You can read the whole article, for free, here.In March of 2008, I sat down in the carriage house with Friedman and two other members of his solar-sailing team: Harris “Bud” Schurmeier, the retired project manager on the old Voyager missions; and Viktor Kerzhanovich, whose long career in both Russia and America has earned him the U.S.S.R. State Prize and more than one NASA Group Achievement Award. If the Planetary Society tends to exhort its more than 50,000 members in sonorous terms, conversation in the carriage house was speculative and playful. Throughout the morning, the years fell away from the three old-timers eager to tell a visitor about how solar sailing works—and to spar a bit.
“Light has energy,” said Friedman. “That you can’t argue with.”
“More important,” said Kerzhanovich, “it has momentum.”
“Therefore it has a force,” added Friedman. “You’re using the energy of light, and the force derived thereof, to transfer momentum of light energy to your vehicle, in order to propel the spacecraft. Basically your spacecraft, your solar sail, looks like a sail, but it really is a mirror. And so it’s reflecting the light, and that reflection is where the momentum transfer occurs.” If the mirror were fixed to a wall, there would be no transfer. But in free space, with no gravity and no air pressure? You’re off to the cosmic races.
“It’s not the solar wind,” Friedman reminded me.
“Things got named wrong,” said Schurmeier. However pretty it sounds, “sailing” is really a metaphor. There is such a thing as solar wind, but as Friedman explained, “Solar wind is electrons and protons that come from the sun, and they have mass, but they go very much slower than light.”
It’s photons, not protons, that we’re talking about?
“Right,” said Friedman. “Photons have no mass, they’re all energy. You do get a force from the solar wind, but it’s about a thousand times less than the force you get from this reflection. You turn your mirror in different directions, you can point the force in any direction you want!”
In 1980, the landmark series Cosmos premiered on public television. Since then, it is estimated that more than a billion people around the planet have seen it. Cosmos chronicles the evolution of the planet and efforts to find our place in the universe. Each of the 13 episodes focuses on a specific aspect of the nature of life, consciousness, the universe and time. Topics include the origin of life on Earth (and perhaps elsewhere), the nature of consciousness, and the birth and death of stars. When it first aired, the series catapulted creator and host Carl Sagan to the status of pop culture icon and opened countless minds to the power of science and the possibility of life on other worlds.The version of the series used seems to be the same as the 2000 DVD version; it's especially nice to have Ann Druyan's introduction at the beginning of the first episode, as well as the 1990 updates at the end of episodes like The Edge of Forever. (I'm guessing that the DVD music changes are still in there.) And unfortunately, the website is restricted to viewers in the United States.
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