The need to conserve

On 7/9/06, sraj wrote:
(...)
> A good thing about travel on our planet is that it does not as a whole lose
> mass each time people travel in a jet from London to Shanghai.
(...)

Well, the original mass of quero used by a plane is indeed lost
forever. Fossil fuels become rarer and rarer as we use them, in no way
different (economically speaking) from the amount of propellant
stockpiled in a habitat becoming less and less as spaceships use them.
Hum... Thinking better, in habitats you would for starts use fuels
that can be replenished by using raw materials and sun energy, while
on Earth you don't have that option for the aviation fuels in use.

Also, not necessarily a spaceship for transit between habitats would
have to spend propellant. Depending on the orbits of the origin and
destiny habitats, some sort of throw-and-catch system would transport
masses between them without use of propellant. I also think that there
will be "grids" of habitats, composed by many rotating structures
plugged into a shared non-rotating transport grid - were again there
would be no need of spending propellants. In fact, those classic pairs
of O'Neil cylinders are a kind of very simple "grid" containing just
two rotating structures.