Nasa has launched a spacecraft whose mission is to crash into an asteroid to test technology in the event that humanity should one day need to deflect a giant space rock from a doomsday collision with Earth.
The Dart (Double Asteroid Redirection Test) blasted off at 6:21am GMT Wednesday aboard a Falcon 9 SpaceX rocket from the Vandenberg Space Force Base in California.
Its payload, about the size of a vending machine, was released from the booster a few minutes after launch to begin its 10-month journey into space.
Moments later the rocket's reusable lower stage flew back to Earth and safely touched down on a landing vessel floating in the Pacific.
The $330 million project to demonstrate a planetary defence system is the first of its kind.
No real threat
Its target is an "moonlet" object called Dimorphos, which measures about 160 metres. It’s circling a much larger asteroid called Didymos, which is about 780 metres in diameter.
Together, they orbit the Sun, but pose no threat to our planet. Dimorphos is tiny compared with the cataclysmic Chicxulub asteroid that struck Earth some 66 million years ago, leading to extinction of the dinosaurs.
They belong to a class of bodies known as Near Earth Objects (NEOs) – asteroids and comets that approach our planet within 50 million kilometres.
While there are some 10,000 known NEOs measuring 160 metres or more, none has a real chance of slamming into the Earth within the next 100 years.
"What we're trying to learn is how to deflect a threat," Nasa's top scientist Thomas Zuburchen told journalists.
The plan is to fly the Dart directly into Dimorphos at 24,000kph, hitting it hard enough to shift its orbital track around the larger Didymos asteroid.
Impact should take place sometime in October 2022, when the pair of rocks are 11 million kilometres from Earth, the closest point they will ever get.
Cameras mounted on the impactor will record the collision and beam images of it back to Earth.