What are the chances of an asteroid hitting earth in 2032?
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Hello and thank you for downloading the More or Less podcast with me, Tim Harford. We're the programme that looks at the numbers in the news and in space.

On December 27th, 2024, when many of us were still in a mince pie coma, astronomers in Chile discovered an object in space with a trajectory that was potentially a bit too close for comfort. The object was an asteroid, which they called 2024 YR4. Cute.

As you probably know, asteroids are basically space boulders and quite probably dinosaur killers. News agencies and social media went wild trying to predict which cities it would hit and what the damage would be. There is a potential danger in space and it's on a collision course with Earth. A city-destroying asteroid that could smash into Earth in 2032. Could wipe out an entire country in a few seconds. People certainly lost their cool. So what was the likelihood of the asteroid actually hitting Earth?

It must be a pretty high percentage to get people so excited. 1%. The initial finding was a 1% likelihood of hitting Earth. So why was this such a big deal? Partly because a 1% chance of something really bad is still quite bad. But mostly because shortly after discovery, this 1% likelihood was upgraded to 3%.

Only one other asteroid in recent times has come close to that. That one won't hit us, don't worry. You should be able to see it pass harmlessly over in 2029. But sometimes asteroids do hit. For example, in 2013, when an asteroid hit Chelyabinsk, Russia. Or the Tunguska event of 1908, also in Russia. It seems asteroids really hate Russia.

So what are the odds that asteroid 2024 YR4 could give us a close encounter of a crashing into our planetary kind? We have a lot of asteroids and comets in the solar system. We actually cataloged more than 1.4 million of those. Some of them have the potential of coming close to the Earth.

My name is Davide Farnocchia. I work as a navigation engineer at NASA's Jet Propulsion Laboratory and I track asteroids and comets for my job. Asteroids are tracked using data gathered by astronomers around the globe. They look for the infrared signal of a nice warm asteroid or by light reflected off an asteroid's surface. They then calculate the speed by measuring how quickly they go past stars. As stars don't really move, they're a good reference point.

Most of us are familiar with asteroids from disaster movies such as Armageddon, Deep Impact and Ice Age 5, Collision Course. Although we haven't had to tackle any Hollywood-sized asteroids as of yet, objects entering Earth from space aren't as uncommon as most of us would believe.

Space is mostly empty, but there is material, and Earth is reached by about 100 tonnes of interplanetary material pretty much any given day. That's the typical rate. But most of that is dust, and so we don't even notice. It's too small to make any effect. Some of these objects include asteroids, which are among the oldest objects in the solar system.

They are kind of the leftovers of the formation process of the solar system. When the solar system formed, there was a lot of material going around. A lot of that material kind of got back together and formed the planets. But some of that material remained between the orbits of Mars and Jupiter. Jupiter was pulling enough with its gravity to prevent the formation of another planet between Mars and Jupiter. And so we were left with a bunch of rocks in that ring

between Mars and Jupiter. Occasionally, some of these asteroids escape this interplanetary ring road due to a mechanism called resonance. Which is similar to what kids do when they go on a swing. If they go on a swing and they move their legs with the right timing as the swing goes up and down, they start going higher and higher.

And so what happens is that if you are in a resonance that your motion has the right timing compared to that of Jupiter or another planet, your orbit kind of gets excited and you might get injected into the inner solar system. Having these intrepid little asteroids enter Earth's atmosphere isn't as rare as you might think. Smaller asteroids, or fragments of asteroids measuring less than a metre across, hit the Earth's atmosphere almost daily.

asteroids measuring a few metres across pass between the Earth and the Moon several times a month. Most don't actually enter into our skies, but some do. We've had in 2024 four asteroids

What about bigger asteroids? Thankfully, they are a much rarer occurrence.

We don't have much evidence for larger asteroids hitting Earth, but scientists have used crater formation rates on the Moon to calculate that over the last 600 million years, the Earth has been struck by around 60 objects whose diameter was 5 kilometres or more. That is 60 different occasions when quite a lot of the creatures on Earth at the time had a very bad day.

2024 YR4 is not as big as this. Its diameter is thought to be between 40 and 90 metres, about the size of the Statue of Liberty, either with or without the plinth. The impact of an asteroid like this wouldn't cause mass extinction, but it could wipe out a city. So...

what are the odds of it actually hitting? Right now it's 5, 10 to the minus 7, so that would be 5 in 10 million. Those are the odds. Very, very small. Very small indeed. So why did everyone get so excited? We actually had a much larger impact probability a while back. It reached a peak of 3.1%, which is the largest impact probability ever recorded for an object of this size. 3%.

That sounds like time to get serious. But wait, that is still a 96.9% chance that it won't hit us. Still, I prefer the much longer odds we now have. So why is the collision now thought to be so much less likely? And why did the risk initially rise before it fell?

Initially, we only had limited observational data. We saw that the impact in 2032 was a possibility, but the uncertainty was so large that the probability of actually hitting the Earth was very small.

As we gathered additional observational data, this uncertainty in position for the asteroid in 2032 became smaller, but the Earth remained inside the swath of possible positions for the asteroid. And so the probability went up because the Earth was occupying a larger fraction of that uncertainty. Think of it as kind of like an intersection, a road junction such as a crossroad.

We know where the Earth is and how the Earth travels. We know at what time and what point the Earth will travel into the middle of the intersection, but we can't predict exactly the asteroid. As the weeks have gone by, they've collected more and more data, allowing them to more accurately measure the asteroid's speed, its path and the likelihood of a collision.

We realised that the object was going to go through it at a different time than the Earth. Excellent news. So what percentage are we at now? It's 99.999999, whatever, it's not going to hit.

Despite it very likely no longer being a threat, Davide and the team at NASA continue to watch 2024 YR4 and other asteroids. So we track all of them. Every day we have a bunch of objects that are discovered. Those are new, they're added to the catalogue. And when that happens, we take a look right away and see whether they might be possible to impact sometime into the future. With so many asteroids out there, is Davide worried about an imminent impact?

Asteroid impacts for asteroids that are capable of causing substantial damage are really unlikely. It's not something that people should lose their sleep over. And with those words of reassurance, we leave you. Thanks to Davide Farnocchia and all the people at NASA who are keeping an eye on the skies. If you have any questions or comments, please do write in to us at moreorless at bbc.co.uk. We will be back next week. Until then, goodbye.

In the 21st century, your home is a symphony of energy. Digital devices, battery backup, EV charging. And you've got the power. With help from Schneider Home. Think about automated energy control. Why do your appliances need a connected system? Well, because your energy use is streamlined. Managing power and charging off peak to optimize your savings and use. Which is good for the planet and our wallets. It's like double savings from your friends at Schneider Home. Design your system at schneiderhome.com today. You've got the power.

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