Predicting When Space Junk Will Come Home To Earth This weekend, a defunct German satellite is scheduled to crash to Earth, just a month after a NASA satellite did the same. NASA orbital debris scientist Mark Matney and Phil Plait, author of the Bad Astronomy blog, discuss whether engineers on Earth have any say when--or where--objects fall.

Predicting When Space Junk Will Come Home To Earth

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IRA FLATOW, host: This is SCIENCE FRIDAY. I'm Ira Flatow. Sometime Saturday night, a defunct German X-ray satellite named ROSAT is going to come screaming through the atmosphere on its way to impact on the Earth's surface. The chances of a piece of this thing, or any piece, will hit somebody, well, somewhere on the surface of the Earth, one in 2,000. The chance it'll hit you in particular, are, of course, much lower. So you can crawl out from underneath that table.

Deja vu? Just last month, a NASA satellite did the same crashing act, diving into the Pacific. We put all this stuff into orbit. You know that old saying about what goes up, has to come down, eventually, right? Or can we do something about it? What do you think? Our number is 1-800-989-8255, 1-800-989-TALK.

Let me introduce my guests. We'll be talking more about this whole event and what we can do and how much junk is there up there. Mark Matney is an orbital debris scientist - get that kind of job - at NASA's Johnson Space Center in Houston. Welcome back to SCIENCE FRIDAY, Dr. Matney.

MARK MATNEY: Good to be here.

FLATOW: You're welcome. Phil Plait is an astronomer and author of Bad Astronomy blog for Discover Magazine. He's based in Boulder, Colorado. Welcome to SCIENCE FRIDAY, Dr. Plait.

PHIL PLAIT: Hi, thanks for having me on.

FLATOW: Here there. Phil, tell us about this German satellite. What did it do? Why - is it still working? Why is it coming down?


PLAIT: Well, ROSAT is short for the Roentgen Satellite. Roentgen, with an umlaut over the O, was a German scientist who discovered X-rays. And the satellite was named after him because it observed the sky in X-rays. And it was an astronomical satellite that observed exploding stars and galaxies and all kinds of high-energy violent events like that.

It was launched by NASA in 1990, served for many years, did great work for the astronomical community. There was a glitch many years ago that shut it down. It's been in orbit ever since. And now its number is up.

FLATOW: And do we know when exactly it's coming down?

PLAIT: Well, we don't know exactly, because it's very difficult to know exactly when a satellite's going to come down. The Earth's atmosphere is hard to model. It's very thin up there, 100 miles or more up, but it exists. And sometimes it's a little bit denser, sometimes not, and the satellite might be tumbling, and so it makes it very difficult to know exactly when it's come down - it's going to come down, excuse me.

But what you said is pretty good: It should be coming down sometime this weekend. As time goes on, the predictions will get more accurate.

FLATOW: Mark Matney, just how much junk is up there?

MATNEY: Well, it turns out that the United States Space Surveillance Network, which is mostly our military asset, spends all of their time looking at space, and they track something - they have a catalog of about 16,000 objects in Earth orbit right now. And those are objects larger than about a softball in size.

FLATOW: And do we expect all of them to someday come down?

MATNEY: Most of them. The ones that are out of geosynchronous orbit probably will not anytime soon. But all of the ones that are in the first several thousand miles, eventually - first several thousand miles from the surface of the Earth, will eventually come down, but it's a very strong function of how high they are. The higher they are, the less atmosphere they encounter, and the slower they'll come down.

FLATOW: So somebody is keeping track of every single one of these little pieces?

MATNEY: Well, down to a certain size. Radars can only see down to a certain size, and it's about 10 centimeters, about four inches. And that's the catalog. And, in fact, we also keep track of those in case we need to move the Space Station out of the way, which it periodically does.

FLATOW: And could pieces be coming down all the time, the tiny pieces that we just don't know about?

MATNEY: About once a day, something in orbit that we catalog and track re-enters the Earth's atmosphere.

FLATOW: Once a day?

MATNEY: Once a day, but most of them are very tiny, and they burn up upon re-entry. About once a week, an intact object comes in, a rocket body or a satellite. So it happens pretty often. I actually did a quick set of numbers because I came - before I talked to you today. And there have been something like 4,000 objects, actually more than 4,000 objects larger than one ton, that have re-entered the Earth's atmosphere since the beginning of the Space Age.

FLATOW: We haven't heard about 4,000 (unintelligible).

MATNEY: Most of them you don't hear anything about.


FLATOW: Mark, what do you think about that? I'm sorry, Phil, what do you think about that?

PLAIT: Those numbers sound about right. It's funny to hear it that way because you start to think oh my gosh, the sky is falling. But, in fact, what this is telling you is that it's very rare for us to even know about these things coming down. And people tend to panic when they hear that a satellite is coming in, something like ROSAT, or the Upper Atmosphere Research Satellite, which came in last month.

But in fact, these aren't that big of a danger. It's certainly something to be aware of, but it's not something to panic over.

FLATOW: Are there big pieces of this ROSAT going to survive the trip through the atmosphere?

PLAIT: Actually yes. ROSAT...

FLATOW: Wait, wait, you just told me not to be afraid.


PLAIT: Well, the point here is that the Earth is really big, and these pieces are really small, and you occupy an extremely small part of the surface area of the planet. So even with 4,000 of these things coming in over the past 50 years, we've only had instances, I think, one person who had a little, tiny piece of metal bounce off her shoulder.

MATNEY: It was a piece of insulation. It didn't even hurt her.

PLAIT: Yeah, right. And I actually, I've seen a rocket booster re-enter. I thought it was just a very bright meteor, but then the next day I found out it was a booster. So these things happen without us knowing about it. And with ROSAT, we do know about it, and some things we do know about in advance, and ROSAT was a famous satellite.

It weighs about two-and-a-half tons, which makes it about a third of the size of the satellite that came in last week – last month, but ROSAT is going to fall apart a little bit more. So there should be 30 pieces or so that will probably hit the ground intact. They have to understand how the spacecraft is put together and make models of what can survive re-entry, but the number they come up with is approximately 30.

Most of these are only going to weigh a couple of pounds. However, the mirror assembly of ROSAT, the thing that actually focused, if you want to say that, the X-rays, the part that saw the X-rays, that will probably come down intact, and that weighs a lot. It's actually well over a ton. And it's going to come in intact and hit the ground.

Now, the good news for that is that's one piece falling over an entire planet, and the Earth is 70 percent water, and what's land is relatively inhabited. So again, like you said at the beginning, the odds of anybody getting hit by any of these pieces is a couple of thousand to one against.

FLATOW: Mark, is there any way to know if, you know, as it gets closer to actually getting close to the Earth, to put out a warning sign if it's going to get over some piece of land, tell people to watch out?

MATNEY: Well, even in the final hours of any re-entry, there's still a great deal of uncertainty, and basically we can narrow it down to the last few revolutions, but those revolutions cover a significant chunk of the Earth's surface. So in a sense - just to give you an example, in the UARS' case, about a day before, we were pretty confident it wasn't going to fall over North America. But then something changed on the spacecraft and changed its drag slightly, and as a result, the final orbit, some of them did pass over North America.

Fortunately, it landed in the ocean, but that just shows you how difficult it is to predict the final stages of these re-entries.

FLATOW: Would it be possible to actually shoot the thing down if you could see it was going to go to a bad place?

MATNEY: Well, in principal you could. If you recall, we did that with USA193, but the concern there was it had a tank full of toxic hydrazine, quite a bit of it actually, and there was a great concern that that would fall in a populated area and create a very large cloud of toxic gas. Because the odds are so small on these re-entries, and the effort and expense to actually intercept something is pretty extreme, these standard re-entries really don't warrant that kind of mitigation.

FLATOW: Well, you know, shouldn't there be a policy that if you put something up in space, you should clean up after yourself?

MATNEY: Well, actually we've been moving in that direction. Both of these spacecraft, UARS and ROSAT, were launched in about 1990. But since about 1995, our knowledge of re-entries has gotten a lot better, and internationally, we've been moving - a number of different space-faring countries have been moving in directions to try and design their spacecraft, that if it has a risk on the ground - we've developed the computational capabilities to compute those risks. If the risk exceeds a certain value – that the value we use in the United States is one in 10,000, that you really ought to think about changing the design of your spacecraft, make it out of metals that won't survive re-entry or perhaps put an engine on it or extra fuel so you can do a targeted re-entry and drop it out over the Pacific Ocean where it won't hurt anyone.

We did that with the Compton Gamma Ray Observatory. We re-entered it over the Pacific Ocean, and yes.

FLATOW: So designing them more to control re-entry a bit.

MATNEY: Yes, to try and target where you're going to put it in. The other thing is something we call design for demise, and that's what I mentioned. You talk to the engineers from the very beginning, and you say look, you need to design your spacecraft not out of these materials like titanium and steel that are likely to survive re-entry but see if you can use some other metals that will disintegrate upon re-entry.

FLATOW: Phil, do the sun cycles - you know, we know the sun has period cycles - does that affect where these things might fall?

PLAIT: Bizarrely, it does. When the sun is active and has a lot of sunspots, there are a lot of solar flares and coronal mass ejections that blast out subatomic particles, the Earth's atmosphere puffs up in reaction to these when they hit us. And it's a tiny effect.

But these satellites are orbiting up there where even a tiny effect over a long period of time can affect them. As I understand it, ROSAT was originally thought to burn up last year, but the Earth's atmosphere has been relatively calm because the sun has been quiet.

We had a very long, quiet period from the sun, much longer than usual. It's just now starting to ramp up. 2013-2014 is when we're going to start seeing a lot of solar activity again, although there's been a little bit.

So yeah, it's odd to think that satellites might come in because of the sunspot cycle, but there you go.

FLATOW: So if you need this weekend to have a party, a ROSAT party, everybody comes as a satellite, some Sputnik or something - Telstar


PLAIT: I'm the one that's having a ROSAT re-entry party, actually.

FLATOW: I know someone would have one. And what would a party like this be like? Do people do come dressed as satellite debris? Could you be, you know, a canister, a booster rocket?

PLAIT: Well, I dressed as Hubble for a Halloween party many years ago. So it's not that hard. But the problem is when do you throw the party? You may not know until a day in advance, as Mark said, and you've got to realize these satellites are moving very rapidly. Orbital speed is about five miles per second.

So if you're off in your calculations by a minute, that's 300 miles. So we don't know where it's going to come down because of that, and in fact these estimates of when it's going to come down are still plus or minus, you know, a day at the moment, although they'll get much better over the next 24 hours or so.

FLATOW: Well, you gave me a good idea. This being Halloween coming up, if you dressed as a piece of the ROSAT satellite, no one could say you weren't, right? I mean, their analysis of your metallurgy...

PLAIT: Well, if you want to walk around with a 1,600 pound mirror wrapped around, yeah, I guess that's your - or 3,000 pounds, I suppose.

FLATOW: All right. Thank you both for taking time to be with us today, and we'll be watching that satellite with you.

PLAIT: Thank you.

MATNEY: Thank you very much.

FLATOW: You're welcome. Mark Matney is an orbital debris scientist at NASA, at the Johnson Space Center in Houston. And Phil Plait is an astronomer and author of Bad Astronomy blog for Discover Magazine based in - he's based in Boulder.

So you now have an excuse of what to do - how to dress up for Halloween with the ROSAT if you'd like to. We're going to take a break and come back and change directions. We're going to talk about the future, a future without oil, running on clean energy, biofuels. Can we do it by 2050? My next guest thinks so. Amory Lovins will join us after this break. Stay with us.

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