Fifty-three years ago this week, Explorer III became the third artificial satellite to be successfully orbited by the United States.  Interestingly, this early trio of successful orbital missions had been achieved in a period of less than 60 days.

The early Explorer satellites (Explorer I, II and III) were designated as Explorer A spacecraft.  Their primary mission was to study the Earth’s Magnetosphere.  Each satellite measured about 81-inches in length and had a maximum diameter of 6.5-inches.  On-orbit weight was close to 31 pounds.

Explorer satellite instrumentation was modest.  The primary instruments carried included a cosmic ray detector and micrometeorite erosion gauges.  Data were transmitted to Earth using a 60 milliwatt dipole antenna transmitter and a 10 milliwatt turnstile transmitter.  Electrical power was provided by mercury chemical batteries that accounted for roughly 40 percent of the payload weight.   

Explorer I was the first artificial satellite to achieve Earth orbit.  The satellite was launched atop a Jupiter-C launch vehicle on Friday, 31 January 1958 from LC-26A at Cape Canaveral, Florida.  The country’s first satellite quickly went to work and discovered what we know today as the Van Allen Radiation Belts.

Explorer II was to verify and expand upon the findings of Explorer I.  However, the craft never achieved orbit after it was launched on Wednesday, 05 March 1958.  The cause was attributed to a failure in the 4th stage of its Jupiter-C launch vehicle.  While the outcome was disappointing, the Explorer Program quickly readied another Explorer satellite for flight.

Explorer III was launched from Cape Canaveral’s LC-5 on Wednesday, 26 March 1958 at 17:31 UTC.  The Jupiter-C launch vehicle performed admirably and delivered Explorer III into a highly elliptical 1,511-nm x 100-nm orbit.  However, all was not well in orbit.  Telemetry data indicated that the pencil-like satellite was tumbling at a rate of about 1 cycle every 7 seconds.

Explorer III performed its intended mission in spite of the anomalous tumbling motion.  Indeed, the craft corroborated the findings of Explorer I and helped verify the existence of the Van Allen Radiation belts.  However, the unwanted tumbling increased Explorer III’s aerodynamic drag and significantly shortened its mission lifetime. 

Explorer III’s orbit decayed to the point that it reentered the Earth’s atmosphere on Tuesday, 27 June 1958.  During its 93 days in space, the spacecraft made approximately 1,160 trips around the Earth.