Solar Sails III (because two just isn't enough)

One thing that I've wanted to quantify since reading Intelligent Life in the Universe, an outstanding book by Carl Sagan and I.S. Shklovskii, is the idea of exogenesis. Exogenesis is the hypothesis that life formed elsewhere in the universe and was somehow transferred to earth in the form of some small seed or spore. Now since E.T. E. coli presumably do not have little tiny jetpacks or other means of active transport, they would need to traverse the cosmos in some passive way. One such way would be solar sailing.

Way back in Solar Sails I, we derived equations describing the maximum speeds and time-of-travel for various distances for a given solar sail. Each of these equations was a function of the surface mass density of the sail, which is just the amount of mass per unit cross-sectional area. All we need to know is the cross-sectional area and mass of a given object and we can apply these equations to just about anything!

Solar Sails II

[NOTE: In my hurry to make up for weeks of non-posts, I managed to almost immediately knock Nic's first post from the top of the page. It's got the LHC, black holes, and about 3 full cups of metric awesome, so make sure you check it out (after reading this one, of course).]

Last time we did some calculations on how fast and far our solar sails can go, but those calculations were just for the sail itself. If you are going to do any science with it, you're going to need a payload. Let's take it a step further and make it an actual spaceship (with people and everything!)

Solar Sails I

Solar sails are in the news again, and this time not just for blowing up. The Japanese space agency is launching what they hope to be the first successful solar sail tomorrow. In honor of that, we will be discussing the physics of solar sails.

First of all, what the heck are solar sails? Solar sails are a means of propulsion based on the simple observation that "Hey, sails work on boats. Therefore, they should work on interplanetary spacecraft (in space)." Boat sails work when air molecules hit into the sail and bounce back. By conservation of momentum, this gives the boat sail an itty bitty boost in momentum. Summing over the large number of air molecules moving as wind, the boat gets pushed along in the water. A similar process works with solar sails, but instead of air molecules doing the hitting, it's photons. Since each photon of a given wavelength has some momentum, by reflecting that photon the solar sail can gain a tiny bit of momentum. Summing over the large number of photons coming from the sun over a long time frame we can get a considerable boost. So let's see how good solar sails are.