Space junk is an ever-growing problem, due both to the increasing quantities of junk and the increasing number of satellites that have to thread their way through it. Yet methods to clean up space seem slow and expensive, as they target only one piece of debris at a time.

Most space junk is metallic (or partly so), and hence conductive although little of it is made of ferrous metal. Non- ferrous metals, passing through a magnetic field, are slowed by virtue of eddy currents (and hence magnetic fields) induced in them.

So, how about a magnetic junk decelerator? Simply set up a huge (but thin) loop of wire, and run enough current through it to generate a decent field. It would be nice if the wire could be superconducting, but not essential. Every piece of junk passing through the loop will experience a very slight deceleration at each orbit, resulting eventually in orbital decay and re-entry to burn up in the Earth's atmosphere. Active satellites could be let through the loop unimpeded by just turning off the current for a moment.
-- MaxwellBuchanan, Sep 04 2019

Birkeland Current Transformer by [Ian Tindale]. Mentioned in my anno [notexactly, Sep 16 2019]

Wikipedia: Electrodynamic tether https://en.wikipedi...ectrodynamic_tether
Mentioned in my anno. A device for converting electric energy to orbital energy or vice versa, inside a magnetosphere [notexactly, Sep 16 2019]

Wikipedia: Van Allen radiation belt § Proposed removal https://en.wikipedi...lt#Proposed_removal
by V. V. Danilov; refined by Robert P. Hoyt and Robert L. Forward. Mentioned in my anno [notexactly, Sep 16 2019]

Numbers would have to run for field strength against a laser ablating a surface on the debris. I feel a large, fine power loop in space isn't right somehow. It doesn't feel very green.
-- wjt, Sep 05 2019


With a sufficiently large loop, perhaps passing through the Van Allen belts, could we create a decent current using the earth's magnetic field? Would doing so slow down the earth?
-- RayfordSteele, Sep 05 2019


Super conducting is easy at space temperatures, you just need a little solar shielding. How will the system cope with the wife variety of junk orbits?
-- bs0u0155, Sep 05 2019


How much would the eddy currents slow down an object moving at miles per second?
-- 2 fries shy of a happy meal, Sep 05 2019


Space junk is orbiting at 7.5km/s. I'm guessing you need to reduce that to 6.5km/s to deorbit.

So you need the space junk to convert kinetic energy to a different form of energy. In this case, the eddy currents will become almost entirely heat energy. So you need to radiate 7MJ/kg in heat. (If you want the eddy currents to form a repulsive magnetic field to repel against the wire, you'll need the wire to be *really* close).

If you could raise the temperature of the junk of mass X kg by YºC, it would radiate that amount of heat in Z seconds. Given a value X, calculating the values of Y and Z are left as an exercise for the reader.

I don't think you're going to get any noticeable effect from a single loop of wire.

Magnetic field strength falls away at 1/r³ so size matters.

Maybe if you made coil tubes (rather than a single wire) with radius at a maximum of about 10 meters you might be getting somewhere.
-- xaviergisz, Sep 05 2019


On second thought I don't think the losing energy via heat would work. You also need to transfer momentum. Although you can transfer momentum with photons, they would all need to be travelling in the opposite direction the junk is moving to slow it down.
-- xaviergisz, Sep 05 2019


//How will the system cope with the wife variety of junk orbits?//

Keeping the various and sundry flotsam of hair products, curling, straightening, and make-up that encircle our sink from overwhelming us and forming a small planetoid is indeed problematic. If there is a solution here to be had, I'm all for it.
-- RayfordSteele, Sep 05 2019


//How will the system cope with the wife variety of junk orbits?// Well, they're mainly between work, Tesco and the local hairdresser so I'm thinking they'll be the easy ones.

More generally, though, the loop (or loops) would just cruise around, slowing the orbits of whatever happened to pass through.

I might also mention that a single pass is not intended to de-orbit a piece of junk. Rather, this would act over months or years, with multiple passes of junk through the loop.
-- MaxwellBuchanan, Sep 05 2019


The question of who would operate it could become a bit of an international bother. Could we perhaps casually 'forget' to turn it off every time certain Russian 'weather satellites' passed by?

While you've been away on your hunt, your doorman left a message with me that there's been some visitors with random and varied accents calling who seemed anxious to interview you. The usual NASA guys with whom you're familiar, some folks from the newly-formed Trump Space Force, a nice fellow from the British Space Program, MI6, several seedy- looking Russian callers wishing to 'escort you' to parts unknown, a Chinese contingency of Huawei executives, an Israeli Defense contractor, some engineers from India, or perhaps it was Pakistan, several fellows who simply wouldn't identify themselves except by number and who worked for a Mr. Blofeld, I presume they were business associates of yours, and Sturton. Naturally I did not disclose to Sturton your whereabouts.

I wouldn't have been over there except that I simply had to complain about the bad parking habits and noisy scuffles raised amongst this motley assortment, which were disrupting a nice game of croquet I had going on with Prince Andrew. I recommend hiring a valet.
-- RayfordSteele, Sep 05 2019


I think the main issue with this idea is that space is just too big for any practically-constructable ring to be very effective.

The ring would take some of the momentum of the deceleratees (?), due to conservation of momentum. If it's orbiting oppositely to them, which would be best for their deceleration, it would also itself be maximally decelerated. If it's orbiting in the same direction, but more slowly (which would be possible at the same altitude if that altitude is the ring's apogee, and the debris has a higher perigee than the ring does), it would be accelerated and thereby kept in orbit by the debri's* momentum, but it would also decelerate the debris much less. It may be possible to work around this by using a "tacking" kind of technique, where it takes debris that's orbiting perpendicular to its own orbit, effecting a (very small) plane change to itself instead of a raising or lowering of its orbit. Then the deceleration of the debris is middling, while the ring's orbit is only affected in direction. That could even be used deliberately, to maneuver into other orbital planes and clean them up too.

*Yes.

// With a sufficiently large loop, perhaps passing through the Van Allen belts, could we create a decent current using the earth's magnetic field? Would doing so slow down the earth? //

For the first question, probably. See related idea "Birkeland Current Transformer" [link]. You could even, maybe, use that to power an electrodynamic tether [link] to raise the ring's orbit. For the second question, I doubt it. I don't see how the momentum would be coupled back to the body of the planet. But it has to come from somewhere, so maybe.

Tangentially, as a secondary purpose for this ring, you could also use it, or said electrodynamic tether, to remove the Van Allen belts: [link]
-- notexactly, Sep 16 2019


//space is just too big for any practically-constructable ring to be very effective// You're probably right. But if it worked, each ring could be very cheap to build, and no more expensive than a small satellite to launch. So, you could have a lot of these things up there at any one time, each of them whittling away at debris in one particular orbit.

Alternatively, you could build a few smaller, very high-field devices with enough propellant (or maybe ion thrusters) to move around. Then you pick one piece of orbiting debris, move a loop to intercept it, and degrade its orbit by a large amount in one go. This still requires "one-by-one" clearance of debris, but seems possibly easier and cheaper than other "one-by-one" proposals like nets.
-- MaxwellBuchanan, Sep 16 2019



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