The answer to this question taught me about the sort of parameters I need to consider if I want to consider how "detectable" an object in space is.
I want to consider the detectability of a spacecraft's exhaust. In the case of a chemical rocket engine, the exhaust is a gas composed of the products of an exothermic chemical reaction. Could I thus model the exhaust as a single low density blackbody? An ion thruster is similar, since the exhaust is a "gas" of ionized particles.
Somehow, though, treating the exhaust as a small blackbody feels like it could potentially be quite problematic. Is the assumption fine?