Xenon: Propellant of the Cosmos
Sam Cassidy, Mechatronics Engineer
5 minute read
It would be awfully convenient if satellites could pull over at a 7-Eleven to fill the tank and get some coffee. Unfortunately, space gas stations haven’t been solved yet (though companies like OrbitFab are working on it) or space coffee to ease the burden of endlessly driving over 15,000mph. For a spacecraft to be effective in its mission, it must secure propellant for extended operations. Satellites require minimal propellant, mostly for collision avoidance and stationkeeping, as they are locked in a predictable orbit. For example, the James Webb Space Telescope (JWST) was placed into orbit at Earth’s Lagrange point L2. In order to continue operating and conducting its mission, the JWST requires occasional maneuvers to adjust for drift and perturbations to its orbit, or possibly even to avoid colliding with oncoming objects. Each of these maneuvers expends propellant, which is a finite resource aboard a satellite. Once this propellant runs out, that object can no longer maneuver itself, meaning it will either continue to drift further and further away from its intended location or may even be destroyed if another object comes too close and collides with it. JWST has a planned mission life of 5 to 10 years, limited by the amount of propellant depleted. At a cost of 10 billion USD, extending the life of this mission would be of great interest to the longevity and utilization of NASA’s budget. If satellites like this can be refueled in orbit, that can extend the useful mission life by enabling it to continue stationkeeping for more years to come.
In the quest for a more sustainable propulsion system, there’s the option of ion thrusters, or more generically, electric propulsion (EP). Ion thrusters are not just for sci-fi novels. While its name sounds futuristic, this technology is quite the opposite. EP was brought to fruition in NASA’s Glenn Research center by Harold R Kaufman in 1959. Mr. Kaufman was expanding on the original idea written by soviet scientist Konstantin Tsiolkovsky in 1929. While Tsiolkovsky did not theorize this concept for space travel, Kaufman was able to adapt the technology to the near perfect vacuum of space. Considering that Kaufman’s work pre-dated Armstrong’s historic moonwalk by a decade , this technology is anything but new. The beginning of space travel did not consist of long term missions, making combustible materials perfectly reasonable. As humanity has evolved in our space interests, such as taking humans to Mars in the next decade, the need for long-term propellant continues to grow. The first NASA satellite to feature EP was Space Electric Rocket Test 1 (SERT-1). Launched in 1964, this mission ran for over 24 years and showed the world the value in low force, long duration propulsion.
The future of EP uses xenon as its consumable propellant. Xenon is an element found in our atmosphere at a rate of 0.0000087%. If you got lost in the zeros, that’s nearly 87 parts per BILLION. To capture this sparse element, xenon is refined as a by-product of the liquid oxygen industry. In general, 1,000 tons of liquid oxygen yields only 1.2kg of xenon. KMI alone is expecting to use more than fifteen times this amount per year. To utilize xenon, this now-captured element is condensed into a liquid form and stowed away on a spacecraft for slow use. EP works on the principle of supercharged atoms violently approaching each other and, with the help of electricity and magnets, being propelled from the tail end of the thruster. Seeing that the propulsion is on an atomic scale, the force output is extremely low, but consistent. The benefit to working on this same scale is the reduction of consumable material needed for propulsion. Currently, modern EP produces an output of 0.5 Newtons, a force equivalent to holding 10 nickels in your hand. While the force is extremely weak at the moment, it can be sustained and compounded over days, weeks, or years of operation. One of the advantages of space is the elimination of Earth-like restrictions such as friction and air resistance. This allows a seemingly weak force to propel something to incredible speeds. It’s estimated that the maximum speed of this technology is 200,000mph, meaning it is well equipped for satellite and small spacecraft usage. Currently, over 100 geosynchronous satellites are held in orbit with the assistance of EP. This effective and efficient propulsion technology has allowed satellites to stay in space longer than previously attempted.
The future of xenon is quite bright (chemistry pun intended as xenon was used in the early days of flash photography). While it is difficult to obtain, there is a significant supply in our atmosphere. Provided there’s solutions to the engineering/extraction problem, there are multiple markets that depend on increased supply. The computer chip manufacturing and medical industries are both large consumers of xenon that are steadily increasing their demand, driven by the CHIPS Act. As society advances, computer chips become more plentiful in our everyday life, meaning the demand for xenon will continue to grow as a result. The medical industry has recognized xenon as a more efficient anesthesia than our current compounds due to the lessened side effects and increased effectiveness. Including aerospace, all the major sectors that depend on this chemical are projected to rise in their demand and, at this time, supply is lacking. Market research shows xenon gas use conservatively growing nearly 39% between 2020 and 2027. This is provided that there are no other market breakthroughs driving further demand in that timeframe.
KMI’s mission is unconventional in the sense that we are an orbital operational craft that not only depends on the orbit around earth, but also uses propulsion to move around in that same orbit. Orbital debris travels in a variety of paths all over the Earth’s orbit. During Active Debris Removal we will need to accommodate for variable flight paths, meaning long sustained propulsion is preferred. We will use this electric propulsion for slow adjustments to our flight, allowing KMI to rendezvous with an object requiring our service with our goal to match the speed of orbital debris over the course of the days leading up to contact. Additionally, we will be operating a relatively small spacecraft, limiting the available storage for large amounts of combustible fuels. Intending to pack 18kg of xenon compressed tightly to a small, manageable size will continue the optimization of maneuvering and mass. This will allow the KMI spacecraft, Laelaps, to remain a compact craft and support efficient movements.
KMI, along with the space community as a whole, will continue to rely heavily on xenon in the coming future. As space travel progresses, and xenon proves to be a rare commodity with invaluable uses, it is the best long-term, low-thrust propulsion that cutting-edge technology has to offer. Xenon’s availability and run duration with EP is unmatched by any neighboring technology. It is commonly said that you can't teach an old dog new tricks, and it appears the same goes for space travel!
Recommended column to read next: Solar Panels and Nuclear Spacecraft