Mercury is the planet closest to the Sun. Its average distance from the Sun is approximately 58 million km; its diameter is 4,875 km; its volume and mass are about 1/18 that of the Earth; and its mean density is approximately equal to that of the Earth. Mercury revolves about the Sun in a period of 88 days. Radar observations of the planet show that its period of rotation is 58.7 days, or two thirds of its total revolution. this means the planet rotates one and a half times during each revolution. As its surface consists of rough, porous, dark-coloured rock, it is a poor reflector of sunlight.

Due to its distance from the sun, Mercury is dry, hot and virtually airless. Although its crated surface resembles that of the moon, it is believed that its interior is similar to that of the Earth's , consisting primarily of iron and other heavy elements.