ocr: Note that all this immense mass of rock would well protect future inhabitants from harmrul radiation. As well as provide a minimal amount of gravity, if it were truly big enough. The ideal spherical region Now we need more mass, of a particular kind. The optimum substance at this point would help us hermetically seal our rock, while also providing the basis of an internal life support system. Too, it would be convenient if it was already present in space, so that we wouldn't have to lift it from Earth's gravity well (The necessity of lifting would make it horrendously expensive; and we're barg ...
