Explorer I was the first satellite to be launched into Earth orbit by the United States. Its official name was Satellite 1958 Alpha (a name which only a computer could love).
It was designed and built at the Jet Propulsion Laboratory (JPL) of the California Institute of Technology. You can see the old JPL logo on the side of the rocket.
Explorer I was officially built for the International Geophysical Year (IGY, 1957-1958), an international scientific effort that focussed on Earth sciences. Unofficially, Explorer I was America's response to the launch of the Soviet Sputnik I (on October 4, 1957).
On January 31, 1958, Explorer I was launched from Cape Canaveral, Florida by a Jupiter-C rocket (a modified Redstone ballistic missile, also known as a Juno I). (Note: NASA wasn't created until later in 1958. The agency in charge of Explorer I and its launch vehicle was the Army Ballistic Missile Agency.)
The satellite carried a Geiger counter (a charged-particle detector) that had been designed and built by Dr. James Van Allen. Sometimes, when Explorer I was at altitudes above 2000 km, the Geiger counter would say that the particle count was strangely low. That happened because there were so many particles that it couldn't count properly. Those regions of charged particles (trapped near Earth by its magnetic field) were named Van Allen Belts.
The four things sticking out of the middle of the satellite are flexible whip antennas for communications with the ground. The satellite would rotate about its long axis, causing those antennas to extend outward in space.
The official Explorer I mission lasted 111 days, but it stayed in orbit until 1970, making over 50,000 trips round the Earth. (Eventually, it re-entered the Earth's atmosphere and burned up.)
For more information:
Author(s): Troy McLuhan (unknown date prior to 2007 December 31)
