Any of a series of unmanned U.S. space probes sent near Venus, Mars, and Mercury. Mariners 2 (1962) and 5 (1967) passed Venus within 22,000 mi (35,000 km) and 2,500 mi (4,000 km), respectively, and made measurements of temperature and atmospheric density. Mariners 4 (1965), 6 and 7 (1969), and 9 (1971–72) obtained striking photographs of the surface of Mars and analyzed its atmosphere and magnetic field. Mariner 10 is the only spacecraft ever to have visited Mercury (1974–75).
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