Interesting documentary on asteroids broadcast on BBC4 yesterday and available on BBC iPlayer until Jan 4 2013: http://www.bbc.co.uk/iplayer/episod...2011_Asteroids_The_Good_the_Bad_and_the_Ugly/ "Famed for their ability to inflict Armageddon from outer space, asteroids are now revealing the secrets of how they are responsible for both life and death on our planet. Armed with an array of powerful telescopes, scientists are finding up to 3000 new asteroids every night. And some are heading our way. But astronomers have discovered that it's not the giant rocks that are the greatest danger - it's the small asteroids that pose a more immediate threat to Earth. Researchers have explained the photon propulsion that propels these rocks across space, and have discovered that some asteroids are carrying a mysterious cargo of frost and ice across the solar system that could have helped start life on earth. "