ISEE 3) in dynamic test chamber

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.[23] 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.

Examples of missions that were not developed or cancelled were:[24]

Examples of recent missions conclusions, sometimes cancelled due to budget constraints:

Launch graphEdit

Roughly the number of launches per decade:[25]

Approximate # of Explorer Launches by decade
1950s Template:Bar chart/bar
1960s Template:Bar chart/bar
1970s Template:Bar chart/bar
1980s Template:Bar chart/bar
1990s Template:Bar chart/bar
2000s Template:Bar chart/bar
2010s Template:Bar chart/bar

See alsoEdit


  1. "Explorers Program". Goddard Space Flight Center. NASA. 2009. Retrieved 2009-12-05. 
  2. "Explorers Program". 1958-01-31. Retrieved 2016-02-24. 
  3. "Explorers Program". Retrieved 2016-02-24. 
  4. 4.0 4.1 4.2 "Explorers Missions". NASA. Retrieved 28 August 2015. 
  5. "Explorers Program". 
  6. 6.0 6.1 "Explorer Program". 
  7. Template:Cite conference
  8. "Welcome to the Small Explorer's Web Site". NASA. 18 February 2000. Archived from the original on 17 August 2000. 
  9. "Explorers Program". 
  10. 10.0 10.1 "Explorers Program". 
  11. "Explorer Program". Retrieved 2016-02-24. 
  12. "Explorer 33 (NSSDC ID: 1966-058A)". NASA / National Space Science Data Center. 2008-04-02. Retrieved 2008-07-04. 
  13. "NASA - NSSDCA - Spacecraft - Details". 2016-02-12. Retrieved 2016-02-24. 
  14. "The SAMPEX Data Center". Retrieved 2016-02-24. 
  15. "SAMPEX | The Aerospace Corporation". Retrieved 2016-02-24. 
  16. "< Welcome to T R A C E on-line >". 2010-06-21. Retrieved 2016-02-24. 
  17. WIRE Web Team. "WIRE: The Wide Field Infrared Explorer". Retrieved 2016-02-24. 
  18. "NASA - IMAGE Science Center". Retrieved 2016-02-24. 
  19. "Missions - HETE-2 - NASA Science". Retrieved 2016-02-24. 
  20. "Mission Complete! WMAP fires its thrusters for the last time". 
  21. Savage, Sam (2008-06-04). "CHIPSat Quietly Shut Down". Retrieved 2016-02-24. 
  22. "Press Release: NASA Decommissions Its Galaxy Hunter Spacecraft". 2013-06-28. Retrieved 2016-02-24. 
  23. "Science Office for Mission Assessments: Explorer 2011". Retrieved 2016-02-24. 
  24. "Explorer Program". Retrieved 2016-02-24. 
  25. "About NASA's Explorer Missions". Retrieved 2016-02-24. 

External linksEdit

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