Working in the background, a relatively small group of U.S. spaceflight engineers has been figuring out just what it will take to get humans out of low Earth orbit to Mars, with stops along the way in cislunar space and perhaps on near-Earth asteroids. Spearheaded by William Gerstenmaier, associate NASA administrator for human exploration and operations, and managed in part by John Shannon, the last space shuttle program manager, the team is developing mission architectures that will guide the elected and appointed politicians who must fund and manage mankind's next steps into the Solar System. With budgets tight for the foreseeable future, a lot of attention is going into affordable technology.

One promising technology finding a big role in future exploration architectures is solar-electric propulsion (SEP). Work is underway at advancing the readiness levels of technologies that covert energy from the Sun into propulsion by using solar-generated electricity to force ions out of an engine at high speeds to produce thrust. It isn't a lot of thrust, but it can go a long way on relatively little fuel. A SEP system weighs a lot less than chemical propulsion. Given enough time it can move a lot of mass through space. That in-space advantage makes it particularly attractive for pre-positioning cargo—supplies, habitats and the like—to keep human explorers alive after they arrive on a faster vehicle to explore a distant location.

“Folks in the human exploration mission directorate have identified all of this need,” says Amy Lo, lead system engineer on a SEP project Northrop Grumman has run for NASA's Glenn Research Center. “We would like to send humans to asteroids. In order to do that we need a big tug. Others have identified the fact that in order to, for example, send humans to Mars, we need a lot of cargo in space, so how do we send those things out with existing launch vehicles?”

Lo's project—one of several funded by Glenn—proposes using a “high-heritage” lightweight deployable concentrator to focus solar energy on a receiver containing a working fluid that, when heated, would drive a Brayton-cycle engine to generate electricity. The electricity would power a Hall thruster (photo) to move the spacecraft. Northrop Grumman and its partners—Sandia National Laboratory and the University of Michigan—believe the approach would support a 300-kw SEP system, and are refining a concept to demonstrate the technology by using it to move a spacecraft from low Earth orbit to geostationary orbit.

Potential applications include deep-space exploration beyond the range of solar-array technology, says Lo. If the mission goes deep enough, the Brayton-cycle generator could be driven with heat from a radioisotope thermoelectric generator instead of the Sun.

Among the trades Northrop Grumman is evaluating as it looks for a flight opportunity is finding the best working fluid. The concept for Glenn would use helium xenon, although some ground-based solar-power plants using concentrated sunlight have used supercritical carbon dioxide.

Aerojet is also working SEP concepts that draw on its experience with solar-array powered Hall thrusters on the U.S. Air Force Advanced Extremely High Frequency (AEHF) spacecraft.

“The Hall Thruster technology to us is the one, because it's here today and has some benefits you could realize today,” says Julie Van Kleek, Aerojet's vice president for space and launch systems.

The company has technology in the 20-40-kw range “that's pretty mature,” Van Kleek says. That is much higher than the station-keeping SEP technology used in part to rescue the AEHF SV-1 spacecraft stranded in the wrong orbit (AW&ST Sept. 6, 2010, p. 34).

Despite the technology's maturity and its advantages in a variety of spaceflight scenarios, Van Kleek says, spacecraft engineers have been slow to adopt SEP technology for applications other than stationkeeping in GEO.

“People don't fully understand the value, and some people get scared, because it's so low-thrust, of the time aspect,” she says. “It certainly isn't something that's going to get you there in 30 minutes. [For] a lot of these cargo-type missions, it would take a year. A lot of missions can afford that, but you have to think differently, because everybody thinks chemical. The amount of weight savings you get is worthwhile. Weight is everything when you're talking about cost.”

Other companies working SEP concepts with NASA/Glenn include Ball Aerospace, Boeing, Lockheed Martin and Analytical Mechanics of Hampton, Va. Overall, NASA is spending approximately $3 million on initial work like the $600,000 job Northrop Grumman did. In addition to that solar-dynamic approach, the agency's Office of Chief Technologist is working on solar-array configuration tradeoffs and other SEP issues that could one day enable the kinds of human-exploration missions the agency wants to keep doing (AW&ST May 28, p. 20).