NASA announced this week that it’s moving forward with developing a solar-powered electric propulsion engine.

The agency’s space technology program selected two companies to design prototypes that will be tested for use. Deployable Space Systems and ATK Space Systems, both California companies, were the two finalists chosen from eight applicants.

“Both companies displayed an excellent understanding of the problem and provided technical solutions that were innovative and could be implemented in a time frame commensurate with NASA’s strategy for the development of Electric Propulsion technologies,” said Davis Steitz, NASA spokesman.

Up until now, the largest solar array in space has been about 30 kilowatts, Steitz said. NASA is looking for a system that will be about 300 kilowatts.

“Arrays this large require completely new structural components and methods to deploy those structures once in space,” he said.

The arrays will power electric propulsion engines that could replace chemical propulsion in space.

“Chemical propulsion is mass intensive,” Steitz said. “We have to bring our fuel with us and it is heavy. While we still need to do so for electric propulsion solutions, that amount of mass is greatly reduced. This in turn provides great savings in the cost of the launch and the mission.”

He said NASA is also considering the use of electric propulsion in a space tug that could move satellites to new orbits on command. That would allow for lighter satellites and long-term cost savings. It could also allow for the continued use of older satellites that have run out of fuel.

Having a ready and inexhaustible fuel source like the sun could also allow for manned space travel outside of the low Earth orbit, Steitz said.

The tools are also of interest to the emerging private space industry.

Solar cell efficiency is important in the array designs as greater efficiency means fewer solar cells and a light overall array. But Deployable Space Systems and ATK are not developing cells, they’re configuring full solar systems that will function for NASA.

Once the prototypes are complete, they’ll be tested in a thermal vacuum chamber on Earth along with several other tests to ensure the system will survive the stress of launch and the space environment.