Samuel Walters wrote:
Yes, but the inhabited planet has a native labor force -- not to mention technology -- which can be exploited to help gather and process the water at a fraction of the cost of building a whole operation and importing the labor into a hostile planetary/cometary environment -- or trying to extract sparsely distributed water vapor in the harsh vacuum of deep space.
Who said anything about "sparsely distributed water vapor?" There are vast amounts of H2O quite nicely concentrated in the form of ice. The amount of ice available on the asteroid Ceres alone is greater than the total amount of fresh water on the entire Earth. Most of Saturn's moons consist primarily
of ice. Don't think in terms of vapor or even liquid. In the outer Solar System, water is a mineral
. It's a rock you can mine, and it's just about the most abundant kind of "rock" available beyond the Main Asteroid Belt.
And the environment of an outer moon or asteroid is probably a lot less hostile than an inhabited planet surface -- no harsh weather, no dangerous animals, little solar radiation hazard, and again, much less gravity to fight. There's also the matter of distance. It would take far less energy and effort for the Visitors to mine the outer moons and comets of their own star system -- which, again, should provide thousands
of times as much water as a single inhabited planet would ever need -- than it would take to travel across light-years to another star system. There's no sensible reason why any species would ever need to leave its own system for a resource as basic as water, because water ice is going to be one of the most abundant geological constituents of any planetary system in the galaxy.
And even if, for some reason, their water demands were so great that they needed more than one star system could provide, there are still at least five star systems
closer to the Visitors' home system of Sirius than Sol is.
Although of course Sirius is only 300 million years old and couldn't possibly support advanced life anyway, unless they weren't native to that system and their world is terraformed to begin with. In which case their technology would be so advanced that they wouldn't need anything as primitive and inefficient as human labor to help them acquire water.