I'm writing a novel set in a post post-apocalyptic world, where society on Earth collapsed a long time ago and has rebuilt itself by the time the novel starts.

I need a reason to keep the people from leaving the planet and using the resources in space. My idea is that there was a world-wide war that triggered Kesser syndrome in the space around Earth, cutting humanity off from space travel for a long time.

Ideally, I'd like to have the events of this novel take place thousands of years after this war that destroyed the world. But I'm not sure how much debris would still be in orbit to threaten space exploration by then.

So my question is, how long could the cloud of debris caused by Kessler syndrome stay in orbit to still be dangerous? Keeping in mind that this debris is not just in low-Earth-orbit but also the higher orbits and this is an extremely dense cloud caused by the collapse of massive space infrastructure.

• I've swapped-out tags to ping the feeds of more people who could answer the question as written, feel free to revert the edit if you want. Apr 20, 2020 at 0:34

The only other current answer (by DT Cooper) is factually wrong. He's misunderstood the link he references and so the 30 - 40 years he's quotes has no baring on your question.

## Orbits

Lets first understand how orbits work. If there are no perturbations (drag, solar pressure etc.,) then objects will stay in the orbit they start in forever. This is an idealised view of orbits as we're assuming there are no perturbations - in practice this never happens.

## Perturbations

These are anything that can alter an orbit, alter doesn't have to mean reduce the altitude/cause reentry - some perturbations can act to have the opposite effect. The key perturbation in calculating how long an object stays in orbit depends on the orbits characteristics - lower orbits are overwhelmingly ruled by atmospheric drag, in higher orbits this can be negligible.

How long an object stays in orbit can usually be simply put down to the drag on the object. Drag decreases exponentially with altitude, so a small increase in altitude results in a significantly longer orbit lifetime. Here's a good rough estimate:

200 km 1 day

300 km 1 month

400 km 1 year

500 km 10 years

700 km 100 years

900 km 1000 years (Satellite orbit lifetimes)

You can see that an increase of a few hundred km in altitude results in roughly a 10 times increase in orbit lifetime. But as I said this is exponential.. so increasing beyond 900 km would give you more than a 10 times increase. For your question you've asked about objects lasting 1000's of years... so I'd suggest looking at altitude beyond 1,000 km.

## How Drag Works

The above data is for satellites, but that's a very specific type of object. Drag is a force it's not a velocity - this means that the effect drag has is dependent on the mass of your object. This comes down to F = ma (or for us: a = F/m). But even this isn't a good enough level of detail because we need to know what the drag area (sometimes called cross sectional area, or wetted area) is for your object(s). Drag force is a function of the size of your object - specifically the area that your object takes up when moving through the fluid. If your object is long, thin and flies like a dart then it's drag area is very small, if it's more like a ball then it's drag area is much bigger. Objects at 1,000 km altitude with a very low mass but large drag area (eg, thin pieces of metal plate) could easily deorbit much more quickly than the graph above suggests - this is all due to the ratio of the drag area and the mass.

## Some final thoughts

You've said it would be an extremely dense cloud at higher orbits. Have a think about how much mass that might be required for this dense cloud. If your objects are, say, 0.1 kg each at 1,000 km altitude in a range of +/- 1km and covering from pole to pole then you're talking about a volume of 1,368,000,000 km3 (that's 1.3 billion km3). If you want a 0.1 kg object every km3 (which isn't all that dense of a could) then you'd need 136,800,000 kg of mass in orbit - 136,800 metric tonnes. That is unimaginable, even for such a low density cloud. That's not to say a cloud of 1 object per km3 isn't prohibitive to launching more satellites - just be careful when you describe a dense cloud in orbit.

• Why would 140 thousand tonnes be 'unimaginable', O'Neill cylinders measures it's mass in billion tonnes. Apr 22, 2020 at 15:50
• @sp2danny I suppose it depends on what the mega structure was made for... I can't think of many reasons you'd need something that massive within earth orbit. Apr 22, 2020 at 16:48
• I like this answer a lot. One thing that you don't address is solar wind. Since the wind would both speed up and slow down the particles at different points in their orbits, it would act to de-circularize their orbits. Some few would get kicked out but most would dip down to higher drag radii. I would add this as a supplementary answer but I don't know any of the math. Apr 22, 2020 at 17:59
• Also, if you are going to give this quality of answer, please change to a more memorable user name so I can recognize it when I see it. :-) Apr 22, 2020 at 18:00
• The main problem with the higher orbits are that they pass through the van allen radiation belts, meaning that its unlikely megastructures would be placed at this altitude. Unless you are somehow farming them for energy?? Apr 22, 2020 at 21:19

Okay so after doing research it seems to me that your scenario is somewhat unrealistic. According to this, Kessler Syndrome would most likely only last 30-40 years. https://www.google.com/amp/s/bigthink.com/how-the-kessler-syndrome-can-end-all-space-exploration-and-destroy-modern-life.amp.html

There is an alternative you can use though. Instead of having Kessler Syndrome stop your post-apocalyptic people from exploring space, use this alternative instead: Orbital Weapons Platform. Maybe in the lead up to the war the great powers created devices that were made to shoot down spacecraft without some type of verification. Since nobody knows how to deactivate them, they stay up there, blasting any incoming ships to smithereens.

• Darn, that's a shame. I would've liked to use a more passive barrier, but I actually think an ancient orbital weapon system could work. Thanks for the suggestion. Apr 20, 2020 at 0:24
• @Dashane Du Plessis: You could fit it into your world, trust me. Maybe a few of the would-be space travelers even curse the souls of the ancients for their stupidity. “First they destroy the world, then their junk stops up from rebuilding it” Apr 20, 2020 at 0:41
• @Dashane Du Plessis Why not just has kessler syndrome as a result of the the collapse of civilization, left to the selves the satellites collide starting the chain reaction. that can happen long after the war itself.
– John
Apr 20, 2020 at 1:36
• The article you link to says that it would only take 30-40 years for Kessler Syndrome to develop once the chain reaction started, not that it would only last for 30-40 years. Once it develops, it will last however long it takes for the orbits of enough of the debris to decay. For lightweight debris in low orbits, that might only be a few months or years, but for pieces higher up, it might be centuries. Apr 20, 2020 at 5:44
• You've misunderstood the link. 30-40 years is how long to get to the threshold of no more space travel once the critical mass has been reached Apr 21, 2020 at 18:47

This is just a supplementary answer.

As @user6916458 mentioned, this can work if something(s) big were in high orbits.

After that long, however, most anything in the inner areas would have fallen out by then. That would leave the lower orbits relatively safe.

According to @user6916458's chart, anything below 500km should be relatively safe. that would allow Mercury and ISS style missions. Our ISS would probably be near the top of the "safe" area.

The debris field would be like an Oort Cloud for Earth. A CME or asteroid close approach would disturb some of the debris and cause it to loop in to lower orbits like a comet from the Oort Cloud.

The debris field would block access to geostationary orbits (35,786km) and the other astral bodies.

However, even at those altitudes the field will get thinner over time.

Reaching the Moon might be doable again at that time but it would be dangerous. That might lead to a Moon base from which the rest of exploration takes place.

Economics would be a more effective way of preventing people from looking off-planet. At present, nobody is interested because it's way too expensive. A future society with technology to extract any needed materials on earth would have no need to look further than this planet.