Critical Mass: A cosmic close call that wasn't

New European

12-03-2025

Relax. An asteroid 40-90 metres wide is almost certainly not going to hit the Earth in late 2032.
Perhaps you weren't fretting about it anyway. The possibility of a planetary impact with asteroid 2024 YR4 wasn't a big worry even for those who knew about it. The chance of it hitting the Earth was always small, and even if it did, the consequences wouldn't have been catastrophic.
An impact of this scale might do considerable damage locally, especially in a populated area. But such an event, were it to occur, would be predictable, so that any necessary evacuation could happen well in advance.
And in any case, it would be far more likely to happen either in the oceans that cover two-thirds of the planet or in an unpopulated wild region. Such an outcome would be of considerable scientific interest, not to mention spectacular.
Asteroid 2024 YR4 is one of the billions of rocky objects orbiting the Sun in the asteroid belt, a ring of debris that mostly occupies the region of space between Mars and Jupiter. Some of these objects follow orbits that can take them close to the Earth – and a few are 'Earth-crossing', meaning that their orbits intersect ours.
That in itself doesn't mean they will hit the planet, however (or at least not in the foreseeable future), because an impact requires that both objects be in the same place at the same time. Tens of thousands of asteroids are known to come close to the Earth's orbit, and it's estimated that there may be around a thousand larger than 1km wide (and many more smaller ones) that are actually Earth-crossing.
2024 YR4 was first observed, as the name implies, on December 27 2024 by a telescope dedicated to such purposes, the Asteroid Terrestrial-Impact Last Alert System (Atlas) in Chile. The instrument's observations are linked up to automated warning systems including the European Space Agency's Aegis, so that anything potentially dangerous is immediately flagged up.
Based on what scientists could deduce about the asteroid's orbit, the chance of an Earth impact initially looked small but not negligible; soon after its detection, this probability was estimated to be as high as 2.8%.
But as the calculations have been refined using data from other telescopes, that chance fell. The probability is now estimated as 0.001% – one in 100,000. This makes it officially no longer on the risk list: it has been downgraded to zero on the Torino Impact Hazard Scale, introduced in 1999 to quantify such dangers.
An asteroid 70 metres or so across is a hefty thing, but might seem minuscule compared with our planet. Yet such objects carry immense energy, and an impact would be more explosive than a thermonuclear blast. In 1908, an object – probably a stony asteroid of similar size to 2024 YR4 – is thought to have exploded 5-10km high in the atmosphere over the Tunguska region of Siberia, flattening around 800 sq miles of forest and rating eight on the Torino scale.
The asteroid that is believed to have struck the Earth around 66m years ago off the coast of Mexico, causing global devastation that probably triggered or accelerated the extinction of the dinosaurs, was probably a whopping 10-15km across.
Asteroid impacts of this sort are not inevitable – we might be able to deflect or even destroy them, as in the movies Deep Impact and Armageddon . The deflection of a 177-metre asteroid called Dimorphos was demonstrated in 2022 by Nasa, which sent the 600kg Double Asteroid Redirection Test (DART) spacecraft to hit the object head-on.
It was a small effect, nudging Dimorphos into a slightly altered orbit around the 800-metre twin asteroid Didymos, which can come within 6m km of the Earth.
2024 YR4 is a reminder that we can't be complacent about the risks of planetary impacts. But in an age when zealots argue that a colony on Mars is an urgently needed back-up for humankind against a civilisation-ending impact, we need to keep them in proportion.
An impact rating 10 on the Torino scale, 'capable of causing global climatic catastrophe that may threaten the future of civilisation as we know it', is thought to happen on average only every 100,000 years at most.
The risk over the next several centuries is tiny – and by that time, who knows what we'll be able to do about such things, assuming we haven't wiped ourselves out in the meantime?