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| Explorer 4 launch cover with Goldcraft cachet. |
Explorer 4 was launched on 26 July 1958. The Department of Defense's Advanced Research Projects Agency (ARPA) had initially planned two satellites for the purposes of studying the Van Allen radiation belts and the effects of nuclear explosions upon these belts (and the Earth's magnetosphere in general), however Explorer 4 was the only such satellite launched as the other, Explorer 5, suffered launch failure.
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| Explorer 3 launch cover with Sarzin cachet. |
Explorer 4 was a cylindrically shaped satellite instrumented to make the first detailed measurements of charged particles (protons and electrons) trapped in the terrestrial radiation belts. The purpose of this experiment was to extend the first measurements of the trapped radiation belt discovered with Explorer 1 and Explorer 3 and to provide measurements of artificially injected electrons from the three high-altitude Argus nuclear detonations.
The mission remained secret from the public for six months. The satellite telemetry was analyzed for three Operation Argus nuclear weapons tests at high altitude. Explorer 4 was in orbit and operational during the three Project Argus launches 27 August 1958 to 6 September 1958, part of the mission objective was to observe the effects of these high-altitude A-bomb detonations on the space environment.
(Reference from
Explorer 4)
