What would happen if a ‘city killer’ asteroid actually hit Earth?

Just after Christmas last year, an incoming “city killer” asteroid was spotted by Chile’s Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope. The early warning system sent out an alert, and follow-up observations were made to figure out its likelihood of actually hitting Earth. Coming in at a probability of around 1 per cent, the results were worrying enough to trigger an international response, involving a coordinated effort by the likes of NASA and the European Space Agency (ESA) to track the space rock’s trajectory and learn more about the risks.

In the following days, the odds of the so-called ‘asteroid 2024 YR4’ impacting Earth – on 22 December 2032 – rose to 2.8 per cent. This was enough to alert the UN and send it straight to the top of ESA’s risk list – and for good reason. It’s the largest object we’ve ever seen with such a high impact probability. And if there’s a one in 36 chance that an asteroid around 50 metres across is going to slam into our planet, it’s worth taking pretty seriously.

If the probability had kept creeping up in subsequent models of the asteroid’s path, it would have kicked a mission-planning task force into action. US and European space experts would have joined members from Japan’s JAXA space agency, Russia’s Roscosmos, and China to consider various options for deflection. These would probably begin with smashing a spacecraft into it (as NASA and ESA’s successful DART mission trialled in 2022) then move on to more desperate, sci-fi-inspired measures like sending nukes or lasers into space.

Luca Conversi is the manager of ESA’s NEO (near-Earth object) Coordination Centre, based in Rome. As the likelihood of the asteroid’s impact grew, he and other space officials “began to get excited,” he tells Dazed, which might sound like a strange reaction to our impending doom. But, as he points out, it gave space agencies some great opportunities to test their coordination capabilities. “We do exercises, but real life is always different than just the exercises. So it was exciting from that point of view.”

Besides, Conversi adds, the asteroid is big, but not big enough to wipe out all life on Earth. “People should not think about the dinosaur extinction event, or an Armageddon-like event.” If it landed in the middle of the Sahara desert, for example, or in the middle of the Atlantic ocean, it probably wouldn’t cause any major damage.

We do exercises, but real life is always different – Luca Conversi

OK… but what if it came down over land, or – in the worse-case scenario – over a city? Well, because it’s a solid rock, it would probably explode on impact with Earth’s atmosphere, Conversi explains. Besides the fragments themselves, this would cause a powerful shockwave that could damage a small city. If this were to happen with asteroid 2024 YR4, he compares the effects to the Tunguska event, a suspected meteor air burst over a sparsely-populated area of Russia in 1908, which felled an estimated 80 million trees over more than 2,000 square miles.

At this stage, it’s probably worth pointing out that ESA reduced the probability of a 2032 impact to 0.001 per cent earlier this week, making it very, very unlikely. Why has the number gone up and down so much since late last year? Apparently, it’s to do with the way the data is collected and what it tells us about the asteroid’s possible trajectory. This is not an easy task.

To illustrate, Conversi paints a picture of a baseball game: the pitcher throws the ball, the hitter whacks it with his bat, and fielders watch it fly through the air to see where it’s going to land. The problem is, an asteroid is only visible for a few days before it goes out of sight again, continuing its orbit of the Sun. “We only observe the object for a few days, and you need to predict where it could be in eight years,” Conversi explains. “So it’s like closing your eyes, and then you need to try to catch the ball.”

In the case of asteroid 2024 YR4, astronomers modelled various locations it could pass close to Earth and the Moon in 2032. These are recorded as red dots on the graph below, with the yellow being most likely; the Earth is blue, and the Moon’s orbit is recorded in grey. As they gathered more data and narrowed down the possibilities, this line shrunk, and because a higher percentage of data points covered the Earth, it seemed more likely that the asteroid would hit us. Now, the line has shrunk beyond Earth, resulting in the rapid decrease in the probability of impact. However, there’s still a decent chance (1.7 per cent) that it could hit the Moon.

The next question is obvious: what would a lunar impact look like, and would it have any knock-on effects for Earth? “It would be great!” Conversi says. “That I can definitely say.” For one, it would be an amazing spectacle, visible through telescopes, binoculars and possibly even the naked eye. Plus, it would “create a nice crater,” he adds, and because the Moon has no atmosphere, the ejected material would fall to Earth at some point, in the form of Moon dust. Most importantly: “There is no danger at all for us.”

For now, then, it seems like we’re out of the firing line. But again, the brief asteroid scare offered a valuable insight into how we might respond to the real thing. People are always asking Conversi if he’s watched the 2021 film Don’t Look Up, he says when I… ask him if he’s ever watched Don’t Look Up. And, for the record, he has. The analogy here isn’t perfect, he says – because the film’s Leonardo DiCaprio-played scientist is dealing with a comet, not an asteroid; comets tend to have a much shorter warning period – but some of it holds up, like how the plan of action moves up the chain of command, all the way to the US president.

You always want to keep the asteroid in one piece, because the moment you make it explode, you have smaller pieces going everywhere – Luca Conversi

What’s less realistic about sci-fi films and disaster movies, he suggests, are the attempts by nefarious governments to mislead their citizens about the risk. In reality, almost all of ESA’s research is done out in the open, down to where (approximately) and when an impact might take place. Then, of course, there’s the actual solution. “One thing I want to underline that’s contrary to Hollywood movies is where they try to nuke and destroy the asteroid,” he says. In reality, this would be a bad idea: “You always want to keep the asteroid in one piece, as much as possible, because the moment you make it explode, you have smaller pieces going everywhere and you don’t have control.”

That said, if the worst came to the worst, scientists might try sending a nuclear bomb to detonate beside an asteroid and deviate it via a massive shock wave. “Then again, it’s not been proven,” he says of this “imperfect” solution. “And launching a nuclear device has lots of political consequences. Plus it’s dangerous, because if your launcher fails and you have a nuclear explosion in the upper part of the atmosphere… it’s not nice.”

If astronomers were to spot a truly dangerous “city killer” asteroid, or something even bigger, then they might have just a few years to figure these problems out. Luckily, it doesn’t seem like that will be the case this time around.