Mariner Program

NASA's Mariner program attempted ten robotic missions to Venus, Mars, and Mercury, combining planetary flybys with Mars orbiting. Mariner 1, 3, and 8 were lost during launch; the other seven completed their scientific missions. The series demonstrated that relatively small spacecraft could conduct interplanetary exploration and revealed major differences among the terrestrial planets, including Venus's dense atmosphere, Mercury's cratered surface, and Mars's volcanoes and valleys observed by Mariner 9.