Following the launch of the Soviet Sputnik I 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 intermediate-range ballistic missile (IRBM). Working closely together, ABMA and JPL completed the job of modifying the Jupiter-C and building the Explorer-I in 84 days.
Explorer I was equipped with a geiger counter for the purposes of detecting cosmic rays. Sometimes the instrumentation would report the expected cosmic-ray count (~30 count/sec) but sometimes it would show a peculiar 0 count/sec. The Iowa group (under Van Allen) noted that all of the 0 count/sec reports were from an altitude of 2000+ km over South America, while passes at 500 km would show the expected level of cosmic rays. After Explorer III, it was decided that the original geiger counter had been overwhelmed by strong radiation coming from a belt of charged particles trapped in space by the Earth's magnetic field. (see: Van Allen radiation belt)
The discovery of the Van Allen Belts by the Explorer satellites was considered to be one of the outstanding discoveries of the International Geophysical Year.
Explorer-I was placed in an orbit with a perigee of 360 kilometers (224 miles) and an apogee of 2520 kilometers (1575 miles) having a period of 114.9 minutes. Its total weight was 13.9 kilograms (30.7 pounds), of which 8.3 kilograms (18.3 pounds) were 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, an internal temperature sensor, three external temperature sensors, a nose-cone temperature sensor, a micrometeorite impact microphone, and a ring of micrometeorite erosion gauges. Data from these instruments were transmitted to the ground by a 60-milliwatt transmitter operating on 108.03 megacycles and a 10-milliwatt transmitter operating on 108.00 MHz.
Transmitting antennae consisted of two fibreglass slot antennae 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-I instrumentation was designed and built with simplicity and high reliability in mind. It was completely successful.
Explorer I ceased transmission of data on May 23, 1958, when its batteries died, but remained in orbit for more than 12 years. It made a fiery reentry over the Pacific Ocean on March 31, 1970. Explorer I was the first of the long-running Explorer program, which as of March 2000 as launched 78 Explorer probes.