Many of the problems are not impossible even by today's technology.<BR><BR>We can build a self-sustaining biosphere. There's no scientific or technological reason why not with today's knowledge. We haven't pulled it off yet, but that's mostly through lack of trying.<BR><BR>We can thus build a self-sustaining biosphere in space. It'd be ferociously expensive, but there's no scientific or technological objection.<BR><BR>What we can't do is move it anywhere.<BR><BR>We use rockets, giant explosions in a tube. They're inefficient, clumsy and unreliable, but great for giving a very high impulse for a very short time. Rockets, of course, use chemical reactions which are a few orders of magnitude less energetic than nuclear reactions. Rockets aren't going to get us anywhere because they plain don't scale well. Adding 1kg of mass to a rocket's payload means adding 40kg of fuel and fuel tank (for the Ariane V design anyway). As the mass to be lofted gets greater, a rocket looks worse and worse.<BR><BR>Part of a rocket's sucktitude comes from its chemical nature, chemical reactions are not very energetic compared to the mass of reactants used.<BR><BR>This brings us back to the biosphere. They're *heavy*. You're not pushing one anywhere with a rocket. Maybe not even with nuclear propulsion and I don't see macroscopic amounts of antimatter anywhere.<BR><BR>Larry, the "musclature falls off while in space" isn't true. It is if you define "space" as "microgravity" but there's no reason to make that definition, a vessel large enough for interstellar transport will have no problem spinning to assert an outward acceleration.