Dr. John Schilling discusses why the exhaust plume of a decoy will have to have the same thrust as a real ship:
Problem is, the rate (i.e. velocity) at which the plasma is coming out, manifests itself as a doppler shift in the characteristic emission lines of the plasma. As soon as a dedicated tracking sensor focuses on the target for a second or two, the game is up. If the plasma is coming out fast, it can't help but produce thrust proportional to mass flow rate (manifested as luminosity) times velocity (doppler). If the plasma is coming out slow (or fast but in opposing directions), it will be seen to be coming out slow and thus be recognized as not a real engine.
This assumes that you can gather enough light in a few seconds to get a spectrograph with enough accuracy to tell if there is a dopplar shift. But if detecting the ship against a noisy background is hard, getting a spectrograph is harder. In detection, you're focusing all the light into a small number of pixels to give it the best chance to trigger some threshold. But in a spectrograph, you have to spread that line out so you can see the absorption lines — each emitted frequency has to build up enough photons to be distinct from the absent frequencies. For stars and things of similar apparent brightness, this takes hours.
Only then can you get the dopplar shift and figure out that you're looking at a decoy, but by then the enemy has gotten a few hours' advantage, and you still have yet to find the little bugger. If burns last only minutes, you are shit out of luck — you don't even know whether that thing you just saw as a decoy or the real mccoy.
Junghalli wrote:The obvious solution would seem to be to have the decoy shine redder light from the front, transitioning to bluer light in the back. Problem is,
...that spectroscopy doesn't work that way. The atoms themselves are moving at some relative velocity, so their absorption lines are shifted. It's not a matter of redder or bluer, but the spectrum of a luminous object has absorption line patterns that resemble sodium or nitrogen or what-have-you, but shifted down a ways.