# How to Effectively Collect and Recycle Space Junk?

## The Question

What would be an efficient and relatively low-cost solution for collecting and recycling of space junk?

The most important criterion of effectiveness is the ability to remove even small pieces of junk from the orbit and then put them to good use. Small is on the scale of millimetres.

Deorbiting should be avoided since it is 1) potentially dangerous for surface installations especially without the protection of an atmosphere, 2) wasteful (unnecessary damage on impact and burning in the atmosphere once terraforming progresses to advanced stages).

## Context

A small group of colonists arrives at a moon of a gas giant with a plan to terraform it and build a new life for the generations to come. They aspire to build a sustainable and environment-friendly civilisation that in the future will span over the star system and perhaps even extend its reach to nearby stars.

They start a big-scale mining operation in the nearby asteroid belt and other gas giant moons. Other on-going projects are development and building artificial magnetic shields and a satellite network to monitor the moon and coordinate operations. The colony ship was never meant to land, so it stays in the orbit. Smaller vessels are used to transport people and materials to a tiny domed (and partially underground) installation on the surface. The space traffic is not overwhelming, yet. Accidents are very rare but still happen.

The moon originally does not have any debris or other junk nearby. The colonists intend to keep things neat and clean forever. Therefore, they want to put in place a system of collection and recycling for the space junk. They believe that it would effectively lower the risks of accidents in the future when the traffic is projected to increase substantially. They also do not want to repeat the mistakes of their own Earth and take a route of 'irresponsible mismanagement of resources' that almost killed the planet.

Deorbiting and burning in atmosphere bigger pieces of debris is not a valid option since there is no atmosphere yet. Moreover, the colonists want to reuse the materials and technology. The Chief Engineer is a neat freak, so he insists on collecting and processing even the tiniest pieces of debris. His team is not sure it is possible to achieve his standards of sparkling-clean space, but promise to do their best.

## Technological level

• fully automated and robotised asteroid mining;
• space travel at 1/10 of the speed of light;
• terraforming technologies (however, only one project has been completed successfully by the time of their departure);
• genetic engineering;
• suspended animation.

Technologies that are envisioned by scientists of today but cannot be built because of technical difficulties (materials, money, political will) are fine. However, something like magic space vacuum cleaner is not possible unless it can be explained by existing science.

## Real-life Proposals

There are some real proposals for my problem. However, I am not satisfied with them because they mainly suggest cleaning the orbit by burning the debris.

Project Pac-Man

An approach and capture system – a so-called “Pac-Man” solution. Credit: 2015 EPFL/Jamani Caillet.
Originally published on Space.com

This system uses a net cone to capture debris as small as 10x10 cm (4x4 in). It can be launched from orbit and relies on optical scanners to find and capture junk.

The downside is that it is meant to deorbit junk and burn with it in the atmosphere. It is very wasteful. Also, I am not sure if all debris can be easily seen. Moreover, the net design does not allow capturing of smaller pieces.

DARPA satellite recycling

This programme is developed by the US Department of Defense to scavenge parts of satellites (such as dishes) and reuse them. Again, this does not help with small pieces and 'useless' junk that cannot be used as parts for new equipment. It is still a much better idea than burning things.

Other approaches include towing junk to other planets (like Mars) to some kind of a recycling facility, using nets and harpoons to capture debris, or just vapourising it with lasers.

This question is not a duplicate of previous questions related to Kessler Syndrome or space debris. My focus is on recycling rather than clearing the orbit. It is also important for me to avoid deorbiting.

• If this is a previously uninhabited system then where is the space junk coming from? They could just immediately clean up after themselves if they lose ships or satellites and then the question of space junk would be solved on an individual basis. If they don't leave space junk in orbit for long, it should not degrade to very small pieces either. Or was this system previously inhabited by an alien species whose tech you are bent on recovering? – Real Subtle Nov 4 '17 at 7:28
• @RealSubtle, there is no pre-existing debris. And the goal is to keep things neat and clean as the day they arrived. – Olga Nov 4 '17 at 12:01

Unless everthing in orbit is magnetic, the only ways to be efficient are:

• Use the pac-man method. It's the one and only way to collect non-magnetic debris from orbit because there's no atmosphere to push things around with a blower. However, rather than using the single-mouth solution shown in your picture, methinks a three-engine net trawler would provide better coverage. It is a 3D problem, after all, and what you want is the biggest honking net you can possibly get out there.

• Use fully automated drones in a controlled flight path such that everybody (especially including pilots) know where they are. As those nets fill up, the trawler teams become catapults full of bullets. When the nets are "full" (measured by the drag on the drones), the drones come together in front to enclose the mess and form a tug.

• Space-station collection. Don't even try to get this stuff to the ground without processing, first. Your drones will want to take advantage of their orbital momentum to hit the target, which would efficiently use hooks and winches to snag-and-drag the material in. Empty the net, and send the drones back to their pre-arranged route.

And remember that this takes time. Cleaning out a messy high orbit could take months to years. Remember, it's a 3D problem. Nets. Big honking nets.

• I was planning on doing most of the construction and recycling in orbit. But this is precisely the reason I want to make sure that there is no industrial space pollution. I am wondering if drones could interfere with terraforming material drop off. Or should they be just temporarily suspended? – Olga Nov 4 '17 at 12:20
• The beauty of software is that the drones can be programmed to do just about anything. At a specific time they can clear a channel. With a specific broadcast command they can do the same. Etc. Anything you do in space requires planning because gravity and mass don't take well to one another (or take too well, depends on your POV), so there's plenty of time for the drones to get out of the way. – JBH Nov 4 '17 at 16:47

You'll meet several significant problems when you try to imagine garbage collector like "Pac-man" - pieces of garbage move with relative speeds so high, that they can turn "Pac-man" itself into more garbage on contact, and if you wish to equalize the difference in speeds, you need ridiculous amounts of fuel.

Using lasers to control garbage orbits may be viable option. You don't need close approaches, all you need is ground-based laser (there are agreements that prohibit launching weapons to space, mind it), that can hit 10cm piece of junk from ~500 km (lower orbits are quickly cleaned by residual air drag). Then you either decelerate and deorbit junk (small pieces can't harm anything, they'll evaporate on reentry) or guide it into different orbit for rendezvous with a garbage processing satellite.

• Welcome to worldbuilding! Would a laser hitting a 10cm bit of rubbish from ~500km away be enough to cause such a large movement of the rubbish? – Mithrandir24601 Nov 4 '17 at 8:32
• Hmm... we are talking about a future colony in 'a far away galaxy'... so there are no agreements prohibiting anything. There is also no atmosphere. Moreover, the conditions specifically state that I would like to avoid burning anything. Recycling is the goal. Nonetheless, I think yours is an interesting idea. Would you mind elaborating on it a bit? – Olga Nov 4 '17 at 12:08
• @Mithrandir24601, amount of movement will depend on the laser power at orbit distance. If you can regulate it, you can partially evaporate object, probably receiving big delta-V in short amount of time, or use light sail approach to slowly, over the course of weeks, move this rubbish. On the second thought, since you can't precisely control thrust (garbage can be of any shape, surface color/finish, rotate, etc) you may have better luck using active light sail microsatellites, which will still rely on ground laser to move, but will control direction of thrust using onboard navigation systems. – A.C. Nov 5 '17 at 9:40
• @Olga, main concern is human time. Even if this community has access to interstellar flight, it doesn't mean they have effective propulsion, magnetic nets and can do clean up in matter of days. So they'd probably prefer simple and reliable automatic systems (lasers can work for a very large periods of time, light sails can not be easily damaged to the point where they stop functioning). If they program some of their computers to deal with junk, they don't have to bother about it themselves and can focus on other things (they are small community with more important things on the list). – A.C. Nov 5 '17 at 10:10
• When I get the opportunity I'll post a link to a question where I go in depth into the question of controlling orbits with lasers. The end result is that it's totally doable though – bendl Dec 29 '17 at 3:10

Gather them
If your civilization has the technology to create a 0.1c propulsion unit then a 0.01c propulsion unit should be readily attainable. With such a powerful rocket a small fleet of robotic craft could match orbits with all of the larger pieces of junk and simply load them on board.

Absorb them
The finer pieces would require a different approach if you wanted the task done in a reasonable time frame. One option would be a very large double skinned sphere which was very lightly pressurised. The gap between the skins being filled with some sort of self-sealing goo. Inside there is something that can generate a large electromagnetic field and strategically placed around the surface are some form of electron beam projectors.

This object would be placed in orbit and would aim the beam projectors at any objects in nearby orbits these would then become electrostatically charged and would be attracted to the sphere and either stick to it, get embedded in it or be vaporized and captured inside it.

A sphere would be needed as objects could approach from almost any direction except directly below and at a wide range of velocities depending on the type of orbit.

Forget them
Perhaps a better plan (not strictly allowed but…) would be arrange for the Chief Engineer to meet someone he admired greatly who could say to him “We have a lot on mess on the surface and we need it tidied up. We could really use all these people you have working on your project like Olga, A.C., JBH and Slarty. Perhaps the orbital clean up can be delayed until we have cleared the surface a bit. Let me buy you a beer and we can discuss it”.

• The Chief Engineer sends his warmest regards and firm 'no' (even after a couple [or more] of beers) to the idea of leaving junk in orbit. His OCD does not let him sleep. And we do not want him grumpy because of sleep deprivation! But he was also very interested in your junk absorption idea. Is there any article or video that could give more info on those beam projectors? – Olga Nov 4 '17 at 12:15
• ooh, OK well I geuss he's the boss... I will have a look to see what I can find – Slarty Nov 4 '17 at 12:31
• I have not done any calculations concerning how large the spherical craft should be or how large the electrostatic forces would need to be but I imagine they would need to be considerable. Even then the orbital sphere would need to be moved into many different orbits at many different orientations and at many different altitudes to be effective, so it would take a lot of time but these ideas would help. wired.com/2013/10/electrostatic-space-junk here are some more ideas - not all good but... treehugger.com/clean-technology/… – Slarty Nov 4 '17 at 12:47

A small fleet of "double net catchers" might do the trick.

You need an elongated satellite with two Pac-Man catchers at the extremes. Mareal forces will orient satellite to orbit "standing up" with one end pointing to center of planet.

Lower end would move slightly slower than orbiting debris, while upper end would move a bit faster.

This arrangement would actually sweep orbits while keeping differential speed at a manageable value; this assuming debris follow a regular pattern, much like rings.

Debris collected by sweeping sails would accumulate at extremes where appropriate containers can hold them till next inspection.

This idea is patterned along Niven's "integral trees".

An answer that will satisfy your OCD engineers requirements is available, but EXPENSIVE.

And it will require significant mapping work of the Lagrange points of the gas giant and all of its moons. (You did not state how many moons the gas giant has. I would be surprised if it is less than four.)

The thousand mile view is that over time, loss of delta-v will result in most of the material being trapped in Lagrange points anyway, but this is not a good enough answer. So the process must be accelerated. This means a network of both optical and radio telescopes must be built that scans to a fine enough resolution to establish trajectory data for the space junk. And there must be some fairly hefty data processing infrastructure to do the calculations on radar and image data.

Once you have a database that catalogs the orbital path and projected decay of all the space trash, you can use a combination of two types of space vehicles to implement a solution. The Pac-man vehicle has already been described in other answers.

Another vehicle that can match orbits with a particle and fire either a laser or plasma burst will result in an EDM effect (electrical discharge machining). This has the net effect of sneaking up on a particle and partially vaporizing it. The sudden conversion from solid to gas or plasma accelerates the particle to a new orbit. Larger delta-v values might even be used to intentionally crash the particle to a destination where future building will occur on the airless moon. Defer the problem of collecting the junk till construction is actually occurring and the waste becomes a resource.

I wanna work for this engineer if he has project management savvy to marshall these kinds of resources.

The most effective and efficient way of dealing with space junk is the same for any waste cycle: Keep everything clean and tidy from beginning to end. Cleaning up after the fact is always much, much harder. So, avoid doing it.

Any technical civilization capable of 0.1 c spaceflight and robotic mining should be able to produce and operate swarms of robotic mini-spacecraft. These would congregate around any orbital site of manufacturing or industrial activity including normal space operations. The sensors of these collector vehicles will be extremely advanced by out standards since they are the product of an interstellar civilization.

Therefore, even the smallest piece of material that wanders away will be detected, pursued and collected. In fact, any reasonably organized interstellar technological civilization should be quite capable of fabricating collector craft to meet their junk collection requirements on demand.

A whizomatic wrench floats away from where maintenance robots were doing repairs and a suitable sized and functional collector drone emerges from the fabricators setting off in hot pursuit. Whizomatic wrenches are too big for the usual collector drones to salvage. The flecks of paint and hull material dislodged by those selfsame maintenance robots (really they need to go back to the workshop for a long overdue service) and the usual micro-drones zero in on them for collection.

The collector drones will place all salvaged items and materials in mobile containers for delivery to recycling units.

Not messing up in the first place is always the best policy. If a mess does happen and space junks results, then make sure there are swarms of collector drones on hand to clean up. Also, ensure there is additional capacity to fabricate additional collector drones to handle any junk of non-standard sizes (both big and small). Follow these sensible rules and the Chief Engineer will sleep easy. Perhaps, a book at bedtime and a glass of hot milk wouldn't go astray either to help him rest.

Space cow magnet.

This would not attract space cows (probably) but would be an orbiting version of a cow magnet. These are smooth magnets cattlemen feed to cows. They attract metal the cow accidentally eats and keep it in the rumen, so it cannot go down into the guts and cause damage.

I envision orbiting electromagnets. There would be many, each responsible for its own strip of orbit. They would be cheap: a solar panel, a battery, a big coil and a detector.

As each one orbits it maintains a small field. If it detects a perturbation of this field (because of a nearby metal object) it can ramp up the field. This will hopefully pull the object in to the orbiting magnet or if not, slow it down so it could be collected by a different satellite later. Ferrous objects would of course be attracted but because of the motion of the object through the field, it would induce a current and consequent magnetic field in anything conductive, and so could also trap or at least slow aluminum, nickel and other metals.

The cow magnet is a permanent magnet but the electromagnet will need some structural way to hang onto nonferrous things it pulls in; they will no longer be attracted once still. Bristles?

Metal objects the wrong shape to produce an adequate field or objects which have only small amounts of metal (the rest being rubber, I presume) would still be detected and the space cow magnet satellite could transmit an estimate of the direction and speed of this object. A separate operation (geostationary trash grabber?) could alter its orbit to intercept these.

This sounds like a good idea for capturing missing bolts and gears. But what about plastic, big pieces of ice, and other non-metal things? – Olga yesterday

I did not like that I did not know about the ice. Then I stumbled across this:

Water is diamagnetic and is weakly repelled by a magnet. I did not know this at all! Very cool. Not so cool for the prospects of the space cow magnet sweeping up chunks of ice, though.

• This sounds like a good idea for capturing missing bolts and gears. But what about plastic, big pieces of ice, and other non-metal things? – Olga Nov 4 '17 at 12:29
• Big piece of ice is an interesting question. Is dirty ice adequately conductive to produce an induced current when moving thru a magnetic field? Bones, plastic, rubber and the like will not be collected. Carbonaceous things generally will not last too long in the UV of orbit, at least on earth. – Willk Nov 4 '17 at 23:06
• The device amazes me, but Olga's question reads, The most important criterion of effectiveness is the ability to remove even small pieces of junk from the orbit and then put them to good use. Small is on the scale of millimetres. Methinks this wouldn't work for that. – JBH Nov 4 '17 at 23:26
• @JBH: Unpack that idea. A millimeter sized piece could not be attracted by a magnet? Or a millimeter piece would not perturb a magnetic field it passed through? Remember these space cow magnets can take a swipe at each passing piece each time it passes. It might take years to get them all. – Willk Nov 5 '17 at 13:49
• @Will, I'm just thinking that a millimeter sized piece of plastic wouldn't be attracted, wouldn't perturb, and couldn't be "tagged" (so to speak) to make it so. ... unless you "spray" everything with something sticky that would adhere and make it magnetic. That might actually be a handy way to solve the problem as passing magnets would sweep up unused "spray." – JBH Nov 5 '17 at 20:03

First before answering the question we first need to consider a few things about space junk.

First lets define it:

• Space debris would be any artificial objects in planetary orbit that do not serve a purpose.

Next lets think about the source of it:

1. satellites that have reached the end of their lives
2. spent rocket stages
3. debris from stage separations (bolts, paint and other various small bits)
4. fragments from the breakup of large objects (such as collisions, accidents)
5. intentional destruction of satellites, such as Anti-Satellite missiles or sabotage.

Now I ask these questions basing on what conditions you set forth originally

• The moon originally does not have any debris or other junk nearby.
• They believe that it would effectively lower the risks of accidents in the future when the traffic is projected to increase substantially
• Accidents are very rare but still happen.
• fully automated and robotised asteroid mining;
• space travel at 1/10 of the speed of light;

Now:

As this seems to be a peaceful situation we can almost entirely remove #5, even if we can't these would be fairly rare events.

It's a reasonable assumption if they are capable of interstellar travel, that they are no longer using multi-stage to orbit space craft. This would eliminate 2 very large sources of space junk, #2 and #3.

These are very important considerations because solving the problem depends on the scope of the problem. Here we can see that the actual problem ( in this case ) is relatively small and if managed early should be trivial. Let's look at the last 2 sources.

1. satellites that have reached the end of their lives

This is either a predictable event, a satellite has been replaced with an upgraded system and so it is now obsolete. Or, it is an event caused by some kind of malfunction/accident/sabotage that renders the satellite inoperable. In each case the "Authorities" would know pretty quickly and could dispatch a robotic probe to go and pick it up.

1. fragments from the breakup of large objects (such as collisions, accidents)

Again this wouldn't be a big mystery when it happens. The challenge here is that the longer it takes to clean up the mess the further dispersed the debris cloud would be. So interest could be added here by highlighting the sense of urgency in the cleanup window. But, even still, it would be a relativity simple matter to collect the "larger pieces" even smaller bits if caught soon enough would be pretty trivial to clean up ( for a civilization at this level of tech ). The DeltaV ( or effort assuming actual fuel is not an issue ) is greatly reduced before the debris has a chance to spread out along individual orbits. I am not sure how long dispersion would take, it's probably a function of the energy involved in the breakup. Even in modernly powerful breakups, the bits would continue mostly on their original orbit. Even if a ship was to breakup on its way to the planet, most of the bits would follow it's original path and eventually hit atmosphere. You may be able to disregard orbital mechanics for things like transfers to other moons or planetary bodies but, orbital mechanics needs to play a large role here, because these objects are drifting un-powered.

For smaller dispersed bits ( or even a cloud of small bits), I think it's entirely reasonable to deorbit and allow them to burn up. I wouldn't be economical to collect these bits, and because they are dispersed you would need to cover a much larger part of the orbits. So you would want something very cheap and mass producible. The environmental impact would be very minor as they would vaporize in the upper atmosphere.

The idea I had to deorbit these types of debris would be a very small well armored space craft, attached to a very large sheet. Think something like the solar sail ideas. Now the sheet would need to be made of a material that allows for many punctures of it without affecting its overall strength. We don't want large chunks shearing off or even tiny parts. Basically you could puncture it but it won't tear. Maybe something with a cellular or honeycomb feel to it.

Now you might think how will a large sheet (maybe the size of a football field or larger) that can be punctured but not tear be used to deorbit small debris. Here is how, you launch it retrograde. Most if not all orbits will be along the rotation of the planets axis and not far from the plane of the equator. Even with higher technology, this is more a matter of orbital dynamics ( think geostationary orbit ) than anything else. So when these small bits puncture the membrane of the craft moving in a retrograde orbit they will blast through the membrane at an incredible speed, but in doing so they will lose quite a large amount of their own velocity. Possibly even enough to cause them to deorbit.

There is some risk with the "screen" for lack of a better name, would suffer some kind of failure it self. In the case it hit some odd bit of large junk (easily predicted with their tech), both parts would probably deorbit. Because of the light weight ( low mass ) nature of the screen it would naturally deoribt over time as it collides with the bits of junk and loses speed. It could also be built specifically with it's eventual deorbit in mind, so that it would burn up in the upper atmosphere as well. This would be done by putting it in a more eccentric orbit that grazes the upper atmosphere on the perigee of the orbit ( the lowest point ), this drag over time would decay the orbit naturally.

Another idea would be to use a elliptical polar orbit to sweep the normal orbits of debris, but in this case a flat sheet probably isn't the best geometry for the screen. A polar orbit would be good because it would cross many other orbits over time, with each intersection having a small chance of colliding with a bit of junk. The effect would be less from the collision, because it's perpendicular not reverse, but certainly some orbital energy would be lost. One consideration is this could actually make things worse by changing the plain of the derbies and actually spreading them out more. But I will let you worry about the fine details like that.

The only real challenge in this would be keeping the screen perpendicular to it's orbit. However, a 3D shape ( such as a sphere ) could also be used instead of a flat plain. To me this is largely a minor detail, given the advanced tech they have and the simplicity of the "screen" craft.

Anyway, those are my thoughts.

• For large bits, clean them up quickly with robots. Recycle units etc.
• For clouds of small bits, hit them with the screen moving in a retrograde orbit which will cause their orbit to decay.

Something that also occurs to me, is that if the main concern is traffic. You should spend some time integrating how their "shipping lanes" and traffic control work. This can take inspiration from many places, like airports etc.

You could work in a few scenes where the "actors" are stuck waiting for a launch window, or have to wait for some large ship to clear the shipping/traffic lane. You could do scenes that actively incorporate the "control room/tower" of the traffic system. It could be a plot item where someone sabotages the traffic system etc. etc.

Basically prevention should be an equal if not larger part of this effort, with mainly "accidents" accounting for most if not all of the "problems". And then the odd malfunction or sabotage here or there to spice things up.

One last thought is that craft traveling at even 0.1c should have some kind of shielding. Think "deflector" but not in the Trek sense. Basically a field to push space dust out of the way, this could even be a byproduct of an advanced engine that warps space. So the idea here is clean up the large bits and just ignore the small bits. As these, would be pushed out of the way of the ships by their space warping drives ( or deflectors ).

Anyway, hope that gives you some ideas.

The most important criterion of effectiveness is the ability to remove even small pieces of junk from the orbit and then put them to good use. Small is on the scale of millimetres.

Differently sized pieces of debris will require different types of "catcher". For large pieces, you will need to match orbits and use some kind of grapnel or robotic arm. And you'll need some means of controlling their rotation/tumbling. For metallic objects you can use Foucault brakes. Otherwise you can try with electrically charging the surface and use electrostatic pulses, or shooting adhesive, metallic goo on the objects and then use Foucault brakes.

For the next step, we need an important condition: all the debris is orbiting in the same direction - either same as the moon's rotation or opposite to it. If you have both then this will not work and you run a considerable risk to operations.

If all objects are orbiting in the same direction, objects in the same orbit are still in respect to one another (otherwise, they're hurling against one another at a speed equal to the square of 4GM/r, with M the mass of the moon and r the orbital radius, and you really don't want to be there).

But in our scenario, all you need to do is enter an orbit a few hundreds of meters above or below the one(s) you want to sweep. Objects above will appear to drift toward you, objecs below will appear to gain ground and approach from behind - all at manageable speeds. You drop a pair of large unfolding butterfly nets made of conducting wire, one above and one below, with lightweight struts to keep them in position. Now you start spraying electrons towards anything approaching. This leads to the building of a large positive charge on the nets' surfaces, and the incoming particles having been negatively charged, they will be trapped.

Electrostatic traps are capable of filtering particles smaller than dust, and at those speeds, larger particles present no problems. So you're pretty much set up for a clean sweep.

There will be an appreciable tidal force trying to push both nets until the craft has rotated 90 degrees and is all contained in the middle orbit; you'll need some ACS, possibly based on a counter-rotating disk, to keep the craft's axis directed towards the moon.

When the net is full, you just bring it in for recycling. But since there are no considerations of air resistance etc., the net might well never get "full", and you could be happy harvesting it once a year or so.

Project Rho has this picture:

The idea was to suck up the space debris, crush it and shoot it out the back. It was not specified how it was powered but if you can go .1 C you'll probably be able to power it. However, this machine is not built to recycle debris and is only for small objects that cannot be easily shot down by lasers.

EDIT:

On Project Rho: Atomic Rockets the page where I found this was http://www.projectrho.com/public_html/rocket/civmilitary.php

• I am going to upvote this answer so you can comment on it. Is there a link to this project? Could you add a bit more information? And what about the recycling part? – Olga Nov 5 '17 at 1:32

## I propose a double light web, and a (quite distant) robotic 'catcher' grid.

Things in orbit move in nice predictable orbits, at least in the short term, and are only slightly affected by the gravitation of other objects in orbit.

### One Light web:

What we want is a pair of bright lasers (not powerful, we don't want them to damage anything) with some hi-res video cameras. They sweep a plane of a large area. If they hit anything, the reflection is picked up as a flash by the video camera. With a few cameras, we can triangulate where exactly in the plane the object appeared.

### Another Light web.

This does exactly the same thing, in another plane a few yards from the first, and parallel to it. The line from Plane 1 to Plane 2 gives you the trajectory of the space junk passing through the planes. The time between penetration of Plane 1 and Plane 2 gives you the speed of the object on that trajectory. At the speed of light, this can be communicated 1000 miles away to a grid of garbage cans, spaced perhaps dozens of yards apart. Exactly when and where the space junk will reach the grid of garbage cans can be computed, and the garbage can closest to where the space junk will be uses retro rockets to put itself in the path, catch it, and return to its original position.

All of that can be roboticized.

Why do you need to collect and recycle? You can't be cost effective when you are trying to catch objects that are millimeter sized. There simply isn't enough material there to justify the effort.

You need to remove the material from orbit but catching and recycling it really isn't cost effective for anything beyond the largest of objects. It's more cost effective to mine another asteroid than collect paint chips and dust.

Now assuming I had to catch and recycle for whatever reason, I'd build a trap which would be like a solar sail which would orbit the moon. It would have to be strong enough not to tear and sticky so small objects would adhere after hitting it, kinda like giant fly paper. I'd then have small spider like robots that roam the surface collecting the bits and returning them to a central repository for collection.

The robot spiders could also maintain the trap should it get damaged.

It might be too hard for your target civilisation or take too long, but this is the ultimate approach.

# Place the moons right

The idea is to place additional larger satellites – not even asteroid size, but larger, Moon or Ceres sized – into the orbit. They will basically sweep all the small space junk either into the atmosphere, or outside Earth orbit, or into the few Lagrange points, actually Trojan points, if I recall correctly.

More or less Jupiter does a similar job for the Solar system.

And this setup will keep the orbit clean. The drawback is, of course, the cost and feasibility of initial setup.

• It is an interesting suggestion. However, 1) we are not on Earth; 2) there is no atmosphere; 3) we want to collect and recycle space junk; 4) we would not want to risk the stability of our system, which is a moon of a gas giant 16 light years away from Earth. – Olga Nov 5 '17 at 1:30