Chris Riley investigates black holes, the most mysterious and menacing phenomenon in the universe. A super-massive black hole lies at the heart of our own galaxy. But Chris discovers that, rather than being a hazard, these bottomless pits of gravity may be an essential part of our universe. Meanwhile, Kathy Sykes meets Francisco Diego, who designs prisms for the world's largest telescopes - and chases eclipses in his spare time. Plus we learn how comets are made, and discover the possible connection between these icy rocks and how life may have come to Earth.