The first spacecraft to visit two planets, Mariner 10 in 1974 flew first past Venus and then on to Mercury to make two flybys of this, the closest planet to the Sun. It went via Venus, not just to collect data, but to use Venus's gravity to accelerate it and redirect it towards Mercury. It was the first time this 'sling-shot', or gravity-assist principle had been used in interplanetary flight. It would later be used to brilliant effect on the Voyager missions to the outer planets. Launched in November 1973, Mariner 10 took three months to travel to Venus, passing on February 5, 1974, within 6,000 km of the planet. Some seven weeks later, on March 29, it swooped just 700 km above Mercury, taking the first ever close-up pictures of the surface, revealing a Sun-baked, lunar-type surface. Afterwards, it looped around the Sun and returned for a second, more distant flyby on September 21, and a third, close flyby on March 16, 1975, approaching to within 330 km of the Mercurian surface.
