I heard that Kessler Syndrome would essentially end space exploration, but how expensive would it be to get past it if Earth really wanted to?

To be more specific about the Syndrome, suppose all the satellites some time in the future, say in 2050, were destroyed.

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    $\begingroup$ Suggest that you define Kessler Syndrome in your question. (A scenario in which the density of objects in low Earth orbit (LEO) is high enough that collisions between objects could cause a cascade.) $\endgroup$
    – Erik
    Mar 17, 2015 at 5:44
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    $\begingroup$ Would this Kessler Syndrome be based on some specific event today (Like if several of the current satellites were to collide ... or get shot by China) or some general situation in future? Without more details, it would be too difficult to answer. As is, this is like asking how difficult it would be to clean up an oil spill without even specifying where and how much oil was spilled $\endgroup$
    – neelsg
    Mar 17, 2015 at 8:03
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    $\begingroup$ Perhaps the cheapest way is to treat that syndrome ... $\endgroup$
    – Antzi
    Mar 17, 2015 at 11:47
  • $\begingroup$ @Erik What exactly should I define it? Wikipedia defines it very nicely for anyone who doesn't know what it is. $\endgroup$
    – Kelmikra
    Mar 17, 2015 at 21:46
  • $\begingroup$ @neelsg Okay. Edited. $\endgroup$
    – Kelmikra
    Mar 17, 2015 at 21:47

5 Answers 5


It is worth reading http://webpages.charter.net/dkessler/files/KesSym.html and some of the linked articles in that article are also very informative reading, altough can be quite technical.

The first thing is that a cascade does not happen in a matter of days or even months. Instead what happens is increasing density of debris reduces the expected lifetime of a satellite.

Not all orbits are affected. The main problematic orbit is low earth orbit. Very low orbits, where atmospheric drag is significant, will not be badly affected because the atmosphere will quickly remove debris. Below around 800 km atmospheric drag will be reasonably effective as the orbits of debris will decay in the matter of decades.

Particularly problematic is low earth orbits in the range 800-2000km, at these altitudes it takes centuries for atmospheric drag to remove debris, the debris in those orbits stay. With nothing removing the debris, the density is free to build up from collisions. The low rate of orbital decay and the fact that they are below the van allen belts are factors which make these altitudes attractive for satellites.

Even in the worst case scenario, the low low earth orbit would remain reasonably hospitable because the atmosphere is quickly removing the debris and the very stability of the higher orbits means the debris flux from higher orbits would be reasonably low. During the worst of a kessler syndrome cascade there would be significant extra debris even at very low earth orbit, but it would improve in a matter of years.

In higher orbits, the orbit doesn't become unusable per se. Instead what you have is probabilities. With a desired mission lifetime in mind, a level of shielding is required for a certain probability of achieving that lifetime. Above a certain projectile mass (~100g) shielding becomes impractical. However, the same debris which are too large to shield against, are also large enough to track effectively, so active collision avoidance could be used. Both shielding and provision for active avoidance adds mass and complexity to the satellite.

Middle Earth Orbits will not be badly affected because the density of satellites is much lower. The density of satellites is much lower because these orbits are not as useful (also the van allen belts are found in MEO, and are problematic for satellites at certain altitudes). A satellite would still need to pass through the LEO debris belt, but it comes back down to probabilities, there is a probability of collision which damages the satellite, and you can add shielding to reduce the probability of damage to a level deemed acceptable. Also, a path can be plotted which avoids trackable debris.

In summary then, even in the case of a very bad kessler syndrome, very low earth orbits, where orbital decay measured on the matter of years, will remain quite usable. Collision avoidance and shielding can be used to reduce probability of satellite destruction to acceptable levels. It may be the case that certain altitude ranges become effectively unusable because the expense of making a satellite which can survive in the debris belt, would be greater than the expense of either having the satellite in a lower orbit (requiring frequent boosting) or a higher orbit (requiring a more powerful rocket and a dangerous pass through the worst of the debris). I think the impact of a kessler syndrome is sometimes exaggerated, although being deprived of the most useful altitudes and the destruction of nearly all existing LEO satellites would certainly be very harmful to humanity's ventures in space.

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    $\begingroup$ Remember that Kyth'Py1k does not want to launch an Earth satellite, but a spacecraft that will go far beyond Earth. You seem to be answering some sort of a different question about launching satellites. $\endgroup$ Mar 18, 2015 at 12:01
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    $\begingroup$ @raptortech97 Maybe I should add a paragraph to clarify, because the relative safety of very low earth orbits is very relevant as often LEO is used as a 'staging' ground before launching a spaceship to GEO or to deep space and without a orbital staging ground some such missions would be basically impossible. For instance a Mars mission generally proposes staging the spaceship in LEO. This was in my mind when I was writing the answer but maybe it is not obvious to everyone. $\endgroup$ Mar 18, 2015 at 16:13

If you are talking about going through the debris, not staying in it (i.e. an escape trajectory or hohmann transfer to cleaner orbit) then the ship would only be in LEO for a few minutes to hours. Only the most severe Kessler Syndrome would cause a problem.


Kessler syndrome describes the cascade effect of debris to debris impacts. One catastrophic impact can cause many smaller object each capable of causing a further catastrophic impact, hence the cascade.

To be clear, if in 2050 all satellites are destroyed this isn't strictly Kessler syndrome. It also depends on how much all the satellites are destroyed. A good measure for this is the median cross sectional area of the resulting cloud of material. If the cross sectional area is very small, then the mass is even smaller! If you assume all objects are spheres then you can see through the area of a circle ($\pi r^2$) and the volume of a sphere ($\frac{4}{3} \pi r^3$) as you get 2 time smaller in radius you get 4 times smaller in area and 8 times smaller in volume (and hence mass). So very small particles actually deorbit pretty quickly because they have a much lower ballistic coefficient.

Now to your question:

If the object are small say 1mm or smaller we can handle the impact of them easily with existing technologies such as wipple shields. Anything larger and you're in more trouble. Assuming you don't want to first remedy the debris situation you're left with a few choices:

  • build a very thick hull, this would increase the size of impact you could withstand but it would increase your fuel costs.
  • search for a gap in the cloud, the exploded debris objects are going to follow a similar path as they did when they were satellites, so if you're lucky you night be able to launch from the north pole for example and thrust directly upward through a nice gap in the cloud.
  • nuclear pulse propulsion - NPP could be used to propel your spacecraft (it's never been tested, but more for social/political reasons and technological readiness) and since you already have the nukes on board throw a half yield nuke out ahead of your spacecraft to blast the debris fragments out of the way (half yield so you still get forward motion). We're really moving towards science fiction now but...
  • you could use laser ablation to change the trajectory of any potential threat in real time (this would be extreme hard for pointing and tracking).
  • an Alcubierre drive contracts space in front of the spacecraft and expands it behind the spacecraft (allowing faster than light speeds from an external reference frame) this sort of technology could be used to create a launch window by skipping some debris ahead of where it would be, but the TRL on this is about 1.
  • wormholes. I guess this is a reasonably obvious metjod if we have the technology; although if you've watched enough stargate you'll at least know to avoid a wormhole that passes through a star that could emit a solar flare. No wait! Do go through those stars, and bring the technology back to today and we can stop the creation of the debris cloud. Although then there would be no need for you to bring it back - ok new rule: please take care of paradoxes before defining your flight plan.

So in terms of expense you're looking at increasing cost the further you go down the list. Whether is be launch costs (at say \$10,000/kg) for extra mass or development costs for wormhole technology (at say \$1 Bazillion). Putting a accurate number is going to be difficult without a narrower question (spatial density of objects, median object size etc.)

  • $\begingroup$ Well- that escalated quickly! But was a good read. $\endgroup$ Sep 23, 2019 at 22:12

I'm guessing that it would be cheaper to clean up the space debris than to build and launch a super strong shield for everything you send into space. I'm also not sure that we have the technology to build a shield like that at the moment.

As pointed out in another answer, orbits decay, so you can also just wait it out. Orbits tend to decay faster for objects in lower orbits, so it would probably be best to focus efforts on cleaning up higher orbit debris and larger debris items first.

What follows is a back-of-the-envelope calculation of what it would cost using small ion thruster "tug" satellites to deorbit all debris larger than 5cm in earth orbit at the moment (Note that this still leaves an enormous amount of debris smaller than 5cm that can still easily inflict a lot of damage as well):

  • NASA estimates 14000 pieces in orbit as of 2009
  • An ion thruster powered 3U cubesat tug could cost about \$65000
  • You can remove at least one debris item per tug using 3U cubesat tugs (No reference whatsoever)
  • Assuming Geostationary Orbit for all objects (Worst case)
  • SpaceX will launch 4850kg to GTO for \$61.2M
  • A 3U cubesat weighs 3kg (I think this is defined in the cubesat spec but it can vary)
  • This means that you can deploy about 1600 tugs in one launch (4850 / 3 ... ignoring a lot of other relevant factors)
  • Including launch, the cost of a tug is then about \$103000 (65000 + 61200000 / 1600)
  • This brings the total cleanup cost to about \$1.4B (103000 * 14000)
  • Projects like this require operations and engineering staff as well as admin, so you can double the number above to be more realistic

The scenario outlined above is probably the least efficient way to do it, but it might give you an idea of the upper limit for cleanup operations.

  • $\begingroup$ You assume geostationary orbit for all debris, but your $61.2M/4850kg only gets you to GTO, not GEO. You still need to circularize the orbit once you get to GEO altitude. I doubt an ion thruster has the power (Isp, thrust, ...) to do that on its own, particularly while manuevering in a debris cloud, so you'd need a secondary propulsion device for that mauever. That costs weight, which costs fuel, and fuel for the circularization burn itself. Plus you need to do something once your bag is full... $\endgroup$
    – user
    Mar 18, 2015 at 10:23
  • $\begingroup$ @MichaelKjörling You don't need a circular GEO to reach an object in GEO, you simply need to intercept it. You can probably even conceivably deorbit debris by a fairly violent collision with the tug if you design it that way. I'm certainly no expert, so there are probably some gaping flaws in my proposal, but it is only meant to be a very rough estimate. $\endgroup$
    – neelsg
    Mar 18, 2015 at 10:32
  • $\begingroup$ I can't see any real way to remove debris other than intercepting, then collecting or capturing, then doing something with it. A GTO can get you the intercept. If you are simply looking to change its orbit perhaps you don't need to actually collect the debris. The "doing something" could be giving it a nudge to alter its orbit, but the velocity needs to come from somewhere and it would be awfully difficult to determine ahead of time what the resultant orbit will be. It would probably be easier to have something scooter around in GEO collecting debris at low relative velocity, but what then? $\endgroup$
    – user
    Mar 18, 2015 at 10:41
  • $\begingroup$ @MichaelKjörling As said, I'm no expert, but there are some options like catching the debris in a net or electromagnetic deflection. Additionally, I mention GEO in my answer, but most debris is between 800-1000km LEO. This would require much less fuel. $\endgroup$
    – neelsg
    Mar 18, 2015 at 11:03
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    $\begingroup$ You assumed you would only need to remove 14000 pieces of space junk. Aren't there 14000 currently in orbit, and wouldn't there by far more in need of being cleaned up if Kessler Syndrome happened? $\endgroup$
    – Kelmikra
    Mar 18, 2015 at 21:30

The Kessler Syndrome does not end space exploration. At worst it delays it. The number of satellites in LEO is self correcting in a few decades see Space debris half life

If the amount of objects in LEO orbit gets high enough, they can become a fuel/reaction mass resource Can I get higher with Space Junk?

With sufficient technology it is not a concern, consider a star trek type shield, you would just turn it on and drive though same as debris in space. (I think we have a question/answer around this also, but not seeing it)

  • $\begingroup$ -1. This answer is basically "just wait a few decades or invent a magic shield!" $\endgroup$ Mar 18, 2015 at 2:14
  • $\begingroup$ James Jenkins, according to NASA's "An Assessment of the Current LEO Debris Environment and the Need for Active Debris Removal" (2010) ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100017146.pdf Kessler syndrome (derbis self-reproduction) already started (even if stop all space launches): "The current debris population in the LEO region has reached the point where the environment is unstable and collisions will become the most dominant debris-generating mechanism in the future." and "25-year decay rule will help, but will be insufficient to prevent the self-generating phenomenon" $\endgroup$
    – osgx
    Mar 18, 2015 at 2:27
  • $\begingroup$ "Another 8-to-9 collisions are expected in the next 40 years (1 every 5 years)" (slide 12/21) - if space launches will be continued. If mankind stop all launches, around 5 collisions per 40 years are expected. $\endgroup$
    – osgx
    Mar 18, 2015 at 2:30

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