你被SpaceX飞船碎片砸中的概率很低，但绝不为零｜科学60秒

依据北美防空司令部（NORAD）数据渲染的太空碎片图像，每个点代表 NORAD 数据目录中的一块空间碎片
图片来源：Yeus, CC BY-SA 3.0, via Wikimedia Commons

Rachel Feltman: If you’re listening to this podcast, chances are pretty good that you’ve heard about the problem of space junk— the countless pieces of trash from dead satellites, old rockets and other assorted space infrastructure orbiting our planet that can travel as fast as 22,000 miles per hour or more. You may also know that even the smallest pieces of debris can damage satellites and space stations at those speeds. But even if you’re aware that hunks of this cosmic trash occasionally crash down to Earth, it probably feels like a fairly abstract problem to you. The world is big and full of stretches of uninhabited ocean, and the odds of space junk falling anywhere near you are close to zero.

That’s how Samantha Lawler, an associate professor of astronomy at the University of Regina in Saskatchewan, used to think about space junk, too. Then a farmer found a huge heap of debris not far from her house.

For Scientific American’s Science Quickly, I’m Rachel Feltman. I’m joined today by Samantha to hear more about her close encounter with parts of an old SpaceX craft—and the perplexing process she went through to try to get someone to deal with the hundreds of pounds of space trash.

How did you first hear about this particular debris?

Samantha Lawler: I heard about this when a journalist sent me an e-mail, and she just asked, “Hey, we heard about this space junk. I’m not asking you to decide if it’s real or not, ’cause it’s probably fake, but can you just talk about space junk?” Sure, okay. And then I just happened to see a photo of the space junk.

And my first thought was “wow, that looks just like the SpaceX junk that fell in Australia a few years ago. I wonder if it actually is real space junk. Oh, my gosh. This is right near me. That’s so crazy.” And I e-mailed my collaborator, Jonathan McDowell, and he, almost immediately, confirmed: yep, there was a SpaceX Crew Dragon trunk that landed over Saskatchewan back in February.

That could be it. So then, by the time I talked to this reporter, who just wanted to ask me general questions about space junk, I was like, “Wow, I found out everything. This is amazing. I know exactly what piece it was and when it fell.” And she just wanted, like, very basic stuff.

So I was way more excited than her at that point.

Feltman: Could you tell me a little bit more about how your collaborator was able to pinpoint what piece of space junk this was?

Lawler: Yeah. So Jonathan McDowell is an astrophysicist who works at [the Center for Astrophysics | Harvard & Smithsonian]. And he’s an x-ray astronomer, but he just happens to love tracking satellite launches and reentries, and he has probably the best civilian database of all of that information, so he just keeps track of it all, so it was very easy for him to, you know—I just e-mailed and asked, Was there anything that fell over Saskatchewan in the last few months because this was over the winter, right? Nobody would have noticed until the farmers went out to do their seeding. So he was able to access that information because he’s the one who keeps track of it.

Feltman: Yeah—not the companies that are dropping the space junk?

Lawler: No, not the companies that are dropping it. There’s no, like, government database that’s public, right?

It’s all just done by Jonathan McDowell, and there’s another really good database, CelesTrak, that's also just run by someone for fun. It’s really wild that there’s not better public information on everything that’s in orbit and everything that’s falling on us from orbit.

Feltman: Yeah, especially given that I, I know the scale of the problem is increasing. You know, what kind of numbers are we looking at in terms of space junk and potential space junk these days?

Lawler: Yeah, there’s more than 10,000 active satellites in orbit, so functional satellites, and then there’s tens of thousands of pieces of large debris. There are hundreds of thousands to millions of pieces that are big enough to do damage, right? Everything that’s in orbit is traveling at several kilometers per second, so faster than a bullet.

So even a tiny screw or a fleck of paint, if it hits something, like a window on the International Space Station, it can do some serious damage, so tiny pieces of debris that we can’t even measure from the ground can really cause serious problems.

Feltman: So going back to, you know, this particular space junk incident, can you describe what this debris looked like when it fell?

Lawler: Yeah. Like, you can see pictures of it, but when you actually see it in person, it looks way bigger, and it’s really terrifying to think about that just falling out of the sky onto the ground, right? So this piece of debris, the first one that, that he found, it’s, like a semitruck hood, right?

Like, it’s really big, right? It’s almost my height, too, like, leaning on a wall. And it’s covered with this carbon fiber that’s sort of unraveling around the edges, which is probably why it didn’t burn up. SpaceX has said that [satellites] will burn up completely when they fall through the atmosphere, but these very large pieces did not, and I think probably part of that reason is because this carbon fiber sort of unraveled on the way through the atmosphere and slowed everything down. There was another piece that one of the neighbors found. It was a very large piece of metal with, like, really long tendrils of carbon fiber hanging off of it. There was another piece that was like a big metal spear, almost. Like, that one really scared me, just thinking about that falling into the ground. It was about eight or nine feet tall.

It weighed 80 pounds. And it’s just, like, a spear made of aluminum. Like, that’s terrifying, terrifying, right? There were five pieces, about 250 pounds in total, that were found by the original farmer and his neighbors, and they had brought it all to farmer, uh, Barry Sawchuk’s equipment shed just to, like, have a nice display for journalists to take a look at, right?

’Cause this is terrifying.

Feltman: Yeah. Well, and did these pieces cause any damage, or were folks fortunate enough that they landed in empty fields?

Lawler: Yeah. So where the pieces fell, it is very sparsely populated, right? This is grain-farming country. There’s fields that are used for growing thousands of acres of wheat and canola and a few cattle fields. There’s only a house every few kilometers, right?

It’s very sparsely populated. So in some ways, it’s a great spot for this to happen, right? It also sort of highlights, like, if it’s this sparsely populated and people still found pieces, like, that’s a lot of stuff coming down, right? There was no damage, and nobody’s house got hit or anything, but there will be pieces found for years.

I have no doubt that there are smaller pieces or maybe even pieces this big that are still out there to be found. One neighbor apparently just seeded over it, right? He drove his big tractor over a piece, and he didn’t even think about it until he saw the news, like: “Oh, there’s space debris in my field that’s now growing canola or whatever,” right?

So people will continue finding pieces. It could cause damage to these giant million-dollar tractors and combines in the future, but, it’s really, really quite terrifying to think about.

Like, that fell, like, right near my house. That, that could have hit me. Like, wow.

I just saw in the news that SpaceX has actually changed how they are letting these particular objects reenter, and they’re going to try to get them to land in the ocean, which is great in terms of not killing people, but it’s still like—they're dropping giant pieces of garbage. Like, why is this okay? The disposability really bothers me.

Feltman: Yeah, well, and that is a great segue to my next question. In the piece you wrote for Scientific American, you kind of brought people along for the journey as you tried to get this debris dealt with. So what surprised you the most about that process?

Lawler: The thing that surprised me the most, that still continues to surprise me, is that nobody in the Canadian government really seems to care. Like, like wait, so SpaceX, a private company, just dropped garbage on Canadian citizens and then came and picked it up and left, and, like, nobody cares? Really?

This is fine? This is how this is supposed to go? So I’m still just kind of shocked that, yep, this is fine. It’s okay for private companies to drop garbage on you from orbit as long as they, like, come pick it up afterwards. Like, it’s just so bizarre.

Feltman: Yeah. Well, and, and what did that process entail?

Lawler: Yeah. What is supposed to happen: anything that goes into orbit and comes back down is covered by the Outer Space Treaty and the Space Liability Convention, which are these big treaties written back in the late 1960s, early 1970s, in the Apollo space race era.

They’re written for a time when governments are the only entities launching stuff into orbit, so they don’t recognize private companies. It doesn’t recognize individuals, so—now a private company has dropped stuff on private citizens. How does that work? What was supposed to happen is that Global Affairs Canada should have talked to the U.S. [Department of State], who should have talked to SpaceX, right?

It should have gone through government-level dialogue. But I think what actually ended up happening is that someone at SpaceX saw the news and got in contact with the farmer. Stuff that falls out of orbit on another country, you’re obligated to give it back to whatever country launched it, right? If you are a Canadian, and you find space junk in your field, you have to...[full transcript]