Explorer 4 measured secret 1958 nuclear blasts in space

Explorer 4, launched from Cape Canaveral on July 26, 1958, carried University of Iowa radiation detectors with a double job: map Earth’s natural radiation belts in detail, and measure the artificial belts made by three secret nuclear explosions set off high above the South Atlantic that summer.

The tests were Operation Argus, carried out by Navy Task Force 88, nine ships and about 4,500 crew, just five months after the idea was conceived. Physicist Nicholas Christofilos had predicted that a blast at very high altitude would inject a shell of electrons into Earth’s magnetic field, with possible effects on radar and communications.

James Van Allen, Carl McIlwain and George Ludwig built the instruments in under three months, using smaller and more discriminating detectors than the single Geiger tube on Explorer 1, chosen with knowledge of how intense the natural belts were.

What happened, and when

• July 26, 1958: Explorer 4 reaches orbit and starts recording the natural background.
• August 1958: A backup, Explorer 5, fails to reach orbit, so Explorer 4 carries the experiment alone, supported by sounding rockets and more than 40 ground stations tracking its telemetry.
• August 27, August 30 and September 6, 1958: Nuclear-tipped rockets are fired from the USS Norton Sound. Each burst leaves a shell of trapped electrons spiralling along magnetic field lines to altitudes of more than 4,000 miles, confirming Christofilos’s prediction.
• December 1958: Pioneer 3 data, together with Explorer 4’s natural-belt measurements, lead to the discovery of a second, outer belt encircling the inner one.
• Spring 1959: The public learns of the tests.

Van Allen later judged the payoff in his own words:

“Analysis of our Explorer IV data on the natural radiation belt as well as on the artificial radiation belts from the Argus bursts propelled the entire subject to a new level of understanding and broad scientific interest.” — James Van Allen