Explorer 3 (Harvard designation 1958 Gamma) was an American artificial satellite launched into medium Earth orbit in 1958. It was the second successful launch in the Explorer program, and was nearly identical to the first U.S. satellite Explorer 1 in its design and mission. Background The U.S. Earth satellite program began in 1954 as a joint U.S. Army and U.S. Navy proposal, called Project Orbiter, to put a scientific satellite into orbit during the International Geophysical Year (IGY). The proposal, using a U.S. Army Redstone missile, was rejected in 1955 by the Eisenhower administration in favor of the U.S. Navy's Project Vanguard, using a booster advertised as more civilian in nature. Following the launch of the Soviet satellite Sputnik 1 on 4 October 1957, the initial Project Orbiter program was revived as the Explorer program to catch up with the Soviet Union. Launch The satellite was launched from Cape Canaveral Missile Test Center of the Atlantic Missile Range (AMR), in Florida at 17:38:01 GMT on 26 March 1958 by the Juno I launch vehicle. The Juno I had its origins in the United States Army's Project Orbiter in 1954. The project was canceled in 1955 when the decision was made to proceed with Project Vanguard. Following the launch of the Soviet Sputnik 1 on 4 October 1957, Army Ballistic Missile Agency (ABMA) was directed to proceed with the launching of a satellite using the Jupiter-C which had already been flight-tested in nose-cone re-entry tests for the Jupiter IRBM (intermediate-range ballistic missile). Working closely together, ABMA and Jet Propulsion Laboratory (JPL) completed the job of modifying the Jupiter-C to the Juno I and building the Explorer I in 84 days. Spacecraft Explorer 3 was launched in conjunction with the International Geophysical Year (IGY) by the U.S. Army (Ordnance) into an eccentric orbit. The objective of this spacecraft was a continuation of experiments started with Explorer 1. The payload consisted of a cosmic ray counter (a Geiger-Müller tube) and a micrometeorite detector (a wire grid array and acoustic detector). The Explorer 3 spacecraft was spin-stabilized and had an on-board tape recorder to provide a complete radiation history for each orbit. It was discovered soon after launch that the satellite was in a tumbling motion with a period of about 7 seconds. Explorer 3 decayed from orbit on 28 June 1958, after 93 days of operation. Mission results The discovery of the Van