Interplanetary Monitoring Platform

The Explorers program is a United States space exploration program that provides flight opportunities for physics, geophysics, heliophysics, and astrophysics investigations from space. 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.

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.

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.

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