A breakthrough in air-breathing electric propulsion (ABEP) research led by Francesco Romano at the University of Stuttgart demonstrates a plasma thruster capable of utilizing residual atmospheric gases as fuel. Designed for spacecraft operating in Very Low Earth Orbit between 100 and 450 kilometers, the system uses an innovative specular intake and a cathodeless radio-frequency helicon design to counteract orbital drag without carrying xenon or conventional onboard propellants.
The Promise and Penalty of Very Low Earth Orbit
Choosing an orbit for a satellite has always been a game of orbital compromise. Very Low Earth Orbit (VLEO), stretching roughly 100 to 450 kilometers above the planet’s surface, offers undeniable engineering perks. Remote sensing cameras capture sharper images, while communications and radar systems require less power to transmit signals through a thinner column of air. The natural atmospheric drag found at these altitudes provides a built-in disposal mechanism, passively pulling inactive satellites down to burn up safely upon mission completion.
That exact same atmospheric density introduces a brutal operational penalty. Air resistance constantly bleeds kinetic energy from the spacecraft, forcing satellites to expend continuous thrust simply to maintain their altitude. Traditional propulsion setups rely heavily on costly stored gases like xenon, capping mission lifespans the moment the propellant tanks run dry. Enter the concept of atmosphere-breathing electric propulsion. Instead of launching with finite propellant reserves, an ABEP system scoops up the extremely thin ambient air directly in front of the moving spacecraft, ionizes those captured molecules into plasma, and expels them out of the rear exhaust nozzle to generate continuous thrust.
Confronting the Corrosive Upper Atmosphere
Turning this theoretical physics concept into functional flight hardware requires solving harsh engineering hurdles. The upper atmosphere subjects exposed spacecraft surfaces to atomic oxygen (AO), formed when ultraviolet radiation splits standard diatomic oxygen molecules apart. Atomic oxygen is aggressively corrosive. It chews through metal electrodes, acceleration grids, and standard cathodes used in conventional ion engines.
The cathode dilemma is particularly lethal to standard ion propulsion. Ion engines rely on an electron gun featuring a cathode to neutralize the spacecraft. Without this neutralization step, the entire chassis builds up an electrical charge, pulling the freshly expelled ions right back onto the hull and completely nullifying the forward thrust. Atomic oxygen attacks and destroys these critical cathodes rapidly.
VLEO presents an additional environmental variable: extreme atmospheric volatility. Air density shifts dynamically based on solar activity, latitude, and day-night thermal cycles. Designing an electric engine stable enough to operate continuously across these fluctuating conditions has historically stalled deployment.
Innovative Intake Architecture in the Lab
To bypass these physical roadblocks, Francesco Romano engineered a contactless, neutralizer-less radio-frequency (RF) helicon plasma thruster paired with an optimized atmospheric collection system during his PhD research published on arXiv. His work forms part of a broader European push, mirroring developments in air-breathing electric propulsion systems backed by the European Space Agency and managed by groups like TransMIT GmbH and IQM.
Capturing particles spread so sparsely across the upper atmosphere demanded specialized intake architecture. Romano evaluated three distinct geometric designs in his research. First, an enhanced funnel design acted as a passive molecular trap. Second, a diffuse intake utilized a compact hexagonal layout built from a coated titanium alloy. Third, a specular intake relied on a parabolic mirror coated with graphite or silicon dioxide to bounce air particles directly into the engine throat.
The specular intake proved to be the standout performer. During wind tunnel testing simulating atomic oxygen, argon, and nitrogen, the parabolic mirror configuration successfully collected roughly 94.3 percent of incoming air particles. Crucially, its collection efficiency degraded by only eight percent when subjected to a 15-degree angle tilt.
Adapting Medical Tech for Space Propulsion
Solving the thruster side of the equation required looking outside traditional aerospace engineering. Romano drew inspiration from medical diagnostic equipment, specifically adapting a birdcage antenna typically found inside MRI machines. This configuration delivered 99 percent of the supplied electrical power directly into the thruster core, eliminating the power losses associated with standard wire coils that burn off energy through their own internal reactance.
The system pairs this antenna with a solenoid wrapped tightly around the engine body. The resulting magnetic field forces the plasma out the rear in a quasi-neutral jet, ejecting positive and negative ions simultaneously. Because both charge states leave the exhaust together, the spacecraft remains electrically neutral, completely eliminating the need for a vulnerable cathode neutralizer.
Infinite Propellant for Future Missions
Laboratory validation inside vacuum chambers simulating VLEO gas concentrations demonstrated steady plasma generation using just 50 to 60 watts of radio-frequency power. That power draw fits comfortably within the energy generation capacity of standard spacecraft solar panels.
By transforming residual atmospheric drag from an orbital hazard into an infinite propellant source, this propulsion architecture points toward a future where remote sensing and communications satellites can operate indefinitely in low orbits without carrying dead weight at launch.
