Explorer 3
Photograph of the nearly identical Explorer 1
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| Mission type | Earth science |
|---|---|
| Operator | Army Ballistic Missile Agency (ABMA) |
| Harvard designation | 1958 Gamma 1 |
| SATCAT no. | 00006 |
| Mission duration | 93 days |
| Spacecraft properties | |
| Manufacturer | Jet Propulsion Laboratory |
| Launch mass | 14.1 kilograms (31 lb) |
| Start of mission | |
| Launch date | March 26, 1958, 17:31 UTC |
| Rocket | Juno I |
| Launch site | Cape Canaveral LC-5 |
| End of mission | |
| Decay date | June 27, 1958 |
| Orbital parameters | |
| Reference system | Geocentric |
| Regime | Medium Earth |
| Semi-major axis | 7,870.7 kilometers (4,890.6 mi) |
| Eccentricity | 0.165894 |
| Perigee | 186 kilometers (116 mi) |
| Apogee | 2,799 kilometers (1,739 mi) |
| Inclination | 33.38 degrees |
| Period | 115.7 minutes |
Explorer 3 (international designation 1958 Gamma) was an artificial satellite of the Earth, nearly identical to the first United States artificial satellite Explorer 1 in its design and mission. It was the second successful launch in the Explorer program.
The satellite was launched from Cape Canaveral Air Force Station, Florida at 17:31:00 UTC on March 26, 1958, by the Juno I vehicle. The Juno I had its origins in the United States Army's Project Orbiter in 1954. The project was canceled in 1955, however, when the decision was made to proceed with Project Vanguard.
Following the launch of the Soviet Sputnik 1 on October 4, 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 JPL completed the job of modifying the Jupiter-C to the Juno I and building the Explorer I in 84 days.
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 June 27, 1958, after 93 days of operation.
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