There is a small amount of air drag in low earth orbit, caused by the very thin gas atmosphere. The solar wind from the sun strips off and ionizes some of this

There is a small amount of air drag in low earth orbit, caused by the very thin gas atmosphere. The solar wind from the sun strips off and ionizes some of this gas. Ions are atoms or molecules with an unbalanced positive or negative charge - contrast that with neutral atoms or molecules without an electric charge. Ions can be accelerated via an electromagnetic field. You can use this as thrust for your spaceship. The faster the exit velocity of the gas, the more efficient your acceleration. (the technical term is Isp or impulse-seconds or something). Normally spacecraft carry their own neutral fuel, ionize it, and then spit it out the back end. This requires a lot of electrical energy but if you get that from big solar panels this tends to be more efficient than chemical energy rockets. More efficient => less mass=> smaller, cheaper spacecraft and rockets. The paper estimates that in very low earth orbit (190km altitude -- for reference the International Space Station is at 400km altitude), there is enough density of ionized gas, that you could suck in ionized gas, accelerate it out the back with ion thrusters, and basically break even or better. This means, at that altitude, with a very specific design, you could use the atmosphere as propellant rather than bringing your own in a bottle. In theory, not needing a fuel supply means you're not fuel limited on satellite lifetime, and fuel supply is normally the big constraint that ends a satellite mission.("We're out of gas for pointing and moving the spacecraft") Takeaway: might be useful for Starlink / Project Kuiper / imaging systems in 5-10 years... Might be useful around other planets with plentiful solar power but it's a very niche solution only usable at this one altitude band currently. See also: Hall Effect Thruster, Electrostatic Grid Thruster. Contrast with: fusion powered Bussard Ramjet.