The Sky at Night team explores the threat of an asteroid impact on earth. Around 2,300 asteroids have been identified as ‘potentially hazardous', and it's thought that a million ‘near-earth objects' are yet to be accounted for. Detecting these possible threats is now a priority for space scientists. And they're developing methods of planetary defence that sound like the stuff of science fiction.Maggie meets Professor Alan Fitzsimmons, expert in asteroid observation, to learn how the latest technology monitors near-earth asteroids. He explains which ones are a current concern and why we missed the dangerous Chelyabinsk meteor – a 9,000-ton fireball that exploded in the sky above Russia. Could it happen again?Chris meets the Open University's Professor Simon Green, who has been involved in Nasa's recent planetary defence mission Dart. In this mission, a spacecraft was flown directly into an asteroid in a successful attempt to change its orbit, and the hope is that this could be repeated if an asteroid was identified as a real threat to earth. Simon demonstrates why smashing into an asteroid is even more complicated than it sounds.Our inhouse stargazing expert, Pete Lawrence, explains how to get a rare sighting of Jupiter passing behind the moon and why it is that we can see the moon in the daytime.And exoplaneteer George Dransfield is at Royal Holloway University to meet planetary scientist Dr Queenie Chan. Her recent analysis of the famous Winchcombe meteorite offers new evidence in support of asteroids bringing life – as well as destruction – to earth.