How new rocket fuel could be made out of thin air

By Warren Miller, contributing writer

Image source: Pixabay.

Satellites propelled by ion thrusters are nothing new, but this is — the European Space Agency, in conjunction with Italian company SITAEL, has developed an electric thruster that uses surrounding air molecules as propellant, cracking the door open for a new generation of satellites that can function in low orbits for longer periods of time.

Until now, satellites equipped with ion thrusters were limited by the amount of onboard propellant that they could store internally. The ESA’s GOCE gravity-mapper satellite circled the planet for four years at altitudes as low as 250 km, but when the 40 kg of xenon that it began its mission with was exhausted, its sojourn came to an end. By ionizing air molecules instead of gases like xenon, satellites would never run out of propellant, provided that they were orbiting at an altitude low enough for air to still be present in the atmosphere.

The new thruster works by ingesting air molecules and compressing them until they transform into thermalized ion plasma. An electric charge is applied to accelerate the molecules and they are ejected from the thruster, creating propulsion. By removing the need for stores of onboard propellant, outfitted satellites could become much lighter. The air intake system also significantly reduces air drag, another benefit.

“This project began with a novel design to scoop up air molecules as a propellant from the top of Earth’s atmosphere at around 200-km (124-mi) altitude with a typical speed of 7.8 km/s (4.8 mi/s),” said Louis Walpot, an ESA researcher on the project.

One interesting potential use for this technology is to create a class of skimming satellites that periodically drop down into the atmosphere, fuel up by using the ion thruster, and then climb back up into a higher orbit. Teams of coordinated satellites could use this technique to make sure that sufficient satellites are in the right orbit and the right position to accomplish the mission. Redundant satellites could also be staged in a waiting pattern to fill any gaps caused by malfunctions or aging. A network of small self-powered satellites might create a mesh, similar to those used for the Internet of Things back on Earth.

Satellites operating in Earth’s orbit may not be the only beneficiaries of this new technology. Researchers believe that the thruster could be tweaked to function in a variety of atmospheres, meaning that they could theoretically be incorporated into fully automated spacecraft designed to survey other planets. Mars’s atmosphere, for instance, could provide enough fuel to power an unmanned satellite mission for years.

The team built a complete thruster to validate the concept and measure the thrust, testing it at SITAEL in a vacuum chamber simulating the environment at 200-km altitude. The system was able to ignite repeatedly using ordinary atmosphere as its sole propellant. According to Walpot, this result means that air-breathing electric propulsion is no longer simply a theory but a tangible, working concept, ready to be developed to serve one day as the basis of a new class of missions.