Satellites in low earth orbit (think Starklink, ISS) orbit around like 300-600 km height. At this height the the atmosphere is pretty thin but not negligible an

Satellites in low earth orbit (think Starklink, ISS) orbit around like 300-600 km height. At this height the the atmosphere is pretty thin but not negligible and these satellites slow down over time due to drag. For this reason they often have to have a method of propulsion for "station keeping" [1]. Now propulsion in space is somewhat different to how we do on Earth (in airplanes). To propel yourself forward, since momentum is always conserved, you need to have something (with momentum) and throw it backward (which requires energy), so you are propelled forward. On Earth Jet engines suck air in, combine it with fuel and throw it all backward to propel themselves forward. A propeller just throws the air backward to boost itself forward, however in space you can't do that, cause there is nothing to suck in. Which means you need to carry that mass with you. There are some very efficient designs (like Ion thrusters [2]) that propel ions of gases to very high speeds (using electric energy obtained from solar panels) to achieve very efficient propulsion. This LEO satellites are not exactly in a perfect vaccuum, what this paper suggests is the viability of a method where a engine sucks air in the thin atmosphere, and uses a (very efficient) engine to propel it backward that it is able to compensate for the drag forces the satellite would endure. 1. https://en.wikipedia.org/wiki/Orbital_station-keeping https://en.wikipedia.org/wiki/Orbital_station-keeping 2. https://en.wikipedia.org/wiki/Ion_thruster https://en.wikipedia.org/wiki/Ion_thruster