Compared to chemical propulsion (the type of propulsion that rockets use to escape Earth’s gravity well and reach orbit), SEP has lower thrust but is more fuel-efficient and can provide thrust for longer periods of time. For these reasons, SEP works well in the vacuum of space, particularly on spacecraft with long mission lifetimes.
SEP engines provide thrust by converting solar energy into electricity and using that electricity to accelerate ionized propellant at extremely high speeds. The iconic blue glow from a SEP thruster is created from photons released by the ions as they lose energy upon leaving the engine.
NASA has been working on SEP technology since the 1950s and they’ve used SEP on prior missions like the Dawn spacecraft, which is currently in orbit around the dwarf planet Ceres and is the first spacecraft to orbit around two extraterrestrial bodies.
Under the new contract, NASA hopes to double the thrust capability compared to current electric propulsion systems and increase the fuel efficiency by 10 times the current chemical propulsion.
One challenge with deep-space missions that use SEP is that as you travel deeper into the solar system (farther away from the sun), it becomes more difficult to effectively capture light from the sun to power the spacecraft. Because of this, NASA stated its current SEP research is funded in parallel with work to advance solar array technology.
During the 36-month contract, Aerojet Rocketdyne is responsible for constructing, testing and delivering an SEP product for testing and evaluation. Eventually, the goal is to have Aerojet Rocketdyne deliver four electric propulsion units that will fly in space.
“Through this contract, NASA will be developing advanced electric propulsion elements for initial spaceflight applications, which will pave the way for an advanced solar electric propulsion demonstration mission by the end of the decade.” Steve Jurczyk, associate administrator of NASA’s Space Technology Mission Directorate
In addition to this particular electric propulsion contract, Aerojet Rocketdyne is responsible for the chemical propulsion — the RS-25 engines — for NASA’s Space Launch System, the rocket designed to be used on missions related to NASA’s Journey to Mars initiative.
Aerojet Rocketdyne’s current contract is part of NASA’s overall push to advance SEP systems. NASA plans to test the largest and most advanced SEP system ever used in space on their Asteroid Redirect Mission, which is designed to capture an asteroid and place it in orbit around the moon. That mission is currently slated for the mid-2020s.