Motherlode Earth (was : Space habitats)

--- In spacesettlers@yahoogroups.com, "hitssquad" wrote:
> Even Venus, Mars, and Earth's moon together don't add up to the mass
> of the earth. Earth is truly the motherlode of the solar system.

Which rests of the strange notion that rocky material is the limiting
factor for supporting human populations. The true limiting factor,
that we know, is energy. The sun pours out 2.16 billion times as much
of this as the Earth receives, and it's going to be pouring it out for
the next 6.5 billion years.

The limiting factor might be nitrogen. If so, then the kuiper belt has
roughly 2000 times as much nitrogen as the earth (2.27e22 kg vs.
1.2e19 kg.).