The Explorer program is a NASA program that provides flight opportunities for heliophysics and astrophysics investigations from space. The program includes 92 missions since 1958, the latest including two missions flown in 2007, one in 2008, and one in 2009.
The Explorer program was the United States's first successful attempt to launch an artificial satellite. It began as a U.S. Army proposal 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 Explorer 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 Explorer program was taken over by NASA, which continued to use the name for robotic spacecraft missions. Over the years, NASA has launched a series of "Explorer" spacecraft carrying a wide variety of scientific investigations.
Explorer 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. Some Explorer spacecraft have even traveled to other planets, and some have monitored the Sun.
The Explorers Program Office at Goddard Space Flight Center in Greenbelt, Maryland, provides management of the multiple scientific exploration missions in the Explorer 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.
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