h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

As far as I am aware, solar sails actually do not primarily make use of the solar wind. The solar wind is a thin stream of particles that are ejected from the atmosphere of the sun and stream away from it at a rapid rate. But their velocity is still very much less than the speed of light, and when you get very far from the sun the solar wind becomes practically nonexistent. A solar sail however should more properly be called a 'light sail'. It uses LIGHT PRESSURE to sail - the actual momentum imparted to it by light particles, or photons, that hit it. That's why a solar sail should have a highly reflective surface - if it reflects the photons back in the opposite direction it is boosted by double their initial momentum.

Note that photons are "massless" but because they do have energy, and Einstein has shown that mass and energy are equivalent, they have a kind of mass and therefore also momentum. So they can actually impart momentum to an object. The solar sail needs to be so very large and very thin so that it will intercept a large enough amount of photons but resist them with little inertia so that its velocity will be changed significantly.

The real benefit of solar sails is that they can reach speeds very close to that of light. The "propellant" is light itself, and light goes at light speed, so this kind of propellant is the fastest kind that exists. The pressure it exerts may be very low, but a low acceleration, if sustained for long periods, can produce a very high eventual speed.

Light sails can also be driven by gigantic lasers specifically designed to project narrow, powerful beams onto them. Lasers of planetary size in orbit around the sun could be used to propell light sail vehicles to other stars within a fairly short timespan.

I have checked through my original source for this (an excellent article in October 2000's Astronomy Now), and you are absolutely correct. It is light pressure, and not solar wind pressure, that is the main force behind solar sails. It is an established fact that light does indeed push against surfaces.

Oops. I have learned something new today!

CR

I have checked through my original source for this (an excellent article in October 2000's Astronomy Now), and you are absolutely correct. It is light pressure, and not solar wind pressure, that is the main force behind solar sails. It is an established fact that light does indeed push against surfaces.

Oops. I have learned something new today!

CR