Moons of Mars formed a large moon. Gravitational interactions between this moon and the outer ring formed Phobos and Deimos. Later, the large moon crashed into Mars, but the two small moons remained in orbit. This theory agrees with the fine-grained surface of the moons and their high porosity. The outer disk would create fine-grained material. While many Martian probes provided images and other data about Phobos and Deimos, only few were dedicated to these satellites and intended to perform a flyby or landing on the surface. Two probes under the Soviet Phobos program were successfully launched in 1988, but neither conducted the
Mars rover Mars rover A Mars rover is a motor vehicle that propels itself across the surface of the planet Mars upon arrival. Rovers have several advantages over stationary landers: they examine more territory, and they can be directed to interesting features, they can place themselves in sunny positions to weather winter months, and they can advance the knowledge of how to perform very remote robotic vehicle control. There have been four successful robotically operated Mars rovers. The Jet Propulsion Laboratory managed the "Sojourner" rover, the "Opportunity" rover, "Spirit" rover, and, now the "Curiosity" rover. On January 24, 2016 NASA reported that