|Mission type||Space physics|
|Harvard designation||1961 Upsilon 1|
|Mission duration||112 days (achieved)|
|Launch mass||37.6 kg (83 lb)|
|Start of mission|
|Launch date||16 August 1961,|
|Launch site||Cape Canaveral, LC-17B|
|Contractor||Douglas Aircraft Company|
|End of mission|
|Last contact||6 December 1961|
|Decay date||1 September 1963|
|Reference system||Geocentric orbit|
|Regime||Low Earth orbit|
|Perigee altitude||790 km (490 mi)|
|Apogee altitude||77,620 km (48,230 mi)|
Explorer 12 (also known as S3) was a United States Satellite built to measure the solar wind, cosmic rays, and the Earth's magnetic field. It was launched on 16 August 1961, aboard a Thor-Delta launch vehicle. Explorer 12 was the first of four S3 series spacecraft. It ceased transmitting on 6 December 1961 due to power failure.
This satellite was launched from the Atlantic Missile Range by a Delta launch vehicle on 16 August 1961. Its objective was the investigation solar wind, interplanetary magnetic fields, distant portions of the Earth's magnetic field, and energetic particles in interplanetary space and in the Van Allen belt.
The spacecraft weighed 37.6 kg (83 lb). Its instrumentation included 10 particle detection systems for the measurement of protons and electrons and their relation to magnetic fields, a solar cell damage experiment, optical aspect sensor and one transmitter.
The spacecraft achieved orbit and all instrumentation operated normally. Its transmitter ceased operations on 6 December 1961, after sending 2568 hours of real time data. During its life of 112 days, it completed 102 orbits and data was acquired approximately 80% of the time. This satellite provided significant geophysical data on radiation and magnetic storms.
Explorer 12 was designed to study space physics, and so had a multitude of instruments including a cosmic-ray detector, a particle trapper, and a magnetometer. Good data was recorded for 90% of the mission.