It is assumed that any kind of ship like that would be assembled in space and if we can't do that we are most likely not ready yet.
Water would be a good candidate for both shield and jacket for the simple reason it is readily available in space in huge amounts and doesn't have to be lifted from Earth. What is necessary is a single remotely controlled ship that will go far enough from Sun to be able to capture and steer one of the watery asteroids. The further you go the slower the orbits the less energy you need to dramatically alter course of any astronomical body.
We don't even need to bring all that water to Earth. Instead, it might be cheaper to fly half-finished craft out, finish assembly by robotic means and then turn it into center of solar system. The craft could then gain velocity, use as many gravitational assists as possible on the way in and then back and pick up the crew before it leaves solar system for its journey.
Water would be a good candidate for both shield and jacket for the simple reason it is readily available in space in huge amounts and doesn't have to be lifted from Earth. What is necessary is a single remotely controlled ship that will go far enough from Sun to be able to capture and steer one of the watery asteroids. The further you go the slower the orbits the less energy you need to dramatically alter course of any astronomical body.
We don't even need to bring all that water to Earth. Instead, it might be cheaper to fly half-finished craft out, finish assembly by robotic means and then turn it into center of solar system. The craft could then gain velocity, use as many gravitational assists as possible on the way in and then back and pick up the crew before it leaves solar system for its journey.