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From: Steve Long [mailto:longsteven@...]

> "Combs, Mike" wrote:
> >
> > This seems to proceed from an assumption that establishing
settlements
> > on planets will be easier than in space. What do you see as the
> > reasons for this?
>
> ::: raising my hand ::: Possibly, because you don't have to start
construction of a planet from scratch?

True, but then we have to ask the question: What does the planet provide
us with for free that has to be engineered in space?

Pressurization? The lunar surface is a near-vacuum, and air pressure on
Mars is 1% Earth-normal. The atmosphere of Mars will in no significant
way mitigate the difficulties of engineering large, pressurized volumes.
On the other hand, the atmosphere is sufficient to raise up dust, so it
can be a nuisance. We can derive carbon from the atmosphere, but is
this a big cost savings? Compared to the total mass of what we need for
a functional habitat, what percentage of it is carbon? The NASA Space
Settlement studies estimated less than 1%, even when leaving out the
mass of the radiation shield.

Protection from radiation? The surface of the moon might see about 1/2
the radiation as free space; the Martian surface might see 1/3 as much.
So in neither case are we freed from the requirement to engineer
radiation shields. Martian radiation shielding might only require about
1/3 the mass as orbital, but how much more will it weigh? (Trick
question; the more-massive orbital radiation shield won't weigh
anything.)

Gravity? Unfortunately both the moon and Mars provide the wrong amount
needed to keep us healthy (or at best, not enough to ensure problem-free
visits to Earth). If there were some extraordinary expense associated
with setting an orbital habitat rotating, or keeping it rotating,
preexisting gravity might be an advantage, but there's no reason to
expect this.

Pre-established ecology? None exists beyond Earth. So all created
beyond Earth must be built from scratch.

Day-night cycle? Major problems with this on the moon, though Mars is
pretty much ideal. But this would only be an advantage if there were
some extraordinary expense associated with artificially providing this
in orbit. In comparison with everything else we need to do in either
case, building mirrors out of aluminized Mylar and then tilting them on
a set schedule doesn't seem an especially-expensive proposition.

I'll just paste in a paragraph from one of my articles:

It's difficult to avoid thinking that surely there's some savings in not
having to build the very ground beneath one's feet. It surprises many to
find that in an O'Neill habitat, the loading on the structure from the
centrifugal force acting on the interior soil and furnishing is
significantly less than the loading from the internal air pressure. Once
you've engineered the required pressure vessel hull (which we must do in
either case), you don't have that much further to go to engineer
something safe to landscape and build houses on.

It's true that Mars already exists, and we don't need to build the
planet. But we do need to build large, pressurized volumes with
habitable conditions inside to live there. So the proper comparison
isn't between the planet and Island One, it's between a pressurized dome
on Mars and a pressurized sphere in orbit. Almost everybody takes if
for granted without much thought that the former is easier or cheaper
than the latter, but I don't think anybody's ever done the side-by-side
comparison on a level playing field (with equal outcomes) needed to put
hard numbers to this.

I can only think of one significant advantage for Mars. The ore needed
can be moved from the mining site to the building site via means that
don't involve space travel. But it's important to remember that moving
a pound of ore from the moon to HEO, or from a NEO to HEO, will involve
a much simpler and easier variety of space travel than the kind we're
usually accustomed to discussing: namely from where we presently sit to
orbit. I'm sure ore transportation costs will be a significant part of
the expense of setting up habitats in HEO. But I'm also sure there will
be significant additional expenses associated with operating at the much
greater distance of Mars. I don't think anyone can authoritatively say
that the latter might not equal the former.

Regards,

Mike Combs