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Mariner 10A US spacecraft that executed the first dual-planet mission by making close approaches to Venus and, three times, to Mercury. Weighing 503 kg, it carried two television cameras that returned the first close-up views of Venus and Mercury. Other instruments included an infrared radiometer to measure Mercury's surface temperature; an ultraviolet spectrometer to detect atmospheric airglow and another to measure atmospheric absorption of solar ultraviolet radiation; and two magnetometers to monitor magnetic field variations throughout the flight.
During the Venus flyby at a distance of 5760 km on Feb. 5 1974, Mariner 10 returned 3500 photographs – those taken in ultraviolet radiation showing the planet's cloud cover and atmospheric circulation in unprecedented detail. Using the gravitational pull of Venus, the probe was placed on course for a Mercury flyby on Mar. 29 1974. After course corrections two more visits to Mercury took place on Sept. 21 1974 and Mar. 19 1975, each after two solar orbits by Mercury and one by Mariner 10. The three Mercury encounters, at minimum distances of 740 km, 48 000 km, and 330 km respectively, provided more than 10 000 photographs of Mercury and led to the discovery of the planet's magnetic field. See also Mercury; Venus.