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Congratulations! You're the new leader of the future UN(Unified Earth Country governing body), and you want to push for space travel for all. But at this point(in the futures), centuries of space travel has led to much debris in space. But long periods of economic depression for the world(due to war? fossil fuel crisis? not important for my story) means that all the large debris has collided over and over to form lots of sub millimeter debris, undetectable until you get hit. This situation is so bad, that the only way to have a chance to make it orbits it to launch with wipple shielding in the fairings, then burn as early as possible to leave orbit and out of the immediate debris field, or else you risk getting hit hundreds of times on the way up. It's your job as the new leader to start cleaning up this mess, and while you some time(at most, a couple of decades), Earth is ready to venture out, to start colonies on Mar and elsewhere. What technologies and techniques do you use? Current approaches seem to locate each objects, but with billions of particles, it wouldn't be practical to locate and track each one. Would you deploy large solar sail like objects, but with wipple shield to "trap" debris slowly? What would be the most cost effective method? Quickest?

Note: I'm not even sure if it would be possible to have a debris field with such a high density that hundreds of hits by debris would be basically guaranteed, but I'm decently sure that if all the debris is moving in one general direction, it should be possible.

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  • $\begingroup$ What's the budget and what's the time limit? Immediate mitigation will be most expensive. The longer you wait, the less expensive. I may want lots of other things with that budget, too. $\endgroup$
    – user535733
    Mar 20 at 0:39
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Laser Broom

If you had a really big system of lasers zapping particles semi-indiscriminately, you could perturb the orbits of these particles enough to decay the orbits. You wouldn't need to be really accurate, because the particles are small and wouldn't take much to nudge them. the concept is designed for slightly bigger particles (centimeter sized) so it might scale down differently than the example. But the smaller the particle, the easier a hit would perturb the orbit by ablating the surface and causing thrust. Wikipedia at least thinks it would be fairly cost-effective. I'm not sure if the concept is targeting individual particles anyway, since hitting something a couple centimeters in diameter specifically with a ground-based laser sounds tricky. Id guess you'd be blasting the heck out of a region of space and hoping to hit as much as possible. In the target-rich environment you describe, I don't think it would be a problem.

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Either A) the debris is so dense that you can laser-broom it out of orbit or B) it is sufficiently sparse that you can work on cladding your spacecraft in suitable protective layers and you'll be OK. If the debris is so small, multilayer thin Whipple shields seems like they should be more than adequate.

Remember that everyone hoos and hahs about the velocity term in the kinetic energy equation, $e_k = \frac{mv^2}{2}$, and relative orbital velocities in a collision might end up as high as 20km/s or more, but your submillimetre dust grains are gonna end up with millgram and microgram masses. A milligram at 20km/s only carries 200J of kinetic energy... that's less than a 9mm bullet, and it doesn't have nearly the amount of momentum that the 9mm will have. Most collisions will be less energetic than that, though, especially if most of your debris is going in the same direction.

A few thin sheets of aluminium and maybe some slabs of dense polyethelene with a decent amount of spacing seems like it'd do an excellent job of protection. So much so, that you could imagine throwing up some dust-absorbers on sub-orbital trajectories whose only job is to spend a little time at orbital heights, soaking up debris in their layers of UHMWPE before falling back to Earth. You don't even need a serious rocket for that; suborbital rocketry is some orders of magnitude cheaper than the real thing, after all. A big dumb payload in a real orbital rocket can do a better job, unfurling a nice big umbrella of tough plastic and aerogel to soak up the worse junk without breaking up, then ditching back into the atmosphere before the wear gets serious enough to risk the shield breaking up.

sub millimeter debris, undetectable until you get hit

If it is dense enough to be dangerous, you can see it by the effect it will have on light transmission and reflection. You'll be able to see it alright... after all, we can see the interplanetary dust cloud in space around Earth, and that's got a density of about 1 fleck of dust per million cubic metres because the cloud is big enough to affect light transmission.

I'm not even sure if it would be possible to have a debris field with such a high density that hundreds of hits by debris would be basically guaranteed,

They might not be guaranteed instantly, but a big cloud of crud in a slightly different orbit is going to hit you hard at regular intervals. It'll grind you down over time, certainly.

but I'm decently sure that if all the debris is moving in one general direction, it should be possible.

If you want to maximise kinetic energy, and hence the hazard presented by your debris, it needs to be in a mix of orbits. Eccentric ones give you higher velocities but reduce the chances of intersection in any given orbit, but to maximise damage you'll be wanting debris orbiting in totally different directions.

When everything is travelling in more or less the same direction, the risk of damage can be much reduced by injecting yourself into an orbit that minimises relative velocities. You'll not be getting 20km/s impactors in that situation, that's for sure.

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It will turn into a ring system within a few decades.

Very few of these particles is static above a fixed point on earth (those few in geostationary orbit above fixed points on the equator) - every other particle is relative to the surface travelling in a roughly sinosiodal pattern that crosses the equator rapidly, with the exact period and phase a property of the orbit.

The particle density will vary base on latitude, two particles of the same period and phase will collide at the equator, exchange momentum, and slowly transition towards an equatorial orbit.

Two particles in equatorial orbit with different phases but very close periods will eventually pass at slow relative speed, and join together, (just like planet formation works). As these clumps accumulate at the equator, their gravity pull will reach further away, increasing the range of orbital periods that are consumed.

Eventually this will form what looks like rings. But I'm assuming the debris isnt one uniform material, and theres some large things still up there like former satellites (if not, launch something disposable) - in which case those satellites will clear the dust clumps from their orbital path, once again like planet formation, creating a ring system with gaps in it. The gaps would correspond to the altitudes where former satellites cross the equator.

Within a few decades the ring system should have enough gaps in it that you can plot a launch that, assuming it needs to cross the equator, can pass through the gaps in the ring system. Your first missions will probably be to replace critical satellites, given the old ones are now the core of many small moons.

The small moons and rocks in orbit could be manipulated into reentering (a small spider-like robot jumping from rock to rock, calculating each jump so the delta v destabilises the orbit), however since it's made of old spacecraft and satellite molocles, its probably disproportionately valuable - leave it in space, mine it, refine it, and build new space craft in orbit, saving on launch costs.

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