The great thing about solutions to space debris is that pretty much every off-the-wall idea has been suggested with some degree of seriousness. (This is also the only great thing about space debris.) Many of these suggestions never make it off the ground, but the suggestions are there. Space debris is a problem with no clear solution - the nature of the debris itself is so varied that no single approach is likely to solve it. However, any solution is better than no solution, and we’re still in a phase of learning what methods are likely to best help clean up after ourselves.
Any solution to space debris has a number of significant hoops to jump through to be considered useful, but the number one rule for debris-hunting missions is “do not generate more debris.” The last thing you want to do is send up a new satellite, crash it into a dead satellite, and then lose control of both the new one, the old one, and have bits break off of both. When two satellites collide, you can generate thousands of new and dangerous pieces of shrapnel, the very definition of not helping the problem. Space debris is moving at an astonishing speed around our planet, and so any collisions between objects up there are extremely energetic. It is very easy to break a rigid object in space by hitting it with another object coming at it from the other direction - even a stray bolt can do a huge amount of damage if it’s moving at more than 10,000 miles per hour.
If you want to catch a satellite, with a magnet or with any other method, you need to do so very, very carefully in order to avoid disaster. This usually means you can’t use the main body of the satellite to do the catching, which in turn means you need some kind of maneuverable grabbing arm, or you need a deployable net. These methods work best with large objects, like satellites which are still intact, and you still have to approach them carefully, because most satellites aren’t designed to withstand sudden changes in velocity. A high velocity whiplash to a satellite could easily snap off solar panels, antennas, or other protruding objects.
A satellite which is defunct but intact (a best-case space junk scenario) has the additional complication of not being particularly regular in shape. Because there’s very little atmospheric drag at these altitudes, the satellites have no obligation to be aerodynamic like airplanes do, and so often have instruments poking out in all directions. A partially destroyed satellite will have the same set of protrusions, but will also have a sharp edge where it broke. On the one hand, these irregularities mean it might be easier to snag the satellite somehow. On the other hand, catching it comes with a much higher risk of damaging whatever’s doing the catching. (This is another reason nets seem appealing; it’s relatively hard to puncture something that’s mostly empty space.)
But there’s a more significant problem with using a magnet on a tether to collect space debris, even if it is maneuverable. A huge chunk of our space debris isn’t magnetic.
Satellites are very finely-tuned computers, designed to make a series of measurements extremely accurately and precisely, but they’re also designed to be light; the lighter the satellite, the less fuel required to push it into orbit, and the less expensive that satellite’s launch becomes. Metal is heavy! And even the metals that are used aren’t necessarily magnetic - aluminum isn't. So a magnet as a collecting device might collect stray bolts, or some of the internal supports to a broken-apart satellite, but it would totally miss the rocket debris imaged above, or a plastic circuitboard. And that’s just the big pieces; trying to catch a tiny fleck of paint with a magnet is an exercise in futility.
The debris that’s most likely to be collected together before being pulled out of orbit are those tiny ones. If you could catch a sufficient number of them, you might be able to gradually clear our skies of some of the tinier pieces of debris which otherwise go pinging at high speeds into our still functional satellites (and sometimes our crewed missions). Plucking tiny flecks of paint out of the sky is impractical in the extreme, but we may be able to take a lazier approach. Something more like very durable flypaper has been suggested for this purpose — anything that a piece of grit could impact, get stuck to, but not break or bounce off of. Aerogel is often the material of choice here; it’s very light, so cheap to launch, and very good at catching tiny pieces of stuff. We used it to catch pieces of a comet and bring them back to earth with the Stardust mission, and we had some up on the Mir space station for a time to help identify what the tiny pieces of space junk were in the first place. Once the aerogel fills, you could send it down Earthwards to burn up in our atmosphere.
We are still working on how to catch space debris. Rather than using a magnet to collect a number of satellites at one time, most of the are focusing on capturing individual, large, dead satellites. Again, it’s not a cure-all, but removing any of these hazards is better than removing none of them. The European Space Agency is working on e.Deorbit, which is taking a fishing net on a tether approach. The irregular shape of the satellites will help trap the net on its surface, and then the net-launching satellite can drag its cargo back into the atmosphere, where it will either burn up or crash safely in the ocean.
Japan’s space agency JAXA has a prototype for a new system of dealing with space junk, which, if it works, will deal with both magnetic tiny objects and one large object per mission. Currently tightly coiled and attached to the ISS resupply capsule Kounotori 6 is a 700 meter long tether made out of conducting metal, called the Kounotori Integrated Tether Experiment (KITE). When the resupply capsule leaves the ISS, loaded up with the ISS’ old batteries and other unneeded goods, but before it re-enters the atmosphere, the tether will unspool, and a charge will be run through it. The motion of the long charged cable through the Earth’s magnetic field will create a force pushing other objects down towards the Earth - hopefully tossing some small pieces of metal down towards the atmosphere. In its final form, the tether will have a grappling hook attached to its other end, which can be attached to a dead satellite and then used to tug the defunct satellite down to Earth. This current test, however, will end with the tether being released, in an orbit where it will fall back to Earth after a few weeks. Hopefully it will have cleared at least a little bit of our mess away with it.
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