In 1877, American astronomer Asaph Hall, discovered that Mars has two travelling companions, Phobos and Deimos, two small satellites that orbit very close to the planet’s surface. Phobos travels at a distance of 9,000 km and has a radius of 11 km and a mass of 1.08e16 kg., while Deimos is at a distance of 23 km and has a radius of 6 km and a mass 1.80e15 kg.