Could we really deflect an asteroid heading for Earth? An expert explains NASA's latest DART mission

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An asteroid that landed in Russia in 2013 injured 1,500 people – and was just 20 metres in diameter. What could we do if a major threat was detected?
A NASA spacecraft the size of a golf cart has been directed to smash into an asteroid, with the intention of knocking it slightly off course. The test aims to demonstrate our technological readiness in case an actual asteroid threat is detected in the future.

The Double Asteroid Redirection Test (DART) lifted off aboard a SpaceX rocket from California on November 23, and will arrive at the target asteroid system in September, next year.

The mission will travel to the asteroid Didymos, a member of the Amor group of asteroids. Every 12 hours Didymos is orbited by a mini-moon, or "moonlet", Dimorphos. This smaller half of the pair will be DART's target.

Are we facing an extinction threat from asteroids?

We've all seen disaster movies in which an asteroid hits Earth, creating an extinction event similar to the one that killed off the dinosaurs millions of years ago. Could that happen now?

Well, Earth is actually bombarded frequently by small asteroids, ranging from 1-20 metres in diameter. Almost all asteroids of this size disintegrate in the atmosphere and are usually harmless.

There is an inverse relationship between the size of these object and the frequency of impact events. This means we get hit much more frequently by small objects than larger ones – simply because there are many more smaller objects in space.

NASA JPL

Asteroids with a 1km diameter strike Earth every 500,000 years, on average. The most "recent" impact of this size is thought to have formed the Tenoumer impact crater in Mauritania, 20,000 years ago. Asteroids with an approximate 5km diameter impact Earth about once every 20 million years.

The 2013 Chelyabinsk meteoroid, which damaged buildings in six Russian cities and injured around 1,500 people, was estimated to be about 20m in diameter.

Assessing the risk

NASA's DART mission has been sparked by the threat and fear of a major asteroid hitting Earth in the future.

The Torino scale is a method for categorising the impact hazard…
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