Space exploration. After World War II ended, the United States and the Soviet Union engaged in an intense rivalry known as the Cold War. The two nations built increasingly powerful rocket-powered missiles, including ones that could fly across continents. The space age began on Oct. 4, 1957. That day, the Soviet Union launched Sputnik I, the first artificial satellite, into orbit around the earth. On Jan. 31, 1958, the United States sent up its first space satellite, Explorer I.

Most space exploration has been conducted by mechanical and electronic robots. Starting in the late 1950's, spacecraft carrying no crew have been sent to the moon and various planets to gather information that cannot be obtained in any other way. In 1977, the United States launched Voyager I and Voyager II, two identical spacecraft with powerful telescopic cameras. From 1979 to 1981, both craft flew near Jupiter and Saturn. Voyager II flew past Uranus in 1986 and Neptune in 1989. Both spacecraft sent back data and photographs that greatly enriched scientists' knowledge of these planets.

Excerpt from the "Exploration" article, The World Book Encyclopedia © 1999