Although NASA this week decided not to contribute funds to the project, it has set aside money in its fiscal year 2000 budget for development of solar cells and related technologies. We want to develop propulsion systems to let us get out of the solar system. And when you look at the options that are available . . . Drawing of the Interstellar Probe, a sailcraft that would venture to the edge of the solar system. More than a century ago, Scottish physicist James Clerk Maxwell realized that light must exert a pressure.
Physicists now think of light in terms of photons, particles that have no mass but carry momentum.
Like ping-pong balls bouncing off a wall, photons striking a perfectly reflecting surface exactly reverse their direction of motion, imparting twice their momentum to the surface in the process. The nudge delivered by a single photon is minuscule, so a solar sail must be large in order to intercept as much light as possible. Indeed, at Earth s distance from the sun, the force applied by light on a square meter of sail is only 9 micronewtons, less than the weight of an ant. To maximize the acceleration from sunlight, the sail must be wrinklefree. The sail must also be lightweight, with a density less than that of this printed page.