Assuming the tech comes together as intended, it has delicious possibilities for unmanned exploration as well, as Nathan Strange from NASA’s Jet Propulsion Laboratory explained in another presentation at the Ames strategy meeting. He calculates that a solar-electric engine mounted on an SLS rocket could ferry 28,000 pounds of cargo (equivalent to the Cassini probe plus eight Curiosity rovers) to Saturn in five years. Ah, the possibilities — and the caveats. The SLS does not exist yet. The SEP is still under development and may never happen. And this new, complex hybrid approach would likely not be cheap.
Even if you don’t have faith, though, there is good news. A more modest version of the solar-electric propulsion technology is already available and hard at work, propelling the Dawn spacecraft currently circling the dwarf planet Ceres in the asteroid belt. Dawn’s xenon engine doesn’t produce a lot of thrust, but it can keep firing for months or even years, building speed and changing course. That engine is what allowed the spacecraft to orbit the asteroid Vesta, break free, then orbit Ceres. There’s no reason why we couldn’t repeat the experiment with a whole fleet of similar probes, sending each one to multiple destinations using simple and cheap xenon engines.
Or we could go further and do away with fuel entirely. By the time you read this, the nonprofit Planetary Society should have launched a test version of a solar sail, a giant space kite that flies by harnessing the pressure of sunlight. NASA and the Japanese space agencies have flown basic prototypes as well. Even more so than a xenon engine, a space sail produces only a feeble push. But it is fundamentally inexpensive, inexhaustible and endlessly scalable. The bigger you build the sail, the more thrust you get. Tack into the photon wind, and you can steer wherever you want.
Speed Up by Slowing Down
If you are reading closely, you may have noticed that I’ve pulled a major switcheroo on you. I started out talking about going faster and ended up describing ways to go even slower. Xenon engines and solar sails are tortoises in the space race, and the low-energy paths that put them to best use can take many years to complete. But paradoxical as it sounds, slowing down might be the most effective way to speed up space exploration.
The long wait between launch and arrival is a big problem, yes, but the chasm of time between mission starts is an even bigger one. Right now there is a frustrating lack of any steady, consistent cadence to our exploration of the outer solar system. “Cadence” is a word I heard repeatedly at Ames, often expressed in a pleading tone. Without it, graduate students move to other fields, engineers head elsewhere in search of work, infrastructure crumbles, public interest drifts and funding becomes even harder to sustain.