Hazard, risk and planetary defence

Small rocks hit Earth every day; large impacts are very rare. This final lesson uses ideas of risk, probability and uncertainty to judge the danger, and looks at how it is managed.

Hazard and risk

A hazard is something that could cause harm. Risk combines how likely the harm is with how serious it would be. Impacts are an unusual hazard: small events are frequent and harmless, while large ones are extremely rare but could be devastating.

Tiny particles hit every day and burn up as meteors. Objects a few metres across hit about once a year, usually exploding high over the ocean. Events like Chelyabinsk (about 18 m) and Tunguska in 1908 (probably 40 to 60 m, which flattened about 2,000 km² of forest) happen on timescales of decades to centuries. A body 10 km across, like the one that made Chicxulub, arrives on average only every hundred million years or so.

Finding them first

Survey telescopes scan the sky every night for near-Earth objects. Nearly all near-Earth asteroids larger than 1 km have been found, and none is on course to hit Earth in the foreseeable future. The search continues for smaller ones, which are far more numerous and fainter.

When a new asteroid is found, its orbit is uncertain because it has been observed for only a short time. Early predictions sometimes give a small chance of impact, which almost always drops to zero as more observations narrow the uncertainty. For example, the asteroid Apophis, about 340 m across, once had a small calculated chance of hitting Earth. Further observations ruled this out, and on 13 April 2029 it will pass about 32,000 km above Earth's surface, close enough to see without a telescope.

If one were coming

With years of warning, a spacecraft could change an asteroid's path, as the DART test showed in 2022. With only days of warning, the response would be to predict where it would fall and move people away. That is why finding asteroids early matters more than any other part of planetary defence.

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