A new design for a plasma engine fuels on only thin air

Choosing an orbit for a satellite always comes with trade-offs. Very low Earth orbit (VLEO), between 100–450 km (62–280 miles), has distinct advantages. Remote-sensing cameras can take better pictures, communications and radar require less power, and atmospheric drag automatically cleans up dead satellites.

But there is also a cost—air friction requires satellites in this orbit to use an engine nearly constantly to stay aloft, which in turn requires fuel—typically expensive gases like xenon. So, as part of his Ph.D. thesis at the University of Stuttgart, available on arXiv, Francesco Romano decided to solve that problem by using the air molecules that cause the friction as fuel for a plasma engine to keep satellites aloft indefinitely in VLEO.

His solution falls into a category of atmosphere-breathing electric propulsion (ABEP) systems. These scoop up thin air in front of a spacecraft (or, in some cases, a missile) and channel it into an electric engine, which then turns the molecules into plasma and shoots it out the back, producing thrust. It’s easy enough to explain in theory, but in practice, there are difficult technical problems to work around.

The first is atomic oxygen (AO). In the upper atmosphere, UV radiation splits O₂ into this aggressive, single-atom form of the gas that we all need to breathe. AO is notoriously oxidative, corroding metal electrodes, acceleration grids and even the cathodes used in standard Hall thrusters or other types of ion engines.

Perhaps most importantly, AO burns through the cathodes used in the “electron gun” that neutralizes the spacecraft so that the whole thing doesn’t become charged and simply suck the charged particles right back to itself, nullifying the thrust they provide. Without that feature, the whole ion-propulsion system fails.

Another difficult feature when designing engines for use in VLEO is the variability of the atmosphere itself. It changes based on the day-night cycle, latitude and even solar activity. Making sure an engine can continually operate in all these different conditions has proven difficult so far.

To solve these problems, Romano developed a contactless, neutralizer-less radio-frequency (RF) helicon plasma thruster and paired it with an optimized atmospheric intake system. Let’s tackle the intake system first.

He tested three different versions of an intake. One, called an “enhanced funnel design,” acted as a molecular trap to capture air particles that are spread so far apart that they never run into each other. Next, he used a “diffuse intake” with a compact hexagonal design made of a coated titanium alloy. Finally, he designed what he called a specular intake, a parabolic mirror coated with graphite or silicon dioxide that bounced particles directly into the engine.

The clear winner, both in terms of collection efficiency and alignment sensitivity, was the specular intake. It collected about 94.3% of air particles (AO, argon or nitrogen in a wind tunnel test), and the efficiency dropped only 8% when subjected to a 15° tilt.

To design the thruster, Romano turned to a medical device for inspiration. Using a birdcage antenna, similar to those used in MRIs, he designed a thruster that ensured 99% of the delivered electrical power entered the thruster. This extremely high efficiency improved on standard wire coils that would lose some power because of their own reactance.

A solenoid wrapped around the engine creates a magnetic field that pushes the plasma out the back in a quasi-neutral jet—both positive and negative ions are pushed out of the thruster, ensuring no neutralizer is needed.

Testing the system proved its reliability. Romano used a vacuum chamber to intentionally simulate a VLEO atmospheric concentration of the three primary gases the thruster would encounter at that altitude. The engine generated steady streams of plasma with only 50–60 W of RF power, well within the capabilities of traditional spacecraft solar panels.

After that experimental validation, he took an additional step and applied models of the propulsion system to real-world use cases. This included the GOCE satellite, which famously launched into VLEO with a xenon ion thruster and eventually ran out of fuel.

According to the thesis’s calculations, the new engine could operate indefinitely between 190–250 km (118–155 miles) using less than 1.6 kW of power, which is still well within the generation limits of standard spacecraft solar panels. But the use cases aren’t limited to Earth. Mars has an atmosphere dominated by CO₂, and, according to the thesis, the engine could support a spacecraft indefinitely above the Red Planet at an altitude of 120–160 km (75–99 miles), much closer than existing orbital satellites.

Ultimately, there is no guarantee this thruster will ever see use outside a lab. But the idea is intriguing, and there are plenty of potential commercial applications if it can be de-risked and proven to work on an actual mission. It’s unclear whether Romano has plans to pursue that track, but his work so far shows that the design has potential—maybe someone out there is willing to pursue it.

Who’s behind this story?

Swati Mestri

Swati Mestri holds a bachelor’s degree in Electronics Engineering and has worked as a content editor since 2019. She has experience editing research documents across technology, health care, and materials science, and has a particular interest in technology and space.

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Andrew Zinin

Master’s in physics with research experience. Long-time science news enthusiast. Plays key role in Science X’s editorial success.

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