Dart mission: why is Nasa crashing a spacecraft into an asteroid?

The spacecraft will travel 6.8 million miles through the solar system in an attempt to nudge moonlet Dimorphos a fraction off course

Nasa is preparing to launch its $330m Double Asteroid Redirection Test (Dart) probe, testing the space agency's ability to alter an asteroid's trajectory with kinetic force.

The plan is to crash a robot spacecraft into the moonlet Dimorphos at 15,000 mph and change its path just a fraction. If the mission is successful, it will mean that Nasa and other space agencies could deflect an asteroid heading towards Earth and avert an Armageddon-style impact.