The x-ray satellite ROSAT, launched in 1990, has made a new universe visible. It has discovered over 120,000 x-ray sources and allowed us for the first time to look in new ways at stellar explosions, galactic collisions, extremely compact pulsars, black holes, and quasars that shine 10,000 times more strongly than the brightest galaxy. It has detected x-rays from Comet Hyakutake and from the Moon. ROSAT is one of the most successful scientific instruments ever launched. In The Invisible Sky, two of the scientists who were instrumental in the design and launching of the satellite team up with a well-known science journalist to chronicle the beginnings, early failures, planning and construction, and deployment of this most famous of x-ray observatories. They describe the cutting-edge science being done with it and show many of the most spectacular color images it has generated. This beautifully illustrated book is the first to describe for lay readers one of the most rmearkable instruments in modern astronomy.