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My idea for protecting whole planets from enemy landing craft are orbital mines. The mines are very small and are located in very low orbits, thus having a larger kinetic energy due to bigger orbital speed. They are not outfitted with any steering mechanism as it would be too much weight, instead they are magnetic and will thus chase any ships that try to land on the planet. There also are thousands and thousands of them, this means that there is a 90 percent chance of colliding with the mines when trying to descend into the atmosphere.

The mines, which would of course often fall to the planet due to orbital decay, are always renewed by small rockets launched from the surface of the planetwhose upper stage, containing many of the mines, explodes when reaching orbit, to let the mines free.

Could this provide adequate protection against incoming alien ships?

Note: I don't care about landing my own ships on the planet. The residents trade the ability to travel into space without danger for protection against invaders.

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    $\begingroup$ Having the mines chase ships because they're magnetic assumes that the enemy builds space ships out of iron (or some other magnetic material). $\endgroup$ – jamesqf Feb 28 '15 at 21:16
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    $\begingroup$ This isn't really a question. $\endgroup$ – smithkm Feb 28 '15 at 21:41
  • $\begingroup$ Hi MedwedianPresident. I added a question that hopefully matches your intent; feel free to either roll back or edit further in case it doesn't quite fit what you are after, but do try to have a question in your, well, question. $\endgroup$ – a CVn Mar 1 '15 at 21:26
  • $\begingroup$ Also, I see you have contributed a handful of questions, a few of which even quite well received. I'd encourage you to register an account on the site, so that you do not lose your earned reputation and privileges should you lose your web browser cookies. See worldbuilding.stackexchange.com/help/why-register and more generally worldbuilding.stackexchange.com/help/account $\endgroup$ – a CVn Mar 1 '15 at 21:27
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    $\begingroup$ What good are mines when I can drop an asteroid on your planet? $\endgroup$ – Schwern Mar 1 '15 at 23:40
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I have a feeling this wasn't completely thought through. Magnets attract more strongly to another magnet than to a ferromagnetic ship (unlikely a ship would be ferromagnetic anyway). Close proximity, free floating, and unmaneuverable magnetic mines will rapidly be attracted to each other and explode. If you place mines far enough apart that they aren't attracted to each other you leave more than enough room for a ship to pass without danger. Magnetic mines are a terrible defence mechanism.

If the people are already able to put thousands of mines into orbit and track and replace the ones falling out of orbit, they should be smart enough and able to simply opt for an active defense system. Use orbiting missile platforms that shoot anything not themselves.

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The level of effort is disproportionate to the results. If I really wanted to attack the planet, I would probably drop heavy Kinetic Energy Impactors ("Rods from God"), which could be made from almost anything. Lunar slag would be quite cheap and easily available.

I could also whip up a large body out of lunar rock and send it in orbit around your planet, clearing out a large path through the orbiting mines, if it was worth my while to actually land Marines or something like that. With a high enough tech level, I could simply park a giant xaser out in the edge of the hill sphere. The Atomic Rockets website and Rocketpunk Manifesto discuss giant x ray lasers, with the conclusion that they would be ravening beams of death from a distance of a light minute, simply sweeping the sky of enemy ships, stations and space debris.

What would be more effective is a way to rapidly shoot something into the path of an orbiting spacecraft. A bucket of ball bearings or even a cloud of sand would do considerable damage if sent aloft unexpectedly by a very fast rocket or perhaps a railgun. If the enemy spacecraft is heavily built, then the last resort might be a "plumb bob" cannon, with a nuclear explosive at the bottom of a shaft filled with water and a heavy containment cap on top. The X-rays from the nuke turn the water into plasma in a fraction of a second, and the plasma couples effectively with the steel cap, shooting it into orbit at something like 6X escape velocity. Making a mobile version is an exercise for the reader...

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There also are thousands and thousands of them, this means that there is a 90 percent chance of colliding with the mines when trying to descend into the atmosphere.

How many? Space is a really big place

There's over 500 million square kilometers of surface area of a a reasonable orbit (easily proven, the earth's surface is more than 500 million square kilometers). If you had not thousands and thousands of them, but 500 million, you could make a grid of them with 1km square holes in it. Not exactly tough piloting.

Worse, you're going to have a hard time with those magnets. The only way you'll get within a few orders of magnitude of the pull you need is with superconducting electromagnets (and probably a nuclear power plant to power them). If more than one lander is sent in, you'll have to energize multiple mines to take them out. Guess which your mine will have more attraction to: a stealthy lander designed to penetrate magnetic defenses, or a giant superconducting electromagnet just 1km away.

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In addition to the precision attack that Thucydides is talking about there's the simple brute force option of dinosaur killers. "Do what we want or else!" In addition, you are setting up a basically guaranteed Kessler Syndrome. It won't be a big threat to a landing craft but things won't last long in orbit in the debris cloud you'll be creating.

Finally, magnets are incredibly useless as a seeker mechanism. To get the sort of delta-v out of them that would be needed means things that vastly exceed anything man has ever built--and even if you could somehow keep them from going for each other there's a nearby ferrous mass they would go for--the planet. They'll keep deorbiting themselves.

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Mines are effective for one reason: they're hard to detect. Your mines are easy to detect, they're orbiting in space, moving in predictable patterns, and putting out an extremely powerful magnetic field. An attacker can easily avoid or shoot them.

Alternatively they can employ the equivalent of a mine clearing device. As others have suggested, just put a few decent sized rocks in similar orbits and they'll start clearing paths. Bonus if they're ferro-magnetic. But as long as you're putting rocks in orbit, why not just drop them as weapons? Despite what Hollywood tells us, mines with enough power to destroy a spaceship won't do squat to an asteroid.

Mines are an area denial weapon. They don't kill everyone, or even very many, but they make it expensive and slow to move through an area. The attacker has to either accept some casualties, take time to find and remove the mines, or take another route. A minefield does not stop the enemy, but it can slow them down giving you time to react, or redirect them towards a more heavily defended route. Think about that when rethinking your planetary defenses.

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