Mercury. Composite of images of Mercury seen from the Mariner 10 spacecraft. Numerous meteorite impact craters are seen. Mercury is the closest planet to the Sun, orbiting at an average distance of 58 million kilometres, less than half the Sun- Earth distance. Mercury is 4880 kilometres across, less than half the size of Earth. It is a rocky planet that orbits the Sun in 88 Earth days. With a rotational period of 59 Earth days, it rotates three times in two Mercury years, with one sunrise in those two Mercury years. Its temperatures range from 400 to -170 degrees Celsius. Mariner 10 flew past Mercury three times from 1974-1975.