We could just watch and shake our fists as they steal our future at their leisure. They wouldn’t even have to send their people. They could use robots on ten systems at once.
The second valuable thing is intelligent life. The intelligent monkeys are worth preserving and talking to. But their advanced civilization has lots of potential dangers so need controlling protectorate.
The third valuable thing is better minerals. The active processes of the Earth concentrates minerals. They could grind up a whole asteroid, but cheaper to take from Earth. Also, biologic processes make minerals; vacuuming up coal is cheaper than making it. These wouldn't be whole civilization but pirates grabbing some cheap loot before the authorities and natives notice.
The fourth valuable thing is Earth itself. Inhabitable planets are rare. The expected way would be apocalypse or annihilation. But I think it would be more interesting to have alien invasion where they ignore us. They settle the arctic, and don't do anything until attacked. Then they start removing CO2 until the temperatures drop and ice age returns.
Worse, they could throw bits of the Oort Cloud at us.
Yes, but they clearly have the capability to get to the Oort Cloud, and we (other than a small probe) do not. Having the high ground (both physically and technologically) makes holding the Oort Cloud a pretty good spot to be in.
(It also takes substantially less ∆V to nudge an existing rock into a nastier trajectory, and we're essentially a big easy to hit static target.)
The whole point of the high ground is someone just needs to ‘drop’ something in, which is cheap and easy, but someone on the ‘low ground’ needs to make up all the energy to get up there before they are even at the same level.
The energy involved in de-orbiting something and dropping it to the surface of earth (at very high speed) is orders of magnitude less than it would take to get the same mass to even earth orbit from the surface.
Holding a brick over the opening of a well is a much more credible threat to someone at the bottom, than that someone with the same brick threatening to throw it back out.
All you need is for orbits to intersect with the surface, not for a zero velocity vector at the surface. That is far cheaper to do than getting to orbit in the first place.
This absolutely applies to orbital mechanics.
Atmospheric braking makes it less likely to punch a hole in a skyscraper in manhattan when de-orbiting. And defacto raises ‘the surface’ in some senses.
It doesn’t change that it’s far ‘cheaper’ to go down a gravity well than go out of one.
Though that atmospheres (and liquid oceans) exist at all does prove exactly the point that I am talking about. The Sun and planets too, come to think of it.
If it was cheaper to ‘get out’ than ‘fall in’, none of those could exist.
Edit: I used a Hohmann transfer orbit calculator, and from an orbit of ~200km above sea level to an orbit intersecting ground level (0km), it only takes 203 m/s.
Without an atmosphere, it would be quite a show of course. But in this situation, that’s the point isn’t it?
if landing gently, then sure. but that is an entirely different problem.
Still, your logic doesn't apply, and hitting something in a higher orbit shouldn't be that much harder. You also only need to nudge it to hit something UP. (though it does get harder with a lesser impact with increasing distances)
I fact there might not be that much of an effect at all when you consider that both Earth and the Oort cloud are in the Sun orbits - you would have to hit something pretty hard in the Oort cloud to create a massive impact on Earth, there would at best be a bit of leverage, which I guess wouldn't outweight the protective effect of Earth's atmosphere, unless you try to send something monumental, at which point you probably can just come down and hit Earth. Consider also the timescales involved.
Anyway, if you really want to destroy a planet, you want sonething small but fast. It penetrates into the planet, and rips surface on the other side.
Dropping things is way easier than throwing things up. This is easy to verify yourself too.
It's much less delta-v to go from the Oort cloud to Earth—to a highly-eccentric orbit that intercepts the Earth's orbit, without matching its velocity.