Could you be hit by a falling satellite?
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Hello and thanks for downloading the More or Less podcast. We're your guide to statistical claims that seem out of this world. And I'm your Space Force Captain, Tim Harford.

Loyal listener Edward wrote to moreorless at bbc.co.uk, questioning a number he heard on our fellow BBC programme Inside Science. At the rate that satellites are being launched at the moment, there are going to be 100,000 up there by 2030, with them coming down at a rate of about one per hour. The 100,000 figure comes from the European Space Agency's recent short film, Space Debris, Is It A Crisis?,

It does seem a lot, but then space is famously quite big. We have questions, and so did Edward. Questions such as, is this really true? How many are up there at the moment? How many are currently re-entering the atmosphere? And do they ever crash into each other? To answer these questions, we spoke to someone who definitely knows more about this than we do.

I'm Jonathan McDowell. I'm an astronomer at the Smithsonian Observatory on the campus of Harvard University. And I've been studying the history of satellites and space exploration for about 50 years. First things first, what are these bits of machinery buzzing around us?

A satellite, in the sense we normally use it, is an apparatus that is falling around the Earth at tens of thousands of miles an hour. That's how fast you need to go to stay in orbit, to not hit the Earth. They're used for taking pictures of the Earth. They're used for scientific research. But they're also used for communications more and more. And in fact, an enormous amount of our lives is mediated without us realizing it

by communication satellites.

Most of us use satellites every day, whether through our phone calls, internet or GPS. They've become an intrinsic part of society and their numbers are growing. People might not think we're in a space race at the moment, but over the last decade, our skies have become a lot more populated. We think of the 1960s, right, as like the height of the space age when we were landing on the moon and Apollo and so on. But there were only a couple hundred working satellites back in those days.

By about 10 years ago, there were just over a thousand.

Today there are over 11,000. We've seen more than a factor of 10 increase in just a decade. Part of the reason for this rapid growth is who is creating these satellites? In the 1960s it was all superpowers. Mainly it was the US and the Soviet Union and most of them were spy satellites looking at each other. Now the vast majority of those 11,000 working satellites

are operated by commercial companies, more than half of them by Elon Musk's SpaceX. China have deregulated their satellite industry. This satellite space race is likely to drive the numbers up over the next five years. Satellites require a licence and agencies such as the UK Space Agency are using requests for licences to estimate how many satellites will be in our skies in years to come.

But is that number really 100,000?

It is very fuzzy because, for example, there's one company that's applied for in total about half a million licenses, but no one believes they're really intending to do it. They don't have the capability to build that many. So you have to use some judgment about, yeah, okay, this application seems actually plausible. And when you kind of make that filter, then there's probably around 80,000 satellites on the drawing board at the moment.

Our listener wanted to know whether there'd be the risk of crashes with so many satellites. And the answer is yes. Worries about space collisions have been growing and growing. We did have a really bad collision back in 2009 with two half-ton satellites smashing into one another and creating thousands of pieces of shrapnel from the satellites that are still orbiting the Earth mostly today.

And so the satellites have to have rocket engines on them that will let them dodge other satellites to avoid running into each other. We're tracking 25,000 pieces of junk orbiting the Earth. All the different countries' satellites are whizzing around next to each other. And all are going in different directions in three dimensions. And so it's sort of like a crazy dodge-ums game.

In the first half of 2024, SpaceX's satellites had to perform almost 50,000 collision avoidance manoeuvres. And that's with only 11,000 satellites up there.

Assuming they don't meet an early demise, these little machines have a lifespan of five years. But what happens when they're decommissioned? If you just switch it off, the drag from the atmosphere is going to bring the satellite slowly down and eventually it's going to start entering the upper atmosphere. As you get lower, the atmosphere gets denser, there's more drag, you get lower more quickly.

And that is a runaway effect. And eventually you're trying to fly through the proper air at 17,000 miles an hour. And so you burst into flames and melt and break up into pieces. If you're a big enough satellite, some of those pieces might actually reach the ground. If you're a smaller satellite, the melted molecules will just add to the upper atmosphere.

Historically, space agencies haven't much worried about re-entry as it was such a rare occurrence. That brings us to the next part of our fact check. Will there really be a re-entry every hour? Let's see how they reach that figure. It's fairly back of the envelope stuff. The agencies looked at the current rate of re-entry, which according to Jonathan's data is around two or three a day. Meaning nearly 10% of the whole satellite fleet will re-enter Earth at some point in the year.

If the number of satellites goes up to 100,000, then assuming re-entry remains at the same rate, you would have 26 a day, more than one every hour. As predictions go, this isn't the silliest one we've seen, but it does seem a bit on the high side. There are doubts as to whether we really have capacity for 100,000 satellites. And the re-entry rate isn't the same every day. There are peaks and troughs, depending on when the satellites are launched and when they're retired.

We've just been through a bit of a peak with Starlink retiring its first generation and that way we were getting like three or four a day. Now it's back down to more like two or three a day, but that's still a lot compared to what we had a few years ago. You're using the upper atmosphere as an incinerator for several tons of material a day, and that's really quite significant.

Burning up in the atmosphere seems like quite a tidy way to get rid of something if you ignore the potential environmental impacts of releasing tons of aluminium oxide and other particulates into the atmosphere.

But not all of the satellite burns up. Some pieces reach Earth. Certainly we've had property damage. We haven't had anyone injured, although there was some poor woman in Oklahoma who had a little piece flutter down and give her a very small bruise. The Federal Aviation Administration predicts that by 2035, one person every two years will be killed or injured by a piece of falling satellite –

So far, there have been no recorded deaths or serious injuries. So how did they come up with that figure? A lot of the information has come from meteorites. So it will be an informed kind of guesstimate. That's Fiona Thompson, based at the Centre of Extragalactic Astronomy and Physics at Durham University.

Fiona has been tasked by the UK Space Agency to look at the current state of knowledge about satellites burning up in the atmosphere. It turns out that that state of knowledge isn't as good as you would hope. They'll have done the numbers projected. They've got to then work out, well, what happens as the atmosphere ablates, it burns up?

and how much of it will actually land on the ground. We don't know any of that at all. We don't know how it bleeds because it's a very new area, relatively. The first piece of satellites

went into plasma wind tunnels, probably in about 2018. The plasma wind tunnels mimic the effects of the Earth's atmosphere. Parts of satellites are placed into these tunnels to see at what speed they need to travel to burn up and how much they do indeed burn up. The idea was that the atmosphere would just naturally recycle in the same way meteorites do.

So there's no observational data on what happens in the re-entry of satellites. It's not that there's a lot of work not been going on, there's a lot of work been going into it, but it takes a long time to develop this knowledge.

Companies currently have to prove that the risk to humans is low. They have to prove that their satellite design has less than a one in 100,000 likelihood of causing injury to humans on the ground. But it turns out we can't really be certain about this figure.

We've realised that some of the composite materials survive re-entry better than we had expected. We're realising we can't just let them re-enter uncontrolled. But there's really very little regulation about it at the moment, and that's something I'd like to see change. Hmm. Watch this space. That's all we have time for this week. Our thanks to Jonathan McDowell and Fiona Thompson. If you have any questions or comments, please write in to moreorless at bbc.co.uk. And until next week, goodbye.

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