Mariner 9 became the first spacecraft to orbit another planet. Launched on May 30, 1971, the 506-kilogram (1116 pound) spacecraft circled Mars twice each day for a full year, photographing the surface and analyzing the atmosphere with infrared and ultraviolet instruments.
When Mariner 9 first arrived, Mars was almost totally obscured by dust storms, which persisted for a month. But after the dust cleared, Mariner 9 proceeded to reveal a very different planet - one that boasted gigantic volcanoes and a grand canyon stretching 4,800 kilometers (3,000 miles) across its surface.
More surprisingly, the relics of ancient riverbeds were carved in the landscape of this seemingly dry and dusty planet. Mariner 9 exceeded all primary photographic requirements by photo-mapping 100 percent of the planet's surface and taking the first close-up photographs of the tiny Martian moons, Deimos and Phobos.