Well...there would be an initial boost phase and deceleration phase to get to 1-G and to slow down from 1-G at the other end of the journey. For solar sails, the maximum speed is determined by the wind itself which travels at about 450 kilometers per second. (Note, most designs being looked at today use the pressure from sunlight itself, not the solar wind, but the following example is meant to be an illustration of some relevant principles which are more easy to think of with actual particles involved...not photons).
Once you accelerate to this speed, you will not be able to accelerate to higher speeds. The amount of time it takes to get to this speed is determined by how large the cross section of the solar sail is which is being impacted by the particles in the solar wind, and by the total mass of the spacecraft. I would estimate things as follows:
The particle flux in the wind is about 5 protons per cubic centimeter at 450 kilometers per second, or 5 x 4.5 x 10^7 = 2.25 x 10^8 particles/sec/cm^2. The momentum in this flow is just this flux times the mass of a proton or 3.6 x 10^-16 grams/sec/cm^2. Now a 1000 kilogram ship with a 5 kilometer sail will intercept its own mass of protons in about:
1000 kg
T(seconds) = ------------------------------------------
3.6 x 10^-16 x pi x (5 kilometers)^2
or 3.5 x 10^9 seconds. This is about 114 years. Obviously, it is going to take a long time to reach exactly the same speed as the wind itself, but this simple example also tells you that you could reach perhaps a few kilometers per second speeds much more rapidly. At, say, 10 kilometers per second, it would take you about a couple of years to get to Jupiter, and about 10 - 12 to get to Pluto. Actual sails use photons of light from the sun which deliver a pressure many times higher than solar wind pressure, and so the acceleration time is shorter.
These are all estimates because I have never had to sit down and really think about this stuff. For information on NASA's solar sail technology visit SunSail
Copyright 1997 Dr. Sten Odenwald
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