A long-time goal of space enthusiasts is about to reach fruition--the first solar sail is about to take flight. The really neat thing about it, to me, is that it's privately sponsored. I remember discussing this at dinner in 1982 with Rob Staehle, the JPL engineer who was planning the project that long ago as an extra-curricular activity (more recently, he was the pre-project manager for the Pluto Express mission and is now the Deputy Project Manager of the Europa mission), and it's great to see it finally happening.

To the degree that many people are aware of the concept of solar sails, they mistakenly believe, taking the nautical analogy, that they are blown by the solar wind. But solar sails, or light sails (the more generic term, because they could be powered with lasers as well as the sun) actually get their thrust from radiation pressure. The solar wind is composed of heavy, highly-energetic particles that would blow right through a sail, destroying rather than propelling it. The sail is instead impinged by photons, the components of light.

The article linked above says that the sail absorbs them, and gains their momentum, but if this occurs, it's actually less efficient. Ideally, the photons actually reflect off the sail, imparting twice the momentum that they would if they were absorbed. Thus, a well-designed sail has a mirrored surface, or at least a surface that acts as a mirror for the frequencies of light for which it's designed. Also, since the lighter the vehicle, the greater the acceleration for a given force, it's made as thin as possible while still maintaining structural integrity. Finally, since force is pressure times area, the bigger the sail, the more thrust can be attained.

Because the solar radiation pressure is so small, even for a large sail, the total force might only amount to a few pounds. But if that's the only force acting (other than gravity), it can still add up, and with continuous acceleration, get you to an outer planet faster than chemical propulsion.

One question often asked is, if the radiation pressure always acts outward from the sun, how a sailing spacecraft can come back into the solar system. Answer: like conventional sailing ships, it tacks (though the analogy is imperfect--being in a vacuum, unlike the water for a ship, there is no medium in which it travels, and it thus has no use for a keel).

Imagine that the sail is at an angle with respect to the sun. Some of the thrust is directed radially along its orbit. Add to orbital velocity, and the energy increases, and the sail heads out to the outer system. Change the angle to subtract from it, and the sail will slow, and fall back in toward the sun. Angle it out of the orbital plane, and you can slowly perform a plane change.

If we really did want to drop nuclear waste into the sun, a sail is probably the only affordable way to do it, with the additional advantage that as the star is approached, the thrust increases as the square of the distance (twice as close means four times the thrust). Unfortunately, because they're such delicate things, the sail might burn up before it had decreased its velocity sufficiently to drop all the way. So a final booster rocket might still be needed.

Here's an extremely little-known fact. Solar sails played a significant role in the conceptualization and development of nanotechnology. Back in the 1970s, a young student at MIT, enamored with space, was trying to figure out how to develop the minimum thickness for a sail. He came up with a concept for laying out an ultra-thin layer of aluminum on a wax, using a technique called vacuum-vapor deposition, in which the metal would be heated to a vapor, and sprayed on a substrate in a vacuum chamber. Afterwards, the wax would be melted away, leaving the thin aluminum foil. He reasoned that he could get a sail that was only a few atoms thick--strong and reflective enough to be a good sail (as long as it was handled properly) while providing maximum performance.

One thing led to another, and he eventually came up with other techniques for building things at atomic-level scale, and gave some serious thought to the implications of such manufacturing. He wrote a book on the subject in the mid-1980s, and eventually, in 1991, received the first doctorate in the field, having played a major role in inventing it, from MIT. His name, of course, was K. Eric Drexler.

It's cool to hear that solar sails are nearly up and running -- yet more proof that the world of tomorrow is going to be run by the sci-fi geeks of yesterday (myself included).

I'm a bit skeptical about "tacking" with a solar sail, though, given my experience with aquatic sailing. When you tack on a sailboat, as I understand it (and I certainly haven't studied sailboat theory or design), you are leveraging the wind, which blows against you, into the keel of the boat, which presses against the water. (Remember, to tack, you never sail directly upwind, always obliquely.) Your keel can't displace the water backwards, so instead it slides forward and cuts through, driving the boat upwind. Not very efficient in water; I can't see how it would work in a vacuum.

The tacking analogy isn't precise. All I'm really describing is tilting the sail to provide a non-radial thrust component. If it adds to the velocity, the craft moves away from the sun. If it subtracts, it moves toward. If it's out of plane, you can actually do plane changes with it. No medium or keel is required in this case.

Ah, finally, a description of solar sails that makes sense to me! Like Mr. Murtagh, I could never reconcile the concept of tacking with the way solar sails were explained to me. That's because every description I ever heard referred to the solar wind - and if so, then tacking would not be possible without something to resist lateral movement, such as a keel or centerboard. Sailboat sails are airfoils and work via Bernoulli's principle (except when going directly downwind). Now I see that solar sails are not, and work entirely differently.

By the way, has anyone managed to see "Attack of the Clones" yet? There's a solar sail powered spacecraft which looks an awful lot like the one in Tron.

Also by the way, I'm sure plenty or folks recall Mote in God's Eye, where the Moties first came out of their system in a laser pushed sailcraft.