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Los Angeles — A SpaceX rocket was set to blast off from California late on Tuesday as Nasa seeks to demonstrate a first-of-its-kind planetary defence system, designed to deflect an asteroid from a potential doomsday collision with Earth.

The Double Asteroid Redirection Test (Dart) mission will test Nasa’s ability to alter an asteroid's trajectory with kinetic force — crashing a robot spacecraft into it at high speed and nudging the space boulder just enough to keep our planet out of harm’s way.

Dart’s target is a tiny fraction of the size of the cataclysmic Chicxulub asteroid that slammed into Earth about 66-million years ago, killing most of the planet’s animal species. It is not on a path that will cause it to hit Earth in the foreseeable future.

But scientists say smaller asteroids are far more common and pose a far greater theoretical threat to Earth in the near term.

Nasa has hired Elon Musk’s company SpaceX to launch Dart aboard a Falcon 9 rocket from Vandenberg Air Force Base on the California coast, about 250km northwest of Los Angeles. If lift-off is postponed Nasa has an 84-day launch window in which to try again.

Once released into space, Dart will journey 10 months to its destination, about 11-million kilometres from Earth. Its target is an asteroid “moonlet” the size of a football stadium that orbits a much larger chunk of rock — about five times bigger — in a binary asteroid system named Didymos, the Greek word for twin.

The moonlet, called Dimorphos, is one of the smallest astronomical objects to receive a permanent name. But at 160km in diameter, its size is typical among the known asteroids — rubble-like remnants left over from formation of the solar system 4.6-billion years ago.

Relative proximity to Earth

Scientists chose the Didymos system because its relative proximity to Earth and dual-asteroid configuration make it ideal to observe the results of the impact.

The key to avoiding a killer asteroid is to detect it well in advance and be ready with the means of changing its course, Nasa planetary defence officer Lindley Johnson told a media briefing.

“We don’t want to be in a situation where an asteroid is headed towards Earth and then have to be testing this kind of capability,” he said.

The team behind Dart has determined that slamming a car-sized projectile into a Dimorphos-sized asteroid at 24,000km/h) should do the trick. The spacecraft, a cube-shaped box with two rectangular solar arrays, is due to rendezvous with the Didymos-Dimorphos pair in late September 2022.

Cameras mounted on the impactor and on a briefcase-sized mini-spacecraft released from DART about 10 days beforehand will record the collision. Observations from ground-based telescopes and radar will then measure how much the moonlet’s orbit around Didymos changes. The team is expecting to shorten the orbital track by about 10 minutes but would consider at least 73 seconds a success.

The entire cost of the project will run to about $330m, according to Lindley, well below that of many of NASA’s most ambitious science missions.



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