On 6 October the Cassini space probe will be shooting off into space with 72.3lb of plutonium compounds on board. Cassini's mission is to explore Saturn and its moons. Rather than heading straight to Saturn though, Cassini will fly twice around Venus and back to Earth in what Nasa calls a "slingshot manoeuvre". This is to maximise the use of the Earth's gravity to increase Cassini's velocity.
If all goes well, it will pass just 320 miles above Earth. Too deep a descent, though, and Cassini could disintegrate in the Earth's atmosphere. Even Nasa's own research says that five billion people could be contaminated if the plutonium comes raining down on to Earth. Thousands could die from lung cancer after inhaling plutonium dust.