# How can I prevent Kessler Syndrome among space stations?

In a world I’m building - the same one as in Is space piracy orbitally practical? - there are many space stations in low Earth orbit, used as orbital spaceports, each about ten times the size of the ISS. Small shuttles bring in passengers and crew from Earth, and larger craft, docked at the stations, then bring this cargo to Venus, Mars or the Moon.

At this point in time - in the year 2100, 35 years after the events of the space piracy question - these stations are ubiquitous and absolutely necessary for Earth to retain its economic and social status in an increasingly populated Solar System. However, with too many space stations, Kessler Syndrome becomes an issue. The breakup of one station through an accident could quickly be multiplied, proving catastrophic for the network of stations.

I think that about 100 stations could be enough to have an extremely strong trading and exploration hub. However, it seems clear that a terrorist could blow one up. Given the size of these stations and the associated ships, this could prove to be a huge problem, taking down other space stations and satellites, possibly crippling Earth's interplanetary trade.

Shields to protect against small pieces of debris (e.g. Whipple shields) are already in use, but they aren't really helpful against larger fragments. Assuming that it is possible for someone to blow up a station, what measures can the station operators take beforehand to minimize the resulting damage? How can they prevent Kessler Syndrome?

I saw Dealing with space debris and Kessler Syndrome, but I believe this is distinct. That question involves clearing up other debris; this question has to do with preventative measures before any additional debris is created. I believe the answers to this question will be much different, as almost all of those answers suggested cleaning debris up. One did not, but I'm already assuming that there are sensors in place to detect any sudden debris. The problem is still one of avoiding it - not easy to do with large space stations.

• Are armour or shields a thing in your universe? If ships/stations can be made resistant to small debris, while large is very proactively chased down and deorbited by automated ships (since you considered space piracy, I assume engines are efficient and robust, so swarm of small garbage collector ships, with enough propellant/efficiency to adjust orbit a lot isn't that far-fetched), that would still prevent Kessler cascade. – M i ech Dec 8 '16 at 17:34
• @Miech Things like Whipple shields are definitely in use, if that's what you mean, but they can only provide a small amount of protection. – HDE 226868 Dec 8 '16 at 17:42
• Remember that space is big, even relatively close to Earth. I don't think that the failure mode of most accidents will cause enough debris to cause a cascade event if there are only 100 stations up there. – ShadoCat Oct 19 '18 at 21:20

So, if I'm understanding correctly, you're wondering how to prevent a catastrophic failure on a space station from creating debris that would be hazardous to other stations? If so, there are a few options:

## 1) Failure Points

This is a development of a technology commonly used on aircraft to minimise the effect of a crack in the skin of the aircraft. The skin of the aircraft is built with deliberate weak points, so if there's a failure at any one point, the entire panel will break out. This prevents the damage from spreading beyond that point.

In our space station attack scenario, your station would be built from modules designed to come apart under stress. Thus rather than the explosion ripping metal into thousands of sharp splinters, it would produce a few hundred soft-edged squares.

## 2) Catch the debris before it leaves

I don't remember where I saw this - it was ages ago, when I was a kid, and it was only a brief mention.

In brief, it talked about dealing with the problem of space junk by requiring any orbiter to deploy a large gel disc, spin-stabilised, near the ship. The idea was that any junk or debris that was jettisoned by the ship would be trapped in the gel instead of flying off who-knows-where.

This isn't perfect, of course, since it only protects against debris in a narrow band, but you could add a couple of discs around the structure.

## 3) Make your ship out of non-shatter materials

This is kind of a sub-set of 1), but distinct enough for its own page.

Different materials react to explosions in different ways. By building your ship from something that does not break up into fragments in a blast - largely plastics - or coating them in such materials - like, say, Line X - you can ensure that while the contents of the station may be destroyed, the debris is manageable.

• A combination of 1 and 3 seem to be the way to go. Debris is going to be a fact of life -- the best you can do is minimize the quantity and hopefully increase the size so tracking (and cleanup) is easier. – Avernium Dec 8 '16 at 17:04
• A space station is subjected to temperature swings of 100C to -100C in low earth orbit. First, I'm not sure plastics are a good choice for structural integrity with that much temperature variation. Second, if you did have plastics strong enough for a hull, I don't know if they would still be shatter-resistant at -100 C. – kingledion Dec 8 '16 at 17:52
• kingledion is right. You should use metal foams instead of plastic to armor your stations. Metal foams have the additional benefit of trapping particles that hit them, so over time your foam-covered stations will mop up any small debris in their orbits. You'll need to replace the armor regularly, since the foam will break down over time from impacts with debris. – ckersch Dec 8 '16 at 18:28
• @kingledion The temperature of the environment around the station is irrelevant, it's the temperature of the materials that counts, and despite popular misconceptions, things do not instantly freeze in space. Space is an excellent insulator; materials keep their heat until it can radiate away as infra-red. Even while a station is in the dark, its internal heat would keep its materials at a comfortable temperature. science.nasa.gov/science-news/science-at-nasa/2005/… – Werrf Dec 8 '16 at 18:31
• Your polyethylene link is about using the short slowing down length of cosmic ray protons in hydrogen heavy polyethelyne to protect astronauts from radiation, not about using it as an exterior hull structure. Plus, insulators work by having a large temperature gradient in a small distance. That way the inside is warm enough for humans and the outside can drop to low temps. A 300K blackbody loses 459 W per m$^2$. The ISS generates about 100 KW. You can't afford to have your spacecraft exterior that warm or you will lose energy way too fast. – kingledion Dec 8 '16 at 18:44

First references. Relevant papers include Kessler and Anz-Meador, 2001 and Kessler, et al, 2010. Where have I heard of this Kessler guy before...

# Strictly limit the number of objects in orbit

This is a 'first principle' to be used in conjunction with one of the other two below methods. If there are 100 space stations in similar orbits around the earth, then, everything else in a nearby orbit down mm size needs to be removed. This can be done with laser ablating, or by having 'space-tugs' go 'trawling' with large nets or whatever. If an investment in near-zero space debris is made, the sudden catastrophic loss of one station is much less likely to cause a cascading failure.

The number of particles caused by the breakup is relevant. Per the 2001 paper, a 1985 Air Force anti-satellite kinetic kill experiment hit a 850 kg satellite with a 16 kg projectile at 7 km/s and generated 285 catalogued particles.

Per the 2010 paper, collisions can be divided into two types, non-catastrphic and catastrophic. A non-catastrophic collision (small particle, large space station) will generate about 100 times in mass of debris as the mass of the impacting projectile. However, most of these particles are too small to catalogue and would not contribute to cascading.

A catastrophic collision would involve a full breakup, and result in 90-100 particles that are themselves large enough to cause cascading effects in other satellites of the same size. For example, the 1985 Air Force test yielded between 80 and 95 particles that could have catastrophically destroyed other 850 kg satellites.

So if there are 100 orbiting space stations, and strictly nothing else above 1 cm, then even a catastrophic explosion will only result in about 200 orbiting objects that could cause further cascading collisions. Since that is less than the number of active satellites right now, cascading failures could be unlikely.

# Orbit your space stations very low so atmosphere removes much debris

Place the space stations in a very low earth orbit, at 200 km or even lower, at an altitude where the stations have to use thrusters periodically to counteract atmospheric drag. Then, a catastrophic breakup will send half of the objects hurtling into the atmosphere, preventing them from being a problem. Fig 1 of the 2001 paper shows the altitude distribution of debris from the 1985 Air Force experiment. The satellite was killed at 525 km, and the debris is in a bell shaped curve from ~400-700 km. If a space station had been killed at 200 km, then it is feasible that half or more of the fragments would be pushed right into the atmosphere and not be available to impact other stations.

# Orbit your space stations in medium earth orbit so there is much lower chance of collision.

This is just simple geometry; the surface area of a sphere. The volume of a 10km deep shell around the earth at 500km orbit is 5.9E9 km$^3$. The volume of a 10km deep shell around the earth at 5000km orbit is 1.8E10 km$^3$.

If the low earth orbit option reduces amount of debris released by a factor of two, then the medium earth orbit option reduces the particle density and thus the probability of being hit by a factor of 3.

Also, in the 2010 paper Figure 5, the authors use model results from Liao and Johnson, 2006 to predict instability of debris fields by altitude. They conclude that the region between 600km and 1700km are unstable, with specific bands within it at increased risk of 'runaway' cascading Kessler syndrome effects. So by going low or high, we can void this dangerous region altogether to increase the chances of survival.

• I do like the atmospheric reentry idea. I also wonder if over time, that could save on shuttle fuel costs. – HDE 226868 Dec 8 '16 at 17:45
• You have additional problems with higher orbits, you are restricted by the van allen radiation belts, so placing space stations there is unlikely due to requirements for heavy shielding causing all round difficulty. Of course, that might make for interesting pirate territory, too, in a heavily shielded stealth base.. – Innovine Dec 8 '16 at 19:18
• The problem with higher orbits is that if you do wind up with Kessler syndrome, it takes longer to clear up. A debris field in extreme low orbit might re-enter in a year or less, while one in geosynchronous orbit will take millions of years to go away. – Mark Dec 8 '16 at 21:58
• It is better than half. A single impulse (like an explosion) from an orbit with altitude X at location Y will end up with an orbit that passes through location Y. It will either go down or up from altitude X; but the other half of the orbit will be under altitude X. And if the debris goes faster than the orbit, it goes outwards (and slows down), if it goes slower (against the orbit) it goes inward (and speeds up). In both of these cases, the debris intersects the atmosphere. It is the sideways cases (with a bit of skew) that present the largest risk. – Yakk Dec 9 '16 at 3:26
• @HDE226868 Lower orbits should instantly save fuel for shuttles in comparison to higher orbits. But you'd have to use more fuel to keep the stations up there. - And you'd have to get this fuel up there with shuttles, thus using more fuel, ... nasa.gov/mission_pages/station/expeditions/expedition30/… – I'm with Monica Dec 9 '16 at 8:57

With the numbers you called, there would probably be no exaggerated risk of Kessler Syndrome. According to the Wiki: Space Debris there are already a few thousand metric tons of debris in NEO.

Your hundred 10x ISS sized stations would have an approximate mass of 400.000 tons. Assuming tracking technology is at least as good as it is today, each piece bigger than ~1cm can be tracked and simply be avoided. Its what the ISS already does today.

In case terrorists or accident would blow up a station, this would add about as much debris as there already is today. Also, retention times in lower orbits range from a few years to a few decades before stuff deorbits on its own by atmospheric friction. Large pieces could also be brought down by automated probes that attach to the piece and then deorbit actively.

By design, stations could also be built to not break into too many pieces, making the debris pieces fewer and easier to remove. Equipment can be hardened against very small pieces (dust).

To actually cause a runaway kessler syndrome it would only suffice for terrorists to blow up a station if nobody acts to prevent the consequences. While the debris from one or two stations might be enough to cause kessler syndrome in the long term, as outline above there are ways to remove debris before it escalates. Thats especially a factor if you assume access to space is cheaper than it is today and technology two decades 120 years advanced, making space technology cheaper as well.

You could have a fleet of small drones with efficient ion or em propulsion whose job is to attach to any dangerous debris and fire their engines enough to de-orbit the debris. Whether the drone goes with it or fires its engine to regain orbit is up to you.

A kessler syndrome does not mean immediate destruction of everything. If a space station explodes, it will send out a cloud of debris, but this debris is unlikely to intersect anything else for thousands and thousands of orbits. Eventually some bit of debris hits another station and the situation worsens, but it's still not a terribly rapid process. Space is BIG and still mostly empty.

Wikipedia page on space debris says that satellite lifetimes in a kessler syndrome would be reduced to years, or months. I think this is reasonable reaction time to be able to identify and track debris, launch interceptors and perform debris avoidance manouvers where necessary. Yes, it requires preplanning and thought, but the OP is looking for preventative measures, and stopping the cascade in the early stages is very important.

• Sending out drones to match orbits with random debris would be a slow way to solve the problem, but if you equip them with enough armor, velocity matching becomes unnecessary! – Maxander Dec 8 '16 at 22:00
• @Maxander And if you equip them with enough velocity, armor becomes unnecessary :) – kingledion Dec 9 '16 at 2:45
• @Maxander This needs to be a video game <.< – StarWeaver Dec 9 '16 at 9:01

Putting 100 space stations into orbit is no small matter, and would be very taxing (titter) for the people of Earth so I assume you've employed nano-bots for construction, with towed-in asteroids for raw materials... or something similar to that?

If so, why not build all the solid bits (external skin, superstructure, internal elements) with fused molecular-scale nano-bots? They would lock together and achieve their rigidity by means of phased power transmission running through the structure. Should any part of the structure be separated from the rest, there would be no active signal to keep the bots together, and they would drift apart, breaking down in the cold of space into individual silicon atoms.

Non-fixed parts (eg tools, etc) can also be constructed of nano-bots, but fused by means of a gel-based power pack that maintains a wireless link to the station (or portable source). Should the object move out of a threshold range, it, along with its silicon-based circuits, would drift apart into silicon atoms. The exposed power gel would become brittle, breaking down into microparticles in the cold of space.

This opens up some comedic possibilities here, where a crew member might go EVA for maintenance, but forgets to "pair" his tools with his suit... you can picture the expression on his face as his drill dissolves away to dust in his hands.

Soft furnishings are not likely to become dangerous debris, but could still be woven partially from nano-fibres that break their bonds at low temperatures.

Unfortunately, food, water stores, waste, and human/animal bodies are another matter. They would quickly freeze into large, dangerous projectiles.

For any stores (food, water, waste) one possibility is to build all containers out of matter-eating nano-bots. They form a rigid container, remaining quiescent on the station, but away from it in space they would devour all contained matter, excreting a dust of component atoms outside the container. When there is nothing more inside the container, the nano-bots dissolve away into atoms. Don't put your school lunch in 'em.

For bodies... well I'm stumped with that one. I cannot think of any nano-tech solution that isn't also the grisly flesh-eating basis of yet another Michael Crichton novel.

Note: power and transmission sources would have to be well shielded of course, to prevent space pirates (or demented hobbyists) from firing off EMP blasts and watching space stations dissolve away to nothing.

Earth will have a rapid detection, rapid reaction, automated system set up.

Within minutes of a station blowing up (maybe even seconds), dozens to hundreds of old-fashioned chemical rockets will be launched with custom payloads on freshly calculated trajectories, to intercept and deorbit all large debris in a matter of minutes, not hours.

If that sounded far-fetched, think about the scores of InterContinental Ballistic Missiles (ICBMs) that the US and Russia keep pointed at each other, ready to fire at a moment's notice. That's a few decades old already. What I'm proposing is strictly harder, but a reasonable extrapolation.

Instant detection and tracking is the only technology that needs large advancement to make that feasible.

Are you running fusion reactor engines? They’d have a rather large and hot tail. You could have cleanup drones with a big wide radiating exhaust. They speed up and slow down in the orbit of that mess. Burn it all to component atoms in a fusion torch. A number of those would be in standard orbit awaiting automated activation upon detection or on the basis of based instructions. Drones would have to dock with your stations semi frequently to refuel. You could have it a requirement to have an X number of them to have a stable orbit assigned to you for a permanent orbiting station.

• Hey Mark, if you need any clarification on the topic, do so by commenting on the question. That'll help to pinpoint a better and more precise answer. Nevertheless, you probably won't get much feedback from this, since it's an old question. – Faed Oct 19 '18 at 22:41