When Satellites Collide

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on February 22, 2009 at 8:00 AM, updated February 22, 2009 at 8:14 AM
By Keith Johnson

The recent collision between two satellites, one American and one Russian, has raised some interesting questions.

1. Why hasn't this happened before?

To quote an anonymous wag, "There's a lot of space out in space!" But our small human brains sometimes find it difficult to visualize these celestial dimensions. Here's a fanciful comparison that may help:

Take out your ballpoint pen. Look at the tip. You'll see (unless you're as old as I am, in which case, use your imagination) a miniscule metal ball about half a millimeter across.

Now take a pair of pliers, dig out the tips from about 10,000 pens, and drop them into a sack. We'll let these represent satellites.

(The Space Surveillance Network maintains a database on more than 10,000 satellites larger than a football. Most of these are dead satellites, old rocket motors, and other debris. There once was a toothbrush, and a glove left outside by spacewalker Ed White. The list also includes an entire toolkit that floated off from a recent Shuttle mission. Nearly a thousand are active satellites, though.)

Take your sack of pen tips out to a well-mown square field about three miles on a side; we'll let this represent the surface area of the Earth. Recruit a couple hundred volunteers, line them up along the sides of the field, and have them shoot the pen tips across it, perhaps with miniature slingshots, in different directions and at various heights above the grass. Tell them they have an hour and a half to complete their task (that's how long a typical satellite takes to complete an orbit).

This is far from a scientific experiment, but the chances of any two tiny spheres colliding is very roughly the same chance of two satellites colliding above the Earth. It's not completely impossible, but it is extremely unlikely. On the other hand, actual satellites don't stop after 90 minutes. They continue circling the Earth, so even if the chance of a collision is tiny on any one orbit, the event gets more likely the longer we wait.

In fact, we've had close calls before. The International Space Station and the Space Shuttles have frequently had to adjust their orbits when a satellite was scheduled to pass uncomfortably close. And a piece of orbiting junk tore off a boom from a French satellite in 1996. There is a lot of space, but there are a lot of human-made things zipping around up there, too!

2. Can we do anything about this?

Since most satellites are classed as debris, you'd think the responsible governments would at least go up and clean out all the junk.

But it's not easy. Launching a satellite in the first place is expensive; the Iridium satellites cost about $50 million apiece to get put in orbit. To remove one from orbit, it appears we would have to launch a second, trash-collecting satellite, and have it attach (expensive) rocket motors to the defunct object to slow it down, sending it into the atmosphere to burn up or fall into the ocean. Then the trash-collector would also have to have extra rockets to bring itself down, so it wouldn't wind up as debris. Disposing of a dead satellite might cost three times what it took to put it up in the first place!

There are other possibilities, including laser "brooms" to sweep satellites into decaying orbits, and aerogel blobs that could move around, collecting a large number of pieces of junk before coming down. But none of these techniques is available yet, and none of them would be cheap.

3. Has anyone ever been hit by space junk?

Considering the size of the Earth, and the fact that satellites tend to burn up when they re-enter, it seems unlikely. But apparently it's happened. In 1997 an Oklahoma woman named Lottie Williams was hit in the shoulder by a piece of blackened, woven metallic material about three inches across. It was later confirmed to be part of the fuel tank of a Delta II rocket which had launched a U.S. Air Force satellite in 1996. She was not injured.

Perhaps we should ask Congress to devote part of the current financial bailout to supplying hard hats for every person in the country!


Comet alert! New Comet Lulin may be putting on a show later this month. For details, see the article at the Sky and Telescope Web site.

Planetarium star show. The new planetarium show "Oasis in Space," along with the laser show "Laser Radiohead," are now playing on Saturday nights. For more information, see the planetarium Web site. Show information is also available by phone at (856) 256-LITE (-5483).


Keith Johnson has never seen an artificial satellite fall out of his artificial sky at the Edelman Planetarium at Rowan University. He can be reached by email at or by calling (856) 256-4389.