Olympus Mon, Mariner 9 image. Olympus Mons is the largest volcano in the solar system at 600 kilometres wide and 24 kilometres tall. Mariner 9 reached Mars on 13th November 1971, becoming the first spacecraft to orbit another planet. Previous Mars missions had only flown past the planet. It used television cameras to return pictures of the Martian surface to Earth, and studied the composition of the planet's atmosphere and surface. Captured in 1971, as the spacecraft orbited Mars.