Electric propulsion of spacecraft Free

Many electric propulsion systems intended for outer space have already been proved efficient and reliable.⁵ If implemented on future missions, those systems could extend the life of prominent, billion-dollar space programs. Examples abound of programs currently limited by chemical-propulsion capabilities. Launched in 2009 with only 12 kg of chemical fuel, the Kepler space telescope, illustrated in figure 1, had to stop its search for exoplanets after nine years in deep space because it no longer had hydrazine propellant. (The mission was renamed K2 in 2014 after NASA had to stabilize the spacecraft’s pointing.) Kepler’s thrusters used that fuel to correct its drift and maintain the telescope’s orientation toward a specific target and its data transmission to Earth.⁶ All other systems, including Kepler’s solar-cell-based power, still operated normally.

Like the Kepler mission, the Dawn mission was forced to end when it depleted the chemical propellant used to orient its solar panels toward the Sun and its communication instruments toward Earth.⁷ Dawn (illustrated in figure 2) was launched in 2007 and cost nearly a half billion US dollars. It was NASA’s first science mission to use solar-electric-powered ion thrusters (see box 1) and reach two of the largest objects in the asteroid belt—the dwarf planet Ceres and the protoplanet Vesta—to collect priceless data on the formation of our solar system.⁸ 

Dawn would have had to burn more than 6000 kg of on-board chemical propellant to reach and orbit those asteroids. Fortunately, it accomplished both by sending only 400 kg of xenon propellant through the electric thrusters. That design decision significantly reduced the size of the launch vehicle and the cost of the mission. Had the Kepler and Dawn missions been configured with an all-electric-propulsion system—either a modern ion or Hall thruster system, for instance—it’s likely that both would still be able to continue their cutting-edge research into the next decade.

Although 10 years may seem like a long time for a spacecraft to operate, their financial costs typically are on the scale of billions of dollars. And their development spans many years; in fact, it often takes decades to design and build them. One of the most famous space-based systems, the Hubble Space Telescope, has been subject to five manned servicing missions since its launch in 1990. The last of those missions extended its life past 2020. But sending a service mission to refuel the Kepler space telescope is technologically out of the question—at least for now. As it follows an Earth-trailing trajectory, Kepler will reach the other side of the Sun by 2035, putting it effectively out of reach. For missions like Kepler, using more efficient electric thrusters would have been the most logical way to extend their lifetimes.

Even for Hubble, its remaining hydrazine fuel is only sufficient to maintain target alignment for a few more years. Its hydrazine thrusters are also not powerful enough to raise Hubble from its orbit of 568 km, and the telescope could potentially spiral back to Earth by 2028. Importantly, Hubble uses reaction wheels and magnetic torquers—devices that interact with Earth’s magnetic field but do not require any fuel—for orientation control. Even so, the devices are efficient only at low orbits, where the magnetic field is relatively strong. An efficient electric propulsion system capable of raising the orbit could have kept it active for several more years, perhaps long enough for the next generation of manned space systems to become available for another service mission.

Beyond the low fuel efficiency and other challenges intrinsic to chemical-based space thrusters, many proposed future missions cannot be realized using traditional, thermodynamics-based thrust systems.⁹ Colonizing Mars and establishing other distant outposts require fast and reliable transportation of large manned and cargo spacecraft over millions of kilometers. As with Dawn, using electric propulsion for those missions can potentially reduce the amount of propellant that has to be launched into space by nearly an order of magnitude.

On the other end of the spectrum in mission size is an emerging industry of ultrasmall spacecraft known as CubeSats. A few kilograms in size, they fly in formation or as constellations that require equally small thruster systems. Those systems must not only be efficient but also deliver thrust with unprecedented precision. That’s also critical for space-based observatories sensitive enough to capture faint signals, such as ripples in spacetime, and provide data to help solve other curiosities of the universe, including how the quantum vacuum works and the mysteries of repulsive gravity and negative energy.

China’s Taiji mission and the European Space Agency’s LISA (Laser Interferometer Space Antenna) mission are examples of formation-flying, network-based space observatories. To a large extent, their success will depend on the ability of individual satellite probes to maintain their position and orientation with respect to the source of a target signal and to each other while millions of kilometers apart.¹⁰ 

Both missions’ satellites have drag-free control with ultrahigh precision and ultrastable operation that cannot be achieved by hydrazine thrusters. Other types of conventional thrusters, such as cold-gas systems, do not have the level of specific impulse and control needed to sustain for years the propulsion requirements of those systems. Not surprisingly, the recently launched Taiji-1 powered by ultrasmall, high-precision electric thrusters, and LISA Pathfinder, shown in figure 3, demonstrated precision attitude control using an electric propulsion colloid-thruster system.¹¹ LISA Pathfinder was launched in 2015 and operated until 2017.

The unique demands of those new ambitious missions—and a significant increase in the population of small satellites during the past decade (see figure 4)—may provide the impetus necessary to drive wider electrification across all sizes of space-thrust platforms.

The path to higher-efficiency, lower-cost missions is not without its challenges. Electric thrusters were first used roughly 60 years ago, but their use was limited until about 20 years ago by the lack of sufficient electrical power on spacecraft. Since that time, improvements in the efficiency and size of solar arrays have increased the power available on both communications satellites and deep-space spacecraft to more than 20 kW. NASA’s next solar-electric mission, the Gateway space station, will include the Power and Propulsion Element (PPE), which will likely have a 60 kW solar array to power the transport of cargo to the Moon and ultimately to Mars. Ion and Hall thrusters now routinely run at power levels of 1.5–5 kW and are in development to run at 12.5 kW for the PPE and up to 20 kW for prototypes at NASA’s Glenn Research Center, so power and thruster technology finally exists that can take advantage of electric power available in space.

Another issue is thruster lifetime and energy storage. The satellite communications industry has embraced electric thrusters for on-orbit station-keeping of geostationary communications satellites because they reduce the amount of propellant needed by a factor of 5 to 10 to provide the required 15-year satellite life. But the application can handle only about an hour of thrusting per day.

Providing 5 kW of power, modern lithium-ion batteries have plenty of capacity to run electric thrusters in most station-keeping applications. One major benefit of the batteries is that they allow the thrusters to make the orbital adjustments required to insert a satellite into its assigned orbit. That orbit-raising function requires operating a thruster for hundreds to thousands of hours at a time. No other battery technology is currently available—or even foreseen—that can provide such energy. Likewise, for most propulsion applications in deep space, the thrusters must run for weeks to years at a time, with lifetimes in the tens of kilohours.

Thus all-electric spacecraft will need either to generate electric power on-board or to receive it—in a form known as beamed energy—from a source that may be millions of kilometers away.¹² The development of compact, light, and efficient sources of electric energy on a spacecraft is far from trivial. Similarly, beaming energy across vast distances requires exceptional pointing precision to minimize losses and on-board infrastructure to convert an incoming beam of light into electrical energy.

Solar cells that use novel materials and advanced architectures currently represent the best and most widely used means of on-board energy generation. Even at their maximum output, however, conventional solar panels are unlikely to supply enough energy to meet all the energy needs of future satellites throughout the solar system. Simply increasing the size of the panels will increase a satellite’s mass, can render it more cumbersome to maneuver, and may restrict the field of view for on-board instrumentation.

Despite plans to construct large solar arrays capable of delivering hundreds of kilowatts of power near Earth for cargo and manned missions, exploring deep space and remote planets using solar panels is infeasible. The reduction in available light to power the spacecraft becomes simply too significant as it moves away from the Sun. (See examples of Sun-powered electric-propulsion-driven deep-space missions in box 2.) Thus missions to deep space and remote planets would require sources of electric power other than solar cells.

The most obvious alternative source of power for deep-space missions is nuclear systems. Yet scaling down fission reactors to a realistic size and mass while maintaining low specific power—the electrical output power divided by the mass of the power system—has been difficult and expensive to achieve. Flying nuclear reactors seem unlikely anytime soon unless substantial investments are made to mature the technology for space applications and change public perception about its safety. The NASA SP-100 (space-reactor prototype) program was supposed to design a fission reactor to become a standard deep-space power system, but it was canceled partly because of its high cost.

Other concepts based on thermonuclear fusion systems may provide solutions for high power in deep space and reduce the travel time of missions by nearly half. But developing them assumes that fusion technology can be perfected on the ground and transferred into space in a reasonable time and cost. One of the main motivations for research into nuclear power for space propulsion is the considerable energy-density gain that can be realized with nuclear fuel in place of conventional chemical combustion.¹³ 

Advanced propulsion systems using high-power solar arrays, beamed energy, or nuclear fusion could help to achieve NASA’s plans for a manned Mars mission. The reduced travel times would effectively cut the harmful effects of space travel, such as radiation and weightlessness, on the crew. What’s more, the ability of a nuclear engine to supply both power and propulsion makes it suitable for a broad range of space missions—robotic and manned.¹⁴ 

Apart from those energy issues, the high price of commonly used xenon propellant—currently more than $2000 per kg—is also a problem. Not surprisingly, SpaceX’s Starlink network uses krypton, which is not as efficient as xenon and requires much larger tanks to store the same mass, but it is much cheaper. Another approach is to use sublimating solid propellants such as iodine, which is predicted to make the whole system simpler and less expensive. Two of us (Levchenko and Bazaka) recently reported on the first demonstration in space of an iodine thruster designed by a team led by Trevor Lafleur and Dmytro Rafalskyi (see “For an efficient electric propulsion system, use iodine,” Physics Today online, 2 December 2021),¹⁵ and we expect further efforts to implement the solid propellant. The Massachusetts-based Busek company already sells iodine-propellant thrusters.

Another challenge for the wider adoption of electric propulsion platforms in deep-space missions is the obvious requirement for safety and reliability. That requirement sometimes warrants opting for simpler, well-proven systems at the expense of their efficiency and cost. Moreover, all deep-space missions go through years of design work. During that process, many trade-offs are made between various systems, including electric and chemical propulsion. To cite one example, for the New Horizons mission—the first probe to Pluto—scientists selected chemical systems for its onboard propulsion.

Electric propulsion thrusters offer several exciting opportunities that have not been sufficiently embraced by current R&D communities. More work is needed in the following areas:

• Higher thrust efficiency produced by higher-power, long-lived electric thrusters to support planned manned expeditions and cargo missions to Mars and possibly other celestial objects.¹⁶ That goal requires developing the next generation of high-power ion and Hall thrusters and alternative electric thruster technologies, such as magnetoplasmadynamic thrusters, to provide the desired combination of high power, high specific impulse, low mass, and small size.

• Ultrahigh precision in controlling the thrust with ultralow noise for highly sensitive space astronomical observatories and for fine satellite control within formation-flying constellations.

• Ultracompact electric thrusters for controlling small satellites—particularly those with masses less than 10 kg—to make them capable of active maneuvering, formation flying, and orbit change.

• Alternative power sources in space beyond solar power, such as nuclear or beamed-power systems coupled to high-power electric propulsion systems.

Igor Levchenko is a research scientist at the Plasma Sources and Applications Center at Nanyang Technological University in Singapore. Dan Goebel is a fellow and senior research scientist at NASA’s Jet Propulsion Laboratory at Caltech in Pasadena, California. Katia Bazaka is a professor in the College of Engineering and Computer Science at Australian National University in Canberra.