Scrapped satellites cause atmospheric metal pollution

A new study has found that space junk is a polluting presence in Earth’s upper atmosphere. When satellites become defunct, those who are responsible for them have three options: leave them up there as junk; let them fall down in bits; or, as is becoming increasingly popular, let them burn up on reentry. But the study’s author, Daniel Murphy, says that this process leaves traces of metal particles lingering in our planet's atmosphere. They’re building up, and the consequences are unknown. He’s been speaking to Will Tingle…

Daniel - You have these satellites reentering and we know they burn up and they produce metals: aluminium, copper, these other metals, and those aren't going to disappear out of the atmosphere. And what's really new here is we have measurements showing that they end up in the stratospheric particles. And that was not necessarily obvious. Now we know where they go, they go into the stratospheric particles. And there's also a background of what's called meteoric smoke, and it's little tiny nanoparticles that are left behind after meteors vaporise. And there's a constant drizzle of these incredibly small particles, tens of nanometers in size. And one of the things we're finding is that the metals from the spacecraft are either condensing on or coagulating with these meteoric smoke particles. So instead of having the composition you'd expect from meteors, which is things like iron, silicon, magnesium, they also have these other metals like aluminium, copper, and other metals like that. The concern is that it's different material, it's now metals that weren't previously there.

Will - When this stuff burns up, it's not gone. It remains up in the stratosphere. But, as we say, it's tiny, it's very difficult to detect. So how did you detect the various metal constituents if they were so small?

Daniel - So they're actually burning up at much higher levels than the stratosphere. The satellites burn up between 40 and 80 kilometres. In the atmosphere, that air descends at some point down into the stratosphere. And as these particles come down, they pick up sulfuric acid, which is a natural process. And we have a mass spectrometer on a NASA aeroplane, and we sampled these particles and blasted them with a pulse of laser light and looked at what comes off. And we can very sensitively tell what the sulfuric acid particles are made of. And in many of those sulfuric acid particles, we see small amounts of metals, either from meteors or now from reentering satellites. Our measurements were showing, we estimate, that right now we can detect aluminium in about 10% of the particles in the stratosphere.

Will - That does sound like a lot of stuff that I personally wouldn't want hanging around in the upper atmosphere.

Daniel - Well, I think right now, on the one hand we don't have concrete evidence that it's causing harm. On the other hand, these are new things we're putting in the atmosphere and we really don't know what they're going to do to the atmosphere. And that's an uncomfortable situation to have. There are plans to put a lot of material in the atmosphere and you don't quite know what's going to happen.

Will - That's a very unnerving concept then, isn't it? That 10% of the stratosphere has this particle and we have no idea what it does?

Daniel - I have to say personally, yes, I find it uncomfortable. And again, it's kind of the trite call for more research. Much of which, incidentally, will probably be in the laboratory. People looking at what aluminium does in a sulfuric acid particle. And a lot of the stratosphere is quite cold, and what's the chemistry and how do the chemicals in the stratosphere then react with that particle? So there's actually a lot of laboratory work as well as measurements in the atmosphere.

Will - Within this decade, we've got Jeff Bezos planning to launch over 3000 satellites to keep up with Starlink. Starlink is aiming for 12,000 satellites. Each of them only have, as a rough estimate, a five year orbit time. So with this exponential rise in satellites and therefore a rise in stuff burning up, are we looking at an increase in these sorts of particles in the atmosphere that we don't really know what are going to do?

Daniel - I think you put it very well. That's exactly what you're looking at. And I just saw an article in Science magazine that, at least on paper, there's plans for up to a million satellites in orbit.