Electric propulsion is a technology that utilizes energy stored in batteries or a supercapacitor to accelerate spacecraft. It is a power source that has been used extensively on satellites, ranging from small rovers to larger, more sophisticated research missions. Since its first use, electric propulsion has become a mature technology. In fact, there are now over 500 spacecraft that use this form of propulsion. A number of deep space missions have relied on the technology. However, electric propulsion systems are limited by the electric power available on board the spacecraft and the power required for short-term load leveling.
Spacecraft that have been using this form of propulsion include NASA’s Psyche mission and Starlink, which runs at low-Earth orbit. The Psyche mission is a deep-space science probe that will be travelling through outer space for several years to examine the history of terrestrial planets. It also has a solar array onboard to supply electricity to its equipment.
Although electric propulsion is an excellent option for long-duration spacecraft missions, it is not appropriate for Earth-launched rockets. However, the technology has been demonstrated on spacecraft, and is increasingly being explored for Earth-launched satellites. Another advantage of this type of propulsion is that it is more fuel-efficient than conventional chemical or thermal systems.
Ion thrusters are one form of electric propulsion. These systems push ions through an electrostatic field. They can be either a hollow-cathode or a hall effect thruster. Hollow-cathode ion thrusters have a low impedance pathway for electrons to neutralize the positive ion beam. Hall thrusters, on the other hand, are designed to drive conducting structures to a high negative voltage. Both methods of ion thrusters have a relatively large surface area, but they are also susceptible to light pressure.
In contrast to conventional chemical and thermal systems, electric propulsion systems have a wide range of applications. Many outer solar system missions are now using electric propulsion, including NASA’s Gateway international space station and the OneWeb. For example, the Psyche mission is a three-to-five-year mission to investigate a unique metal-rich asteroid.
Electric propulsion is also being considered for in-space mobility systems. This technology has already been tested on a battery-powered vehicle in Germany and Hong Kong, and energy-saving rail vehicles have been developed in Paris and Geneva. Some of the advantages of this technology include reduced fuel consumption and emissions.
However, these systems are not suitable for Earth-launched rockets because they have very low thrust. If an Earth-launched spacecraft is going to be using electric propulsion, it is important to ensure that the system can be operated under favorable conditions, such as near the Moon. Even though the most advanced electric thrusters can deliver a delta-v of over 100 kilometers per second, this is not sufficient for interstellar travel. Therefore, there are still many proposed future missions that cannot be realized with thermodynamics-based thrust systems.
As more ambitious missions emerge, it may be necessary to expand the electrification of spacecraft. While electric propulsion has proved itself to be useful, it has limitations when it comes to fuel efficiency and propellant usage.