Comment Re: Umm no (Score 5, Insightful) 470
Well, but consider: space is big. Detecting things in it is hard, unless they've giving off light or radiation that you can detect more easily. For example, today, it is very common that we don't notice near-earth asteroids until they're less than a day away, and asteroids are a lot bigger than missiles.
Atmospheric missiles must maintain constant thrust to keep flying, in order to counteract gravity, air resistance, and to maintain course skimming the ocean, as you mentioned. That makes them easy to spot as soon as they cross the horizon, giving you that 10-20s warning.
But in space, constant thrust is not necessary. The missile can be fired initially just like a dumb projectile, and only engage its thrust once it's very close to the target and needs to adjust course to hit it. Until that time, it just looks like a very small rock hurtling through the void, giving off very little energy, making it very hard to spot.
Even if you had radar or some other kind of active sensors to detect incoming missiles before they engage their thrusters and give away their position, the attacker could simply fire their missiles inside a cloud of other flak to camouflage them. So you can see a cloud of thousands of tiny objects coming in, but you can't tell which of them are missiles with warheads until the whole cloud is close enough that those missiles activate and start homing in on your position.