The thing that makes it hard to go fast is not the energy needed but the mass you need to toss out the back. This is because you need to carry the mass you throw out the back. It adds to your total mass and slows you down. In space generally you carry all that toss out the back mass and once you use it, you can't get more. You can get more energy by using solar panels. Going the speed of light would require very little energy if you could just convert that energy directly into forward direction.

Solution: If you are close enough to Earth that you experience drag from the atmosphere, use the atmosphere as the mass you toss. Now you have a really efficient long term way of going fast because you can always get more mass to toss from the atmosphere and you can always get more energy from solar panels. You no longer need to bring mass to toss to orbit and you never run out of mass to toss.

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 2. https://en.wikipedia.org/wiki/Ion_thruster

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.