The European Space Agency has hailed the successful test of an air-breathing engine that works in space.
The engines don’t need the oxygen found in air to burn. Instead, as the ESA has explained here, the idea is to collect air, compress it, give it a charge and then squirt it out to provide thrust.
The engine has no moving parts and all that’s needed to power the engine is electricity. Spacecraft can generally harvest that from the Sun.
The concept’s been used before by the ESA’s GOCE gravity-mapping mission, but it carried 40kg of Xenon gas to provide it with thrust so it could change altitude when its orbit became low. And once it ran out of propellant … you can guess the rest.
Hence the interest in an engine that can harvest air to keep a satellite aloft and in very low orbits. Anything in such an orbit that wants to stay there will need a periodic boost, as the drag caused by the outer reaches of the atmosphere slow spacecraft and degrade their orbits.