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.
Anyway, how the heck does one lob a comet? If throwing one was so easy, why wouldn't they just throw it right back at you?