Explorers program

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 a NASA program, and they have worked with a variety of other institutions and business, including many international partners.

History

Explorer 1's Jupiter rocket ignites

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 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 science focus. Over the years, NASA has launched a series of Explorers spacecraft carrying a wide variety of scientific investigations.

Explorers satellites have made important discoveries: Earth's magnetosphere and the shape of its gravity field; the solar wind; properties of micrometeoroids raining down on the Earth; much about 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 multiple scientific exploration missions in the Explorers space flight program. The missions are characterized by relatively moderate cost, and by small to medium-sized missions that are capable of being built, tested and launched in a short time interval compared to the large observatories.^[2]

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

Launchers have included Jupiter C (Juno I), Juno II, various Thor rockets such as Thor-Able, Scout, various Delta and Delta II rockets of the Delta rocket family, and Pegasus.

Spacecraft by year

Explorers name numbers can be found in the NSSDC master catalog, typically assigned to each spacecraft in a mission. However, one source says they were not officially assigned after 1975.^[4]

Spacecraft by name

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

ISEE-C in a dynamic test chamber, 1978

Explorer missions in the NSSDC master catalog listing. This does not include Missions of Opportunity (MO), but does include some spacecraft with launch failures or were cancelled.

Other missions

Examples of cancelled missions, include:^[15]

• Owl 1
• Owl 2
• MSS A
• CATSAT (STEDI 3)
• IMEX (UNEX 2)
• FAME (MIDEX 4)
• SPIDR (SMEX 8)
• GEMS

Many more missions are proposed, but not selected. For example in 2011, the overall explorers program received 22 full missions solicitations, 20 Missions of Opportunity, and 8 USPI.^[16] Another type of program called missions of opportunity (MO), which adds small missions to other spacecraft, such as an additional instrument. Examples of this include Astro-H, CINDI, TWINS, and HETE-2.

Mission endings

• GALEX - 2013
• RXTE - 2012
• WISE - 2011
• WMAP - 2010
• TRACE - 2010
• FAST - 2009

See also

• Small Explorer program
• Discovery program
• New Frontiers program
• Cosmic Vision
• Tom Krimigis

References

External links

• NASA Explorers program missions page
• NSSDC List of Explorers missions from 1 to 77
• Explorer Program Profile by NASA's Solar System Exploration
• 1957 Video (30 minutes) on launch and impact of Explorer
• Master list of Names, Initialisms, and Abbreviations for un-manned satellites
• U.S. Space Objects Registry
• Explorer Program at Gunters Space Page (detailed list of explorers missions
• NASA Science - Explorers

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