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I have a low tech planet (think late space race levels of tech) that needs to protect itself from an alien invasion.

I'm wondering if it would be practical to mount a defence (beyond more conventional spaceships and orbital platforms) by launching large numbers of heavy rockets into orbit and then blowing them up to create debris fields in strategic orbits to make it harder for the enemy to certain orbits and undock landing craft.

The main problem is I don't know how many tons into low orbit would be needed.

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  • $\begingroup$ think about how big the planet is, then think about how big the atmosphere is, then expand that.. that's the area you need to cover, bigger than the entire surface of the planet. Also debris will attract itself and fall back to earth relatively quickly. $\endgroup$ – Chris J Sep 9 '16 at 9:30
  • $\begingroup$ What is a "strategic orbit"? $\endgroup$ – MichaelK Sep 9 '16 at 9:31
  • $\begingroup$ @MichaelKarnerfors By strategic orbits I mean orbits that would be useful to an attacker, polar orbits for observation or orbital bombardment, inclined orbit paths that go over strategic areas or cities and of course equatorial orbits. $\endgroup$ – Exostrike Sep 9 '16 at 10:00
  • $\begingroup$ @Exostrike Well the problem is that there are so many options for that. :) $\endgroup$ – MichaelK Sep 9 '16 at 10:02
  • $\begingroup$ You are encountering a spacefaring civilization, that managed to cross lots of space, inevitably encountering lots of debris moving at enormous speed relative to their ships. Pebbles in orbit will most likely not harm them. $\endgroup$ – Burki Sep 9 '16 at 14:21
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My idea would be to fill the space with debris without the use of explosive.

First, you need to lift the weight of the explosive, and the remaining piece would be in short number.

My idea is to fill a rocket with chunks attach around a small explosive. Once you are in orbit, release this block of chunk in space then trigger the explosion, nothing deadly, just enough to spread the piece around the planet.

Of course, to cover the place, you will need to repeat this again an again. (Try to fill a bucket of water with a tea spoon ;) )

Where are we going next ?

The main problem is that you will be stuck on the planet for a long time since you won't be able to pass this debris field with inhabitated vessel.

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  • $\begingroup$ That won't be a problem as this is a race who lack interstellar/long range interplanetary travel yet due to alien radio transmission are terrified of alien contact, assuming that any contact will result in invasion. $\endgroup$ – Exostrike Sep 9 '16 at 10:19
  • $\begingroup$ To bad for the science ;) $\endgroup$ – AxelH Sep 9 '16 at 10:26
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No, it will be not practical

Main reason for that will be no the concerns you mention, but because of difference between civilization.

One barely can reach the orbit of the own planet, but other managed to make the interstellar journey, with military goals I guess. Sure it is in the case when they (aliens) do not have some strange deviation, assuming that sending interstellar probe like voyager will be enough to defeat planet civilization.

More general it is expected that they can plan ahead.

More wise to invest efforts into ground to orbit missiles, and use warheads not to create debris field, but as missile warheads potentially it will have more impact. Or if you like that debris idea - create it where you need it and When you need it - to fight particular target.

Debris is just not efficient

It might be considered as a danger for satellites, but we talking about constructions you might break with a screwdriver(by stabbing, multiple times, or once if you more careful stabber).

Our biggest construction in space(earth orbit particular) is ISS with 400tonne of mass. Would it be capable of interstellar travel - I would discourage to use it as the military vessel to defeat a planet based civilization at the stone age level of development. And the level of threat which is dangerous for ISS probably is not significant for possible construction of offender side. Bigger construction is less percentage by mass will be that layer which protects the construction, and more efficient this layer will be.

Another moment of inefficiency - we tended to use lowest orbits which are suitable for our tasks. We do that because of serious limitations of our launch technologies. Not the case with aliens because they are already in space - they can choose any orbit from above, not from below as we do. So to have hope to affect them you should make a thick layer of debris, up to GEO - and you still can't be sure it will be enough, even in the case that debris would be a treat for their crafts.

Not viable, not practical, not smart, not efficient.
For humans, the planet is their stronghold, and hope, hope for the aliens will engage them on the planet, and to be not completely destroyed(in a military sense) until aliens begin ground-planetary operation, and pray they will.

Aliens in that situation more than capable of destroying planet population, and actually planet it self - just because one reason, they will have so much time as they wish or need to make suitable space infrastructure for that task. Our current abilities to project power is limited to low orbits.

If creating debris field is considered as the option - it means humans have time to prepare, my recommendation is - go for the moon, but do not stop for 50years, go farther create you own space infrastructure, this is their only hope to be able to defend themselves and pray to make it in time.

Minimum requirements are to be able to project power up to 5 a.u., prevent aliens to use Jupiter as the source of matter and prevent them from using the sun as an energy source if they need it and preventing them from preventing us to use it, because we definitely need that source.

Easiest way for alien to defeat your humans is to block sunlight to planet, by placing primitive construction in L1

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The problem is space is wide: you will need several year of world weapon production to create enough rocket.

Next, explosion tend to spread debris in every direction, so a lot will be send to outer space or on atmosphere, reducing the rocket efficiency.

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    $\begingroup$ your second point is not correct. Look at modern fireworks, for example: you can place the (future) debris around your charge in a manner that allows it to be blown (pretty much) in the directions you need, which would be something like a flat disk. $\endgroup$ – Burki Sep 9 '16 at 13:16
  • $\begingroup$ Yes, but in space, there is nothing to prevent debris to move in a straight line. So even if this method can prevent debris loss in first time, the majority of debris will eventually deorbit on the long run. $\endgroup$ – Metushael Sep 9 '16 at 14:09
  • $\begingroup$ Actually, there is. it's gravity, and it keeps sattellites where we want them. The same prnciple will apply to your debris. $\endgroup$ – Burki Sep 9 '16 at 14:11
  • $\begingroup$ Sattellisation require a precise velocity on a precise direction, otherwise you will face deorbitation at some point. $\endgroup$ – Metushael Sep 9 '16 at 14:16
  • $\begingroup$ That is of course correct, but this will take many orders of magnitude more time than what your answer suggests and has nothing to do with debris being spread in every direction. $\endgroup$ – Burki Sep 9 '16 at 14:20
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Space is really BIG, which is the main problem. creating a dense shell even in LEO on an earth-planet would take millions if not billions of tons of rock.

What you could do is narrow the field that needs to be covered, with the existence of a natural feature. Something like extra-powerful Van Allen belts that produce high levels of radiation everywhere outside of a few degrees from the equator.

Another alternative would be to blow up a moon or two - plenty of drama available as you try to avoid sending large chunks falling towards your planet. Here you have an orbital problem though, as the moon debris is going to follow the same orbit as the moon, it's not going to be a shell around the planet but more of a streak rotating around the planet.

One thing you need to consider is that objects in the same orbit are all motionless relative to each other - they may be flying around the planet very fast, but they're not going to bump into each other very fast, so the enemy will have an easier time of it if they just match orbits with the debris cloud and descend gradually.

The strategic orbit idea is decent, however as others have pointed out there are a lot of options for an orbit that crosses a given point, once you take into account planetary rotation. You also need to consider the fact that orbits will be largely irrelevant to space ships with no Delta-V limitations. If you're using chemical rockets, you have a limited amount of fuel, and a limited amount of orbit you can change. If you've got Weber-esque gravity wedges, you don't give a crap about setting up an orbit unless you really want to, you can just fly wherever you want.

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