[brmj]

Here's a thought I've had in the past after encountering a document of this type: Stealth in space is trivial is what you are trying to keep hidden doesn't need to accelerate, support human life or do anything much that generates significant heat, especially if it can stay far away from what it is hiding from. Space is big and hard to hide things in, but it is full of all kinds of smallish debris in the vicinity of a solar system. If you have something of about the right size, orbit, albedo, radar reflectivity and temperature, it ought to be hard to distinguish it from a stray bit of rock or ice too small for anyone to have paid attention to it in the past.

Bearing this in mind, suppose that you built a small, camouflaged satellite which would just sit there at ambient temperature until it received instructions, upon which it would point itself at a given patch of sky, do the finest bits of the aiming using its own passive sensors and then fire a nuclear bomb-pumped x-ray laser at whatever has the misfortune to be there. With something like this, one could probably swat any reasonable craft out of the sky before it had time to respond. You could perhaps also use such things offensively by putting them on orbits that will take them within their effective range of whatever you want them to shoot, though they would stand out a bit more that way.

The only real countermeasure to these I can think of would be to move your spacecraft rapidly back and forth at all times, which is probably infeasible given the amount of reaction mass it would take.

I'm not sure what the implications of all of this are, but I suspect they would be interesting.