10 November 2009
Posted by Ryan Anderson
In case you were wondering, the title of this post is the name of a short story by Arthur C Clarke in which solar sail-powered spaceships race each other around the moon.
Ok, that’s cool, but why do I bring this up? Because the Planetary Society is going to be launching a solar sail spacecraft in about a year! An anonymous donor contributed enough money to jump-start the program. The Society had a previous solar sail spacecraft attempt, but the Russian rocket it rode to space malfunctioned and instead of sailing through space, Cosmos 1 ended up in the Bering sea.
Solar sails are a very cool idea, but they have never been tested before, even by NASA. They work because sunlight is capable of transferring momentum, particularly to shiny surfaces, so a solar sail spacecraft just extends it giant silvered wings and hitches a ride on sunlight. It’s a “free” form of propulsion that could be used to accelerate probes without the need to launch heavy rockets and fuel tanks.
Needless to say, people at the Planetary Society are pretty excited about this opportunity to make space exploration history. To read more about the mission, check out this New York Times science article.
(PS – did you remember to vote for my article about MSL over at scientificblogging.com? You can vote daily until the 23rd!)
Wait, why can light transfer momentum to shiny surfaces particularly well? I’m assuming the answer is something like “magic!” Is it because less light is converted to heat? (Argh! light having momentum makes my brain hurt… what happens to the light once it transfers momentum? Can’t it only transfer momentum by slowing down? And can’t it only have momentum because it’s traveling the speed of light?)
It’s because instead of being absorbed, it is reflected. If you picture a photon with momentum p, when it is absorbed, the photon’s change in momentum is -p. And so the reflective surface gains a momentum of p to conserve momentum. But if the photon is reflected, it’s change in momentum is -2p, one to stop the photon and one to send it back the direction it came from. So the sail has to gain 2p of momentum so that in the end, the photon is heading backwards with -p and the sail is moving forward with 2p, for a total of p going forward, which is the same as before the photon was reflected.