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Mariner 10 was the seventh successful launch in the Mariner series and the first spacecraft to use the gravitational pull of one planet (Venus) to reach another (Mercury). It was also the first probe to visit two planets. Launched on November 3, 1973, it reached Venus on February 5, 1974. Using a gravity assist from this planet, Mariner 10 first crossed the orbit of Mercury on March 29, 1974 and did so a second time on September 21, 1974. A third and last Mercury encounter took place on March 16, 1975. It measured the environments of both Venus and Mercury. It then undertook experiments in the interplanetary medium. Mariner 10 showed that Venus had at best a weak magnetic field, and the ionosphere interacted with the solar wind to form a bow shock. At Mercury, it confirmed that Mercury had no atmosphere and a cratered, dormant Moon-like surface. This flight spare was transferred from NASA, Jet Propulsion Laboratory to the Museum in 1982. Spacecraft, Mariner 10, Flight Spare, 1973. Creator: Boeing Aircraft Co.. (KEYSTONE/HERITAGE IMAGES/HERITAGE ART)