Assuming a planet of similar mass and atmosphere as earth, how much and how often do I need to add orbital debris to prevent space travel?
It may be possible to use Kessler syndrome to stop them from placing stable orbiting satellites (see: Can I render satellite deployment impossible, or at least impractical, by exploiting the Kessler syndrome?). I have an answer there calculating that a deployment of around 12,000 is the minimum for LEO only.
To stop people being able to leave? Nope. The escape vector to leave a planet you need a very slim window to directly go through which you can't stop outside of a Dyson-sphere like construct, which isn't really a satellite at that point. Sure, maintaining a stable orbit isn't really viable and without stable orbits, space flight isn't that useful, but you can't stop space travel just by caking a planet with a minor debris field.
Not to mention that space is huge, there's not really an effective way of stopping a satellite with has decided to orbit past GEO. Sure, it's not a great place to orbit, but it can orbit there. And, remember, once you start oversaturating the debris field, it will hit itself and start knocking itself from the sky, so there's a limit to how much you can saturate that,
This is my second most frequently used quote on this Stack:
Space is big.
Really big. You just won't believe how vastly, hugely, mind-boggling big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space. Listen; when you're thinking big, think bigger than the biggest thing ever and then some. Much bigger than that in fact, really amazingly immense, a totally stunning size, real 'wow, that's big', time. It's just so big that by comparison, bigness itself looks really titchy. Gigantic multiplied by colossal multiplied by staggeringly huge is the sort of concept we're trying to get across here.
(Douglas Adams, The Hitchhiker's Guide to the Galaxy)
All the crapola that humanity has put into orbit in the last 70 years hasn't made a dent in our ability to launch ships. But what really stings is that you can't just dump trash — you'd need to actually power your orbital constipation.
The natural spin of things creates rings
If you put enough stuff into orbit that it would stop launches, that stuff would begin bouncing off each other like a drunken llama at a rave. You see, all those stable orbits in places other than the equator exist in no small part because nothing's getting in their way. As soon as you start introducing chaos, the result is for things to settle down and get swept along by Earth's gravitational tidal forces.
Oh, it may take a bazillion years, but it would happen. The only way you can avoid that is to make your blockage intentional with powered satellites that remain in their assigned courses to ensure no launch can happen.
And if you're going to do that, you might as well save yourself some dough and simply arm some of the satellites so they can shoot down the launches.
But I love the idea!
And why do I love it? Because everybody on the planet would be constantly dodging bits and pieces falling out of the sky. It would create an entirely new cottage insurance industry.
It would also cause a boom in investments into geothermal energy, because enough stuff in the sky to stop all launches would (IMO) seriously block out the sun. That means growing mushrooms in caves located under the Yellowstone Caldera to feed the people.
In the meantime your civilization devoted to blocking the planet's inhabitants from leaving has made me rich because I sold all their stock short due to the economic depression caused by the cost of moving so much junk into another planet's orbit! I mean, think about it. How much junk would you need to drop onto the surface to keep people from driving their cars? Now multiply that by a bazillion because the surface area in low orbit is so much larger than the surface area on, well, the surface.
Big. Really, really, really big.
Short answer it won't work
Reason for that is that at 200km orbits things deorbit quite fast, but it still a viable orbit just more expensive one, from which one can sync with stuff flying around and leave the orbit.
- different attack and counter attack strategies are possible, but it pretty much all the same as a result debris is not that effective.
It means you have dump a lot quite often and this may be a more of an attack on the surface of the planet as effects go - so in a sense it easier to kill all the life on a planet than block space travel by debris.
Active means like sleeping missiles are much more effective
Based on other answers saying it's basically impossible to do what you want to do, here's my suggested evil space overlord alternative:
Assumptions: the evil space overlord does not want the civilization to know this is happening. He just wants them to think that space is really dangerous and full of small rocks. The evil overlord also has some kind of scanner/radar-type technology that the civilization is incapable of detecting. (I feel like these are reasonable assumptions otherwise why do any of this. Just shoot them all down blatantly.)
- Find out how small of an object the alien civilization can track. NASA currently tracks objects "as small as 2 inches" in low earth orbit. Let's assume the civilization is at the same level here:
- Litter a good deal of 1 inch debris around.
- Throw in a lot of space guns that fire 1 inch pellets at tremendous velocity.
The idea will be that the civilization will encounter the 1 inch debris and while, as per other answers, it's not able to be dense enough to really create a reliable hazard, it's dense enough that they will notice it.
The guns will then fire on some rule basis:
- Any ship reaching velocity to escape the planet's gravity well gets shot. "Oh wow bad luck we managed to hit one of those little rocks and it went straight through the engine."
- Any ship in orbit gets a random chance to get shot, per day. It's a low chance but nothing stays in orbit for years without taking multiple hits.
We have created a vague hazard that's impossible to accurately track and then weighted the die by setting up some guns, which just look like debris, that will fire automatically to keep the threat inflated. It will probably really confound their scientists but short of discovering one of the guns it should keep them contained and befuddled. Might even be most governments just opt out of the space program because it's "too risky" and public sentiment turns against it.