“Sunjammer”, a story by the noted science fiction author Arthur C. Clarke, appeared in the March 1964 issue of Boys’ Life. I’m not sure how soon after that I read the story, but I’m certain it was before I left my teens.
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The story depicts a race between spacecraft powered by solar sails, using only the pressure applied by sunlight. If all goes well, a large solar sail will soon unfold itself in Earth orbit and test out this old-but-new means of propulsion.
Although analogies with wind powered sailing on Earth are quite useful, let’s be clear that we are NOT talking about the “solar wind.” That term refers to a stream of charged particles flowing out from the sun. Those particles possess mass. Instead we are talking about using only photons, those “particles” of light that possess no mass, but counter-intuitively DO possess momentum. When they reflect off an object, they can transfer that momentum. And that can, with enough photons, a large enough collection area, and a small enough mass, create a change in both speed and direction.
As with so many concepts in space travel, it was the Russian rocketry pioneer Konstantin Tsiolkovsky who first proposed using the pressure of sunlight to propel spacecraft through space and suggested, “using tremendous mirrors of very thin sheets to utilize the pressure of sunlight to attain cosmic velocities”.
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Let’s look at the factors that have to be considered for this to work.
Area: The sail has to have a lot of surface area to collect a lot of photons.
Mass: It has to be really, really light. The lighter it is, the greater the velocity change a given number of photons will be able to create. It follows that whatever payload is attached to the sail must also be quite low in mass.
Reflectivity: A photon that passes through a transparent sail is wasted.
The planned launch on (as of this writing) June 24, 2019 of LightSail-2 employs a 32 square meter (344 square feet) sail of aluminized mylar plastic that is thinner than a human hair.
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Just as with a sailboat, you can tack, sail before the “wind” or against it, and with skillful deployment of your sail, change both your direction and your speed. If you only wish to move away from the sun, the only limitation to your acceleration—the rate of change of your velocity—would be the decreasing number of photons hitting the sail as you move farther away.
LightSail-2 is a project of The Planetary Society, an organization co-founded by Carl Sagan, Bruce Murray, and Louis Friedman, of which I am a proud member. Barring any delays, I plan to be as close as safety allows to watch the world’s currently most powerful rocket, a SpaceX Falcon 9 Heavy, light up the Space Coast skies as it lifts off from Complex 39-A, the departure point for the Apollo 11 mission 50 years before. Wish me and LightSail-2 luck!
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