Physicist Stephen Hawking on stage during an announcement of the Breakthrough Starshot initiative, held in New York on April 12. At the event, investor Yuri Milner unveiled plans, supported by Hawking, to send a fleet of robot spacecraft no bigger than a cellphone to Alpha Centauri, the nearest star system, but still 4.37 light-years away. Even moving at a fifth of the speed of light, it will take 20 years for the surviving spacecraft to reach their destination.