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Assume that the with the technology level of the near future a self-sustainable colony has been established on the moon. For some reasons a conflict between this colony and some powerful nations on Earth arose. Now the lunar colony has to set up defences against aggressions from Earth.

They set up stationary (with respect to the lunar surface) satellites with telescopes which should recognize missile launches on Earth's surface and other satellites to launch missiles to intercept them. In addition they build a few carrier ships carrying smaller fighters to intercept more complicated attacks.

In addition they consider building ballistic attack capabilities against Earth, something similar to what has been suggested in this answer.

Would it be feasible to defend against attacks from Earth using the above methods? Which additional measures should they take or should they try to rely on mutually assured destruction?

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    $\begingroup$ Read Heinlein's The Moon is a harsh Mistress. Your answers are there. $\endgroup$ – ShadoCat Apr 2 '18 at 22:57
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    $\begingroup$ A siege of the moon colony by any earth power will probably end with the moon colony starving either economically or literally depending on how far along the colony is. The moon barely has any resources so whoever is on it will be the loser in any extended conflict. $\endgroup$ – A. C. A. C. Apr 2 '18 at 23:30
  • $\begingroup$ Your lunar-stationary satellites will have to be pretty far out, as they would need 27+ days for each orbit. $\endgroup$ – Paul Sinclair Apr 3 '18 at 3:45
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    $\begingroup$ @A.C.A.C. I suspect that if the colony was that dependent on Earth, there wouldn't be a reason for aggresion from Earth (assuming it's the whole Earth). $\endgroup$ – Spencer Apr 3 '18 at 10:15
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    $\begingroup$ There are no lunar stationary orbits as the required distance is larger than the Moon's Hill Sphere: astronomy.stackexchange.com/questions/20499/… $\endgroup$ – dmckee Apr 3 '18 at 19:26

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They would throw rocks, literally.

The fuel needed to push any given mass out of the moons very small (relative to the earths) gravity well & into the earths (& it's all down hill after that) is going to be negligible in comparison to what it takes to get stuff off of the earth. A big rock inbound from orbit isn't going to need any explosives.

And there are a lot of rocks on the moon.

It's something I think Robert Heinlein covers well in The Moon Is a Harsh Mistress.

I don't think there's a lot they can do to stop incoming missiles, but do they really need to? They can dig down; it's not like there's anything they need on the surface (like water or an atmosphere). They can maybe move around to the far side too.

Which (as Harabeck reminds me) despite also being called the dark side gets as much light as the one facing us, more even, it doesn't have the earth getting in the way of the sun & casting a shadow. So they'll still have that for solar panels & power.

Normal missiles don't do U-turns in a vacuum, unless Earth expected trouble & has been preparing for years it's unlikely they'll have any that can.

If you want to hit the far side of the moon with a normal missile you'll have to pass close enough to the moon at a slow enough speed that the moons gravity captures the missile & pulls it down to the surface (much like indirect fire over a wall with a sling shot).

A normal missile can't brake to achieve this so it's going to have to dawdle the entire way, it'll take days to get there, days in which it might be spotted, days in which it might be shot down, days in which it's target can be calculated from it's trajectory & days in which any personnel & equipment can be evacuated from it's target.

By the time Earth has new missiles with braking rockets (which is what you need if you want to get there fast then hit the far side) they'll be dug in so deep nothing can touch them, the only thing on the surface will be solar panels & those (likely) designed to be moved when needed.

And besides any of that Earth won't know where the targets are because they can't see them.

so the far side is likely to be a relatively safe location (especially to start & most likely later as well).

The moons tiny gravity makes some form of rail-gun to catapult chunks of ordinary rock at the earth feasible & even a mobile platform for such a gun becomes reasonable & realistic, plus being mobile is going to make it very hard for earth to target & destroy the launch platform.

I would expect the stones used as ammunition by these rail guns to be long thin cores of basalt shaped into pointed spears & jacketed in steel or aluminium to hold them together if they crack when fired or during atmospheric entry with a ceramic shell to prevent burn up on entry.

To help them fall point first & not tumble (which could cause them to break up & burn up) when they enter the atmosphere they may be weighted & have fins (spring loaded to pop & lock after leaving the barrel), there may be some advantage to rifling them (a screw thread) so they spin.

A solid projectile with no electronics or payload made from available material that uses as little refined material as possible & is both relatively cheap & easy to turn out in quantity.

Detecting what's inbound shouldn't be a problem for either side. It took around three days to reach the moon last time we went there: with three days for your projectile to be spotted chances are slim it won't be. Stopping what you see is another matter but if the moon's launch systems are all mobile and its population & manufacturing has been moved to the dark side it won't need to stop anything.

I'm not sure about it but the long delay between launch and impact might actually make interception or shooting down missiles viable.

In summary, I think Heinlein got it right: assuming the moon has become self supporting through hydroponics (etc) for food & oxygen they'll win due to the simple economics of the situation. Every rock they launch at earth (which will impact with the force of a nuclear explosion) will cost next to nothing in resources while each rocket earth launches against them will cost millions.

EDIT: This answer has attracted a lot of useful comment so as I find time I will edit this properly to tidy up one daft thing I said (@Harabeck special call out to you there), incorporate some of that & give a little further expansion & clarification, I don't want this to get too long though.

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    $\begingroup$ I'd just like to point out that the "dark side" may be often used to refer to the side of the Moon that always faces away from Earth, but that it does in fact receive about the same amount of light as the side that faces Earth. It's only "dark" in a metaphorical sense because we can't see it from Earth. $\endgroup$ – Harabeck Apr 2 '18 at 21:16
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    $\begingroup$ @Harabeck, your completely right of course, I won't edit to hide my daftness on that point though, I think I've edited enough for now & editing that would leave your observation without a cause, which would seem rude. $\endgroup$ – Pelinore Apr 2 '18 at 21:25
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    $\begingroup$ Hitting targets on the lunar farside isn't much harder than hitting them on the nearside. Sure, you don't have direct line-of-sight, but the Moon's got plenty of gravity to give the missiles the U-turns they need. $\endgroup$ – Mark Apr 2 '18 at 22:18
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    $\begingroup$ @RichardTingle It's 2.3km/sec to throw something off the moon. Another 1km/sec (actually, less due to Oberth) will zero out it's orbital velocity as well. It smacks Earth at 11 km/sec. Linear motors can easily do this. (Note that Heinlein got it wrong--you don't need a steel can for it. A bucket works fine--and some extra motors on the catapult wired backwards lets you recover the bucket.) $\endgroup$ – Loren Pechtel Apr 3 '18 at 1:27
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    $\begingroup$ @Pelinore Please don’t leave problems in answers out of deference to comments! The entire point of comments is to improve answers; by doing that, you are robbing of their only purpose, and you are making your answer worse as a result—which makes the site overall worse as a result, if only by a tiny fraction. The entire premise of the Stack Exchange network is to build a respository of well-written, well-considered, and well-polished answers. Comments only exist to serve that goal—to encourage you to edit your answer to fix problems. (Originally SE didn’t even have comments.) $\endgroup$ – KRyan Apr 3 '18 at 14:14
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Missile detection is a nightmare on Earth already. And we are talking about a planet that is only ~12.7 thousand kilometers wide. Doing so from the moon, which is 384 thousand kilometers away, would be hellish.

Also notice that Earth nations have enough firepower to destroy the Earth multiple times in a row as is today. The moon would be much easier to level.

The only thing lunar residents can reliably count on is MAD. But it's pretty easy. All they need to do is to threaten spreading a few thousand tonnes of dust on Earth's orbit, specially low orbit. That is going to expletive so many satellites (as well as any space stations) that the cost of the war will make it... Undesirable. Seriously. Imagine a world without space based telescopes, comm-satellites, nor GPS. Imagine the cost to replace all of that.

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    $\begingroup$ Missles on Earth usually don't need Hohmann transfer to reach the destination so the period during which they can be detected is small (just after lunch). Detection of transfer stage might be much easier as there is no stealth in space and they will need to adjust orbit. $\endgroup$ – Maciej Piechotka Apr 2 '18 at 23:01
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    $\begingroup$ New Horizons made it through Mpon's orbit nine hours after launch, and it isn't a weapon. A proper Earth-to-Moon missile might do so in much less time - given the technological advancements achieved by the time we colonize the moon, a ballistic missile could probably do it in minutes. $\endgroup$ – Renan Apr 2 '18 at 23:35
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    $\begingroup$ I don't think time matters that much. For such short period missile is an object which is 300 K hotter than background so it literally glows (in IR). You need the acceleration to be short enough to allow it to be masked by background radiation of earth - this is possible with E2E as the period of boost can be much shorter and than be cold during travel through atmosphere. E2M missile will be IR lightbulb on the other hand. $\endgroup$ – Maciej Piechotka Apr 3 '18 at 1:41
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    $\begingroup$ Also Missile-defense system will work better from the moon. Range of Lasers is not limited by atmosphere and rockets are not hindered by drag. Plenty of time to shoot incomings down. Destroy the earth may be misleading. You wont have a problem with fallout on moon (because there is no environment) and Live in a Bunker already anyways. So you only have to fear more or less direct hits! $\endgroup$ – Daniel Apr 3 '18 at 11:12
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    $\begingroup$ @MaciejPiechotka No, missiles are not generally 300K hot after they leave the atmosphere: Many rockets use liquefied gases as fuels, and virtually all rockets worth their salt use liquefied oxygen as an oxidizer. And the oxygen tanks are the bigger ones. You just need to scrap the insulation (which is just there to avoid chunks of ice detaching at launch anyway), and you get a rocket that's a chilling 90K cold. Now try detecting that with your heat-camera. And I'm not even talking about rockets running on sub-cooled liquid methane, like what SpaceX is currently working on... $\endgroup$ – cmaster Jul 31 at 1:08
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With great difficulty

My orbital mechanics are weak but it'd be really difficult to detect weapon launches against the Moon, from the Moon. Most of the lunar transfer orbit diagrams that I can find show the firing of the rocket motors happens on the far side of Earth. After the engine burn is complete, the rocket follows a ballistic course from Earth to the Moon. It won't take too long for the rocket to cool off to space ambient temperature, so thermal imaging won't do much good. Paint the warheads in matte black paint and visual acquisition is incredibly difficult.

Further, let's assume that any nuclear strikes against the moon will just launch straight into lunar transfers, without any preliminary orbits around earth.

Compound this with the extremely small size of the W88 nuclear warheads; a mere 1.75 meters (69 in) long, with a maximum diameter of 55 cm. (22 in). At this size, you'd have to know exactly where to look and when in order to detect any incoming warheads then get super lucky.

Now, the launch sites of most nuclear weapons will be known and observable to those on the moon. This means there will be launch windows where weapons can be launched and preferred lunar orbit paths. A careful lunar base commander will work out the most likely launch windows and watch those approaches.

MAD

The threat of counter-attack might be enough to prevent launches in the first place. The US-Russian Cold War ran on that principle. While there may not be enough uranium on the Moon to run a nuclear weapons program, there may be asteroids floating around that can be used as giant planet smashers should the Moon be attacked. If the asteroid is big enough, there won't be anything that Earth can do about it.

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  • $\begingroup$ The reliance on attack is their best form of defence, maybe take out a small island somewhere to prove that they are serious. $\endgroup$ – JeffUK Apr 2 '18 at 20:28
  • $\begingroup$ With orbital velocities in the picture, do we even need to consider nukes? Why wouldn't the combatants just throw rocks at each other? $\endgroup$ – Harabeck Apr 2 '18 at 21:18
  • $\begingroup$ @Harabeck they certainly could but the OP only mentions missiles launched from earth, not asteroids controlled by earth set to a collision course with the Moon. Certainly, if you're throwing big rocks, nukes are somewhat superfluous. $\endgroup$ – Green Apr 2 '18 at 21:58
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    $\begingroup$ If you're looking for incoming, you don't use optics, you use radar. Unlike on Earth, there's no horizon to hide behind, no atmospheric clutter to obscure things, and you've got lots of room to lay out your radar antennas. A W88 might have a radar cross-section of only 24 square centimeters, but a good Moon-based radar should be able to track paint flecks and ball bearings. $\endgroup$ – Mark Apr 2 '18 at 22:22
  • $\begingroup$ @Mark the problem with any EM detection scheme is return from the target. Radar stealth is a well understood technique now. How hard would it be to design a W90 that builds in stealth capabilites such as special paint and proper geometry to reflect the radar energy somewhere else? For a well motivated nation-state, my guess is, very easy. $\endgroup$ – Green Apr 3 '18 at 0:40
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If you assume, as I did, that we're talking about a United Earth, read from this point. If you carefully read the OP's post, which I finally did, skip to "EDIT" and read from there.

The historian Shelby Foote while speaking about the U.S. Civil War once concluded that the Southern Confederacy never had a chance. The Northern Union with its greater manpower and industrialization fought the war "with one hand behind their back." Had they suffered greater losses, they simply would have brought the other hand out.

Your lunar colony has the same problem in spades. No fundamental resources and dependent on Earth for almost everything (no matter how much handwaving is asserted, there's no such thing as 100% recycling and the next baby born would require new resources)... no matter how many weapons had been stockpiled, the moon would fight a valiant war and die in a blaze of glory... and would then be forgotten as a darkened cinder in the sky.

If it appeared the lunar colony was winning, the Earth would simply bring the other hand out... and then another hand... and another hand... having basically unlimited resources compared to the moon.

Further, it's unbelievable Earth would be so blind, so ignorant, that it would allow the moon to stockpile enough weapons to actually invoke Mutually Assured Destruction. Someone in the Earth bureaucracy would eventually say, "hmmmm.... you've been importing about 100X the putonium you need for your nuclear reactors... care to explain, or should we send in troops now?"

Now... if the Moon had somehow developed agriculture and asteroid mining... that might (might) change things, but it's still a bit hard to believe. Why wouldn't the greater manpower and governmental interference of Earth be watching all of that very closely?


EDIT: @JeffUK caused me to reread the OP's question and I must apologize that I misunderstood a critical point: I jumped to the conclusion the colony was defending itself against a united Earth — which for the reasons I state above I believe would be impossible. In reality, the Earth still has nations with competing interests and the colony will be attacked by a coalition.

(If, after reading the rest of my post you think a defense against a United Earth is possible, remember... unlimited resources. Missle after missle after missle until all the defensive tech is exhausted.)

MAD is irrelevant

In this situation Mutually Assured Destruction is irrelevant. If the parent nation cannot sustain it, the addition of the colony will not create it. The parent nation will always have greater and more expedient access to resources.

Besides, we're forgetting that missles take time to get to the moon. Using this somewhat unscientific assertion that missles travel at 4 miles-per-second (14,400 mph, I was too lazy to prove the article's veracity) they would require 16.6 hours just to get to the moon — or, more importantly, to get from the moon to Earth. That's a lot of warning.

It's easy to shoot down a missle if you have the time

Which brings me to my "new answer." Defending the moon from Earth-launched missles (or the Earth from moon-launched missles) is trivial (OK, it's not, but you'll see what I mean). Remember, the difficulty we have on Earth is that we have a window of only minutes to destroy the missle.

But lunar-bound or -launched missles have hours and hours. That's a long time to track trajectories, to launch counter-missles, and for those counter-missles to loop around a few times before they actually hit the incoming ETBM (extra-terrestrial ballistic missleTM :p).

As I think this new situation through, stealth would be critical, which means you need a means of propulsion that doesn't create a heat bloom that can be seen from Mars. The worry isn't a full-out war, it's a first-strike. A patient enemy may decide to declare war only after the two months they patienty took placing a hundred stealth missles in near-lunar orbit.

Now the moon has the same problem Earth has, minutes to fix the problem. If they can't stop the missles on Earth, they can't stop them on the moon.

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  • $\begingroup$ You don't need to be able to totally destroy earth to invoke MAD. you simply need enough firepower to make them think twice. Besides, t's possible that the weapons were put up their on purpose (E.g. by a super-power wishing to protect themselves) and captured by the moon colonists. $\endgroup$ – JeffUK Apr 2 '18 at 20:30
  • $\begingroup$ @JeffUK, the Earth's surface area is 13.5X larger than the moon's and it has infinite resources, which means it can boast 13.5X the number of weapon emplacements and can replace them quickly.. Perhaps the only way MAD is possible is pre-existing weaponry, but that would require a whomping good explanation as to why they were left in place when the colony was established. In the reality of a story with a backstory, left-over MAD-capable weapons are as unbelievable as uncontrolled/unsupervised mining. Against one nation I could believe. Against the planet I cannot. $\endgroup$ – JBH Apr 2 '18 at 21:56
  • $\begingroup$ @JeffUK, I reread the OP's question. I made the assumption that the moon was defending itself against a united Earth. It is not. It's only defending itself against a coalition of nations. In this case MAD doesn't apply because the colony's fatherland would have more than enough weapons to justify MAD. Ergo, this isn't a question about MAD. I'll modify my answer accordingly. $\endgroup$ – JBH Apr 2 '18 at 22:04
  • $\begingroup$ MAD situation is asymmetrical. It is not possible to destroy lunar biosphere (there is none). It is relatively very easy to lob rocks from Moon to Earth and cause kiloton class explosions, no high tech warheads required. The infrastructure to launch lunar capable rockets from Earth is very vulnerable, while mass driver able to hit Earth could even be on a mobile platform (possibly standard mining equipment used to deliver resources for space construction). Earth population centers are very vulnerable, while lunar population centers would be underground and small meteor proof by default. $\endgroup$ – hyde Apr 3 '18 at 11:35
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    $\begingroup$ I think you're dramatically wrong here. Take the basic concept of "high ground is an advantage" and multiply it times a trillion - earth is stuck at the bottom of a giant gravity well. The earth can win in a "traditional war" but you strongly underrate the advantages the moon has in a total war scenario. The farside of the moon is basically impossible to profitably attack (delta V = \$$$$), meanwhile the earth is totally exposed. The moon can just drop dumb mass until there is no infrastructure or ecosystem left. If the moon is not self-sustainable, yeah, they die too. $\endgroup$ – John K Apr 3 '18 at 18:33
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While in other answers I have been rather enthusiastically describing energy and kinetic based attacks on the Moon colony, looking at it from the other way around, you can see there is no possible way of matching force for force; the resource balance is far too great.

The Lunar colonists all need to change tack and focus on fighting a "4GW"

Fourth-generation warfare (4GW) uses all available networks — political, economic, social, and military — to convince the enemy’s political decision makers that their strategic goals are either unachievable or too costly for the perceived benefit. It is an evolved form of insurgency. Still rooted in the fundamental precept that superior political will, when properly employed, can defeat greater economic and military power, 4GW makes use of society’s networks to carry on its fight. Unlike previous generations of warfare, it does not attempt to win by defeating the enemy’s military forces. Instead, via the networks, it directly attacks the minds of enemy decision makers to destroy the enemy’s political will. Fourth-generation wars are lengthy — measured in decades rather than months or years.

The Sling and the Stone: On War in the 21st Century by Colonel Thomas X. Hammes USMC (Author)

Diplomatic channels will have to be opened to various powers on Earth, and deals brokered. Nations will become recipient of high tech Lunar goods and services if they in turn provide trade goods like water and life support materials and exotic elements not common on the Moon. These nations will also apply pressure against the nations which are opposing Lunar independence, and serve as conduits for Lunar diplomats and business people to message other nations which may be hostile or neutral. Playing off nations against each other is also a viable tactic, if China is looking over its shoulder every time they make a move against the Moon because Australia and India have vested interests in protecting the Moon, then they will have to move more slowly and cautiously to protect their Earthly flanks.

At the same time, the Lunar colonists will need to disperse their base to be less vulnerable to attacks of various sorts, and consider moving off the Moon altogether. Sending expeditions to NEO's to mine for water and other life support elements provides cover for evacuating portions of the colonists, and the NEO provides a much better source of materials and living space. If necessary, they can power the NEO and create a space ark, capable of moving them slowly away from the Earth and towards Mars or the outer Solar System.

enter image description here

From this....

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To this. A graphic of where water bearing objects are in the Solar System

Each step makes the job of the Earth military more difficult, provides greater uncertainty in the political and economic affairs of Earth and makes delivering a knock-out blow to the Moon and space colonists that much more difficult. If the Lunar colonists are skillful enough, they could create enough time and space to eventually convince and even prove there is no way for the leaders of Earth to achieve their political and military goals at any reasonable cost.

The main thing is to move slowly and carefully, prevent provoking the Great Powers, and constantly work to strengthen the hand the Lunar colonists hold.

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Build a space-debris barrier!

As the Answer from @MikeB Pointed out. gravel at high velocity is quite dangerous in space. It is also very hard to detect. So the lunar colony could just "poison" the earth´s orbit with tons of gravel - resulting in a barrier that has a high chance of downing any spaceships and missiles.

As a side-effect, you would also destroy earth´s GPS, Satellite-communications and space-based surveillance capabilities and probably make falling stars an everyday-sight.

If Lunar uses small gravel-cannons this may well go undetected from earth until their satellites start failing - when it is already to late to counter.

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It seems as though your question has both defensive:

Now the lunar colony has to set up defences against aggressions from Earth.

And offensive components:

In addition they consider building ballistic attack capabilities against Earth, something similar to what has been suggested in this answer.

We'll start with the first point. As others have mentioned here, the simplest and easiest thing for lunar colonists to do is fling rocks at incoming enemy ships or missiles. NASA has recently proven that the moon holds significant reserves of water ice, which can be used to make (nearly) unlimited quantities of rocket fuel. The moon also receives roughly 30% more intense sunlight and solar panels can be manufactured largely using in-situ resources, so power generation isn't a problem. Your colonists will be able to hurl as many rocks as they can collect.

How effective are rocks against missiles? Well, you need to remember that orbital mechanics take all the fun out of space combat. A solid object the size of a BB traveling at orbital speeds can tear a hole several inches wide and deep through armor. It doesn't take a large or explosive payload to destroy a ship or a missile in space -the relative speeds alone will do the job. For example, during the Apollo missions, the Apollo capsule/LEM assembly traveled toward the moon at a stately 10km/s and took a whopping 2-3 days to travel from Earth to the moon. For Earthling missiles to do the job in mere hours, they'll have to travel much, much faster. At those kinds of speeds, even a paint fleck is a hazard.

Your colonists would also potentially be able to use high-power lasers to intercept incoming ships or missiles, but their ability to manufacture the precise optics will depend on how advanced the colony is -and if rock throwers will work, why bother?

Now, attacking Earth from the moon is a far more challenging problem. Ballistic missiles launched from the moon would be only slightly more effective than those launched from Earth, because although the Earthlings can't simply hurl rocks at them as easily as the lunar colonies can, you still have many hours or even days to detect an incoming missile, track it, and intercept it with other missiles. And while it would be substantially more expensive for Earth to intercept those missiles, Earth has more than enough resources to intercept any number of missiles the lunar colonies could muster, just because Earth has such a tremendous industrial base and military establishment.

But ballistic missiles may not be possible for your lunar colonists anyway, because the precious Uranium needed to make nuclear warheads is not prevalent in the lunar crust. You might be able to extract enough to make a handful of warheads, but not anywhere near the numbers that Earth could throw at you. Now, if you're far enough in the future that fusion reactors are available, that might be a moot point. Otherwise, the best your lunar colonies could do would likely just be interfering with Earth-orbiting satellites. Fling a few tons of gravel through the geosynchronous orbital band where most of the telecom and military satellites reside, and you could disrupt their communications systems and do a fair amount of economic damage.

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    $\begingroup$ The moon rebels don't need nuclear warheads, the speed a projectile from the moon will smack the earth at means ordinary inert rock is more than adequate. $\endgroup$ – Pelinore Apr 3 '18 at 1:48
  • $\begingroup$ Ordinary inert rock will burn up in the atmosphere -unless you're hurling rocks the size of school buses or larger. If you want to make a pure kinetic kill with the force of an atomic explosion, you'd need to send solid metal projectiles, likely made of tungsten or an Inconel-like alloy, of considerable size. $\endgroup$ – MikeB Apr 3 '18 at 23:15
  • $\begingroup$ @Pelinore you seem to have missed the "...force of an atomic explosion" part of my comment above. But I'll humor you: a solid-iron asteroid the size and shape of a bus (~896,000kg) has a terminal velocity of about 2.3km/s. Coincidentally, the total temperature due to atmospheric heating at this speed is right around the melting point of iron. But let's say the rock maintains terminal velocity and doesn't lose any appreciable mass: hitting the earth at 2.3km/s will release roughly as much energy as a 0.6kT nuke. That isn't much damage potential, and we're talking theoretical max yield here. $\endgroup$ – MikeB Apr 5 '18 at 0:05
  • $\begingroup$ Suppose it's going faster than 2.3km/s when it hits the atmosphere: drag is proportional to the square of the speed, as is atmospheric heating, so in addition to burning off more of your 'roid, you'll also be subjecting it to increased stress as it tries to slow to terminal velocity. The likelihood that it remains in one piece diminishes as speed increases. I'm too lazy to compute the mass loss vs. entry speed, but it's safe to say that you'd get a minor increase in explosive yield up to a certain speed, beyond which the asteroid would burn up or break apart before hitting the ground. $\endgroup$ – MikeB Apr 5 '18 at 0:08
  • $\begingroup$ Simply jacketing your projectile isn't enough; it has to be aerodynamic to survive reentry, and have the right heat-resistant materials. Check out the "rods from God" weapon concept. My point is, simply hurling rocks won't do much damage. You'd need actual shaped projectiles to even reach the ground. businessinsider.com/… $\endgroup$ – MikeB Apr 5 '18 at 0:11
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Lunar colonists need to build controllable set of mirrors to focus the solar beam on strategic objects of the Earth

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    $\begingroup$ Can you flesh out a bit more this comment-like one liner and make an answer out of it? $\endgroup$ – L.Dutch - Reinstate Monica Apr 3 '18 at 6:38
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The most likely reason for a moon base is to harvest Helium-3 which (theoretically) is a clean source for fusion energy. The moon base would be fitted with a fusion reactor that runs of He3 so it would be extremely well stocked for energy.

With a virtually endless supply of clean energy, you can run all your hydroponics, recycling and life support systems forever. Earth isn't going to damage anything that they need should they win so they won't do anything that might affect the He3 supply or the ice supply at the poles otherwise the war would be pointless.

The best defense is to dig down and spread out so they cannot do any major damage. It's not like they need surface access for air, food and water.

Earth's hunger for He3 means the moon will win the siege eventually forcing Earth to the negotiating table. If fossil fuels are depleted, the Earth cannot afford to lose it's supply of clean energy.

Starvation is always a very effective weapon

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There are many ways to defend the moon in the short term, but space is big and earth has pretty much all the resources.

Missiles launched in a straight line at the moon could be intercepted without a lot of problems, because detecting the heat is easy. Earth doesn't need to launch directly or quickly, it can take it's time, because in the end success is guaranteed.

As an example, take a space shuttle filled to the brim with soldiers and launch it away from the moon, i.e. out of sight of your lunar satellites. Accelerate it, maybe use the earth for a fancy slingshot maneuver and stop accelerating early enough to allow it to cool down. You now have a fast, dark and cold chunk of metal filled with angry earth soldiers, hopefully in survival gear so they don't arrive frozen.

Using math, you can have the soldiers land anywhere on the moon and they can decelerate and land at a blind spot and move on the ground to your lunar outpost and capture it before you know what happened.

If earth is lucky, they can even time the approach so the chunk of metal and soldiers approaches the moon from the sun, masking any potential heat signatures even further.

The lunar colony needs to have full 3D coverage of the moon to defend against indirect and purely ballistic attacks, which is possible, but expensive.

So how could the moon still achieve independence or whatever they want to achieve?

Depends on how committed earth is. If earth really, really doesn't want the lunar colony to have independence, the lunar colony is doomed. If earth is more driven by economic considerations, making it too expensive is the way to go.

Defend as long as possible while you train and build up a thoroughly prepared resistance network, including secret tunnels, caves, weapon stashes, communication networks and such. Eventually the lunar colony will get conquered, no way around that. That is when your real plan kicks in. Terror attacks, raids, hit-and-run attacks, everything to make it expensive for the conquerors to stay. Eventually political pressure back home will force them to pull out.

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  • $\begingroup$ "move on the ground to your lunar outpost" OP didn't say that lunar colonies are easily accessible from the surface. Colonies need radiation shielding from day one of their existence, so it's quite probable they'd dig in. $\endgroup$ – kubanczyk Apr 4 '18 at 9:15
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    $\begingroup$ @kubanczyk There will be some connection to the surface which can be used as an entrance, otherwise the colony would be quite useless, disconnected from the outside. $\endgroup$ – Morfildur Apr 4 '18 at 11:48
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It depends on how the colony on the Moon gets its energy. If they use solar panels, they are doomed. These can be destroyed easily by launching lots of small projectiles from Earth's orbit. They need not be bigger than grains of sand, they would be almost invisible, until they impact, and they would arrive like micrometeorites and slowly wear off the solar panels. Repairing or replacing them would be difficult. Those artificial micrometeorites can kill people too. The shortage of energy would make the colony surrender.

Anything on orbit around the Moon or around the Earth would not last long either. It will be a costly war for both sides.

However if the colony on the Moon has fusion reactors deep underground, and powerful railguns shooting from underground, they can continue fighting indefinitly. They can keep throwing rocks out into space from deep bellow surface using those rail guns. Even without much aiming, most of these rocks would end up around the Earth. Some would hit the Earth, some would burn up in Earth's atmosphere, some would fall back on the Moon, eventually making the surface of both planets inpassable. For the people on the Moon it is not a problem, they always lived underground, they would just dig deeper. Gravity is low, digging is easy, and the Moon has a thick crust. Gradually the people on Earth would lose their ability to launch anything into space through the hot, poisonous atmosphere and the space debris.

Earth has plenty of resources, but the colony would have access to the space. Even with lots of debris circling around, some robotic spaceships may get through. Those spaceships can deliver the coup de grâce to the overheated and dying Earth by redirecting a large meteorite towards Earth which would break through the thin crust and cause devastating lava flows.

Landing parties and other forms of close quarter combats would be impossible, except at the very beginning of the conflict. Even then the defenders would have the upper hand. Landing on Earth in enemy fire would be impossible. Fighting in the endless tunnels of the Moon would be a nightmare for the attackers.

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