Bruce, in between posts about Indian women buying negligee online and East Asians cooking in our brand new climate, posted on what was my favorite thing to worry about in 1988: space junk.

As you may (or may not) be aware, the skies above us are a flying trashheap. More than 110,000 known, tracked chunks of litter are hurtling around the Earth at more than 17,500 mph. There may be more than a million bits and pieces.

Some are shards of broken satellites (though there are hundreds of whole, dead satellites zinging around up there too), some are pieces of equipment lost by astronauts (or just dumped out the window) -- an estimated 300,000 fragments were thought to be created by the explosion of one Pegasus rocket alone.

This stuff is dangerous. Forgetting for a moment the danger posed to astronauts in the International Space Station (a one-in-ten chance of a debris accident over the next ten years), these junk particles are a hazard to the low-orbiting satellites most useful for studying Earth. Having our planet ringed in fast-moving debris presents some serious challenges to doing the kind of space science that we need to tackle environmental and social challenges here at home.

In 1983, a paint chip almost too small to see cracked the windshield of the space shuttle. Satellites are pretty routinely damaged by unseen high-velocity grit and grime. As the BBC reminds us, "A pea-sized ball moving this fast is as dangerous as a 400-lb safe travelling at 60 mph." It's hairy up there.

But what can be done about all this junk? There are several mutually-compatible options. First off, we could prevent further accumulation of space junk by using the "Terminator Tether" to "deorbit" satellites at the end of their useful lives. Secondly, we could clean it (slowly) using a laser broom to "sweep" debris from the skies, mounted on the space station, the shuttle, and perhaps even on dedicated "sweeper" satellites. Lastly -- and most ambitiously -- we could recycle it for use as a counterweight for an orbital sling (sometimes called a "skyhook"): an orbiting, spinning tether, sort of a cousin to the space elevator, designed to lift-and-fling satellites from low orbit into higher orbit without requiring added propellant.

Or, we can go back to forgetting that it even exists, and hope for the best.

Let's get some perspective. Space is big. How big?

Well, there are a million objects or more orbiting, but many of those are 1 mm or smaller, which "don't generally pose a large threat to spacecraft." And "one study showed that a craft that had been in space for more than five years was struck by particles more than 30,000 times, with no ill effect."

So how many objects are large enough to do damage? Let's estimate high, say 200,000. And how much space are they in? Small junk is especially prone to atmospheric drag, and probably more likely to be in erratic orbits... I'm going to guess orbits from 200 to 1000 km.

That's roughly half a trillion cubic kilometers of empty space. On average, each piece of debris is a speck, just a couple of millimeters, floating by itself in a cube 130 kilometers on a side. So calling space a "trashheap" may bring to mind an inaccurate picture. A satellite with a 20 square meter cross-section sweeps through about one cubic kilometer on each orbit. While I'm simplifying an awful lot, unless I dropped a zero, I think that means the average satellite will smack such an object on average once every 2.5 million orbits, or about every 500 years.

(I'm sure some of my guesses are off, but I've tried to be conservative. Probably my worst estimate is that the 200- to 1000-km altitudes are evenly populated, when I'm sure there are high- and low-traffic zones where density varies greatly. But as the article points out, we already know how to nudge the space shuttle to avoid debris. We have excellent ground-based radar tracking of space junk down to a very small size, and great collision prediction, so my guess is that the law of averages works out in our favor, not against us.)

If the average planned lifetime for a satellite were 20 years -- a figure I suspect satellite engineers would give their right arms for -- that'd still mean only 4% of satellites would have their planned life cut short by debris. My guess is that there are plenty of other hazards in the orbital environment that, over 20 years, are more than 4% likely to kill a bird.

Space engineering is all about cost-efficiency, so before we start using a space elevator or laser brooms to marginally reduce a 4% risk, we need to make sure the same money can't be spent more effectively on the other 96%.

Which is not to say that the problem should be ignored; clearly the clutter problem is getting worse, not better. But I don't think it's getting worse very quickly, since we've been trucking stuff up there routinely for a quarter-century now. My guess is that "hoping for the best" -- combined with the excellent ground-based radar-tracking systems we already have for essentially all the damaging-sized objects -- will be the best risk-minimizing strategy for quite some time.

Posted by: Jamie McCarthy on October 7, 2004 1:08 PM

The other fact is alot of this trash is deorbiting all by itself and will clean itself out over time.

The only things that wont deorbit are in geosncych orbit and thats very little of whats out there now.

as for using lasers they already have in tests and will likely use them later on the clean up mid sized trash thats in orbits they want clear.

Posted by: wintermane on October 7, 2004 1:26 PM

Working with some Russian Satellite designers on a proposed constellation of 12 store-and-forward email satellites back in the early 90's, I was shocked when I saw the reliability numbers on their satellites, which were MUCH worse than typical american numbers. There response was interesting, it was a lot more cost-effective to build cheap satellites, and throw a couple of extra into higher polar orbits than to squeeze those few extra percent out of the reliability.

Of course, that isn't much comfort to MANNED objects like the ISS, but for the bulk of unmanned flight it made plenty of sense.

- Mitra

Posted by: Mitra Ardron on October 7, 2004 3:49 PM