This article originally appeared in the July 3, 2017 issue of SpaceNews magazine.

For the last few years, NASA has promoted solar electric propulsion (SEP) as a key aspect of its long-term plans to send humans to Mars. While SEP systems can’t generate much thrust, they can run for long periods and are far more efficient than chemical systems. That makes SEP useful for sending cargo, rather than crew, where travel time is less of an issue.

“SEP systems are equivalent to the cargo ship for deep space missions,” said Joe Cassidy of Aerojet Rocketdyne at a June 29 House space subcommittee hearing focused on in-space propulsion technologies.

Electric propulsion is not new: it has been used on commercial communications satellites for years for stationkeeping and, more recently, orbit raising. Those thrusters, though, have been far less powerful that the SEP systems needed for interplanetary missions. NASA has been working with Aerojet and other companies on advanced, higher thrust SEP technologies.

NASA’s Asteroid Redirect Mission (ARM) was going to demonstrate one such advanced SEP system, with several 12.5-kilowatt thrusters. ARM’s robotic spacecraft would use SEP to rendezvous with a near Earth asteroid and then, after grabbing a boulder off the asteroid’s surface, return to the vicinity of the moon.

But ARM is no more. To the surprise of no one, NASA’s 2018 budget request released in May included no funding for ARM. The agency is already moving ahead with plans to end the mission.

“We are in an orderly closeout phase,” Michele Gates, program director for ARM at NASA Headquarters, said a June 13 meeting of the agency’s Small Bodies Assessment Group at the Goddard Space Flight Center.

Gates said the ARM team was “transitioning activities as appropriate to other potential missions” and archiving the rest for future use. “Many of the aspects that we were working towards in ARM will all be part of continued work in human exploration,” she said.

That includes the work being done on SEP. Even as NASA confirmed ARM would be canceled, agency leaders emphasized that solar electric propulsion still had a future within the agency.

“While we ended the Asteroid Redirect Mission going forward, we will take those technologies, with solar electric propulsion as probably the best example, and move forward with them and morph those into a different mission,” Acting Administrator Robert Lightfoot said in a May 23 speech at NASA Headquarters to discuss the budget request.

Uncertain futures

But what will that different mission be? So far, there are only ideas, and not firm plans, for flying a large SEP system on another mission.

One potential use would be on NASA’s Deep Space Gateway, the outpost in cislunar space NASA has proposed developing in the 2020s as a testbed for future Mars missions. Concepts for the Deep Space Gateway NASA discussed earlier this year include, as one component, a “power/propulsion module” that would employ SEP.

Lightfoot told House appropriators June 8 that this module would take advantage of the SEP system that was planned for ARM. “It would build right off of the bus that we had for the Asteroid Redirect Mission,” he said, adding it might be smaller than the one envisioned for ARM to make it “more commercially viable for other uses.”

The Deep Space Gateway, in NASA’s planning, would be followed by the Deep Space Transport, a vehicle that would demonstrate the technologies needed for a Mars-bound spacecraft. It would use an even larger SEP system, perhaps approaching the 300-kilowatt system NASA expects is needed for Mars missions.

However, the gateway and the transport are not yet official NASA programs. While NASA is studying the concept of a cislunar habitat that could use SEP, it was not formally included in the agency’s fiscal year 2018 budget request.

Andrew Hunter, NASA’s acting chief financial officer, told reporters in a May 23 briefing about the budget proposal that NASA was still working to win over the White House and Congress about the concept. “I think it has some introduction activity to go through with the administration,” he said.

Even before NASA started talking about the Deep Space Gateway, others were looking at alternative uses of SEP. A study by NASA’s Mars Exploration Program Analysis Group, presented at a March 2016 meeting, suggested a high-powered SEP system could be used for a future Mars orbiter mission.

The study looked at various options for that orbiter, some using SEP systems like those proposed for ARM. In its most powerful version, SEP could allow a Mars orbiter to not only adjust its orbit around the planet as needed to support science, but also play a role in sample return, capturing samples launched into orbit and returning them to Earth.

While that study looked at using SEP for a Mars orbiter launching as soon as 2022, there is no funding in NASA’s budget for any kind of Mars orbiter, regardless of propulsion, in its 2018 budget request. “We don’t have known plans at this point beyond Mars 2020,” Hunter said. “That’s still TBD in this budget.”

On to Mars

A lack of certainty about how NASA will use SEP has not stopped work on the technology itself. NASA awarded a contract to Aerojet Rocketdyne in April 2016 to develop a 13-kilowatt thruster. That thruster will be completed in 2019 for an unspecified “flight demonstration” — originally, ARM — in 2021 or 2022.

Aerojet and two other companies, Ad Astra Rocket Company and MSNW, also received awards from NASA’s Next Space Technologies for Exploration Partnerships (NextSTEP) program in 2015 to work on technologies for advanced, higher-power solar electric propulsion designs that could be used on Mars missions.

At the House hearing about the in-space propulsion, representatives of all three NextSTEP companies said they were making good progress on their technologies, and emphasized the importance of SEP for Mars exploration. Members at the hearing seemed fascinated with those efforts, and largely unconcerned that, for now, it’s not certain how those technologies will be tested.

An exception was Rep. Don Beyer (D-Va.) “Was it a mistake to cancel ARM and to defund it?” he asked.

Anthony Pancotti, director of propulsion at MSNW, didn’t think so. “I like to use the term, ‘keep your eye on the prize,’ and that prize is Mars,” he said. “The most direct approach to do that is the best path forward.”

That direct approach may end up being the most efficient one, like SEP itself. But also like SEP, it might take a while.