||The lead section of this article may need to be rewritten. (September 2009)|
|Operator||Army Ballistic Missile Agency|
|Major contractors||Jet Propulsion Laboratory|
|Mission type||Earth science|
|Launch date||March 26, 1958 at 17:31 UTC|
|Launch vehicle||Juno I|
|Mission duration||93 days|
|Orbital decay||June 27, 1958|
|Homepage||NASA NSSDC Master Catalog|
|Semimajor axis||7,870.7 km|
|Orbital 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.
Following the launch of the Soviet Sputnik 1 on October 4, 1957, 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 and building the Explorer I in 84 days.
Spacecraft design 
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.
Mission results 
The discovery of the Van Allen radiation belt by the Explorer satellites was considered to be one of the outstanding discoveries of the IGY.
Explorer 3 was placed in an orbit with a perigee of 186 kilometers and an apogee of 2799 kilometers having a period of 115.7 minutes. Its total weight was 14.1 kilograms, of which 8.4 kg was instrumentation. The instrument section at the front end of the satellite and the empty scaled-down fourth-stage rocket casing orbited as a single unit, spinning around its long axis at 750 revolutions per minute.
Instrumentation consisted of a cosmic ray detection package and a ring of micrometeorite erosion gauges. The Explorer 3 spacecraft was spin-stabilized and had an on-board tape recorder to provide a complete radiation history for each orbit. Data from these instruments was transmitted to the ground by a 60 milliwatt transmitter operating on 108.03 MHz and a 10 milliwatt transmitter operating on 108.00 MHz.
Transmitting antennas consisted of two fiberglass slot antennas in the body of the satellite itself and four flexible whips forming a turnstile antenna. The rotation of the satellite about its long axis kept the flexible whips extended.
The external skin of the instrument section was painted in alternate strips of white and dark green to provide passive temperature control of the satellite. The proportions of the light and dark strips were determined by studies of shadow-sunlight intervals based on firing time, trajectory, orbit, and inclination.
Electrical power was provided by nickel-cadmium chemical batteries that made up approximately 40 percent of the payload weight. These provided power that operated the high power transmitter for 31 days and the low-power transmitter for 105 days.
Because of the limited space available and the requirements for low weight, the Explorer 3 instrumentation was designed and built with simplicity and high reliability in mind. It was completely successful.
Explorer 3 decayed from orbit on June 27, 1958, after 93 days of operation.