Just a thought that came to mind ...
The rockets on interplanetary scouts fire to impart motive/braking force to the craft. This force, I understand, is simply a reaction engine - the exhaust departs out the nozzle, and drives the engine & attached craft in the desired direction. In an atmosphere this exhaust would eventually disperse. In space one would expect the exhaust to continue in the direction it exited the nozzle. But space is cold.
Assuming the rockets fired in space far enough for any gravity knot to have negligible effect, how far from the nozzle would the exhaust cool to near CBR temperature?
For instance, the hydrazine thrusters on the Voyagers provided necessary motive force at critical times during it's transit through Our solar system... albeit this was necessarily within a gravity knot.