Following the launch of the Soviet satellite Sputnik 1 on 4 October 1957, Project Orbiter was revived,[4] with two shots authorized as a back-up to Vanguard in early November.

[4] An experiment developed for Vanguard by George Ludwig,[2]: 238  comprising an Anton 314 omnidirectional Geiger tube detector for measuring the flux of high energy charged protons and electrons, was adapted for Explorer 1.

[5] Because of the high spin rate of the Explorer 1 rocket, the experiment's tape recorder had to be omitted, which meant that data could only be collected when the satellite was in sight and range of a ground station.

The payload consisted of a micrometeorite detector (a wire grid array and acoustic detector) and the same cosmic ray counter (a Geiger-Müller tube) experiment included on Explorer 1, but this time with an on-board tape recorder to provide a complete radiation history for each orbit, Ludwig having had time to accommodate for the spin-stabilization of the satellite.: 241 [1] Its total weight was 14.1 kg (31 lb), of which 8.4 kg (19 lb) 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, to be spun around its long axis at 750 revolutions per minute.

[6] 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.

Because of the consistent results, Van Allen hypothesized that the satellites' equipment might have been saturated by unexpectedly high radiation concentrations, trapped in a belt of charged particles by the Earth's magnetic field.