The first US spacecraft was Explorer 1. The cylindrical 30 pound satellite was launched (above) as the fourth stage of a Jupiter-C rocket (a modified US Army Redstone ballistic missile) and achieved orbit on January 31, 1958. Explorer I carried instrumentation to measure internal and external temperatures, micrometeorite impacts, and an experiment designed by James A. Van Allen to measure the density of electrons and ions in space. The measurements made by Van Allen's experiment led to an unexpected and startling discovery - an earth-encircling belt of high energy electrons and ions trapped in the magnetosphere - now known as the Van Allen Belt. Explorer I ceased transmitting on February 28 of that year but remained in orbit until March of 1970.