"The Mystery of the Milky Way," chronicles the history of telescopes, from Galileo's refractor to Newton's reflector and beyond. It looks at key discoveries, such as those made by William Herschel and his sister Caroline, including the discovery of the planet Uranus. Hour 1 also looks at recent missions: the voyage of the Cassini spacecraft to Saturn, the Kepler telescope's search for planets beyond our solar system, and the Herschel Space Observatory's examination of the Milky Way, which is so large that it would take 100,000 years traveling at the speed of light to cross from one edge to the other.