The United States and the Soviet Union began exploring space in the 1950s. Both nations strived to be the first to get spacecraft into Earth orbit and to the Moon. Some efforts succeeded. Some failed. As successes mounted, momentum built for attempting the next great leap: going to another planet.

A robot named Mariner 2, about twice as tall and three times as heavy as a human adult, coasted noiselessly past cloud-wrapped Venus on Dec. 14, 1962, and became Earth's first emissary to successfully examine another planet up close. NASA's Jet Propulsion Laboratory designed, built and operated Mariner 2, which found that Venus has cool clouds and a very hot surface.

A series of other JPL-managed Mariner missions in the 1960s and early 1970s paid initial visits to Mars and Mercury plus follow-ups to Venus and Mars. The first spacecraft to orbit another planet, Mariner 9, discovered dry riverbeds, massive volcanoes and vast canyons during its orbital tour of Mars beginning Nov. 14, 1971. Besides its scientific discoveries, the Mariner series built up expertise for planning and managing future planetary exploration. Mariner 10, launched in 1973, visited two planets, pioneering the use of "gravity-assist" maneuvers by using Venus' gravity for a boost toward Mercury.

Other early JPL spacecraft inspected our own planet's Moon. The Ranger series of crash landers and Surveyor series of soft landers in the 1960s supplied detailed photographs and surface analysis that scouted the way for the bootprints of Apollo astronauts.

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