The first artificial satellite was Sputnik 1, launched by the Soviet Union on October 4, 1957. Sputnik weighed 184 pounds (83 kilograms) and remained in the sky for only a few months. The first successful U.S. satellite was Explorer 1, launched on January 31, 1958. Nowadays, hundreds of new satellites are put into space every year.

Some satellites are launched directly by rockets. Others are carried on space shuttles. The path a satellite takes around the Earth is called its orbit. Satellites orbit at different heights. A satellite that takes pictures of the ground might stay less than 150 miles (240 kilometers) above the Earth. In contrast, a satellite that relays signals from widely scattered points on the Earth might be placed 22,000 miles (35,000 kilometers) high.

Satellites are put to a wide variety of uses. Weather satellites photograph clouds and moisture patterns and provide other data for weather forecasters. Communications satellites relay messages from one ground station to another. Telephone conversations, television programs, and computer data are among the kinds of messages that are handled. Spy satellites are sometimes equipped with cameras to take sharp pictures of enemy bases on the ground. Other spy satellites have radio equipment to pick up the enemy's secret messages. Satellites are also used for global positioning. Picking up signals from groups of these satellites, navigators on ships and airplanes and surveyors on the ground are able to determine exactly where they are.

Scientific satellites collect data of various kinds. The most famous scientific satellite is the orbiting observatory called the Hubble Space Telescope. The Hubble sees the sky more clearly than telescopes on the ground. A space station is a special kind of satellite that astronauts can use as a base. (See also space exploration.)