The United States successfully launched its first space satellite, Explorer 1, on January 31, 1958. It was part of a program for the International Geophysical Year, as well as responding to the Russian Sputnik I launched earlier. With the Explorer’s launch, the Space Race began between the United Stattes and the Soviet Union. Explorer 1 was designed and built by the Jet Propulsion Laboratory and attached to a Jupiter-C rocket modified by the Army Ballistic Missile Agency. The satellite’s payload consisted of the Iowa Cosmic Ray Instrument, without a tape data recorder that was not ready in time for the launch. Explorer 3 included the recorder, which confirmed sparse real-time data received from Explorer I.