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Background

A relatively recent planetary colony (<50 years from colonization) on planet A1 of star system A, that has just a few years ago become self sufficient (Can produce every product needed by its people on its own planet) has been attacked (as part of a conflict including multiple systems) such that several of its satellites/stations in orbit have been destroyed, triggering a Kessler Syndrome completely annihilating all orbital facilities and making it impossible for any current spacecraft to leave the planet. How can the people of this colony clean up their orbit and enter space?

Sci-fi tech

Other than the below-mentioned FTL and spaceship drives capable of propelling spaceships at fast enough (as is required) sublight speeds, all technology in this setting is as "near-future" as possible. That is, something is allowed only if it is supported by our current understanding of physics.

FTL

FTL is only possible a certain distance away from a star. Basically, the "edge of the system" (~40-50 AU) This is true not only for mass but also signals. Thus, during colonization multiple stations were set up at this distance. However, the presence of FTL signals is easy to detect at huge (100s of AU) ranges, so FTL stations can't hide. Consequently, they were all destroyed during the attack.

The Attack

The system A has very little interstellar infrastructure built in it (Comparatively. They still would have at least as many satellites in orbit as Earth, if not more. This is about infrastructure not in orbit). So, an enemy force gains very little advantage from conquering it as they would have to build a lot of infrastructure. Even occupying it would noticeably reduce their fleet strength, which they desperately need.

So they sent an unmanned suicide attack. These drones destroyed their FTL stations, cutting the system off, they destroyed most space structures (and no remaining structure is self sustaining in the slightest), and they then attacked the orbit of A1, getting destroyed in the process. This attack cut off this system from any allies, but the enemy didn't send any actual fleet here.

That is what the colonists are afraid of - that an enemy fleet will come here when they can finally spare the resources, and they want to make sure they are as prepared as possible. If they do come, if they are prepared, they can send an FTL signal to allies. If they are not prepared, no one will know they have been occupied.

Communication

The system is very far away from any other inhabited system, allied or otherwise. Lightspeed signals will take decades to get there, while a signal being sent from under atmosphere might not even be strong enough.

Though the cessation of communication is a giveaway to allies that this system has been attacked, they are too focused on fighting the war to spend much resources on reestablishing contact, unless the enemy fleet actively occupies this system. Moreover, this system is just one of many colonies on both sides that have been attacked with the goal of cutting them off.

All to say there is no help coming from outside, the colonists must do everything themselves

The Question

As the leader of the task force formed to clean up the orbit of A1 and re-establish space operations, what can I do to achieve this goal as soon as possible?

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  • $\begingroup$ The two halves of "Lightspeed signals will take decades to get there, while a signal being sent from under atmosphere might not even be strong enough" are written as if they conflict, but (1) they do not conflict, and (2) " signal being sent from under atmosphere might not even be strong enough" is manifestly not true: a planet-based antenna has a access to a lot more power than does a space station. $\endgroup$
    – RonJohn
    Mar 2 at 15:09
  • $\begingroup$ How does it matter that this is a colony, or planet-bound? In your what but the military triggering is different from the situation imminent for Earth, where simple junk piles seem certain to become a problem very soon? Even Google should show that harpoons, magnets and nets are among the suggested answers… $\endgroup$ Mar 3 at 0:01
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Important things to remember about orbital mechanics:

  • not all orbits are equally useful
  • where less useful orbits intersect more useful ones, the less useful ones will not be used
  • changing the plane of an orbit requires a lot of energy

What this means is that surprisingly big chunks of sky will not have much debris in them... especially directly over the poles. If you had access to suitably powerful rocketry (and if you can engage in interstellar colonisation, you almost certainly do have this) then you can potentially squeeze some flights out into odd trajectories from where they can escape or punt themselves into more useful higher orbits which will be debris-free.

If even these windows aren't safe, you can reasonably assume that they will have much, much less debris in them (because most stuff will be in equatorial orbits and even really big bangs can't spread it very far polewards).

So what you need to do first is to find out where all the debris is.

Space debris around earth Source Depicts objects larger than 1cm, in 2012

You'll be needing a set of powerful, high-resolution radar stations capable of imaging millimetre-sized debris. These will be pretty specialised bits of gear, and won't always look much like regular radar facilities. Here's one NASA uses to track things as small as 2mm in LEO. Its a 70m dish!

NASA Goldstone 70m antenna

In combination with slightly less specialist gear for tracking larger objects, a detailed catalogue of debris orbits can be made, then a plan made to clear out some launch windows, at which point you can break out the laser brooms. It probably won't look quite as awesome as this, but you've got to admit that the picture is pretty:

Laser broom debris clearance

Note that you want to deorbit stuff, not blow it into more pieces! Toast gently, don't blast.

Once you've made a little launch window you can start launching small, heavily armoured satellites into high orbits to make use of high intensity 24hr sunlight to drive lasers unobstructed by atmosphere to do the real serious work of deorbiting debris, and get back to filling up your orbits with your own rubbish again.

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    $\begingroup$ I wish I could give you a second upvote just for the quip at the end... $\endgroup$ Mar 2 at 12:40
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    $\begingroup$ Could you elaborate on "changing the orbit plane requires a lot of energy"? When two things collide, the debris can fly in all directions even if no energy is dissipated. $\endgroup$
    – Kostya_I
    Mar 2 at 22:10
  • $\begingroup$ @Kostya_I only if the colliding objects have similar mass. Most of the collisions will be a small fragment hitting another satellite and then the debris will continue as a cloud still roughly in the same direction. So the normally unused orbits won't be completely clean, but will still have much less debris on them. $\endgroup$
    – Jan Hudec
    Mar 3 at 8:29
  • $\begingroup$ @Kostya_I from a circular equatorial LEO, it takes about ~3.5km/s to escape into interplanetary space and ~5.8km/s to change your orbital inclination by 45 degrees and ~10.8km/s to change it into a polar orbit. Obviously some debris from a suitably energetic collision will be thrown into more inclined trajectories, but you can see that massive changes of inclination are energetically unfavourable. $\endgroup$ Mar 3 at 8:35
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They can fire very powerful lasers to the sky, either from ground bases or from air carried systems: some of the targets will simply be vaporized by the beam, some other will get a jolt which would alter their orbit, resulting in an increased drop rate. Rinse and repeat.

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  • $\begingroup$ @PcMan, already presuming that in 50 years they can worry about orbiting debris is a huge leap, but, yeah, I see your point $\endgroup$
    – L.Dutch
    Mar 2 at 5:48
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Two main ways:

  • Lasers fired from the ground. By slightly applying thrust to each chunk via lasers fired from the ground the orbits can be tweaked. Eventually causing the chunks to burn up in the atmosphere.
  • Tiny robots that know orbital mechanics. If your robots can get to one chunk and there are lots of other chunks to choose from, the robot can pick a course that allows it accelerate itself off chunk 1 towards chunk 2 such that chunk 1s orbit changes enough that it will eventually reenter. The robot then repeats jumping from chunk to chunk, bumping each chunk into an orbit which reenters via the combination of momentum applied by it arriving and departing from it.
    • Assuming all the orbits are randomly distributed, I reckon theres a optimal path allowing a single robot to de-Kessler a planet of all objects over size N without expanding any propellant after arriving in orbit, just jumping from chunk to chunk. I lack the patience to prove such an assertion, so just launch 50 robots and odds are good itll be done.
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  • $\begingroup$ The robots have even more degrees of freedom. They don't need to jump continuously. Instead, they can jump, and then wait for an opportune moment for the next jump, so that the first object will deorbit and the second object will be captures. $\endgroup$
    – MSalters
    Mar 2 at 9:55
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    $\begingroup$ Forget trying to find an optimal path -- that's a variant of the Traveling Salesman problem, which is NP-hard. $\endgroup$
    – Andrew Ray
    Mar 2 at 13:58
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Use an asteroid as a vacuum cleaner

That asteroid those colonists where slowly maneuvering into orbit around the planet so they could mine it, they decide instead to put it into a high eccentricity orbit around the planet. As the asteroid goes barreling through the debris field fun things start happening.

There will be a minor benefit from some debris colliding with the asteroid (so pick a target orbit for the asteroid that intersects with the biggest chunks of debris). But the main aim is to start scattering the debris. The planet, asteroid, and any single piece of debris now make a three body system and three body orbits can easily scatter the smallest object into a new orbit.

So we will either end up sending the debris inwards (which will burn up in the atmosphere), outwards (easier to avoid when launching your next spaceship), or into an eccentric orbit itself which will rapidly de-orbit due to gas drag as it approaches the planets atmosphere.

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Kessler Syndrome does not pose a significant problem.

  • In general, the problem is overrated, in some sense, sure it a big deal when if happens, but as aftermatch...
  • Also, it requires a particular set of initial conditions to be in place to happen at all.

We can imagine these conditions to be fulfilled. Yeah, invasion blasts everywhere, can it wreck the whole satellite system - yeah, why not, but it won't be for the reasons Kessler imagined, it'll be some external factor. And that external factor may clear itself soon after, like a cloud of debris fly by, meteor shower, etc. Yes, it wrecked the system, but then the factors are not sufficient for Kessler to continue - if you put some satellites back as an example, yes, there can be plenty of debris there, and you need to put replacement sats more often - but it not like you're out there and 5 second later your satellite or station is hit dead.

As Starfish pointed it out in his answer: not all orbits are the same.

So it needs to be understood what that debris field potentially is, and deduce that. Depending on the situation, different orbits will have different lifetime, which will depend on how robust the crafts are, relative velocities, densities of debris, sizes of them, and so on. In general, there will be some average lifetime of the orbits you need, so there will be similar orbits which are safe enough.

It needs to be noted that some low orbits - in case of our planet these are 100-400km - are naturally self-cleaning: no small debris will orbit there for long. A 400km circular orbit will decay into the atmosphere in less than a year for objects with density 300 kg/m3 and crosssection around 50 square meters (Mir space station), or - if we recalculate it - a bolt made out of metal, with a mass around 750kg, will enter the atmosphere in about a year. Those which are smaller will decay faster, because of volume/surface dependencies. (These numbers are very approximate, I got them just by eyeballing some Orbital decay pictures for space stations.)

So, even if the situation happens, it does not prevent access to space. Orbits we consider as passable qualities -like the ones for current space stations - will self-clean quite fast from small particles, and you need to monitor and track big enough particles.

I need it to be cleaned by yesterday

Depends on how much effort you put into it, but not impossible. And presumably, your launch capacities and satellite manufacturing are not affected.

No problem then, launch capture satellites, detection satellites, etc. The more you put of them, the faster you can clean it up. Satellites do have a different resistance for impacts, so use a bit reinforced one, some whipple shield ones - they are not necessarily heavier, they just have a simpler function to which a shield is not a problem, as it would be for some space telescope or transmission satellite. These can catch the debris. Utilize close range detectors for small debris, etc.

Facing such a problem less efficietly, with a success rate of, let's say, 80% , aren't game-breakers, as failures do not spill more fuel on the Kessler fire, if youu do that strategically at lower orbits, and other smart ways.

There are other ways to clean stuff, but it needs to be understood that no matter the severity of the Kessler syndrome, it doesn't cur your abilities to solve the problem by launching satellites that do the cleaning job, so you don't have to jump right to some laser solutions.

There are satellites that obit under propulsion, at lower orbits, and which do change orbits - some experimental/testing(?) military satellites. And there are air-breathing ion engines, and this combination allows you to build a system that will wipe your orbits clean as new, over a reasonable time, like a few years. It's just a matter of effort to launch them. Any space-faring colony should have no problems with doing that and face the debris problem.

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  • $\begingroup$ The big risk from Kessler Syndrome is to geostationary orbit: it's a small, high-value chunk of orbital real estate with a very long self-cleaning time. A single satellite placed into a retrograde orbit at geostationary altitude could leave the place unusable for millions of years. $\endgroup$
    – Mark
    Mar 2 at 23:53
  • $\begingroup$ @Mark you just can't place a sat in retrograde orbit and be it geostationary satelite, u can place a sat at the same altitude inclination and be it retrograde, but it more some external debris, not the chunks of existing satelites which were in the orbit. Relative velocity of parts which fly nearby, pieces of sats which were there won't be that high. I pointed out which orbits will be self cleaning, and it is enough to have a strong start, with others it needs some actions and there are multiple ways to clean them, with time and effort it can be done. $\endgroup$
    – MolbOrg
    Mar 3 at 2:30
  • $\begingroup$ But again, themost important aspect I like to adress is a misconception that Kessler prevents activity in space, it is not like a wall wich cuts off all the options, some wind which blows away anyting u put in orbit in very short time. Even with full strength of Kessler at hands it may take months years before random sat be blown away, depends on severity, but for it to be a wall the debris field has to be enourmous, and then Kesler will begin to eat itself - hmm, intersting subject to simulate, hmmm... $\endgroup$
    – MolbOrg
    Mar 3 at 2:39
  • $\begingroup$ A satellite in geostationary is one orbiting at zero inclination at an altitude (on Earth) of 35,786 km. A satellite at that same altitude with an inclination of 180 degrees is going to be passing through at a relative velocity of around 6150 m/s, which will generate a cloud of high-speed debris once it hits something. $\endgroup$
    – Mark
    Mar 3 at 2:39
  • $\begingroup$ @Mark yes, correct. It means some collision near that orbit, in some sense, has to produce 6km/s change in speed, that's a lot. It means something has to move at this or greater velocity, in this orbital plane to produce significant amount of sizeable debris. Or it may happen by some specific conditions, a narrow set of possibilities, to produce some fraction of that collision energy to propel fraction of mass to create some chunck which moves retrograde. Those fractions of fractions not likly to produce too much of those. Or u asking me to add how to deal with all those? $\endgroup$
    – MolbOrg
    Mar 3 at 2:55
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They don't.

If your colonists are deathly afraid of the enemy coming to invade them, then the debris serves as a shield while the planet develops surface weaponry. If anything, it lets them build more effective weaponry since they don't need to worry as much about adding debris to their orbits. Then they wait for reinforcements (and for space based cleanup from the homeworld).

This is a siege. One where the castle doesn't need to worry about growing enough food or finding enough clean water.

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For a less scientific answer:

The KatamariDamacy@home project. Launch balls of solar powered 'sticky stuff' into orbit that can be controlled by users at home. You have large number of students and others spending countless hours trying to maximize the path to increase their 'ball size' to make the top rankings.

Note that it doesn't necessarily have to be actually sticky. It could be inertial dampeners or some advanced tech like that.

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    $\begingroup$ This could actually work, if your sticky balls had a solar-powered ion engine attached to them for maneuvering. I suspect that the solar panels would be pretty fragile to impacts by orbital shrapnel, though, reducing the effectiveness of the robot and also creating additional debris that would need to be cleaned up. $\endgroup$
    – nick012000
    Mar 3 at 2:43
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Similar to Telastyn's answer, they sit tight and build an on-planet intrasystem missile system to defend A (i.e. when the enemy tries to build infrastructure somewhere in A, it is destroyed from A1 with a missile or three. One other thought I had - the problem could be a designed Kessler syndrome producing system. Not just satellite debris, but an enemy-made high-tech satellite system which actively goes after launched rockets.

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