NASA is hard at work developing what they believe is the best space engine for future missions to Mars and beyond. It's not warp drive. No, nothing so exotic or dreamy. In fact, it already exists. The challenge is to enhance it for our needs in space in the coming decades. That is the hope, and the goal, of NASA's continuing development of solar electric propulsion (SEP).

Solar electric propulsion uses electricity generated from solar arrays to ionize atoms of the propellant xenon. These ions are then expelled by a strong electric field out the back of the spacecraft, producing thrust. So, in short, SEP is a propulsion system that is a combination, or coupling, of solar array technology and ion thruster technology.

The NASA Glenn Research Center in Cleveland, Ohio, has been a leader in both ends of this technology for decades. Its work with ion thruster technology began with the Space Electric Rocket Test 1 in 1964. Today, ion thrusters are used to keep over 100 geosynchronous Earth orbit satellites in their locations, a process called station keeping. The Deep Space 1 mission, which made flybys of asteroid Braille and the comet Borelly between 1998 and 2001, used the NASA Solar Technology Application Readiness (NSTAR) ion propulsion system.

"The greatest example of the technology we've developed here that is currently in space is the Dawn mission," said NASA Glenn senior propulsion engineer Dave Manzella told SpaceFlight Insider.

The Dawn spacecraft, launched in 2007 atop a United Launch Alliance (ULA) Delta II booster, used three NSTAR ion thrusters to propel it on its voyage to the asteroid belt where it has orbited the giant asteroid Vesta and, currently, the dwarf planet Ceres. Dawn was the first spacecraft to go into orbit around two extraterrestrial bodies – an impressive feat that, if it had used chemical rockets, would have required a much heavier spacecraft and a tremendous amount of fuel to achieve.

"The characteristic of SEP that makes it so attractive is basically its fuel economy," Manzella said. "We talk about a parameter called specific impulse. It is a measure of the average exit velocity of the gas that's coming out." SEP can provide specific impulses that are an order of magnitude higher than can be achieved with chemical systems. That means SEP can get ten times the fuel economy, which means it can use an order of magnitude less propellant to do the same mission. Why is that important? "Because with all our missions, much of what we launch is just propellant to go other places," Manzella said. "If you can impact the amount of that mass, you can really impact the economics of doing things in space."

So why isn't SEP being used everywhere?...