Won’t the sail approach the speed of light if it stays on course in line with the sun for an exceptionally long time?
No, because the solar wind drops off around 100 AU, and the power of the solar wind is going to reduce the farther out you are. These kinds of craft would get much more acceleration from a laser array that can put much more concentrated energy into the sail. But just like the solar wind, it will lose power the farther away from the array it is, along with any kind of intermediary debris attenuating the beam or unfavorable angles between the array and the craft.
So you can get these to an appreciable fraction of the speed of light, but I don’t think we’d be able to get anywhere close to c with this kind of a setup.
Edit: I was wrong about the solar wind above, it’s only like .5% as powerful as the photons emitted by the sun, and that energy drops off at only 1.5 AU, so they’ll get much less energy than I thought without an external power source like a laser array.
Doesn’t “solar wind” refer to the physical particles emitted from the sun? Like hydrogen, helium, etc ejected from the sun’s outer layers?
My understanding is that the solar sail is propelled mostly by the photons themselves, not the atomic particles that may also be reaching it.
Of course this probably doesn’t change your argument at all, since the intensity of light drops off precipitously as you fly further and further from the sun.
I suppose it can’t build up enough speed by going in direct line out of the solar system as the falloff would mean there’s not enough “runway” so to speak. But I wonder if you could circle the sun and “tack” like a ship does sailing into the wind and build up more momentum that way?
You could have an eccentric orbit that swings far out into the solar system and then when it approaches the sun again accelerates to reach the escape velocity of the sun. But that would take years.
And I think still only be rather slow once it escapes the solar system since the escape velocity would be almost used up.
There is the concept of a nuclear photonic rocket - Wikipedia which I think of as a light sail and a white hot glowing lump of nuclear fuel on a string.
I believe this is one of those things that benefits from scale. Theoretically, the larger you make the sail, the better the thrust to mass ratio you can achieve (even before calculating a better payload mass to sail mass ratio). With improved materials, we can make stronger and lighter sails and support structures, and this will in turn result in higher velocities by the time the vehicle has left the effective range of the sun. I think speeds truly approaching c are unlikely, but they can still achieve “really freaking fast”.
But then new advanced materials could also change that, we’re developing metamaterials with some fascinating properties, carbon nanotubes are just the tip of the iceberg. Who’s to say that we couldn’t some day achieve those speeds.
