I postered a comment on the essay, I'll repost it here:
You assume the interstellar species will be living on a planet. What if they hollowed out their planet of resources long ago and now live in a cloud of asteroids instead? Asteroids are hollowed out as giant fortresses in space, and a bio dome sphere thingy built inside. As they are living in an asteroid cloud, to prevent impact they would have to attach thrusters to each asteroid they live on to dodge other asteroids.
So the asteroid from kuiper belt attack won't work... but what if the asteroid was launched at close to speed of light? 1. That would take a very big amount of energy. 2. They would've had to carry the energy to the kuiper belt. 3. They would've been noticed and a fleet would've sent in to capture it before the asteroid was launched.
What about the nuclear bomb? Well they live deep in the asteroid, it'll protect at least against some nuclear hits. If one asteroid blows up there are still 499 others. Still it would be great to prevent the loss of sentient life... Also what happens if 500 nukes were launched at each asteroid? I've got no answer to that.
I do wonder if there could be a device that would force nuclear explosives to be triggered prematurely, lets say, 2km away. That would be sufficient to protect the asteroid.
What about a shotgun approach? Take a comet, accelerate it up to a significant fraction of lightspeed (after all, you you have lightyears in which to do so), and trigger a bomb in the comet to blow it apart into a few smaller pieces, each traveling at a significant fraction of c. Rinse and repeat.
So the invading force goes into the Oort cloud, picks a chunky rock, attaches a giant thruster to it and begins accelerating it towards c. A day later, the speed of the rock has reached 0.5c.
Hours later, a defending fleet arrives, shoots and kills all the invading bds. The rock however, continues at 0.5c even though the thruster and guidance systems were destroyed. It reaches the asteroid cluster, and hits... nothing. It keeps going to the other side of the solar system, and as its speed was so high, exits orbit.
Accelerating a giant rock to c takes throwing out alot* of gas. The asteroid based civilisation, to prevent random impacts with random asteroids, would've been tracking every object in the belt. One of the rocks suddenly accelerates, I think it will be picked up.
Also, when the asteroid was accelerating, its direction would've been picked up. Once accelerated to c, it will still take hours to arrive at the designated target. The home asteroids simply have to maneuvre away from an intercept course. And no, guidance systems on the accelerated rock won't help. It's travelling at c, and by the time the system finds out the target has moved away, the asteroid would've missed it. Unless the asteroid have 100% maneuverability, and energy to change course whenever, however it likes. But if the asteroid can do that, any asteroid can do that. ;)
So your only option is accelerating the rock from the next system onwards. It will now take years for the asteroid to arrive. To reduce the probability of a successful attack, just have a "random thruster day" every 30 days where all the home asteroids just move randomly to different locations. By the time the enemy asteroid arrives the location of all the home asteroids would've been different. The probability of hitting anything worth hitting will be very low. Even if you have 1000 incoming asteroids.
Also you're assuming you won't get picked up accelerating an asteroid to c in the next system; Remember, asteroids have no atmosphere and 100% visibility. You might miss 1, but 1000? I'd doubt it. if 1000 asteroids are really incoming then you just make every day random thruster day. ;) They have so much energy, you should too.
Generally I agree with this analysis, but I think there is an attack that would work:
Strap small N-bombs (or conventional bombs?) to as many as asteroids as possible, launch them at the target area, and then blow them up ~500,000km away. Suddenly you have thousands (millions?) of significant chunks of rock travelling at 0.5c and spreading out. If you got the explosion point right and had enough asteroids you could cover a reasonable amount of the likely positions, even based on random movements.
It's unlikely to destroy an entire asteroid-belt based civilization, but wouldn't be trivial to evade either.
It seems you and Heinlein are of like mind; remember Mycroft's conclusion in The Moon is a Harsh Mistress that the best way to fight the Earthen forces was to "throw rocks at them"?
Physics. Attacker gets arbitrary mass (reactant, etc) and time to accelerate. Defender needs same mass/time to decelerate... after discovering the attack.
Note that you could literally prep an attack for millennia which would be discovered a matter of hours prior to ending a civilization.
Physics makes for sucky narratives. Good thing it also makes for a vast, dead, boring universe.
I don't think you would try to slow down approaching meteorites to zero speed, only deflect them a little so that they miss the planet. The further away you catch it, the less energy it takes (I assume, didn't do the maths).
Debatable. On the one hand, the farther the projectile is from the planet, the less it needs to be deflected. On the other hand, you have to expend energy to get to the damn thing in the first place.
I suppose the survival of your species renders moot most energy concerns though.
It depends on the momentum of the asteroid. If I've accelerated an asteroid up to .5c, then even if you discover the attack years ahead of time you might still not have the energy to deflect the asteroid by enough to save your planet.
You also have to remember that the attacker can launch multiple simultaneous asteroids. You might have enough nukes to deflect one asteroid. Do you have enough to deflect ten? A hundred?
If you can accelerate an asteroid up to .5c, you have harnessed energy resources to power the earth for a gazillion years. You would be so rich that you dont need to be a warlord. You could become ... a banker.
Actually, at .5c from the Kuiper belt, the warning time is only 30 hours or so and the momentum is such that deflection is impossible. I'm too lazy to do the math but the collision would be spectacular.
Momentum doesn't really work this way. The only things that change the difficulty of deflecting something are it's mass, and the remaining time to impact.
Having a lot of momentum doesn't change how much force is required to give something a given acceleration until relativistic effects start to matter.
(I guess distance also makes it harder, because you need to travel further to start deflection, but that's not really a momentum thing)
Well, since you're going to skirt around how the acceleration actually happens, I'm just going to counter with: The defender also has arbitrary time to defend. Couldn't you "store up" a defense over time somehow? Maybe have a counter-projectile orbiting really fast that can be brought out of orbit to hit the incoming one? Or, I know we don't have energy shields, but with the amount of assumption going on in this whole article, I may as well assume we do ...
The physics on storing kinetic energy like that just don't work.
One intriguing possibility I thought of is the possibility of creating a black hole[1] in the path of the projectile. I have no idea how the energy calculations work for creating a black hole, and what exactly happens to one if a large mass travelling at high speed hits it - I'm guessing if the black hole was small it would just slice the object in half. If it was big enough to "swallow" the object then I don't know what happens - if I'm reading [2] correctly its mass-energy increases, but what that means in practical terms I'm not sure.
Of course, the issue with this plan is before you had a fast moving massive object targeted at planet, but now you have a black hole nearby.
That is an oxymoron. You are either orbiting or going "really fast" but you can not be doing both. (In the context in question single-digit miles per second is not "really fast", it's just the cover charge.) You can pour arbitrary amounts of energy into a kinetic kill projectile, and there is no known reasonable way of defending against such a thing except to not be where it hits.
Just as people worry about nuclear weapons and biological weapons, we get planet-killing kinetic projectiles rather earlier than we get anything else useful from space. In fact if we put our minds to it, and were willing to be patient, we could probably do it today. The same tech we're looking at to divert existing planet killing asteroids can just as easily be turned to tweaking one of their existing orbits towards us instead of away.
Well, both gwern's essay and the essay linked by OP assume that both sides have ships capable of interstellar travel. In gwern's case the idea is to not bother with the ship; just bolt an interstellar drive to a comet and point it at the enemy's homeworld.
As for them throwing comets back at you, that is addressed in the essay. In short, there is nothing stopping them from throwing comets back. Space war will resemble the Cold War much more than any active engagement. Mutually assured destruction will be the overriding doctrine.
The fighting methods in that essay still require ships (to sneak up into e.g. Kuiper's Belt). In any case, a big, dumb projectile can have its course changed with much less energy than required to fire it. The bigger it is, the earlier it will be detected; and the faster it goes, the smaller the angular deflection needed to miss its target.
If that’s not bad enough, one could envision flinging appropriate small moons or large asteroids from other solar systems. Why not? Boosting a large body to fractional c velocities and aiming it at a far away planet isn’t inherently any more absurd than building a ship and boosting it to fractional c. The motion of stellar bodies is famously predictable out for many centuries. The body could guide itself: add some small motors, and it could even correct for small errors in prediction. It would be the ultimate fire-and-forget weapon of mass destruction.
So you don't have to sneak up at all. You just need to have a solar system with a Kuiper Belt, and you've got all the arsenal you need. In fact, launch attacks from remote solar systems is preferable, since it gives your bullets more time accelerate before becoming visible to the enemy. They may have enough enough energy to stop an asteroid traveling at .5c. Do they have enough energy to stop one traveling at .9c?
Reaching even a small proportion of the speed of light requires prohibitively high amounts of energy. Let's consider the energy required to accelerate Haley's Comet (relatively small at 2.2e14 kg) to half the speed of light (v = c/2):
For comparison, the Earth absorbs each year about 3.85e24 J of solar energy, so we're talking about six million years of total incoming solar.
Assuming two societies with similar technology, it will cost vastly less energy for the defender to give the object a small nudge than it cost the attacker to propel it in the first place. The attacker would do much better to invest into a fleet of ships that can respond to contingencies and adapt to the defender's response.
One of my favorite "ohh, that's interesting" facts about space combat, is that a kinetic kill weapon (ie, .9c rock) being shot at you. By the time you see it, it's already 9/10s the way to you...
While it is true that it should be easy to deflect, it is only easy to deflect if you can detect it. The essay assumes that the projectiles would be pushed to a significant fraction of the speed of light. At that speed there is no time to launch any type of deflection.
The energy required to accelerate a big object to even half the speed of light is prohibitive. Even if the process was feasible, it would be a vast cosmic event bound to emit some radiation, which would still arrive much sooner than the object itself.
Well you don't have to accelerate it all at once. Remember, this is space war, and we're playing for keeps. Spending a millennium to accelerate a rock up to .5c might be worth the trouble.
If you take longer to accelerate it, that just increases the probability of detection by other means. It still doesn't change the argument that it should be much cheaper for the defender to detect and deflect your projectile than it was for you to launch it in the first place. Not to mention that an unmapped interstellar gas cloud, or an undetectable asteroid, is all it takes to deflect the projectile a few millionths of an arcsecond and make it miss its target.
This is why Carl Sagan argued against projects like Spaceguard that purported to defend against rogue comets or asteroids. He felt that the risk of a space collision was much smaller than the threat of humans turning the technology around to hurl asteroids at our enemies.
Tl;dr: it's much cheaper and much more effective to lob comets at your enemy's homeworld than it is to fight with ships.
[1] http://www.gwern.net/Colder%20Wars