A Satellite Engine That Runs on the Air That Is Trying to Drag It Down. A Stuttgart Design Scoops Up 94% of the Molecules in Its Path and Turns Them Into Thrust on 50 Watts.

Satellites flying in very low Earth orbit, between roughly 100 and 450 kilometers up, get sharper pictures, need less power for radio and radar links and clean up after themselves when they die, because the thin air at that altitude eventually drags dead hardware back down. The catch is that the same drag pulls live satellites down too, so they have to fire an engine almost constantly, and the engine needs propellant, usually an expensive gas such as xenon. When the tank runs dry, the mission ends. Europe's GOCE gravity-mapping satellite, which flew at about 255 kilometers, met exactly that fate in 2013.

A doctoral thesis from the University of Stuttgart, posted to arXiv, describes an engine designed to make the problem feed itself. Francesco Romano's design belongs to a class called atmosphere-breathing electric propulsion, or ABEP: a scoop at the front of the spacecraft collects the sparse air molecules that cause the drag, an electric thruster turns them into plasma, and a magnetic field fires that plasma out the back. The fuel is the atmosphere itself, so in principle the tank never empties.

Two things have made that simple idea hard to build. The first is atomic oxygen. Ultraviolet sunlight in the upper atmosphere splits ordinary O2 into single oxygen atoms, which are ferociously corrosive. They eat metal electrodes, acceleration grids and, most fatally, the cathodes that conventional ion engines use to spray electrons into the exhaust. Without that neutralizing stream, the spacecraft charges up and pulls its own ions back, cancelling the thrust. The second problem is that the air at those altitudes changes with the day-night cycle, latitude and solar activity, so the engine has to work across a wide range of densities and compositions.

Romano attacked both. For the intake, he tested three geometries: a funnel that acts as a molecular trap for particles too sparse to collide with each other, a compact hexagonal "diffuse" intake made of coated titanium alloy, and a "specular" intake, essentially a parabolic mirror coated with graphite or silicon dioxide that bounces incoming particles straight into the thruster. In wind-tunnel tests with atomic oxygen, argon and nitrogen, the specular design won decisively, capturing about 94.3 percent of the incoming particles and losing only about 8 percent of that efficiency when tilted 15 degrees off the direction of flight.

For the thruster, he built a contactless radio-frequency helicon design with no electrodes in the plasma and no separate neutralizer at all. The antenna that drives it is a birdcage coil, the same geometry used inside MRI scanners, chosen because it delivers about 99 percent of the electrical power into the plasma instead of losing some to the coil's own reactance. A solenoid wrapped around the chamber shapes a magnetic field that pushes the plasma out as a quasi-neutral jet, positive and negative charges together, so nothing needs to be neutralized and there is no cathode for atomic oxygen to destroy.

In a vacuum chamber configured to mimic the gas mix at very-low-orbit altitudes, the thruster produced steady plasma on just 50 to 60 watts of RF power, well within what ordinary solar panels supply. Romano then ran the design through models of real missions. Using GOCE as a reference case, the calculations indicate the engine could hold a satellite between 190 and 250 kilometers indefinitely on less than 1.6 kilowatts. Because the physics does not care which gas it ingests, he also modeled Mars, whose atmosphere is mostly carbon dioxide, and found the engine could sustain an orbiter at 120 to 160 kilometers, far lower than any spacecraft currently circling the planet.

None of this has flown, and the thesis is careful to say so. A laboratory plasma source is a long way from a flight-qualified thruster that survives years of atomic-oxygen bombardment and a solar cycle's worth of atmospheric swings. But the combination of a high-capture intake and a thruster with nothing in it to corrode removes the two obstacles that have kept air-breathing propulsion on paper for two decades, and it opens a lane for permanent, ultra-low satellites that no one has been able to keep up there before.