ROSAT, the ROentgen SATellite launched in 1990, has revealed an entirely new aspect of the night sky - that of objects emitting X-rays rather than the rays of light visible to the human eye. This lavishly illustrated book is the first to describe one of the most remarkable instruments in modern astronomy. It offers fascinating images and engaging accounts of a wide range of Solar-System and deep space objects such as Comet Hyakutake, the Sun, the Moon, and objects outside the Milky Way.