[Nachum]

The primary reason for the use of asteroids is to avoid the danger of cosmic rays. Why not develop a proper shield for out ships in space?

[aruanan]

The primary reason for the use of asteroids is to avoid the danger of cosmic rays. Why not develop a proper shield for out ships in space?

Because it's cheaper to modify an asteroid than to lift all that mass to orbit? If they started with the moon, then there's plenty of stuff there to lift out of a shallower gravity well.