[font=courier new][size=8pt]It seems to me that the sails would solve two problems.
1)
You don't have to carry the fuel. The energy-weight ratio for gasoline is relatively terrible. But, I think that even if you used the most efficient nuclear drive, almost all of the mass you are accelerating, would be fuel. Presumably, the farther you went, the more fuel you would use, and the less your vehicle would weigh. On the other hand, if you are making a round trip, then you would still have half of your fuel when you reached your destination.
2)
If the sail is pushed by starlight, then you get the energy for free. (But, apparently, interstellar light is too weak (and it comes from all directions!). Most designs rely on lasers or masers, aimed at the sails from Earth. In that case, you still have to find the energy somewhere - but the energy required would be much much less, since you would not be carrying the fuel, so the mass of the vehicle you would be accelerating, would be much much less.)
(Similar to gravity, the force (attractive for gravity, repulsive for light) is proportional to the inverse of the distance squared. So, if for instance, the power source is a laser or maser, and, at distance 1 light year from Earth the force on the sail is, "F", then at distance 2 light years, the force on the sail would be F/4, at distance 3 light years, the force on the sail would be F/9, etc. (Maybe light sails will be useful, but I don't think anyone will mistake them for warp drives.))
So, say that a craft from Earth lands on the Earth-like planet which is 20 light years away. Then, it would take 20 more years before any transmission from the craft reaches us. I think that the only way humans will survive, is if they fix this planet. I think imagining that we will escape to a better place, is a form of escapism. (On the other hand, who wouldn't want to wave goodbye forever to this place? Wouldn't it be great to see Earth growing smaller and smaller, and then disappear? It's become like a tree covered with insects, only, we're the insects.)
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