A Nasa mission that collided with an asteroid didn’t just leave a dent – it reshaped the space rock

By Ian Whittaker, Nottingham Trent University

A frequent idea in sci-fi and apocalyptic films is that of an asteroid striking Earth and causing global devastation. While the probabilities of this kind of mass extinction occurring on our planet are incredibly small, they are not zero.

The results of Nasa’s Dart mission to the asteroid Dimorphos have now been published. They contain fascinating details about the composition of this asteroid and whether we can defend Earth against incoming space rocks.

The Double Asteroid Redirection Test (Dart) was a spacecraft mission that launched in November 2021. It was sent to an asteroid called Dimorphos and commanded to collide with it, head on, in September 2022.

Dimorphos posed and poses no threat to Earth in the near future. But the mission was designed to see if deflecting an asteroid away from a collision course with Earth was possible through “kinetic” means – in other words, a direct impact of a human-made object on its surface.

Asteroid missions are never easy. The relatively small size of these objects (compared to planets and moons) means there is no appreciable gravity to enable spacecraft to land and collect a sample.

Space agencies have launched a number of spacecraft to asteroids in recent times. For example, the Japanese space agency’s (Jaxa) Hayabusa-2 mission reached the asteroid Ryugu in 2018, the same year Nasa’s Osiris-Rex mission rendezvoused with the asteroid Bennu.

The Japanese Hayabusa missions (1 and 2) fired a small projectile at the surface as they approached it. They would then collect the debris as it flew by.

High-speed collision

However, the Dart mission was special in that it was not sent to deliver samples of asteroid material to labs on Earth. Instead, it was to fly at high speed into the space rock and be destroyed in the process.

A high-speed collision with an asteroid needs incredible precision. Dart’s target of Dimorphos was actually part of a double asteroid system, known as a binary because the smaller object orbits the larger one.

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