The Odds of the Unthinkable

As you read this, an asteroid named 99942 Apophis is spiraling along a trajectory that will bring it uncomfortably close to Earth in about five years’ time. Thankfully, astronomers are very confident that this 340-meter ball of rock will miss us and instead will squeak between Earth and the Moon on 13 April 2029 before continuing on its way through the solar system. But such a close brush with disaster inspires reflections on worst-case scenarios. Even though everything we know about the laws of physics tells us that Apophis won’t collide with Earth on its current trajectory, could anything happen between now and the flyby that would change that?

Unlikely Odds

Recently, Paul Wiegert, The University of Western Ontario, took up a specific version of this question: if a separate smaller object crashed into Apophis, could that impact nudge the asteroid off its current path and onto a catastrophic collision course?

Luckily, almost definitely not. But, unsurprisingly given a situation as complicated as orbital dynamics and city-destroying impacts, there are caveats. While Wiegert estimates that the odds of Apophis stumbling into an asteroid larger than 3.6 meters across — large enough to divert Apophis into Earth — are about two in a billion, the odds of it encountering a smaller asteroid are higher. He estimates there’s about a one in a million chance that Apophis could strike an asteroid bigger than 60 centimeters, which in theory would be large enough to nudge Apophis onto an orbit that would still miss us in 2029, but would then lead to an impact later (in 2036, for example). We’d have to be doubly unlucky for this to happen, since even in this unlikely event, most small impacts leave Apophis on paths that take it farther from Earth in the future, not towards it. So, there’s certainly no need for alarm, but it will be worth monitoring Apophis in the coming years to make sure it hasn’t strayed from its safe path.

Sneaky Asteroid

There lies an unfortunate twist to this story, however: we can’t actually observe Apophis until early 2027, at best, so we have no way to know if it’s been jostled onto a dangerous trajectory until then. The asteroid has been lurking too close to the Sun for telescopes to safely see it since 2021, so our next look at it will come after a significant gap in time.

Wiegert, acknowledging both the extreme unlikeliness of a deflection but also the dire consequences of misplaced complacency, also calculates how we might tell if Apophis is on a new path once it finally reappears in the night sky. He finds that if the asteroid is even a few tenths of an arcsecond away from its predicted position, we should immediately attempt an intensive observing campaign to check its new trajectory. A deviation wouldn’t automatically spell danger, but it would indicate that something happened while we waited for Apophis to emerge, and that there’s a chance that we could be in trouble.

When it comes to something as serious as asteroid impacts, it’s good to double-check every assumption and to investigate even the most unlikely scenarios. Thankfully Earth is almost certainly safe from an asteroid impact in 2029, though up until then and beyond, astronomers will be sure to keep checking for signs of danger.

Citation

“On the Sensitivity of Apophis’s 2029 Earth Approach to Small Asteroid Impacts,” Paul Wiegert 2024 Planet. Sci. J. 5 184. doi:10.3847/PSJ/ad644d