On October 4, 1957, the USSR put into orbit a tiny sphere with a radio transmitter that beeped its way into history.  Known as Sputnik, this satellite started the "space race" between the Soviet Union and the United States.  On January 31, 1958, Explorer 1 was launched and became the first U.S. satellite, using its single instrument to send back data about the radiation environment high above Earth's surface.  Since then, about 8,100 satellites from more than forty countries have been launched. According to the United Nations Office of Outer Space Affairs, 5,051 objects are currently in orbit.