In July, the Planetary Society’s Lightsail 2 spacecraft demonstrated the viability of “solar sailing,” becoming “the first spacecraft in Earth orbit propelled solely by sunlight.”
But the practicality of solar sailing was first described six decades earlier by physicist Richard L. Garwin.
“It is difficult to exaggerate the importance of solar radiation pressure for the propulsion of satellites or space ships within the solar system,” he wrote in the Journal of the American Rocket Society in March 1958, when he was 30 years old. “Although the acceleration is numerically small, the velocity changes in reasonable times by significant amounts.”
This week, Garwin reflected on this and other episodes in his lifetime of problem solving and technical innovation. He spoke to post-doctoral researchers from the Harvard Physics Department. See Serendipities from Long Ago by Richard L. Garwin, keynote address, September 11, 2019.
How did he come up with solar sailing?
“As physicists do, I had been thinking about how things worked or could work and learned about radiation pressure, as did everybody in high school,” he said.
Not everyone grasped the concept immediately, Garwin noted.
“I recall that when the Chief Scientist of the U.S. Air Force was asked about this proposal at a press conference, he explained that even if it would work, it could only be used for going outward beyond Earth orbit around the Sun and not for going inward, because radiation pressure was radially outward from the Sun.”
“What he missed, of course, was that the fact that the sail was in Earth orbit or, for that matter solar orbit, meant that a reflective sail could be angled so as to provide a force perpendicular to the sail, that would have a component either along the velocity vector or in the opposite direction, so that the orbital velocity component could be increased or reduced; thus, the SS could either gain or lose energy and so spiral in or out from the Sun, or in Earth orbit.”