An atmosphere-breathing electric propulsion system developed in PhD research at the University of Stuttgart uses thin upper-atmospheric gases as fuel for a radio-frequency helicon plasma thruster, potentially letting satellites stay in Very Low Earth Orbit indefinitely without carrying a conventional supply of propellant.
VLEO Satellite Benefits and Atmospheric Drag Challenges
Operating a satellite in Very Low Earth Orbit offers distinct technical advantages for communications and Earth observation. Spanning roughly 100 to 450 kilometers above Earth, VLEO allows remote sensing cameras to capture sharper images while communications and radar systems need less power. Atmospheric drag also helps remove inactive satellites from orbit naturally.
That same air resistance creates a major challenge. At these altitudes, air resistance slows spacecraft down, meaning they must produce thrust almost continuously to remain in orbit. Conventional propulsion systems require onboard fuel—often costly gases such as xenon—which ultimately limit a mission’s lifespan once tanks run dry.
RF Helicon Plasma Thruster Design
To bypass the limits of stored propellant, Francesco Romano explored an atmosphere-breathing electric propulsion system during his PhD research published on arXiv. The concept captures incoming air molecules in VLEO, converts them into plasma via an electric engine, and expels the ionized gas out the back to generate thrust. Turning this theoretical approach into a practical propulsion system required overcoming two major environmental obstacles: highly corrosive atomic oxygen and wide atmospheric variability.
In the upper atmosphere, ultraviolet radiation splits O2 into aggressive single atoms of atomic oxygen. This reactive element corrodes metal electrodes, acceleration grids, and cathodes used in standard Hall thrusters or other types of ion engines. The atmosphere changes based on the day-night cycle, latitude, and solar activity.
Birdcage Antenna and Neutralizer-Less Operation
To solve these problems, Romano paired an optimized atmospheric intake system with a contactless, neutralizer-less radio-frequency helicon plasma thruster. Inspired by a medical device, the design incorporates a birdcage antenna similar to those used in MRIs. This setup ensures 99% of the delivered electrical power actually entered the thruster, improving upon standard wire coils that would burn through some of the power because of their own reactance.
Did you know? By wrapping a solenoid around the engine, the system creates a magnetic field that pushes the plasma out the back in a quasi-neutral jet. Because both positive and negative ions are pushed out of the thruster, no neutralizer is needed.
During experimental validation inside a vacuum chamber simulating a VLEO atmospheric concentration of the three primary gases the thruster would encounter, the engine produced steady streams of plasma using 50 to 60 watts of RF power. This energy draw is within the capabilities of traditional spacecraft solar panels.
Real-World Orbital Modeling and Future Applications
Following laboratory testing, Romano applied his propulsion models to actual real-world use cases. His calculations indicate that the engine could operate indefinitely between 190 and 250 kilometers of altitude using less than 1.6 kilowatts of power. A notable comparison is the GOCE satellite, which launched into VLEO with a xenon ion thruster and eventually ran out of fuel.
The architecture is not restricted to Earth missions. According to the thesis, the system could support a spacecraft indefinitely above Mars between 120 and 160 kilometers, using the Red Planet's CO2-dominated atmosphere to maintain orbits closer than existing orbital satellites.
Engineering Comparison: While standard Hall thrusters or other ion engines use cathodes and require onboard fuel, this RF helicon concept gathers its fuel directly from the surrounding air and ensures no neutralizer is needed.
Practical deployment outside of a lab remains unproven, and commercial applications depend on the technology being de-risked and proven to work on an actual mission.
Frequently Asked Questions
What is an atmosphere-breathing electric propulsion system?
It is a system that collects the thin air in front of a spacecraft (or, in some cases, a missile) and directs it into an electric engine, which converts the molecules into plasma and expels it from the rear to generate thrust.
How does the birdcage antenna improve thruster efficiency? Borrowed from MRI medical devices?
Adapted from MRI machines, the birdcage antenna ensures that 99% of delivered electrical power enters the thruster, improving upon standard wire coils that lose power because of their own reactance.
What causes atomic oxygen corrosion in low Earth orbit?
Ultraviolet radiation in the upper atmosphere splits O2 into aggressive, single atomic forms of the gas that corrode metal electrodes, acceleration grids, and cathodes.
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