In Andy Weir’s sci-fi thriller Project Hail Mary, an extraterrestrial star-eating microbe called Astrophage is harnessed to propel spacecraft on interstellar missions to save civilizations from the chilling effect of Astrophage’s diet. It’s a fun read that brings into focus the real-life challenge of how to fuel interstellar travel.
Progress on that front may come from the chemical element iodine. A telecommunications satellite with an iodine-based electric propulsion system has been successfully tested in space, according to a paper in the November 17 issue of the science journal Nature.
Iodine differs from the noble gases xenon and krypton, the current propellants of choice for spaceflight, because iodine’s solid form transforms directly into a gas when heated, a process known as sublimination. This means iodine propellant can be placed directly into a spacecraft’s thruster, removing the need for bulky high-pressure storage tanks.
Propulsion system miniaturization could unlock new capabilities for communications satellites and space exploration, according to the paper.
“We believe that our first in-space demonstration of the iodine electric propulsion system will not only bring more attention and funding to this topic, but also encourage researchers,” Dmytro Rafalskyi, the paper lead author and chief technology officer and co-founder of the French startup ThrustMe, told me in an email exchange.
At a high level, electric propulsion uses electric power to generate thrust from a propellant. The electric power is often harvested by solar panels attached to a spacecraft and is used to ionize the propellant. The charged particles – ions – are further accelerated within electromagnetic fields to form beams of ions.
“As the ions go one way, the spacecraft goes the other,” Dan Goebel, an expert in solar electric propulsion at NASA’s Jet Propulsion Laboratory at the California Institute of Technology, explained in a blog post as part of outreach for the space agency’s Psyche mission to study a metal rich asteroid that may hold clues to Earth’s origins.
Once the Psyche spacecraft is launched beyond Earth’s gravity in August 2022, a xenon-based solar electric propulsion system will fuel the spacecraft’s 1.5-billion mile, 3.5-year journey to the Psyche asteroid in the asteroid belt between Mars and Jupiter.
In addition to storage concerns, xenon is rare and relatively expensive to commercially produce, according to the team from ThrustMe, which spun-off from École Polytechnique and the French National Center for Scientific Research and is supported by the European Space Agency. Xenon is also used for competing applications in the medical, lighting and semiconductor industries, which has furthered the search for an alternative propellant.
“Iodine, having similar or better ionization performance to xenon and no pressurization requirements, coupled with very high storage density, opens new horizons to the use of space,” Rafalskyi said. Kyrpton-based thrusters are beginning to gain traction, he added, though that noble gas has similar storage and usability concerns.
The use of iodine thrusters is not without challenges, noted Igor Levchenko of Nanyang Technological University in Singapore and Kateryna Bazaka of The Australian National University in Canberra in an accompanying news perspective in Nature. For example, iodine is highly corrosive, which presents a potential danger to electronics and satellite subsystems. Iodine also requires about 10 minutes to be heated to sublimination temperature, which may be too long for collision avoidance maneuvers.
“These challenges need to be addressed before this technology can be incorporated safely into working satellites,” Levchenko and Bazaka write. “Nevertheless, now that it has been validated in space, the system developed by Rafalskyi and colleagues is an impressive contribution to the rapidly changing landscape of electric propulsion technologies.”
In the immediate term, the ThrustMe team is focused on the market of next generation communications satellite constellations where iodine-based propellant systems could help satellites maintain their orbits and avoid collisions while in use and be used to decommission the satellites at the end of life to reduce the ongoing and growing challenge of space junk.
Further out, Rafalskyi added, iodine could be the propellant of choice for space missions to Mars and beyond the solar system.
“One can imagine compact refueling cargoes at certain critical points of the solar system. In addition to being compact, such cargo could keep the propellant stock over very long periods – maybe 10s or 100s of years – without any maintenance needed due to low pressure storage,” he said. “I would say that the important thing to solve here is the possibility of in-situ mining of iodine from asteroids or other space objects, which seems to be a very challenging task, though much easier than mining xenon or krypton.”