Mariner was the first United States space exploration program to visit planets other than Earth. Mariner spacecraft traveled to Venus, Mars, and Mercury, proving that relatively small and cheap probes could successfully explore other planets. The National Aeronautics and Space Administration (NASA) launched 10 Mariner probes between 1962 and 1973.
Most Mariner probes had the same basic design. The spacecraft had a boxy, octagon-shaped body with various attachments. Solar panels powered the scientific instruments. An antenna transmitted data to Earth. Some Mariner probes carried cameras. The lightest Mariner probes weighed about 200 kilograms (120 pounds). The heaviest—Mariners 8 and 9—weighed 1,000 kilograms (600 pounds). Mariner spacecraft were designed and managed by the Jet Propulsion Laboratory.
For many years, NASA’s important space missions often included two nearly identical probes with similar flight paths. If one probe failed, the other could still carry out its objectives, and the expensive mission would not be lost. Most Mariner missions were originally planned for pairs of spacecraft. The pairs include Mariners 1 and 2; 3 and 4; 6 and 7; and 8 and 9. Mariner 5 was built as a backup for Mariners 3 and 4, but after Mariner 4 was successful, the backup spacecraft was modified for a Venus mission.
Venus was the first target of exploration for the Mariner program. When Mariner 2 reached Venus in 1962, it became the first successful space mission to a planet other than Earth. (Mariner 1 had exploded shortly after launch.) Mariner missions showed that Venus has a hot surface beneath thick clouds. They also found that the planet lacks a magnetic field.
Mariner missions to Mars greatly expanded knowledge of the planet’s surface and atmosphere. Mariner 4, the first successful Mars mission, took the first close-up pictures of Mars. Mariners 6 and 7 observed few of the planet’s most interesting surface features because they happened to fly by only heavily cratered regions. But Mariner 9 mapped about 80 percent of Mars and made the first discoveries of the planet’s striking canyons and volcanoes. In 1971, Mariner 9 discovered an enormous system of canyons on Mars. Scientists named the canyons Valles Marineris (Mariner Valleys) after the spacecraft. Mariner spacecraft also made important observations of the Martian atmosphere. This information enabled mission developers to plan safe landings for the first surface probes on Mars. 
Mariner 10 was the first spacecraft to reach Mercury. Visiting Mercury is challenging because near the center of the solar system, powerful gravity accelerates spacecraft toward the sun. Without carefully planned flight paths, probes would zoom past Mercury at high speeds. Mariner 10 used Venus’s gravity to slow down enough to collect data. It was the first mission to use such a maneuver, a type of gravity assist. During several Mercury flybys, Mariner 10 photographed much of the surface of Mercury. It also detected Mercury’s magnetic field.
During the 1970’s, NASA worked to develop a pair of Mariner spacecraft for a mission to Jupiter and Saturn. As plans advanced, the mission became more ambitious, and the spacecraft were renamed Voyager. The Voyager probes launched in 1977 and continue to operate today.
See also BepiColombo; Space exploration (Exploring the inner planets); Space exploration (Exploring Mars).
