Earth: Rock City (was Carrying capacity of the solar system)

On Jan 26, 2012, at 7:48 AM, William wrote:

> They won't be building planets, they'll be building space habitats.
> Which will use many orders of magnitude less material per person.
>
> A Bernal Sphere I think weighs 4 megatons and houses 140,000 people.
>
The target population was 20,000-30,000 (http://en.wikipedia.org/wiki/Bernal_sphere
)