Lou Friedman’s work on solar sails dates back to his days at the Jet Propulsion Laboratory where, in the 1970s, his team began work on a rendezvous mission with Halley’s Comet. It was a mission that never flew, but you can read about its planning stages in Friedman’s book Starsailing: Solar Sails and Interstellar Travel (Wiley, 1988). That title is, as far as I know, the first book-length study of this technology, though it has since been joined by Colin McInnes’ key text Solar Sailing: Technology, Dynamics and Mission Applications (Springer/Praxis, 1999).
Now executive director of The Planetary Society, Friedman’s interest in solar sails led to his work on the Society’s Cosmos I mission, unfortunately lost during the launch attempt in 2005. His interest in interstellar issues remains keen as well, as evidenced by an article he recently wrote for Professional Pilot magazine. “Making Light Work” runs through solar sail basics for an audience that may seem surprising, but I can tell you from my own flying days that as we used to wait in the pilot’s lounge for students to arrive, we would often kick around outlandish concepts like deep space missions (and there were always a few dog-eared copies of Professional Pilot scattered around the room, out of date and thoroughly read).
Friedman speculates about sails kilometers wide in the area of 0.1 microns in thickness, ultralight films that would, when the photons from sunlight lost their punch, take advantage of huge laser installations that could be focused for interstellar distances. Now we’re into Robert Forward territory and also in range of feasible interstellar missions. For as Friedman notes, solar sails are the only technology we currently have that could complete such missions in a single human lifetime:
What is exciting is that we know the way forward. We don’t have to invent some new physics (like matter/antimatter engines) and we don’t have to conjure up new technologies from science fiction (such as interstellar ramjets scooping up and using interstellar hydrogen molecules). Rather, it’s all a matter of engineering—make the light sail materials thinner, the spacecraft lighter and the lasers more powerful.
Of course, the demands are still huge, power on the order of 100 gigawatts, which means power stations located in space, assembled in the inner solar system where solar radiation is much higher than here on Earth (presumably sails would be involved in ferrying the needed materials). And then there’s the problem of sail construction, conceivably handled by making the sail out of plastics whose evaporation would leave only the needed molecules to reflect sunlight and laser photons. Imagine a square kilometer sail weighing just a few kilograms, its electronics sprayed onto the sail rather than flying as a separate payload.
Solar sail technology is no idle dream. After extensive study at Marshall Space Flight Center, NASA’s basic sail design has reached the point where space testing is the logical next step even as research continues in European venues like Germany’s DLR. When we begin a serious push into solar sail technologies, we’ll need to test these designs in near-Earth orbit, and then move out into the Solar System. A logical mission for early sails will be, as Friedman notes, a replacement for the Advanced Composition Explorer (ACE), a mission nearing the end of its lifetime.
ACE operates at a libration point where the gravitational forces of Sun and Earth balance, some 1.5 million kilometers from Earth. A sail mission that could monitor solar weather (and warn us of solar storms) could offer a new kind of station-keeping, one that uses the momentum imparted by photons to stay in position closer to the Sun without the need of remaining at the libration point. Such a position would, among other things, allow greater early warning of potential ionospheric disruptions.
The range of sail missions available in coming decades will be huge, but if we keep at it, we may get to the point where building the kind of laser we’ll need for an interstellar mission becomes possible. Solar sailing is the kind of next-step technology that moves us from one-shot mission spectaculars like Apollo into the realm of a stable and long-term human presence expanding into the Solar System. For the short term, we need to keep doing what Friedman and sail advocate Gregory Matloff are doing, explaining and arguing for the needed steps to get sails into nearby space where their value for more complex missions will be obvious.