Sue Nelson explores how asteroids can inform our understanding of the Earth's past – or threaten our future.

Each year, according to Nasa, a chunk of rock the size of a car hurtles through space on a collision course with our planet. Fortunately, thanks to the Earth's atmosphere acting as a natural shield, instead of crash landing on the ground, the asteroid burns up and produces an impressive light show, streaking across the sky as a meteor or fireball.

Unfortunately, other, much larger asteroids have the potential to be far more threatening than entertaining. "Asteroids come in all sizes," says Michael Küppers, a planetary scientist for the European Space Agency (ESA). "The really big ones, like the 10km (6.21 mile) [wide] or so asteroid that we think led to the extinction of the dinosaurs, happen maybe once every 100 million years."

The asteroid 2024 YR4, which was discovered in December 2024, has been making headlines around the world recently. At around 40-90m (131-295ft) wide, it is larger than a 12-storey building. In January this year, the ESA calculated the rock's trajectory and initially predicted that there was a 1.2% chance of impact with Earth on 22 December 2032.

This officially crossed the comfortable threshold of risk for a near-Earth object – 1% – and triggered the need for investigation by several planetary defence organisations, as well as the US President and US Congress.

Luckily YR4 is not large enough to be capable of making our species extinct but it could still be a "city killer", according to some experts, if it is at the upper end of the estimated size range and landed in a heavily populated area.

In February 2025, the risk of the asteroid hitting the Earth briefly climbed to 3.1% or one in 32. Luckily, humanity managed to avoid mass panic and the risk has since reduced to a more reassuring 0.001%. But where did this asteroid come from in the first place? And how concerned should we be about a similar scenario emerging in the future?

When it comes to understanding asteroids, astronomers and scientists are still being dealt the cards in order to understand the rules of this potentially risky celestial game.

"Scientifically there's a huge amount we can learn from asteroids," says Alan Fitzsimmons, an astronomer at Queens University Belfast and a member of one of Nasa's sky surveys that searches and tracks Near Earth Objects (NEOs). These are asteroids whose orbit brings them within 195 million km (121 million miles) of the Sun.

"Any asteroid we detect is generally a fragment of a much larger body that was formed at the birth of our Solar System" says Fitzsimmons. "So by studying their chemical make-up we get some idea of the conditions in the initial Solar System as since then, over 4.6 billion years, it has evolved dynamically."

These ancient rocky remnants are sometimes referred to as minor planets. Often irregular and cratered, they can also be spherical. They can spin slow, fast or tumble. Usually solitary, they can sometimes be found in pairs (binary or double asteroids) and some even have their own moon. Nasa's Jet Propulsion Laboratory's website keeps tabs on their numbers and at the last count there are over 1.4 million in our Solar System. The majority are located within the asteroid belt between Mars and Jupiter but millions more are too small to be detected.

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