Future interstellar travelers who hope to reach distant star systems will likely need better propulsion technologies than the chemical rockets humans currently rely on for our exploration of space.

Among the range of proposed alternatives, one potential candidate involves what are called solar light sails, a theoretical propulsion system that relies on pressure generated from solar radiation against a large surface to move a spacecraft.

Such concepts have been proposed since the 1980s, some of which combine the idea of using sunlight as a power source with the implementation of lasers, which could propel such futuristic starships to speeds our best current technologies cannot attain.

However, not all spacecraft propulsion experts are necessarily sold on the idea of solar light sails just yet. According to new findings detailed in a paper by researchers Jiaze Li and Chao Shen of the Harbin Institute of Technology in China, this novel form of propulsion may have a few hurdles to cross before it can attain the near-light-speeds many experts had hoped for.

A Tale of Three Forces

According to Shen and Li’s recent research, a key problem arises once a solar light sail spacecraft begins nearing the speed of light: drag forces resulting from the very source of energy used to propel the devices could place significant limitations on their maximum attainable speeds.

A combination of different forces can be expected to occur as sunlight strikes a light sail, the authors say, which includes incident light, specular reflection, and diffuse scattering.

Incident light describes the physical momentum generated by light energy in the form of photons as they strike the sail, whereas specular reflection describes the momentum produced as photons reflect off the sail. Finally, diffuse scattering is a separate form of momentum generated by photons, which, rather than bouncing away upon impact, is actually absorbed by the sail, the energy from which is thereafter released in random directions.

New Limitations on Solar Light Sails

Based on these primary forces, the authors identify that once a light sail begins reaching relativistic speeds, it also becomes subjected to intense Doppler effect—the phenomenon that occurs as changes in the frequency of waves result as their source and an object or observer’s distance changes in relation to it.

In the case of solar light sails, the more the frequency of light waves striking the sail decreases, the less the amount of thrust generated by the three forces the paper identifies. In simpler terms, the faster a solar light sail spacecraft attempts to travel, the harder it becomes for it to accelerate.

Relativistic Light Aberration

At approximately 75% of the speed of light, the researchers say that an additional hindrance emerges, as what physicists call relativistic light aberration takes over. This phenomenon, first described by Einstein more than a century ago, involves relativistic corrections of light sources for observers who move at velocities approaching light speed.

In the case of our prospective future solar light sailors, this would mean that the weakest of the three forces impacting the sail (diffuse scattering) would begin to impose an active drag once the sail reaches approximately three-quarters of light speed. While the overall force of lasers helping to propel the light sail would remain positive at this stage in the system’s acceleration, the authors note an inevitable decline in efficiency.

The authors do concede that while their paper examines the radiative dynamics of solar light sail propulsion, it does not account for other factors that may include drag resulting from encounters with materials like gases, dust, or other physical sources (i.e., non-radiative sources), which could also impact overall speeds.

Additional factors worthy of consideration, but which the authors do not directly address in their paper, include the melting point of any materials used for solar light sail propulsion, which could be heated to extreme temperatures due to combined exposure to solar radiation, laser energy, and friction from movement.

Fundamentally, the team reports that their paper “provides a rigorous and in-depth theoretical foundation for lightsail dynamics,” which illustrates that while this future propulsion concept has its merits, we may still be far from seeing its realization for practical space travel.

By outlining the factors that contribute to constraints on light sail propulsion, Li and Shen aim to improve our overall understanding of this prospective future technology, thereby helping engineers to realize its potential for use in space exploration at some point in the future.

The team’s paper, “Relativistic Lightsail Propulsion Dynamics,” appeared on the preprint arXiv.org server.

Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.

Massachusetts Institute of Technology (MIT) engineers are testing a new category of advanced-propulsion “electrospray” thrusters that combine the benefits of modern chemical and electrical thrusters used to drive satellites and other spacecraft into one hybrid system.

The team behind the novel propulsion technology said that electrospray thrusters could allow mission planners to design smaller, more nimble satellites and spacecraft capable of performing faster, more powerful maneuvers or slower, more precise adjustments, depending on mission requirements.

Electrospray Combines the Best of Chemical and Electric Propulsion

Launching rockets into space still requires chemical thrusters, but once spacecraft and satellites reach orbit, engineers continue to develop novel propulsion methods that are more efficient than chemical propulsion technologies. Given the strict weight requirements of spacecraft launches, finding alternatives to chemical propellants has opened up a wider range of mission profiles.

Among the most common alternatives to chemical thrusters are designs that combine electrical energy with small amounts of propellant to produce small but sustainable thrust. Although electric thruster designs require less fuel, saving significantly on launch weight, they also lack both the power and the rapid, quick burst performance of chemical thrusters.

According to study author Amelia Bruno, a former postdoc in MIT’s Department of Aeronautics and Astronautics (AeroAstro), combining chemical and electrical propulsion into a single system is “the best of both worlds.”

“This opens the door for small satellites to do even more science, more observations, and more interesting missions, all on a smaller and cheaper platform,” Bruno explained to MIT News.

Study co-author Paulo Lozano, the Miguel Alemán Velasco Professor of Aeronautics and Astronautics at MIT, agreed, noting that combining both technologies could enable a CubeSat mission to Mars “where they could make the journey slowly, using electrospray thrusters.”

“You could then use your chemical thrusters to quickly move to look at interesting features,” the professor added. “You could have a lot more flexibility to do a lot more things.”

ASCENT Rocket Fuel as the Thruster’s Ionic Liquid Propellant Enables Hybrid Design

According to a statement detailing the new electrospray thruster’s design, the ones developed by the MIT team are about the size of a thumbnail. Each thruster is also placed directly above a small fuel reservoir. For the MT electrospray thrusters, the fuel is an ionic liquid propellant, a liquid composed of salts that remain dissolved in a solution.

“Ionic liquids are very stable and can even remain a liquid in space, which not a lot of materials can do,” Bruno explained. “And it’s basically a sea of ions, which is why we base our technology around it, so we can pull those ions out into an electrospray.”

When a spacecraft’s battery delivers an electric charge to the fuel reservoir, the ions within the fuel also become charged. The thruster’s design channels the charged particles from the reservoir and through the thruster tips, expelling them into space.

Although the team has tested several types of ionic propellant in its electrospray propulsion thrusters, Bruno and Lozano have collaborated with the U.S. Air Force to synthesize an all-new ionic liquid propellant, which they are calling the Advanced SpaceCraft Energetic Non-Toxic propellant, or ASCENT.

Originally designed as a “green” alternative to extremely hazardous hydrazine rocket fuel that has been the staple in the rocket industry for decades, Bruno notes that ASCENT “happens to be an ionic liquid mixture.”

“And we said, hey, that’s the stuff we typically use,” the researcher added. “Theoretically, this should work. Let’s go figure out how.” 

Tests: Chemical Propellant Equally Efficient as Conventional Ionic Liquids

To test their electrospray design, each tested thruster was attached to a small reservoir about the size of a single Lego brick, and then the reservoir was filled with ASCENT. Next, electrospray thrusters were mounted on opposite sides of a traditional CubeSat.

To simulate the microgravity environment of Near-Earth Orbit, the team placed the thruster-equipped CubeSat on a MagLev stand. The team noted that the MagLev in Lozano’s lab is installed inside a large vacuum chamber. This design allows the researchers to fine-tune the environment to best mimic the conditions in space.

During a series of tests, the team applied varying voltage levels to the electrospray thrusters. As expected, the reaction produced a spray from the thruster nozzles, which spun the CubeSat around on the MagLev stand like a freely spinning top.

During each thruster firing, the team measured the amount of thrust produced. This data, combined with the voltage and fuel data, allowed the team to calculate ASCENT’s fuel efficiency during continuous operation. Some tests lasted up to 100 hours.

When the MIT group evaluated the results, they first determined that the choice of ASCENT was correct, as it successfully fueled each electrospray thruster. Even more significantly, the tests revealed that the readily available chemical propellant was equally efficient as conventional ionic liquids.

“Compared to our normal electrospray propellants, ASCENT can provide similar performance in terms of thrust,” Bruno explained. “Now that we know our thrusters work with ASCENT, we can start thinking of all the ways we can make them even better.”

Testing Electrospray Thrusters on NASA’s Green Propulsion Dual Mode Mission

When discussing the benefits of their successful electrospray thruster tests, Lozano noted that a spacecraft with electric and chemical thrusters can share a single fuel tank. Due to its compact design, this hybrid electrospray thruster could be mounted on a small satellite.

Fortunately, the team won’t have to wait long to see their thruster in action. According to the research team, whose work was partially supported by NASA, their design will be incorporated into NASA’s upcoming Green Propulsion Dual Mode mission scheduled to launch in November. The Green Propulsion Dual-Mode mission will carry a briefcase-sized CubeSat with a primary chemical thruster and specially designed electrospray thrusters, making it the first attempt to combine electrical and chemical propulsion in a small spacecraft.

“This will be the first time that a satellite will have a shared propellant tank,” Lozano explained.

Although a large portion of proposed electric propulsion missions that might employ the new electrospray thruster are designed to explore deeper into the Solar System and beyond, Lozano also highlighted the technology’s possible applications closer to home. For example, satellites equipped with both technologies could rapidly respond to environmental or climate changes in real time with a speed and efficiency unavailable with current propulsion options.

“Say there’s a storm coming, and you’d want to deploy your constellation of small satellites to observe over one location,” the researcher explained. “You could choose to send them quickly or slowly, depending on the nature of the observation.”

“And the only way to do that is if you have two propulsion systems, which is now possible,” he added.

Although the mission will not launch before November, Bruno noted that a successful test could allow engineers to consider missions they never thought possible.

“Now that we know our thrusters work with ASCENT, we can start thinking of all the ways we can make them even better,” the researcher explained.

The study “Performance Characterization of Electrospray Thrusters with Energetic Ionic Liquid Monopropellant” was published in the Journal of Propulsion and Power.

Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.