Electromagnetic tether for space debris removal

[SiberianTiger]

Several times I came across the idea of using an electromagnetic tether for removal of space debris. According to the most rough descriptions, it's a long wire connected to a spacecraft to be removed. Moving through Earth's magnetosphere would induce a current inside this wire, which would cause the kinetic energy of movement to dissipate.

The questions: Are there good and bad areas of Earth's magnetosphere to try the principle out? How can the braking momentum be estimated? Won't Coulomb force cause the wire to tangle, ultimately making it useless?

Thanks.

 

[RisingFury]

Wish we could just get a giant magnet to suck the spce debris from orbit, just like in cartoons...


Give me a moment though and I'll write a simple model.

---------- Post added at 07:43 PM ---------- Previous post was at 06:46 PM ----------

Ok, here we go.

You cannot produce a magnetic force in the same direction as the magnetic field. You need an "up" or "down" component of the field, to produce a force for breaking or acceleration.

The problem is that the same effect will rotate your space craft and thus stabilisation becomes a problem. If you wanna cancel out the torque, I think that'll also cancel out the net force...

You could use some other form of stabilisation though. A long, light boom, with a weight at the end, pointed towards Earth could produce gravity gradient stabilisation...


Now, there's always at least a little bit of the "up" and "down" component of the field, if not for anything else, it's because Earth doesn't only have one magnetic dipole... but the effect would be most noticable over the poles, where the cross product and the field density are greatest.

 

[Dig Gil]

Then using a the gravity gradient for stabilisation could become useless at the poles because of the flattening of the surface there due centrifugal forces. Plus: if you need to thrust debris to make them meet the poles so to de-orbit them, then it would be more efficient to just stick retro-thrusters to the debris an de-orbit them right over the equator where they are.

 

[tblaxland]

It is definitely doable. Certainly such tethers can be used for increasing a satellites orbital energy, so I see no physical constraints as why it couldn't be used in reverse. It should be able to provide enough power for some spacecraft control so you don't need to worry too much if your spacecraft's batteries have died.

I do wonder if a large sail would be more effective though, in terms of mass/simplicity/cost.

 

[RisingFury]

Then using a the gravity gradient for stabilisation could become useless at the poles because of the flattening of the surface there due centrifugal forces. Plus: if you need to thrust debris to make them meet the poles so to de-orbit them, then it would be more efficient to just stick retro-thrusters to the debris an de-orbit them right over the equator where they are.

It won't work just over poles. It'll just be most effective there, because the cross product and filed strength are greatest.

 

[tori]

There's actually a company that does exactly that - sat deorbiting using tethers, tapes or webs. You just buy a module that's triggered once your sat reaches its end of life and it reenters in a couple of weeks.

The downside is that it's useful (as in "takes under a couple of years to deorbit an object") only if well within LEO, anywhere higher and you're looking at centuries at best, which is both impractical and uncertain, as tethers eagerly snap when exposed to micrometeor strikes. Hoytethers and similar multi-tether concepts tend to survive longer, but ultimately it's just a matter of time for any tether exposed to space to break. Our best tethers with months of maximum survival time look pretty stupid when talking about centuries.

So we still need tugs to pull sats from GEO :]

 

[SiberianTiger]

It won't work just over poles. It'll just be most effective there, because the cross product and filed strength are greatest.

My idea was proposing an alternative simpler solution to the latest "GEO garbage scow" design proposed by Energia, which doesn't feel to be very attainable. I thought that simply attaching a really long wire to a defunct sat would cause it to drift off the GEO into lower orbits. But from your math I see that GEO should be among the least favorable orbits for this kind of propulsion.

However, I don't know how symmetric the magnetic field at the GEO altitude really is. Perhaps, interacting with the solar wind already brings in some sensible distortion to the lines along one orbit? I wasn't able to find any data sheet direction/magnitude of the Earth's magnetic field along one GEO orbit.

 

[Loru]

However, I don't know how symmetric the magnetic field at the GEO altitude really is. Perhaps, interacting with the solar wind already brings in some sensible distortion to the lines along one orbit? I wasn't able to find any data sheet direction/magnitude of the Earth's magnetic field along one GEO orbit.

On the side facing the Sun, the distance to its boundary (which varies with solar wind intensity) is about 70,000 km (10-12 Earth radii or RE, where 1 RE=6371 km; unless otherwise noted, all distances here are from the Earth's center). The boundary of the magnetosphere ("magnetopause") is roughly bullet shaped, about 15 RE abreast of Earth and on the night side (in the "magnetotail" or "geotail") approaching a cylinder with a radius 20-25 RE. The tail region stretches well past 200 RE, and the way it ends is not well-known.

The dipole field has an intensity of about 30,000-60,000 nanoteslas (nT) at the Earth's surface, and its intensity diminishes like the inverse of the cube of the distance

Ftom that I think at GEO orbit magnetic field is quite symmetric.

 

[JamesG]

Also you can't predict precisely how rapidly the orbit will decay and as it does so when it will cross into other object's oribital paths, potentially causing the accidents deorbiting is intended to avoid.

Besides, a lot of money and energy was spent putting those things up there. A much better use will be to leave them there until they can be collected and recycled.

 

[tori]

I thought that simply attaching a really long wire to a defunct sat would cause it to drift off the GEO into lower orbits.

It would. But is that really a good idea? You'd basically have a swarm of uncontrolled objects on "rapidly" decaying orbits that pass through the entire equatorial orbital plane (i.e. potentially intersecting all Earth orbits below GEO). When the tether breaks (it takes a long time to drop from GEO) you end up creating debris, potentially striking more objects.

 

[ar81]

How does electromagnetic tether look like?

According to a book by Miguel Bautista Aranda who has worked for NASA for 30 years, we are creating a "prison" of debris as jail bars. Is this idea accurate nowadays?

 

[tori]

They're basically long (10s of km) and thin steel wires. This (STS-46 tether) is how they look when deploying - you can see both parts of the tethered spacecraft. As it goes through Earth's magnetic field, current gets induced and it's own magnetic fields starts interacting with Earth's and forces start pushing on it.

And yeah, Miguel Bautista's right (Google Earth file with realtime satellite and debris tracking).