Dart spacecraft blasts off on mission to knock asteroid off course

Nasa has launched a spacecraft to crash into an asteroid in a bid to knock it off course. Nasa’s double asteroid redirection test (Dart) is the first test of defence technology designed to prevent a space rock potentially colliding with Earth in the future. It aims to prove that a spacecraft can autonomously navigate to a target asteroid and intentionally collide with it – called a kinetic impact – at roughly four miles per second