Explorer 1

Shortly after Sputnik, on 31 January 1958, the US Army launched the first US artificial satellite, Explorer 1 ¹. Explorer was a 2-m long, 15-cm diameter, 14-kg cylinder (perigee ≈ 360 km, apogee ≈ 2,550 km), which only operated for about four months until May 1958 when its batteries were depleted. Among its instruments was a Geiger–Müller cosmic-ray counter (designed by Van Allen and colleagues), and anomalies in the captured data ultimately led to the discovery of the Earth’s Van Allen radiation belts. Explorer 1 remained in orbit for more than twelve years until gradual orbital decay eventually caused it to re-enter the atmosphere on 31 March 1970.