A team of Penn State researchers are developing solutions to some of the biggest problems currently facing satellite technology with the help of a Defense Advanced Research Projects Agency grant. Credit: Kate Myers/Penn State.
Q&A: The new, shorter space race
Apr 14, 2025
By Ty Tkacik
UNIVERSITY PARK, Pa. — Another race to space is on, but the competitors aren’t reaching for the moon. Instead, multiple national agencies and private companies across the world are aiming for the edge of Earth’s atmosphere. Launching satellites into this very low Earth orbit (VLEO) environment — the altitude between 60 and 280 miles above Earth — could solve spacecraft crowding in traditional orbits, according to Sven Bilén, Penn State professor of engineering design, of electrical engineering and of aerospace engineering.
Bilén said satellites in traditional orbits face three challenges: crowding, lower resolution imaging and longer distances for data transmission to and from Earth. Orbiting closer to the ground helps satellites capture higher resolution images and send transmissions with shorter delays, improving both surveillance and communications applications, according to Bilén. The problem, though, is keeping satellites in VLEO.
Bilén is the principal investigator on a $1 million grant through the Charge Harmony research program, an initiative facilitated by the Defense Sciences Office of the Defense Advanced Research Projects Agency (DARPA). His team, in collaboration with researchers at the Georgia Institute of Technology, is in their second year of developing a novel thruster system to help VLEO objects stay in orbit.
Bilén shared details of the research, which could address many VLEO challenges and help facilitate widespread adoption of the technology, in a Q&A below.
Q: The idea of VLEO technology has been around for decades — why is it suddenly making headlines now?
Bilén: Spacecraft crowding is quickly becoming a problem in low Earth orbit, with thousands of communications satellites like Starlink and OneWeb already in orbit and many thousands more to come. The growing density of these satellites increases the risk of collisions between satellites or orbital debris already in these orbits, which is why we are seeing this push for VLEO technology now, rather than 10 or 20 years ago.
Additionally, due to their increased surveillance and communication capabilities, low-orbiting satellite platforms have the potential to become a lucrative business for the companies that can manufacture them, which is why we are seeing increased interest from the private sector. Advancements in electric propulsion over the last several decades have made the prospect of orbiting in VLEO technologically feasible, which also plays a role in the current interest in this technology.
Q: What are the biggest problems facing VLEO technology today?
Bilén: The biggest challenge of orbiting in VLEO is staying in VLEO. At these altitudes, satellites orbit within the outer edge of Earth’s atmosphere, causing them to experience aerodynamic drag. This means any spacecraft or satellite at that altitude deorbits very quickly and requires a constant push from a thruster to stay in orbit. If you did this using traditional propulsion methods, you would quickly run out of fuel. However, if you capture the very thin air, known as “rarefied air” around the satellite, you can repurpose that as fuel. These thrusters are called air-breathing electric propulsion systems.
Another major challenge is how power-starved these low-orbiting satellites are. The propulsion systems alone require a lot of power, which would normally be provided entirely by solar panels, but the lower the satellite’s orbit, the more the Earth blocks sunlight from getting to the satellite during a part of each orbit. Our research addresses both of these challenges.
Q: Your team is currently researching a thruster technology that could revolutionize thruster systems. Could you explain what that entails?
Bilén: My team — which includes Ethan Kravet, a doctoral candidate in aerospace engineering; John Auerbach, a graduate research assistant in aerospace engineering; Mitchell Walker, professor and head of the aerospace engineering department at Georgia Tech; Den Lev, aerospace engineering research engineer at Georgia Tech; and Julian Lopez-Uricoechea, a graduate student in aerospace engineering at Georgia Tech — is developing a self-neutralized air-breathing plasma thruster, a novel electric propulsion technology. This type of propulsion technology uses the surrounding air as a propellant; the air is collected, superheated with microwave energy and then expelled from a nozzle to generate thrust. Other VLEO thruster technologies incorporate an external device to neutralize their thrust-producing charged gas after it is expelled from the thruster, whereas our mechanism is inherently self-neutralizing.
The most common type of electric propulsion device is the Hall-effect thruster, which normally relies on a complex series of electromagnetic interactions inside the propulsion system to produce thrust. In addition, the electron-emitting cathodes they use can erode in oxygen-rich environments like VLEO. Our thruster gets around these challenges by not requiring a cathode and by employing a thermal heating process to generate thrust.
From left to right, John Auerbach, Sven Bilén and Ethan Kravet are the Penn State researchers collaborating across universities to innovate VLEO satellite thrusters. Credit: Poornima Tomy/Penn State.
Q: What has your team achieved so far? What’s next?
Bilén: The first year saw the development and testing of an entirely new type of thruster that utilizes thermal plasma generated by high-power microwaves to operate. We call the system the “air-breathing microwave plasma thruster” (AMPT) and held the first round of testing in vacuum chambers at Penn State and Georgia Tech. We collected thrust measurements and plasma diagnostics on this proof-of-concept laboratory prototype. According to our tests, the thruster can produce more thrust per kilowatt than typical electric propulsion thrusters. In some cases, the thrust-to-power ratio is hundreds of times higher than typical electric propulsion thrusters.
Looking forward, at the request of DARPA, we are scaling the thruster system down in size so that it can fit on a smaller satellite platform. This scaled-down version of the AMPT is a refined design that could eventually be integrated with a VLEO satellite. Additionally, we are developing the concept of a satellite platform to support our thruster and explore practical mission applications. This proposed satellite will orbit lower than any satellite has before.
Q: What do you believe is the most exciting thing about this technology?
Bilén: The most exciting thing is the prospect of this technology being both the highest-flying air-breathing object and the lowest orbiting satellite. This technology could allow us to explore orbiting in a region of space where no satellite has before.
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