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Let's say aliens were trying to drop rocks on the Earth to kill us all. What could we do so we don't die?

  • Aliens are orbiting the Earth
  • They are dropping materials they have mined from the moon.
  • The rocks are fired from a high orbit, and they spirl inward.

What can humanity do?

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    $\begingroup$ I don't feel like I have the stats to post this as an actual answer, but: unless the rocks are incredibly big or made of a specific material, they would burn up on entry. Merely dropping things is more complicated than it might seem, which is why the "tungsten rod" idea is typically floated these days. $\endgroup$ – user5083 Aug 12 '15 at 2:36
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    $\begingroup$ I doubt there is any material from the moon that can resist high temperature of 1650℃ upon re-entry! $\endgroup$ – user6760 Aug 12 '15 at 10:36
  • $\begingroup$ How much advanced is the aliens? Any defense system you build can as well be used to shutdown their spacecraft. Right now our best bet would be using our nuclear arsenal on them and not trying to destroy rocks $\endgroup$ – Freedo Aug 12 '15 at 13:05
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The best answer is to get higher up on the gravity well and blast their ships out of the sky.

Defending from KE bombardment from the ground is very difficult, since the incoming rounds are generally very small and moving very fast (typical studies like Project THOR were looking at tungsten rods about the size of broomsticks, so the cross section would be very small indeed). The rods would be best attacked by shooting down the carrying satellite or warship, with the second best option trying to hit them as they re enter and are still highly visible inside the plasma sheath. Remember, this is a "broomstick" sized object moving at Mach 25, so you'd better have good aim! Railguns firing clouds of pellets might be able to disrupt the incoming rounds enough to make them tumble and break up in flight, but you still have pieces of metal falling at random at high mach numbers, so you are not really that much ahead of the game.

Improbably powerful laser weapons are another means of fighting back. At the Atomic Rockets website, there is some discussion about massive FEL Xasers which can fire deadly coherent beams out to a light second (almost the distance from the Earth to the Moon), but since this uses X-ray frequencies, the atmosphere would absorb much of the energy, and you would have a bolt of plasma running back to the laser emitter... oops!

A massive laser weapon like this in orbit is another matter. The accelerator for the electron beam would be a racetrack over a kilometre in diameter, but with the amount of laser energy being deployed, you could vaporize the incoming rounds in space, plus damage the launching mechanism on the moon and even spacecraft out to the edge of the Hill sphere (while the laser is a Ravening Beam of Death (RBoD) out to a light second, it will still be dangerous out to a light minute. It just won't vaporize steel in microseconds like it does at one light second...) In a high enough orbit, the RBoD will be able to shoot "down" at enemy spacecraft and rounds, and also be able to cover a significant fraction of the Earth and orbital space. Three in orbit should cover the entire globe with some overlap, plus provide for mutual coverage (one shoots at incoming spacecraft to protect the other one shooting projectiles on the way to Earth). Having more is better, since even saturation attacks can be split between multiple battle stations carrying RBoDs.

Once you have the ability to build these sorts of weapons installations in orbit, it is a short step to turning them from battle stations to battlestars and being the ones going after the aliens instead.

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  • $\begingroup$ So humans today are dead then, I presume? $\endgroup$ – PyRulez Aug 12 '15 at 2:22
  • $\begingroup$ +1 for reference to Project Thor. It's one of my favorite KE weapons and one of the scariest. $\endgroup$ – Green Aug 12 '15 at 2:32
  • $\begingroup$ Yeah, imagine if a disgruntled employee of the ISS dropped on of those things over New York. $\endgroup$ – PyRulez Aug 12 '15 at 2:38
  • $\begingroup$ If you could build a weapon that can shoot anything at Mach 25 you might as well fire on the aliens itself. $\endgroup$ – Freedo Aug 12 '15 at 12:12
  • $\begingroup$ @Freedom I'm pretty sure that the Mach 25 object is what the aliens would be shooting at us. If that's what you meant, then just being able to hit something moving that fast on entry doesn't mean you can also hit something in orbit with the same weapon. $\endgroup$ – AndyD273 Aug 12 '15 at 12:55
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Most of the answers seem to be focused on offensive methods against the aliens in orbit, which would be a great way to stop the orbital bombardment, but are not strictly defensive measures.

Some purely defensive measures to survive orbital bombardment:

Stay on the Move: Depending on the targeting and deployment methods the aliens are using it could easily take the aliens near a full orbit (90 minutes at LEO) to move an impactor into a decaying orbit and toward an intended target. So you just need to move fast enough that you won't be there when the bombardment hits, you would likely want to be several hundred miles away depending on the size of the explosion (quite easily on par with large nuclear weapons) so a speed of 100s of mph would be necessary mostly limited to airplanes, something like flying Air Force One and not landing for an extended period of time.

Hide: the Earth is big, so depending on the aliens agenda (are they here to exterminate all humans or only the troublesome ones opposing them) you may be able to hide. Large population centers are going to be pretty easy to see, but small bases located underground or with camouflage may work, or you could hide your operations around not target humans. This is potentially a lot easier if the aliens have limited numbers of ships or observation satellites, as this would limit the times they are overhead and able to observe. Another good hiding spot would be under the sea, submarines on the move would be hard to spot and depending on the depth and size of the aliens bombardment offers some shielding.

Dig Deep: The response by a number a number of Earth governments over the last century when faced with potential large nuclear explosions was to dig bunkers, this could likely work as a defense against orbital bombardment as well. This also has the potential benefit of the hide option above if your bunkers are in hidden locations and difficult to detect from orbit.

Spread Out: This is the internet style of survival, originally design to survive a nuclear war, have enough nodes that if one goes down the system survives. As noted above the Earth is big, and if the aliens have a limited number of impactors they want to deploy, having a dispersed population would allow some to survive even a large scale bombardment. Survival could be improved by not only disseminating the population, but also infrastructure to support the population as widely and redundantly as possible.

Ultimately if the aliens are intent on humanities extinction and don't want or care about the Earth as a place to live an orbital bombardment could go to the level of melting the crust into a molten cinder which I don't think has any effective defense, but if they are looking for a limited victory you could likely employ some of these defensive strategies to save some population of humans.

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Best bet, put a few nuclear tipped missiles on the Falcon 9, SpaceShipOne, A soyuz spacecraft, or whatever and get into orbit.
Being at the bottom of the gravity well has us at a big disadvantage.

Kinetic kill missiles could also be effective. Basically a KKV doesn't worry about a warhead, and instead relies on delivering the kinetic energy of an object traveling at thousands of miles per hour to the target.

As @WilliamKappler said, you can't just drop any old rocks down and expect them to reach the ground. You need something very large, with a high metal content, and preferably a metal with a high melting point.
But the aliens probably know that if they can travel across interstellar space.
They could probably grab a few of the larger asteroids and divert them toward Earth. It would be a be a lot easier than mining the moon for the right kind of materials, and it's worked a few times before.

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Why not just use the simple solution? Shoot a gun at the sky! I oversimplify, as it's obvious that a regular handgun round won't go into orbital height, but all you need is a device that will shoot large aerodynamic projectiles at a high speed. This would be relatively cheap (quite a bit cheaper than massive laser arrays), and there wouldn't be much they could do about it, short of starting to throw rocks at it. But if you make enough of those installations, in a matter of minutes, at least one will hit the enemy ship hard enough to cause a hull breach. This velocity doesn't have to be very high, especially if an explosive is mounted on the projectile, or just a sharp, durable tip.

The only major concern about this is that they may start to bomb out the installations on the ground, but, as mentioned by Jim2B, there's enough clutter on Earth to hide the installations, until the moment of firing. In fact, the entire acceleration mechanism could be kept deep underground, with only a small bit poking out. This would be effectively invisible from space.

Furthermore, you have to consider the "rock-paper-scissors" of aerial warfare:

Ground anti-aircraft installations beat fighters (usually, or at least are armored enough that fighters can't do much about them)

Bombers can reliably destroy ground installations, assuming you have enough so that a few can come close enough that if they are shot down, the explosion of their contents once it hits the ground it is still close enough to assure destruction of the target.

Fighters can reliably destroy bombers, as bombers are meant to be anti-facility, not anti-plane, and fighters are designed to be anti-plane.

Thus, a small force of fighter jets, even available by today's technology (though it depends how high they are) should be able to make it high enough to attack the "bombers", as fighters CAN make it to orbital height (they just need to get enough speed and then pull up). All you need is one or two fighters to make it within firing range, and then they fire everything they have at the bomber, which probably would be enough to breach the airtight hull. If that fails, ramming the ship at full speed while simultaneously detonating all the on board missiles would almost certainly be enough.

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    $\begingroup$ I think you are underestimating the amount of energy needed to reach orbit. As a quick thought experiment the space shuttle is launched with large external thrusters and a massive secondary fuel tank. IF any old fighter jet could reach space wouldn't we not waste all that money on external thrusters? $\endgroup$ – sdrawkcabdear Feb 15 '17 at 0:23
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Would Aliens use rocks as a bombardment method, consider the following: (not mine, all credit to whoever wrote the original)

Rocks are NOT ‘free’, citizen.

Firstly, you must manoeuvre the space vessel within the asteroid belt, almost assuredly sustaining damage to the ship’s paint from micrometeoroids, while expending the fuel.

Then the rock in question must be inspected to ascertain its condition. Should it pass muster, the ship must prepare the potential ordinance for movement. Finally, the ship may begin manoeuvring the warship to abut the asteroid at the prepared face (expending yet more fuel), and then begin boosting the stone towards the offensive planet.

After a few days of expending a prodigious amount of fuel to accelerate the asteroid into an orbit to hit the planet, the ship may then return to the planet via superluminous warp travel and await the arrival of the stone, still many weeks (or months) away.

After twiddling away the time and consuming food in the wasteful pursuit of making sure that the planet does not launch a deflection mission, they may finally watch the ordinance impact the planet (assuming that the ship does not need to attempt any last-minute course correction upon the rock, using yet more of the fuel).

Given a typical (class Bravo-CVII) system, we have the following:

Two months, O&M, Titan class warship: 5.2 Million Credits

Two months, rations, crew of same: 1.2 MC

Paint, Titan class warship: 2.5 MC

Dihydrogen peroxide fuel: 0.9 MC

Total: 9.8 MC

Contrasted with the following:

5 warheads, “Planetary Destruction Grade”: 2.5 MC

One day, O&M, Titan class warship: 0.3 MC

One day, rations, crew of same: 0.0 MC

Dihydrogen peroxide fuel: 0.1 MC

Total: 2.9 MI

Given the same result with under one third of the cost, will have saved a massive amount of money and almost a full month of time, during which the warship may be bombarding an entirely different planet.

Bursarius Tenathis,

Purser Level XI,

Office of Interplanetary War Costings.

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  • $\begingroup$ Interesting answer but the question asked how to prevent the orbital bombardment that you described. How do you stop someone from dropping rocks on you from space? $\endgroup$ – Green Aug 12 '15 at 16:34
  • $\begingroup$ Very true. But prevention can start far before the event. In the big picture of things if rocks were more expensive to drop from space, then you've prevented someone from dropping rocks at you from space. From an entirely economic point of view rather than a physical one. $\endgroup$ – Phil Hodgson Aug 12 '15 at 16:49
  • $\begingroup$ If the spaceship has enough fuel to accelerate up to speeds sufficient for interstellar travel and then decelerate on arrival, the fuel needed for a bit of intra-system travel will be pocket change, especially since fusion fuel can be obtained from the gas giants or from comets easily. $\endgroup$ – Richard Smith Oct 25 at 13:51
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Super massive laser weapons

Lasers

Since we are familiar with our planet and know how to tune our weapons to work on it, we can assume that a sufficiently warned humanity has the capacity to build and deploy soft UV laser weapons with the capacity to vaporize any ship it can hit.

The range limitation will not be one of our laser's power, sensor targeting, or cooking ourselves with our own waste heat. It will be the limitation imposed by light speed delays and alien ship maneuverability. For a first order approximation, assume this to be several light-seconds to several light-minutes (it depends upon whether you assume high acceleration or low acceleration alien ships).

Probably the laser weapon of choice will be the free electron laser tuned to fire beams in the highest frequency that passes out of the atmosphere with minimal absorption (probably something in the soft UV band).

These installations will be over 1 km long and buried under lots of protective ground. Since once we fire the weapon, the aliens will try to target and kill our laser, it makes sense to have as many beam paths as possible under ground with many pop-up mirror turrets that fire the beam into space. The aliens will kill our cheap and easy to fix turrets but they won't know where the actual beam generator is.

Missiles

But our problems start after we zorch several of the alien ships. Assuming we don't manage to kill them all in a surprise attack, as soon as they realize near Earth space is deadly to them, they'll attack through asteroid bombardment staying well outside of our laser range.

Our massive lasers have little chance of doing significant damage to their ships at the range of our asteroid belt and even less of actually hitting their ships.

At ranges beyond a few light minutes, the only thing that is going to work are missiles, or even better missile buses.

A forewarned humanity will boost these into space and hide them throughout the solar system, especially in locations we think the aliens might use for bases. A surprised humanity will just have to clear out near Earth space and launch them with the hopes that the aliens can't scrag our missile buses before they get out of the atmosphere and can maneuver.

The missile bus will use Nuclear power propulsion (it will have to be nuclear pulse propulsion or nuclear salt water reactor based to get both the necessary acceleration and $I_{sp}$). That'll be bad for our environment but at least we can fight back.

The bus will contain multiple independent seeker warheads (probably kinetic but depending upon the planned engagement velocity, it may require nuclear warheads). During main boost and prior to the terminal engagement, the bus will provide propulsion for all of the seeker heads. As the bus approaches the target and prior to becoming vulnerable to counter fire, the seekers will separate. It is likely the bus and one or more of the seekers will contain high end seeker programming to integrate all sensor input (all seekers will contain sensors) and determine an engagement strategy for the whole cluster.

But we're ignoring something

We've been lead to believe the space is the ultimate high ground and that someone bombarding a planet from space has the advantage. But there are some aspects to this we have ignored and even more things to consider

  1. In space, there's no hiding, cloak, stealth
  2. But the "clutter" on the Earth's surface provides plenty of hiding places.
  3. In space, heat is a big issue and the more powerful your weapons, the bigger the heat problem
  4. On the surface, the whole Earth acts like a giant heat sink - meaning surface dwellers can use much more powerful weapons since we have, practically speaking, an unlimited heat sink.
  5. In space, every gram counts so ships won't have any armor
  6. On the surface, Earth installations can be well protected or buried.
  7. An invading force only "has what they can bring with them" and over interstellar distances, that won't be very much
  8. The defenders have the all of the Earth and our tremendous industrial capacity to support war fighting.
  9. In space, your sensor are limited to the size of your vessels (this directly affects the resolution and accuracy of your targeting systems)
  10. As long as surface communications continue, we can integrate sensor data from all across the globe to get much more precise targeting data on the alien ships - we can see them MUCH better than they can see us

Why Bother?

Most of the details of how a space bombardment would proceed will depend upon the alien's objectives. As has been answered elsewhere, there are precious few reasons for aliens to bother coming here the first place. There are fewer to cause them to want to attack us and there are almost none that will cause them to attack us using anything but overwhelming bombardment from space.

About the only ones I can think of are:

  1. They want to use our industrial capacity (in which case, they will want to spare a significant portion of our industrial centers)
  2. They want our real estate (in which case, they won't mind killing us or destroying our industry but will want to preserve the biosphere)
  3. Then want to use humans as some sort of slave/warriors (in which case they won't care about the biosphere or industrial centers but they will want to spare a significant portion of the population)
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  • $\begingroup$ Firstly, I would argue that the only reason to invade is for religious reasons, to eliminate future competition, or because you enjoy killing. Anything found on earth can be found on uninhabited asteroids and planets or created through fusion with matter obtained by starlifting much more easily. $\endgroup$ – Richard Smith Oct 25 at 14:16
  • $\begingroup$ It takes enormous amounts of energy to travel through space, and Earth would contribute little, given how much energy it takes to travel here. According to some of my earlier calculations, to get an aircraft carrier (100,000 ton) sized spaceship up to 0.1c, you need 12577118.204822823 terawatt hours of energy, not counting efficiency (probably below 50%, not a physicist), which is over 600 times more than the combined annual electricity usage of all of modern humanity, and you would need almost as much to decelerate. $\endgroup$ – Richard Smith Oct 25 at 14:17
  • $\begingroup$ To them, our industrial capabilities would be worth absolutely nothing, our biosphere will probably be poisonous, and human slaves and warriors would be much inferior to robotic or cyborg ones, so genocide is the only possible goal. $\endgroup$ – Richard Smith Oct 25 at 14:20
  • $\begingroup$ Thirdly, I don't think our missiles would be all that effective. As you said, there is no stealth in space, so the aliens would see us moving them into position from a long way off. To have survived travelling through interstellar space, they much have had great point defenses to vaporize incoming debris with lasers, so destroying our missiles would be pretty easy for them. $\endgroup$ – Richard Smith Oct 25 at 14:20
  • $\begingroup$ Also, you say there is no stealth in space, but I would argue that refers more to advanced spacefaring civilizations with sensors everywhere than to humanity. By approaching from an unexpected location and putting all the radiators on one side of the ship, they could make themselves very hard to detect. $\endgroup$ – Richard Smith Oct 25 at 14:23
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Firstly, why?

Ask yourself what the aliens want to do.

  • They're just plain evil and want to exterminate humans - Orbital bombardment isn't the way to go. It is a useful supplement and a relatively cheap but effective way to destroy primary targets, but for widespread destruction you'll need something more thorough. Nuclear weapons are much better for the purpose.

    • They want Earth's resources - Now we're talking! In this case, this strategy is actually pretty good because it paralyzes the humans' abilities to organize an attack, so your aliens can swoop down and clear out any remaining humans with their futuristic laser weapons or whatever.

Be hidden.

If you're well-hidden it could be very hard to make a precise strike. This goes very well with the strategy of building underground bunkers, which not only obscures your location but may also protect you from the blast if they do find out where you are.

Stay on the go.

Their orbital weapons can't just drop out of the sky. It could potentially take a long time to align them and then deorbit onto Earth. Even when executed with maximum precision there is still a practical barrier to rapid strikes barring an absolutely massive network of orbiting weapons. As long as you constantly move, it will be very hard to kill you.

Hide. I'm not kidding.

Earth is really big. It's hard for humans to perceive how big it truly is, but you can take my word for it. This works well with the underground-bunkers strategy because if you can avoid detection then there's simply no way for them to hit you. If you do detect a strike incoming, you still have a considerable amount of time to evacuate, so a highly mobile base will be a necessity.

Don't centralize.

These weapons are great for knocking out a dense settlement fast. Avoid grouping together. Big cities cannot be quickly evacuated and can be wiped off the map with significant casualties easily. Instead, have a large number of low-lying bases. If this strategy is employed, avoiding conspicuous targets is even more important.

Salt the earth!

This strategy is a little stupid, but before you evacuate all of Earth's former cities and go into hiding, launch thousands of pounds of debris into space. Pray that it creates a Kessler syndrome-esque cascade of destruction that makes launching orbital weapons very hard. Launching thousands of nuclear weapons and detonating them is also a great idea because it will increase the amount of protective shrapnel floating around in LEO, forming a sort of smokescreen consisting of hypersonic radioactive space junk. The downside is you may never be able to go to space again.

Ronald Reagan is your friend.

During the cold war the Americans experimented with all sorts of ways to destroy ICBMs from space in an attempt to render them obsolete. One system that may be of interest is the Brilliant Pebbles. They serve a similar purpose, destroying projectiles in space, and there is already a lot of research on it (though most of it is classified) so it could be very helpful in defending against the orbital weapons.

There is also a myriad of other bizarre high-tech anti-ICBM technologies from the Cold War Star Wars era. These are all potentially great ways to destroy reentering orbital impactors.

A Side Note: Moon rocks are not the way to go.

Blunt moon rocks burn up fairly quickly in the atmosphere. For an effective orbital strike ammunition, you'll want as much mass and velocity as possible. You can achieve both by making your weapons more aerodynamic, meaning more mass arrives to the surface intact and it loses less velocity to friction. You'll want long rods of something with high heat resistance like tungsten rods.

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