How worried should you be about an asteroid smashing into Earth?

Somewhere, out in the cold depths of space, is a space rock that could destroy a large chunk of life on Earth. Is this fate inevitable? Could we find a way to stop it, or will we eventually suffer the same fate as the dinosaurs? And should this existential threat be keeping you up at night? Here’s what we know.

The asteroid that killed the dinosaurs 66 million years ago was at least 10 kilometres across, big enough to cause megatsunamis, ignite enormous forest fires and darken the skies the world over. Asteroids of that size are estimated to hit Earth about every 60 million years, based on the crater record. For the next size class down, asteroids about 1 kilometre across, estimates suggest they hit Earth about every million years, and the most recent one was about 900,000 years ago. Those numbers are enough to make you nervous.

But one of the things that sets humanity apart from the dinosaurs is our ability to look out into space and interpret what we see there. Naturally, researchers around the world have used this ability to attempt to learn how many asteroids are out there and what proportion of them are on trajectories that could be dangerous.

The good news is that, of the thousands of near-Earth objects (NEOs) astronomers are tracking, there are only about 35 with more than a 1-in-a-million chance of hitting Earth in the next 100 years. The even better news is that all of those still have extremely small chances of coming for us, and nearly all of them are less than 100 metres across. So, will an apocalyptic asteroid hit in our lifetimes? Almost certainly not.

Nevertheless, the eagle-eyed reader will have noted caveats like “of the asteroids we’re tracking” and “small chances” and “nearly” and “almost” and, accordingly, won’t have heaved a sigh of relief just yet. That is in large part because we can’t be sure that we have detected every asteroid, as is regularly proven by breathless headlines stating that a newfound rock is headed straight towards Earth – although they are usually not-so-near misses and pass harmlessly by.

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To calculate the proportion of asteroids we have found, astronomers use three figures: the number that we have found, the volume of sky that has been searched and the strength of our telescopes. Using those, it is estimated that we have spotted all of the asteroids 10 kilometres across or bigger that could pose a danger to Earth, so now you can breathe that sigh of relief: it is deeply unlikely we will suffer the same fate as the dinosaurs.

Of the asteroids that are 1 kilometre across, we have detected about 80 per cent, so it is fairly unlikely any of those are going to pop up unexpectedly. Anything smaller than 100 metres poses negligible danger and would probably burn up in the atmosphere on its way in or cause minor damage if it hit, like the Chelyabinsk meteor in 2013.

The 100-metre “city-killers” are more of a problem, however, as we have detected less than half of them that might be lurking around. If you are going to be worried about asteroids, those are the ones of concern.

Thankfully, there is another thing that sets us apart from the dinosaurs: the technology we have developed to actually go to space. The first way that protects us is through space telescopes keeping an eye out for any space rocks that might be on their way towards Earth. All sorts of telescopes keep watch while going about their other observations, but a dedicated one called NEO Surveyor is planned to launch next year, and that should drastically improve the numbers of asteroids that we can keep track of.

The second way spacefaring protects us is by providing options should we actually find something heading for impact. NASA’s 2022 Double Asteroid Redirect Test smashed into an asteroid to push it off course, demonstrating that we can, in fact, move one of these space rocks if we have to. If we spot one coming towards us with sufficient time to spare, which would mean at least a couple of years, we should be able to shift its trajectory so it passes by.

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If we did fail to prevent the asteroid hitting Earth, it would be a natural disaster, but a predictable one. Chances are it would hit the ocean or an uninhabited area – after all, according to the World Economic Forum, less than 15 per cent of the world’s land area (which is less than 4.3 per cent of its total surface area) has been modified by humans, much less inhabited.

If the asteroid were heading for one of those few inhabited areas, we would have the same options we do for any natural disaster: evacuate, mitigate, shelter in place. Shoring up our disaster response capabilities would help prepare for that possibility, with the useful side effect of helping us respond to the many other disasters that are both more likely and harder to predict.

So, back to the questions that started all this off. Is the asteroid inevitable? Absolutely. Is there a solution? Very possibly. Are we eventually to suffer the same fate as the dinosaurs? If we are, it will be in the far-distant future. But worrying about that won’t change any of it one bit. Instead of stewing in our anxiety, we can prepare now by learning how best to deal with natural disasters more generally – and letting the astronomers keep their watchful eyes on the skies.