Could a moon called Europa hold clues to alien life?
16 July 2014 Last updated at 01:35 BST
The search for life on other planets has long inspired science fiction novelists and Nasa missions but could one of Jupiter's moons, Europa, provide proof?
Bill Nye of the US Planetary Society - also known as The Science Guy - is among those who believe it is worth finding out.
He is hoping to persuade Congress to spend $2bn (£1.2bn) exploring Europa.
He explained his case to the BBC's Laura Trevelyan.