Yesterday, Stephen Hawking and Yuri Milner made the mind-blowing announcement that they want to build a fleet of interstellar spacecraft that can travel at relativistic speeds -- up to 20 per cent the speed of light. But it's not just about reaching our nearest star system, Alpha Centauri, although that is the new Breakthrough project's moonshot. The technology Hawking and Milner are proposing could revolutionise the search for alien life within our solar system.