Mercury has a diameter of about 4,900 kilometres. It has a cratered surface similar in appearance to the Moon but it behaves more like the Earth. This hot world has surface temperatures of 470 degrees centigrade, nearly five times the boiling point of water. Mercury is just under half the diameter of the Earth and has a large iron core, possibly molten, and responsible for weak magnetic fields.

The planet orbits the Sun at about 60 million kilometres once every 88 days and spins on its axis once every 58 days. It has just a trace of atmosphere – mainly made up of sodium, helium and hydrogen.