Today marks the 50th anniversary of the launch of the first US space satellite Explorer 1.
In 1957 an "International Geophysical Year" (IGY) was organized, later extended to 1958, and both the Soviet Union and the USA announced their intention to launch that year artificial Earth satellites. The USSR was first, sending off its first "Sputnik" ("satellite") on October 4, followed by Sputnik II on November 3. However the official US entry, the Vanguard satellite, went up in flames in a launch failure in December. The US then authorized a back-up spacecraft mission, initiated unofficially a few years earlier by Wernher Von Braun. Von Braun had built large missiles for the US Army and had all the hardware ready, but until then was given no permission to launch a satellite.
The spacecraft, named Explorer 1, was launched 31 January 1958 and was designed and built by a group of scientists from the University of Iowa, led by James Van Allen. That group had been previously credited with the first observation of auroral electrons from a rocket; incidentally, the idea of the IGY itself started in 1950 at a dinner party at Van Allen's home (at the time, near Washington).Unlike the orbits of the Sputniks, that of Explorer 1 was quite elliptical and it rose to an altitude of about 2500 kilometers. Furthermore, since it had been decided to omit the spacecraft's tape recorder on the first flight, data could only be collected when Explorer 1 was within range of a tracking station, for at most a few minutes each time. The data were puzzling. At low points of the orbit the number of energetic particles was near the expected value, but at the high portions of the orbit none were counted at all.
Story courtesy of http://www-istp.gsfc.nasa.gov/Education/wexp13.html