The momentum of the mass expelled in the opposite direction ... the rocket-engine exhaust, or the ionized matter expelled from an ion drive.
THAT's why every propulsion engine has outlet nozzles designed with super-high-intensity math, to achieve the highest possible velocity for the mass that gets shot out the back ... so that it will carry the maximum possible momentum.
if there was no atmosphere,the tiny molecules of gases and dust particles will gather together and rain down while the sun shine on the particles making the color glow because the colors are combining into various of colors looking like the galaxy.
