hi again i really think the solar sail people are a bit optimistic about its use for interstellar travel the reason is just that as the acceleration is slow you have to wait a long time and travel a long way to reach high speed at a uniform acceleration a you reach velocity v after traveling a distance s according to v sqrt as in newtonian approximation which is applicable at the speeds we could hope to reach then s v a and for a given v s gets very large if a is small but since the intensity of sunlight decreases like s with distance from the sun a does not remain constant but drops rapidly making things very much worse once you get out to well beyond the orbit of pluto where the sun begins to look like a very bright star the photon drag of the light of other stars begins to become significant as well as the cosmic background radiation which has an energy density comparable to that of starlight finally there is the problem that at really high speeds the light of the sun would begin to be red shifted so you lose power from that effect too some of these problems can be solved by beaming light or microwaves out to the ship with a huge power station a trillion watts say near the earth and a large aperture mirror to focus the beam on the ship then as long as you can maintain the focal spot as small as the ship's sail you could get constant acceleration a calculation i did years ago which i will not try to reproduce now suggested to me that speeds of maybe c could conceivably be reached in this manner which is very exciting but then somehow you have to stop for example if i recall if the aperture of the beaming mirror is r the ship sail dia r the distance from the sun d and the wavelength then you need r r d and acceleration g will get you to c in years at a distance d of light years cm which makes the mirror r km in diameter for a sail r km with a um nb the mirror needs to be optically perfect to but the sail does not of course c is a useful speed and you might split the problem between a beamed laser sailer with a fusion rocket to stop but the trillion watt figure i remember was for a fairly small ship or tons if i recall and i think a nuclear electric ship capable of reaching even a few percent of c is going to be much larger than that for a payload of or tons i cannot imagine a decade ship smaller than an aircraft carrier so it sounds like a pretty tough way to go though possibly competitive with other approaches what i personally think we need to do is work the problem modestly hard from an engineering standpoint considering all approaches producing realistic design mission studies that give us the best trade off solution with current technology right now the trip time for a program we could realistically sell to the combined economies of the planet would probably last a thousand years or more so in ten or thirty years we should redo the study and see how that number has changed due to technological advances at some point the curve of reasonable trip times will either get down to the order of the time between studies or else it will flatten out when we hit the wall of feasible engineering or maybe there will be a huge breakthrough when somebody figures out how to create a wormhole to a nearby star or something equally revolutionary anyhow only when the trip time is getting down to the time between studies which would depend on the rate of technology change would it be sensible to start a serious program with what nasa would call a phase a study leading to a program start then a preliminary design review and then a critical design review which is when you can start cutting metal solar sailing is probably a competitive way of getting around the solar system though i think nuclear electric ion drives are likely to prove better the latter can certainly take us all over the solar system in the time scales of the great explorations of the th and th centuries then in years the solar system will be ours and we will have a far better idea of exactly what is out there in the dark between the stars possibly mini solar systems like jupiter its moons if such systems are common we might find useful places to go only as far as alpha centauri which would change everything and the human species would be safe at last from the many disasters which threaten us now sorry i have been inattentive to wikipedia for a month or two due to other responsibilities which may increase or diminish in