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The Physics of Space Battles
An anonymous reader writes PBS' It's OK to be Smart made this interesting video showing us what is and isn't physically realistic or possible in the space battles we've watched on TV and the movies. From the article: "You're probably aware that most sci-fi space battles aren't realistic. The original Star Wars' Death Star scene was based on a World War II movie, for example. But have you wondered what it would really be like to duke it out in the void? PBS is more than happy to explain in its latest It's Okay To Be Smart video. As you'll see below, Newtonian physics would dictate battles that are more like Asteroids than the latest summer blockbuster. You'd need to thrust every time you wanted to change direction, and projectiles would trump lasers (which can't focus at long distances); you wouldn't hear any sound, either."
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.
What gets me is when the author is painstaking in attention to detail in describing realistic ship movements in a 0 G vacuum, including relativistic effects, but then describes maneuverability as somehow being tied to a ship's "speed". It's another carryover from ships that maneuver via control surfaces that interact with the environment, and thus feels natural. But in a 0 G vacuum it doesn't matter at all whether it is ship A that just finished accelerating and ship B is "sitting still" or vice versa. Both situations are completely equivalent unless there is something else nearby, like a planet, that adds gravity or an obstacle to the situation.