Asteroid impacts have influenced every large body in the solar system. They shape their appearance, alter their chemical abundance and – in the case of our own planet at the very least – they helped kickstart the formation of life. But these same events can also disrupt ecosystems, wiping out life, as they did 66 million years ago when a 10km space rock contributed to the extinction of the dinosaurs (excluding birds).

Asteroids are the material left over from the formation of our solar system, that was not incorporated into planets and moons. They come in all shapes and sizes. Their paths are determined by gravity and can, to some extent, be predicted. Of particular interest are the objects that are close to Earth’s orbit – called near-Earth objects (NEOs). As of Sep 2024, we know of approximately 36,000 such objects, ranging in size from several metres to a few kilometres.

But statistical models predict nearly 1 billion such objects should exist and we only know of very few of them.

Asteroids less than 25m in diameter generally burn up before they can cause any damage. But objects of 25-1,000 metres in diameter are large enough to get through our atmosphere and cause localised damage. The extent of this damage depends upon the properties of the object and the area where it will hit. But an asteroid of 140 metres in size could cause widespread destruction if it hit a city.

Luckily, collisions with asteroids in this size range are less frequent than for smaller objects. A 140 metre diameter object should hit Earth every 2,000 years.

Asteroids larger than 1 kilometre in size have the ability to cause damage on a global scale, similar to the one that helped to wipe out the dinosaurs. These asteroids are much rarer but easier to spot. Since 2011, we think we have detected 98% of these objects.

Less comforting is the fact that we have no current realistic proposal for diverting its path – though missions like Dart are a start.