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I'm looking for a few weapons which I can use to attack a planet from orbit.

I'm not looking to destroy it or make it uninhabitable, just wreck a few things until the occupants stop launching anti-orbital missiles. This question had some good answers, but those were with an advanced civilization and the goal of killing everyone. The technology level I have in mind is basically what we have now, but maybe a few decades ahead. Smaller nuclear generators, a rather small space fleet, and stuff like that.

I know we have some weapons already that can hit earth from space, like ICBMs. I've also heard from somewhere that there might be tungsten rods that we can drop from space. I can't remember where I heard that from though, so it might be nothing.

What are the most realistic/plausible/scientifically possible ways to attack a planet from orbit without killing everyone or making it uninhabitable?

If you can, find a weapon for different fallout levels (house, city, country, and world), so I don't have to be wasteful when destroying rebels.

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    $\begingroup$ Once you're in space, all you really need are large rocks... $\endgroup$ – Matthew Aug 3 at 15:27
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    $\begingroup$ References: - Niven/Pournelles "Footfall" - how to do it and how to fail. An excellent study on this subject. || Orson Scott Card's Ender's Game - It can't be done and, oops, it can't be done partially. $\endgroup$ – Russell McMahon Aug 4 at 2:53
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    $\begingroup$ @RussellMcMahon Ender's Game isn't exactly realistic though. The "Little Doctor" only worked at short range, whereas rocks can be thrown from anywhere; in other words Card changed his fictional physics to force this. And the point of Ender's Game anyway is that the human leaders intentionally chose total planetary destruction because they wanted to completely annihilate the Buggers. Ender was "gamed" to stop him caring about the human spaceship crews he sent to die (which was necessary for the final attack on the planet), not to stop him caring about the enemy. $\endgroup$ – Graham Aug 4 at 12:46
  • $\begingroup$ @Graham We are going to have to agree to nearly totally disagree, unless you change your mind :-). (2) Your conclusion re the "point" of Ender's Game is at complete variance to any reading I could reasonably imagine. The decision to attack the planet was unexpected and against any "Doctrine" he or they held to. More could be said but it has the same bent. (1) Card changed no physics in the context stated. "Throwing rocks" (even uber high tech ones) would have had utterly no chance of success in that context. ... whereas $\endgroup$ – Russell McMahon Aug 5 at 0:02
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    $\begingroup$ @Blueriver The "rods from God" concept has been around a lot longer than The Expanse. It was an actual Cold War concept (see en.m.wikipedia.org/wiki/Kinetic_bombardment), but ICBMs were a more practical solution to mass bombardment. Heinlein used the idea in The Moon is a harsh mistress. $\endgroup$ – Graham Aug 5 at 12:38

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Kinetic Bombardment (AKA "rods from god"):

Kinetic bombardment (https://en.wikipedia.org/wiki/Kinetic_bombardment) takes advantage of the Earth's gravity well to drop things on people from space. Sounds simple and primitive, right? Well, much like a meteor (in fact, at it's simplest level, it IS a meteor) gravity does all the hard work and you just need to aim your mass at the planet and keep it from burning up in the atmosphere. If you aren't fussy and don't car too much about casualties, asteroids are a cheap, non-radioactive alternative to nuclear weapons. Just grab one and aim it. With an additional generation of sophistication, the aiming should be the simple part.

Jerry Pournelle https://en.wikipedia.org/wiki/Jerry_Pournelle actually championed the development of these systems before becoming a successful sci fi writer. I think project Thor is what you are specifically referring to. If you have the mass in orbit, and and the guidance to steer it, you have one of the deadliest weapons in the world. The hard part is getting the mass in orbit, but if you are already in space and can exploit asteroids, that's not a problem. Small problem? Small projectile. Big problem? Big projectile. Losing the war? Drop Ceres on Earth and come back in a few thousand years when the climate recovers.

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  • $\begingroup$ I have a question re: Rods From God. All the literature seems to imply that you just need to aim the rod and just "brake" it from orbit,. However, how energy efficient is that? BoE calc shows a 6.1m × 0.3m diameter tungsten rod decelerated to 0 from geo-synchronous orbit requires a change in momentum ~= 24,900,000 kgm/s ! $\endgroup$ – BM- Aug 4 at 3:31
  • $\begingroup$ Wolfram Alpha shows more 2.552e7 kgm/s, which is not far off (25.5 million kgm/s) $\endgroup$ – BM- Aug 4 at 3:38
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    $\begingroup$ @a4android it would be appreciated if you would quit quoting the same thing on every RfG answer. That analysis is solely based on the fact that we would have to launch the payloads in the first place IRL, but that's not necessarily the case for any arbitrary civilization that's likely to want to engage in orbital bombardment of a planet (there's no reason to assume they couldn't build them in orbit using resources from asteroid mining). $\endgroup$ – Austin Hemmelgarn Aug 4 at 12:39
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    $\begingroup$ @a4android That doesn't apply in this case - the problems raised in that paper only apply if you are trying to build one on the ground and then launch it into space - in the framework of this question they can be built in orbit, which solves those problems. Also, it's really annoying that you are pasting the exact same thing on every Rods from God answer. Doing it on one answer raises a concern, doing it on every answer is just spam. $\endgroup$ – The Daleks Aug 4 at 15:14
  • $\begingroup$ @AustinHemmelgarn The problem is that every RfG answer is saying almost the same thing. Posting the comment to each is one way of highlighting the problem. Can't some of the posters see they're adding little that's new. $\endgroup$ – a4android Aug 5 at 7:37
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Kinetic Energy Weapons

The tungsten rods that you've heard about are what are generally termed 'kinetic energy weapon', or to use the slang term 'Rod from God'. Basically, you just get a really heavy metal rod (tungsten works fairly well, though there are alternatives), fire it into the atmosphere, and let kinetic energy and gravity take over from there. All the power of a nuclear bomb with none of the fallout, very difficult to detect and very difficult to deflect, and not to mention that all it requires in the way of manufacture is getting the weapon up there in the first place - no need to fool around with uranium enrichment plants which can be bombed / blow up in your face. The US government has considered the possibility of placing a satellite in orbit with just such a weapon code-named 'Project Thor'.

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    $\begingroup$ @Trioxidane If you're using a position which is already within orbit of the Earth, you actually do need to fire it, thanks to conservation of energy. Merely pushing it out the the satellite won't be good enough as it will retain the orbit it's headed in. It'll get to Earth eventually, sure, but it's better to just fire it into the atmosphere. Carefully calculation will be necessary to make sure it hits its target. $\endgroup$ – Halfthawed Aug 3 at 16:44
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    $\begingroup$ @Trioxidane I deleted your edit, but please don't do it again. Edits are for improving clarity of answers, not for adding things that original author never touched upon. $\endgroup$ – Halfthawed Aug 3 at 16:45
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    $\begingroup$ @a4android: I don't see that describing it as a dud concept at all. It says that it would be difficult to hit a particular target, and that for Earth-based powers fighting eachother it would be easier/more cost-effective to just lob a normal bomb, but neither apply here and it doesn't seem to question the destructive force of just "dropping things from orbit". $\endgroup$ – Nick Matteo Aug 4 at 5:56
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    $\begingroup$ @a4android It's neither 'dud' nor 'debunked', it's merely impractical given the current span of technology, though a space-faring civilization which has access to asteroid mining should be able to solve most of the issues. $\endgroup$ – Halfthawed Aug 4 at 6:03
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    $\begingroup$ Kinetic weapons are already in use, but in the form of GPS-guided chunks of concrete dropped from planes. If you can land a lower stage of a rocket on a barge you could aim a Rod. $\endgroup$ – Borgh Aug 4 at 9:49
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Broadly, there are three categories of weapons. Directed energy weapons which deliver energy to the target, kinetic weapons which deliver mass to the target, and ordinance weapons which deliver a payload to the target. Let's break it down:

  • Energy
    • Lasers/Masers: Laser weaponry is currently on the cusp of being integrated into the modern military. Already some navy ships are equipped with lasers, albeit mostly for point-defense roles against drones and incoming ordinance and not offensive roles. In the near future (<10 years) these laser systems will also find their way into land-based applications, although still primarily in defensive roles. Extrapolating into your future setting, I don't think it would be unreasonable to have an orbital laser which is capable of striking targets on the ground or in the air. Primarily, this would be a precision weapon as the beam needs to be quite small in diameter. It could be used to opportunistically take out exposed and underarmored ground targets or aircraft, but would not be usable for causing widespread destruction.
    • X-Ray: During Regan's "Star Wars" program, nuclear-pumped X-ray laser weapons were considered. These consisted of a nuclear bomb with long metal rods to lase the x-ray radiation. The advantage is that they are very powerful and destructive and could theoretically create quite a powerful beam of radiation strong enough to melt through obstacles. Unfortunately, this type of weapon is not very efficient for sub-atmospheric targets because the atmosphere eats away at the beam's strength and it is a "one-shot" weapon--after the nuke goes off, the weapon is destroyed. This type of weapon would be cheap and effective against space-based or high-altitude atmospheric targets.
  • Kinetic
    • Rods from god: The US military has actually considered this one but deemed it too expensive at the time (I find it likely that it will eventually be built though). The concept is simple: large kinetic impactors are launched from orbit to strike ground targets with sub-meter precision. There, the sheer speed and mass of the armament causes massive amounts of localized and non-radioactive damage. This concept is liked by the military, because unlike nuclear weapons, RfG can serve a tactical and a strategic role in contrast to nuclear weapons which are exclusively strategic. Furthermore, a country could get away with launching a RfG without escalating the conflict to nuclear levels. Destruction-wise, these compare well with conventional bombing attacks. A RfG could be used to take out a single tank, an entire military compound, or a bunker that's deep underground.
    • Repurposed asteroids: Provided you're willing to play the long game, it is possible to shift the orbits of near-earth asteroids so that they impact at a specific time and location. Doing so would require months or years of preparation, and it would probably be hard to keep it stealthy. Still, if you're able to coat the asteroids in stealth-paint or whatever, they provide a good alternative to nuclear weapons because while they have the destructive potential, they don't have the radiation. Unfortunately though, the explosions caused by meteors most likely relegate the into the "strategic weapon" category as it would be difficult to avoid civilian casualties.
  • Ordinance
    • Conventional warheads: Just like ICBM's can be launched from the ground, you could launch them from space too. These could be equipped with all sorts of munitions ranging from simple high-explosive to war-crime cluster mines. This probably won't be very common as it would be much simpler and cheaper to deploy these weapons from drones or sub-orbital weapons platforms.
    • Nuclear/WMD warheads: Not much to say here, but nuclear, chemical, or biological weapons are almost exclusively strategic because they do not distinguish between military and civilian targets. Nuclear weapons in space ship-to-ship combat might be useful, but in terms of leaving the planet whole and habitable, these probably won't be used for bombardment.
    • Combat robot payloads: Military forces around the world are hard at work on so called "canister drones" which are essentially drone-swarms-in-a-box. The idea is that you drop a hundred or a thousand drones from an aircraft or launch them out of a van or whatever and then they go and cause mayhem. These drones could be equipped with small explosive charges, firearms, or other weaponized payloads. Theoretically, large amounts of combat drones could be dropped from orbit ("on-demand drones") but it's likely that these could be delivered to the target area without needing to go to space first. Damage potential for these is variable, but mostly suited for surgical strikes.
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  • $\begingroup$ Great ideas, but some of these sound a lot more advanced than "what we have now, maybe a few decades ahead" $\endgroup$ – Mr_Bober Aug 3 at 15:37
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    $\begingroup$ @a4android I read the relevant parts of the PDF but I don't see anything that makes kinetic impactors a "dud concept" or "debunked". Yes, there are some engineering and fiscal challenges and it's clear that RfG aren't the penultimate superweapon but there's no smoking-gun-gotcha. Also, if the RAND corporation and US military think it can be done, I think I'll rather believe them than one guy at an anti-space-weapon workshop. $\endgroup$ – Dragongeek Aug 4 at 5:17
  • $\begingroup$ Highly impractical & clunky, that's enough to spell dud. Ordinary weapons work better. The military are often seduced by fancy concepts. Also, it could be a way of tricking the Opposition into wasting resources. Garwin is competent physicist. & skeptics often are right. An easily repeated analysis. Enough to sink them. $\endgroup$ – a4android Aug 5 at 7:30
  • $\begingroup$ To quote TH Huxley: "The tragedy of science is a beautiful hypothesis slain by an ugly fact." RfG is a beautiful hypothesis. By the way, they'd work a treat of an earth-mass planet without an atmosphere. $\endgroup$ – a4android Aug 5 at 7:33
  • $\begingroup$ @a4android wait, wait wait wait. Did you seriously just said "it's impractical" as your basis of argument, when the only impractical part of it is "getting it up there" and our question is "what can a spaceship, already in space, use to attack a planet"? $\endgroup$ – Raestloz Aug 6 at 11:14
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Here is a real-world report on space weapons by the RAND Corporation. They do consulting work for various US government agencies. They found that ground-based weapons are cheaper for most purposes, but then they assumed a ground-based industry.

  • Yes, tungsten rods are a viable concept. To make them practical one would have to preposition them in orbit and deliver enough delta-V do de-orbit them. Lifting them into orbit becomes "free" if the attacker arrives from outer space. Simply leave them up there when the ground troops drop.
  • In most cases a nuclear warhead will be more lethal than a kinetic warhead, so replacing the tungsten with nukes is also an option.
  • Laser weapons may be feasible. Again, there is the cost of lifting them into orbit, and again that is a non-issue for attackers from space.
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    $\begingroup$ Sounds silly I know but how about a giant orbital magnifying glass with an array of mirrors to direct sunlight through it :) $\endgroup$ – Pelinore Aug 3 at 16:21
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    $\begingroup$ @Pelinore, that has actually been suggested by Nazi scientists, but it was utterly impractical (the main purpose of many Nazi "inventions" was to explain why the inventor was not toting a rifle at the eastern front; they did not have to be practical). en.wikipedia.org/wiki/Sun_gun $\endgroup$ – o.m. Aug 3 at 16:53
  • $\begingroup$ Laser weapons are very hard to work with. They loose strength as they pass through the atmosphere. Also they are very hard to aim, both because atmospheric distortion will bend the beams, and because being off by even one degree may mean missing the target by miles. Additionally, heat from the laser can actually warp the mirrors/lenses which can de-focus the beam, or change its angle. $\endgroup$ – user4574 Aug 4 at 1:41
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    $\begingroup$ @a4android, that analysis fits if the orbital weapon must be launched first. It does not apply if both ground and space weapons are in orbit and the ground weapons need to be soft-landed first. $\endgroup$ – o.m. Aug 4 at 6:09
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Frame Challenge:

Don't attack the planet.

Your goal is: "the occupants stop launching anti-orbital missiles".

Cool. Set yourself up in a high orbit above the planet.

Hit a bunch of satellites with normal, garden variety missiles.

Done.

You've just set off Kessler Syndrome. The satellites you've blow up have scattered into hundreds of pieces, which in turn will hit other satellites, breaking them into smaller pieces in an endless cascade.

LEO has become a rotating, orbital shrapnel cloud. No missiles are going to get through. It's now impossible for the groundlings to put anything in space at all.

But you can go get up and down. You're in a high orbit, with heavy (but basically "modern") military hardware. You can use high energy lasers to punch a hole in the orbital death cloud, and go down to Earth. The chaotic orbits means that hole will "heal" quickly, but for a while there was a window.

The groundlings can't do this since:

  • The goal of shooting a laser at the cloud is to push the pieces into the atmosphere, not destroy them outright - shooting up doesn't work
  • Atmosphere absorbs laser energy anyway, so Earth bound lasers can't make it to space

If the density of the cloud gets too low, you can always insert more shrapnel whenever you want. Blown up asteroids are probably the cheapest option for more mass.

You now control all orbital launches and landings, and have achieved your goal.

Congratulations.

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    $\begingroup$ Good base idea, but the laser clearing part is just wildly unrealistic. Orbiting debris travel at kilometers per second. Any clearing would close before anything could pass. $\endgroup$ – Emilio M Bumachar Aug 4 at 18:53
  • $\begingroup$ @EmilioMBumachar - ground based radars can currently track objects down to about 2 inches. As range and atmospheric attenuation are reduced by operating in space, I expect spaced based radar to get a better picture using the same tech. You need to predict which objects will interfere with the landing / launch window, and remove them. Obviously, gravitational interactions between the debris make long term predictions impossible... but you just need to make a window. If you make the groundlings do most of the travel, the extra danger is a FEATURE - it reminds them it's dangerous to go to space $\endgroup$ – codeMonkey Aug 4 at 19:17
  • $\begingroup$ "Blown up asteroids are probably the cheapest option for more mass". That's a delicious sentence. Take that out of context somewhere and see how much fun it is to say! $\endgroup$ – rosuav Aug 5 at 0:43
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    $\begingroup$ @rosuav Perhaps the first sentence of Seveneves? $\endgroup$ – Michael Aug 5 at 1:38
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Since we are talking about technology only slightly better than our current ones, I'd say explosives are exactly what you're looking for.

The fact that you're shooting from orbit doesn't really matter, since you are looking for the effect on earth. Sure it would require some adjustment to the weapon delivery system, but the weapon per-se would be the same as you would use if it dropped by a plane.

  1. Let's begin with small targets: houses. Hellfire missiles are probably your best bet. It's what we currently use in drone strikes. They are great for smaller targets.

  2. Cities: you have a few options here. If you only want to target a specific city in the world (or even a few), then I'd say a nuclear device is your weapon of choice. But if you want to destroy hundreds of cities, then you might want to avoid to deal with radiations. Carpet bombing would be best. It's messy, but also pretty efficient, considering you're not looking to destroy something too specific, but to level the whole town.

  3. Country: I'm not entirely sure what you mean here, but I'd assume destroy both cities and population. If that's the case, I would go with carpet bombing for the cities, and MOABs (Mother Of All Bombs) for the country. MOABs are very effective against soft to medium surface targets in covering in large areas, and even more in contained areas. That means you could easily destroy large areas of a country with it. Not technically a weapon, but hacking could also be an option here; destroying a country's network, taking control of their defences, electrical grid, and so on, can cause serious damage and might push them toward surrendering.

  4. World: Right now there isn't a weapon capable of "destroying the world" (as in killing everything on the surface) without making the world inhabitable. Nukes would do the job, but as said above, it would make the planet a no-go zone. If your goal is to kill everyone, without destroying the environment, then I'm sure some genetically engineered virus might do the job (possibly more than one). Bioweapons are currently not allowed, but then again you're destroying the world, so who's gonna stop you?

A few things I'd like to mention:

  • Most of the world runs on a few basic supplies, with oil, gas, and coal being the most commonly used for energy. Considering oil and gas mostly come from few places in the world, making those spots inhabitable could do some serious global damages.
  • There have been rumours of a "nuclear torpedoes" being developed by Russia a few years ago. The idea was detonating it off the coast of a country as retaliation for a second-strike (as in, Russia strikes first, Country strikes back, Russia uses the torpedo). It is thought to be extremely destructive, provoking a radioactive tsunami. That means having the destruction of a tsunami, plus making the whole area uninhabitable due to radioactive residue.
  • EMP weapons would allow you to deal an incredible economical and logistical damage to a country, without doing much of physical damages (assuming you don't target a nuclear reactor), but as far as I know there aren't EMP weapons that cover large areas yet (closest thing would be to detonate a nuke, but that defeats the point).
  • Targeting food supplies or water supplies is also a great way to deal damage to a single city or a country, but if you're against a whole world it becomes less and less effective.
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I'll write some fiction and then describe weapons.

Some country or country on the planet has started the long, slow climb to becoming a K-level-1 civilization by exploiting space.

Doing this in the next few decades is economically crazy (the ROI is really, really negative for a long time), so I'll assume the nation has aligned around a religious requirement to move civilization to space.

To do this, they started building a self-reproducing manufacturing base in space.

The first problem with space is that it is expensive to get there. So if you can get materials in space and use them, they are going to be insanely cheaper.

Solar powered ion tugs were sent to get high metal asteroids. Decades of effort was put into building vacuum forges capable of refining the materials found. This provided bulk.

Simply insane improvements in 3D printing, and these bulk refined materials, permitted printing 3D printers with reasonable fidelity. These were then used to print further forges and more ion mining drones, which produced more raw materials.

All of this is automated or telepresence friendly. But for religious reasons, the nation wants humans to live in space. So settlements are built, and many many people die.

There is some modest conflict by rogue states and terrorist groups, so small amounts of defence tech is also included in the settlements (point defence drones and the like).

After many decades, there are some non-horrible rotating habitats with organic life in them. They are leaky as hell, so they rely on a constant stream of new volatiles from asteroid mining past the "snow line".

...

In this case, the best weapons that such a culture would have are the asteroid deliveries. Drop medium size rocks on the planet.

This lets you exploit the solar orbital energy of the asteroid and convert it to delivered KE on the ground. It is a long-delayed attack, but the energy amplification is really big.

Traditional attacks also work from orbit. You have line-of-sight to everyone, if a bit of air in the way. Just build cannons and shoot stuff down. Those cannons can be drone-controlled.

Defense against counter attacks is going to be hard. You might have to resort to nuclear interceptors; someone shoots a missile at you? Drop a nuke on it. It is plausible that the amount of uranium easily available in asteroids is probably higher than the Earth's crust, and nuclear reactors are a plausible source of power generation for a space civilization (especially for mining past the snow-line). (dumping heat is going to be as big a problem as making energy for space civilizations, which makes them less practical)

The book The Moon is a Harsh Mistress has another idea; launch rocks from the Moon. You can convert the KE of getting out of lunar gravity well into the KE of falling into Earth's gravity well. That is a modest amount of amplification. It suffers from a single point of failure problem; a launch facility on the moon has to deliver energy fast to efficiently get it off the Moon, which makes it a target.

Asteroid weapon launchers can be slow, and most of the heavy lifting can be done with a portable drone that pushes the asteroid, then detaches (leave a small engine on it for final aiming).

"Rods from God" have the problem that rods are not stable (making them not-tumble is going to be hard), deorbiting requires a lot of energy, wind resistance is really strong. If you rely on wind resistance to deorbit, your orbital energy is wasted on the atmosphere. And your "rod from god" is a lot like dropping a rock from a few 100 feet up. I mean, it hurts, but not that much. If you don't rely on atmospheric braking, then you need to put almost all of the KE of impact into the orbiting body to get it to stop orbiting.

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Most plausible near-future weapon systems to wage war from orbit:

Nukes delivered in ballistic missiles

Against: City or large military/industrial complex

  • Simple
  • High yield

Ballistic missiles with conventional explosive payload

Target: Small military base/factory

Proven technology, but a lot less cost effective than the nukes. But also less investment required.

Drones

Target: Individuals/vehicles

Dropped in (non-reusable) re-entry vehicle (glorified heatshield and some parachutes).

With drones as small as possible, just capable of carrying the required weapon system. Plausible weapons:

  • Missiles (armored vehicles/small buildings)
  • Machine guns (potentially electronically ignited/no reloading required). See Metal Storm.
  • Multiple small "Suicide drones" against lightly armored targets

Microwave cannon

Target: Unshielded electrical installations, possibly individuals

Large amounts of electrical energy and extensive orbital infrastructure required. Firing rate limited by energy availability and (waste-) heat management.

Possibly low orbit required (polar orbits offer good global coverage) to hit hard enough.

Could possibly completely destroy electrical infrastructure (transformers) on a global level in a short time, but would otherwise be more surgical/terror inducing than a device of mass destruction.

NOT plausible:

Dropping kinetic penetrators from orbit

This is an exceptionally stupid concept. Consider: Everything you want to drop you have to get there first-- this means lugging tungsten projectiles from somewhere. Might as well load the vehicle transporting your projectiles with explosives and send it towards the ground.

Asteroids

Same as the above-- anything worth dropping will require so much fuel to get in a collision orbit, that you might as well drop that fuel on target (or nuke things instead). This is even more impractical because it will take YEARS to actually hit (because planets typically clear their orbit of any viable candidates and you would have to fetch asteroids from half-way to Jupiter-- really impractical).

I'd advise you to play Kerbal Space to get an intuitive understand of orbital mechanics (for science, of course ;)

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OPTION 1) Temporarily destroy the ozone layer
"The ozone layer absorbs 97 to 99 percent of the Sun's medium-frequency ultraviolet light" Therefore, deploy a cloud of CFCs into the upper atmosphere. After a few weeks most of the lifeforms on the surface of the planet will begin to die due to having 30X to 100X more UV exposure.

After that, deploy another chemical to clean up the mess.

You don't have to wait for everyone to die, you just have to wait until they give up.

OPTION2) Block out the sun with a dust cloud. Deploy a cloud of dust that orbits the planet at the edge of the planet's atmosphere. You don't need to completely blot out the sun, just dim it. The dust will block enough sunlight that the weather will start getting very cold. Once that happens the inhabitants will run out of food or freeze within a few months.

If the dust is deployed low enough, then atmospheric drag will cause it to naturally de-orbit itself a few months later. You can control how long the cloud stays up by controlling the initial deployment height.

OPTION3) Block the sun with a giant space blanket
Position a large circular space blanket between the sun and the planet. The diameter of the blanket needs to be similar to that of the planet.

If the blanket were 1 mil thick, had radius of 4000 miles, and had a density of 1g/cm^3 (similar to plastic) then it would likely weigh around 3 billion tons (or 30,000 aircraft carriers). Therefore the key to making this work is to find some very lightweight materials to make it from (perhaps woven carbon nano-tubes?).

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    $\begingroup$ The question was about targeting specific parts, not the whole thing. $\endgroup$ – Erik Aug 4 at 6:58
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Both @Dragongeek & @Mr_Bober have a useful list of ideas. Both exploit the fact that a planet attacked from space really has no "rear area": the front is above the inhabitants' heads, with nowhere truly good to run & hide at.

Any such attack, with the goal of conquering a planet with minimal damage to its inhabitants, would run pretty much along the lines of the following:

  1. Put space fleet in visible orbit around planet;
  2. Drop a few bombs, "rods from God", or other destructive devices here & there on the planet;
  3. Explain to the inhabitants you have more of these packages, & demand their surrender;
  4. Repeat steps 2 & 3 until they do surrender.

(Yes, this is roughly the same tactic the US employed in WWI against Japan. The alternative was an invasion of the Home Islands (Operation Downfall), which would have resulted in many, many more casualties. Sorry, a massed interplanetary version of D-Day would be a worse choice for both sides.)

After the planet surrenders -- on terms or unconditionally -- the real challenge starts. Any invasion fleet will only transport a finite number of occupation troops. (Let's say as many as 2-300,000.) The planet will easily have 100 million + inhabitants, the overwhelming majority determined to resist their new alien overlords. Your occupation force will need a plan of how they will maintain control over the planet, determining which carrots & sticks you use. (Unless the inhabitants have a warrior ethos where surrender is worse than death, & the shock they have surrendered destroys their ability to resist.)

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Cyber weapons. No need to resort to physical weapons in this situation. A concerted cyber attack should be able to bring any advanced civilisation to its knees without necessarily killing anyone or destroying anything; and that surely work in advance civilisation would be aiming to do with salt force

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Sun Gun!

Sun Gun has a lifetime membership to the orbital weapons club!

sun gun

https://www.damninteresting.com/the-third-reichs-diabolical-orbiting-superweapon/

What interested the Nazi scientists, however, was his suggestion that a specially engineered 100-meter-wide concave mirror could be used to reflect sunlight into a concentrated point on the Earth. But whereas Oberth’s design had peaceful intentions⁠—to use the intense heat to produce electricity with steam turbines⁠—the nefarious Nazis envisioned a colossal heat ray which could vanquish humanity.

The Sun Gun concept was essentially a scaled-up version of Archimedes’ ancient and oft-debated “Death Ray.”...

Yes, yes, the environmental friendly Sun Gun. All that space energy free from old Sol. You just need to reallocate it. The Nazi Sun Gun is good fun but there are many other sun guns. https://en.wikipedia.org/wiki/Sun_gun

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