It's a famous trope of science fiction: a huge asteroid on a collision course with Earth, and the daring attempts to divert it before a devastating impact.
There are impact craters across the Solar System, revealing how chaotic a place it can be.
But what are the chances of an asteroid hitting Earth? Are asteroids really a threat, and something we should be worrying about?
Famous Earth impacts
It’s estimated that about 90 tonnes of dust and rock from space hit our planet every day.
An object doesn’t have to be particularly large to cause substantial damage.
In February 2013, the Chelyabinsk meteor exploded in the atmosphere above Chelyabinsk Oblast in western Russia.
The 18-metre (59ft) asteroid weighed 9,100 tonnes, entering Earth’s atmosphere at a shallow angle, travelling 69,000km/h (42,690mph).
The explosion created a shockwave equivalent to up to 33 times the energy released from the atomic bomb at Hiroshima, seriously injuring 1,491 people and damaging 7,200 buildings in six cities across the region.
Two billion years ago, a 15km-diameter (9.3-mile) asteroid struck Earth at Vredefort near Johannesburg, South Africa, creating a 300km-diameter (185-mile) crater.
Simple life forms may have existed then, but not so 66 million years ago, when an asteroid 10km (6 miles) across struck Chicxulub, off Mexico.
This created a 200km-wide (125-mile) crater and caused the extinction of the dinosaurs.
Potentially hazardous asteroids
A potentially hazardous asteroid is defined as a space rock with a size larger than 140 metres (459ft), or a brightness of magnitude +22.0 or more, which also has a trajectory that brings it within 0.05 astronomical units (AU) from Earth, or 19.5 lunar distances.
An organisation known as the Minor Planet Center uses data from space and ground-based telescopes to work out asteroid and comet orbits, and make the call on whether an object is a potentially hazardous asteroid or not.
This essential monitoring is complicated by the fact that asteroids’ orbits can vary over time, as they are affected by the gravitational influence of other bodies or impacts with them.
At the time of writing, NASA estimates that over 90% of potentially hazardous asteroids have been found.
They have identified 35,376 near-Earth asteroids (NEAs) of all sizes, 863 of which are larger than 1km (0.6 miles) across, while 10,909 are 140 metres across or more.
Asteroids over 140 metres across hit Earth about once every 25,000 years; those over 1km strike our planet every 500,000 years.
Here are the top asteroid threats, their size and how likely they are to hit Earth.
| Name | Mass | Size | Earth impact probability | Year of potential impact | |
|---|---|---|---|---|---|
| 1 | Bennu | 74 million tonnes | 0.49km | 0.037% | 2182 |
| 2 | 29075 | 78 million tonnes | 1.3km | 0.0029% | 2880 |
| 3 | 2023 TL4 | 47 million tonnes | 0.33km | 0.00055% | 2119 |
| 4 | 2007 FT3 | 54 million tonnes | 0.34km | 0.0000087% | 2024 |
| 5 | 1979 XB | 390 million tonnes | 0.66km | 0.000055% | 2113 |
This article appeared in the October 2024 issue of BBC Sky at Night Magazine
