any of a series of unmanned U.S. space probes sent to the vicinities of Venus, Mars, and Mercury. Mariners 2 (launched 1962) and 5 (1967) passed Venus within 35,000 and 4,000 km (22,000 and 2,500 miles), respectively, and made measurements of temperature and atmospheric density. Mariners 4 (1964), 6 and 7 (1969), and 9 (1971) obtained striking photographs of the Martian surface and made significant analyses of the atmosphere of that planet. Mariner 10 (1973), which flew by Venus once and Mercury three times, came within 330 km (200 miles) of the latter planet on its third pass; it transmitted back to Earth the first close-up pictures of Mercury's surface, as well as analyses of its atmosphere and magnetic field.