A map of all known human-made objects in orbit around the Earth. 65 per cent of them are debris. Image credit: ESA.

Sep 8, 2015 What do you do about a problem like space debris?

Sen—Space is a dangerous place to be. Sure, there’s no air, and a good solar flare can irredeemably irradiate you. But on top of that there’s also the threat of being punctured by a hypervelocity fleck of paint.

Although it seems from videos that things move pretty leisurely in orbit, at the height of the International Space Station orbital velocity is about eight kilometers per second. If you’re in the same orbit and moving in the same direction, your relative velocity might be pretty slow (which is good news if you’re trying to dock with ISS and resupply the astronauts on board).

But not everything up there is so benign. An object in an orbit tilted with respect to that of the ISS might be moving at the same absolute speed of eight kps, but in a different direction. If it hits, that relative speed can still be several thousand kilometers per second, faster than a rifle bullet.

Collisions with large objects are unlikely. But smaller junk can be a problem, and there’s a lot of it. We call this stuff space debris, and examples can be a lost tool from a repair EVA, parts from boosters or satellites deployed into orbit, or even very small objects like nuts, bolts, and yes, flecks of paint.

You wouldn’t think those tiny bits of flotsam would be a problem, but remember, the relative speeds are huge. Think of it this way: A rifle bullet doesn’t weigh much, either, but it’s incredibly dangerous when it’s moving at a kilometer per second, let alone several times that speed. You wouldn’t want one hitting the ISS.

Debris smaller than about a centimeter can be handled by ISS shielding. Pieces on the meter size scale are detectable in advance, and the ISS can be moved to a slightly different orbit to avoid impact.

But material in the centimeter size range is a big problem. Too big to let hit, and too small to see easily from the ground. What can be done?

An international team of scientists may have a solution. Their idea is to first detect the debris with a telescope mounted on the outside of the ISS, and then if they find something, they’ll zap it with powerful lasers. Not to destroy the debris—Star Trek is fiction, folks—but to push it down into Earth’s atmosphere where it’ll burn up. Here’s how this would work.

First, to detect the debris, they propose using a telescope with a 2.5-meter mirror that’s already planned to be put on board ISS in 2017. Designed to look for flashes of light created when subatomic particles from space slam into Earth’s atmosphere, it can also be used to look for space debris near the ISS. It has an extremely wide field of view, and will be very sensitive to light. The team calculates that they’ll be able to detect objects one centimeter in size up to a distance of about 100 km.

Once an object is detected, the next step is to get rid of it. For this they’ll use a powerful fiber optic laser. Each fiber generates a fast high-energy pulse of laser light. Individually it’s not enough to do much, but a bundle of 10,000 of these fibers can deliver a serious amount of energy. They’ll target the debris and hit it with a rapid burst of energy.

This will flash heat the debris on one side, causing it to vaporize. The vapor expands rapidly, pushing on the debris. This acts like a tiny rocket engine, changing the velocity of the object. If the geometry is correct (and this can be set by the direction the telescope and laser system is pointed) then this will slow the orbital speed of the debris, dropping it into a lower orbit. A series of pulses can slow it enough to force it into the upper part of Earth’s atmosphere. Drag takes over, and the debris burns up harmlessly.

This sounds like scifi, but the beauty of this idea is that it uses technology we already have. The group plans on creating a demo of the concept (using a smaller, 20 cm telescope) that could be tested on the ISS, and it can then eventually be scaled up for routine use. They also envision a free-flying system that could be launched into a polar orbit, where a lot of this debris can be found.

I like this idea. It’s very difficult to track debris this size from the ground, and if the response time is rapid enough—the debris is only in range for zapping for a few seconds—it could prove very useful, even critical, for long-term habitability of space.

There are thousands of tons of space of space debris in orbit around the Earth, and more generated all the time. Satellite collisions are extremely rare but do occur. In 2009, two satellites (the Iridium 33 and Russian Cosmos 2251) collided, generating two large clouds of debris moving in different directions, and in 2007 the Chinese purposely blew up a satellite to test on-orbit warfare, which also created an expanding cloud of thousands pieces of debris.

This debris is a serious threat to other objects on orbit—and while the movie “Gravity” was fictional, the basic premise was on target. You do not want this stuff up there, and remediation techniques have to be tested.

Collisions are rare, but if we do nothing about them, they are also inevitable. This ‘scope and laser system sounds like it would be a good first step. 

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