Fortifying a planet

[Darnok]

51 minutes ago, radonek said:

Could work in kerbal space, but IRL orbiting objects are not on rails and can't pass through each other. 

Why would they need to pass each other? Just spray gas cloud on same orbit with same speed.

[swjr-swis]

Based on what I learned from KSP, I'd install a cosmic Custom Asteroid mod and spawn a dense spherical 'belt' of asteroids in low-to-med orbits. The asteroids would have to be composed of pure diamond to be strong enough to kill other stuff in collisions, and simultaneously still allow sufficient (refracted) sun light to reach the planet surface for survival.

 

As an entirely unintended secondary effect, our planet would become every other alien species' female best friend.

[Apexazimuth]

Assuming your're a type 1 civilization, realistic planetary defense would be a gigantic project involving many layers of early-detection systems throughout the solar system, and a constellation of probes with big-ass mirrors in low solar orbit.

The easiest way to destroy a planet's surface is to throw a rock at it, really.   If I were determined to wipe out a civilization's habitat, then I'd grab a few dense, trans-neptunian or Oort cloud objects and accelerate them toward the planet(s).   Given enough time and/or acceleration with a massive enough object, there would be no defenses within that target planet's orbit that could deflect the object soon enough, so early intercept would be necessary.

To do that, you need a network of autonomous probes that monitor deep space in a wide sphere around the solar system so you can react quickly.

Stick a bunch of thin, deploy-able reflective surfaces onto probes in low solar orbit, which are networked and capable of directing sunlight at a point in space in unison.  A few hundred of these with a reflector about a kilometer wide would make a solar beam that would easily vaporize most small objects.  It'd be the solar-system scale version of the ole' "ant under a magnifying lens".   The sun beam is luminous so it wouldn't be detectable until it's too late.  They'd just see a brilliant flash coming from our star before they're snuffed out.  Any object they'd try to throw at us would easily be deflected by the solar beam, which would superheat the surface of the comet/asteroid and effectively turn that superheated surface into a rocket engine that deflects the object.

The heliocentric mirror probes and early detection network would be all you'd need for planetary defense, or system-wide defense for that matter.

[radonek]

29 minutes ago, Darnok said:

Why would they need to pass each other? Just spray gas cloud on same orbit with same speed.

That could make nice planetary ring, but I got the idea you wan to obscure surface.

[fredinno]

51 minutes ago, Apexazimuth said:

Assuming your're a type 1 civilization, realistic planetary defense would be a gigantic project involving many layers of early-detection systems throughout the solar system, and a constellation of probes with big-ass mirrors in low solar orbit.

The easiest way to destroy a planet's surface is to throw a rock at it, really.   If I were determined to wipe out a civilization's habitat, then I'd grab a few dense, trans-neptunian or Oort cloud objects and accelerate them toward the planet(s).   Given enough time and/or acceleration with a massive enough object, there would be no defenses within that target planet's orbit that could deflect the object soon enough, so early intercept would be necessary.

To do that, you need a network of autonomous probes that monitor deep space in a wide sphere around the solar system so you can react quickly.

Stick a bunch of thin, deploy-able reflective surfaces onto probes in low solar orbit, which are networked and capable of directing sunlight at a point in space in unison.  A few hundred of these with a reflector about a kilometer wide would make a solar beam that would easily vaporize most small objects.  It'd be the solar-system scale version of the ole' "ant under a magnifying lens".   The sun beam is luminous so it wouldn't be detectable until it's too late.  They'd just see a brilliant flash coming from our star before they're snuffed out.  Any object they'd try to throw at us would easily be deflected by the solar beam, which would superheat the surface of the comet/asteroid and effectively turn that superheated surface into a rocket engine that deflects the object.

The heliocentric mirror probes and early detection network would be all you'd need for planetary defense, or system-wide defense for that matter.

I have a feeling that would not be able to be done quickly...

[Darnok]

44 minutes ago, radonek said:

That could make nice planetary ring, but I got the idea you wan to obscure surface.

If planet has atmosphere we can use it...

http://www.geoengineering.ox.ac.uk/what-is-geoengineering/what-is-geoengineering/

 

[Kuansenhama]

I think the best defense would be to not have one system, but have many less advanced systems of defense. Space travel is hard enough as it is, and if you add numerous small annoying obstacles to even an overwhelming attacking force, they might just give up, say "it's not worth it going after that planet, It's not that it's impenetrable, it's just a headache!". In other words, your defense is going to be a series of ankle-biting traps.

 

-Obviously, detection is a start. nanobots, telescopes, scanners, and a lot of them. In fact, this might be the most important step, particularly if you're dealing with a cunning faction. It would only take one disease bomb snuck past the gate to murder your entire planets' population and allow the aliens to take over. Depending upon the amount of time it takes them to get from the edge of your detection range to your planet, and how much you know about them, you might have time to prepare defenses suited to their approach.

-Harass them in space. Solar powered laser weaponry seems to be the way to go here. If this is softer sci fi, you could fire something that moves faster than light. Some kind of electromagnetic pulse/ion cannon to disable their electronics might be nice too.

-Orbital harassment. Guarding your planet you want your defensive space stations, missile platforms, your space battle fleet, more lasers (whatever you can muster, really). I'd stay away from the 'Kessler bomb' Idea since you would also be trapping yourself on the planet in the process.

-Planetary Sheilding. In softer sci fi, where you have energy shields, you could consider putting a shield around the entire planet. Sheilds usually use a lot of power, so you might need a Phlebodium reactor or something.

-Underground Defenses and bases. You need to go deep to stay away from 'Rods from the gods' orbital strikes and the likes. If you're rooted in anywhere from dozens to thousands of underground bunkers, it's going to devolve into a ground game where you have the home field advantage. This will probably take the longest and be the most annoying for your enemy.

(I also doubt any enemy with sense would hurl a large asteroid at your planet. Doing so would destroy the biosphere and leave nothing for them to take over.)

Other ways around having to defend a planet.

-Diplomacy.

-Keep your entire (or a majority) of your assets on spacecraft and just move them to where you want to go.

-Spread your assets out across multiple worlds, that way there isn't one planet that gets singled out for attack. If you controlled a system of planets, and each one had their own set of annoying traps, nobody in their right mind would mess with you.

[Bill Phil]

Well, we need to ask ourselves how useful the planet is, and then compare the benefits of defending with the costs, as well as the cost of different levels of defense.

For a capital planet (Coruscant, Trantor, etc.) you would want maximum protection. Other planets that are of high importance need high defense, medium importance gets medium defense, and so on.

In the book Ender's Shadow, planetary defense is discussed a bit.

Castles Brian. They allow advanced guerrilla tactics on the ground, making it a hassle to take the planet.

Castles Brian are from Battletech.

[Apexazimuth]

8 hours ago, fredinno said:

I have a feeling that would not be able to be done quickly...

Which is why hopefully it would be something you already have, because without infrastructure, you're a sitting duck. 

Beyond that, you need to be a type 2 civilization e.g. Star trek's federation of planets, or you need handwavium made from unobtainium.