On 2013 February 15, a meteorite travelling at more than 30,000 miles an hour crashed into Russia's Ural mountains, injuring more than 1,000 people. Just a day later, an asteroid passed within 17,000 miles of Earth. These were reminders that the planet's journey through space is fraught with danger. Professor Iain Stewart explores what meteorites and asteroids are, where they come from, the risks they pose and the role they have played in Earth's history.