Mercury

Mercury is the closest to the Sun it is also the fastest-moving Planet, whizzing round the Sun in just 88 days. Being close to the Sun, Mercury gets extremely hot. Its surface is covered in thousand craters, making it rather like the moon.

Billions of years ago, all planets were bombarded by huge meteorites. On Earth, most craters made by the meteorites have been worn away by the action of the weather because it has almost no atmosphere. So all the craters that form ages ago remain, and the whole planet is covered with them. A huge one, called the Caloris Basin, was made by a giant meteorite that sent shock waves throughout the planet

Only one space probe has flown to study merury. Named Mariner 10, it flew to the planet in 1974, after visiting Venus. Its pictures revealed for the first time that Mercury looked rather like some part of the Moon. Mariner 10 flew past Mercury twice more. On the last occasion, in March 1975, it skimmed only about 300 kilometres above the surface.