John Savard wrote:
but, as Robert Zubrin notes, it does seem to be the best place to set
up a colony.

However, are there any other alternatives that might be even more
attractive?

One of the objections Dr. Zubrin gives to O'Neill colonies is that a
mirror area comparable to the crop area is required for agriculture.
As it would seem to me that aluminized Mylar is easier to construct
than the *land area of the colony itself*, that seems to be an odd
objection.

You aren't the first (or the last, I imagine) to raise this point. There
are many things Zubrin says I disagree with. (Although I still admire
the man).


But the mirror area could be smaller if the colony was closer to the
Sun.

Venus' atmosphere has about the same percentage of nitrogen in it as
Mars', but it is many times denser. A well-shielded O'Neill colony - I
have a design for one, shaped like a wine bottle, with a further
shielding slab out past the mirrors putting light down the neck of the
bottle,

Described on this page?:
http://www.hypermaths.org/quadibloc/science/spaint.htm

You call your mirror system "Cassegrain"?
After the parabolic mirror concentrates it into a focus
it looks like a convex mirror redirects the converging rays
back into parallel rays.

My system is somewhat similar:
http://clowder.net/hop/railroad/ChengHo.html
But where you have a convex mirror redirecting converging rays to
parallel, I have a smaller parabolic mirror sharing the larger
parabolic's focus.

Also I have a 4th mirror that reflects the parallel rays onto the sides
of the cylindrical hab. It seems to me your system would also need such
a mirror as parallel rays would just pass through the bottle's neck and
land on disk at the bottom instead of illuminating the bottle walls. Or
am I missing something?

In my design the hab is built from asteroidal materials atop the north
pole of the asteroid. So the asteroid provides radiation shielding over
almost 2 pi steradians. The walls of the hab have a mixture of water and
dirt that I hope would be adequate radition shielding from those
directions. The the 4 mirrors (including the axial mirror) would shield
some from the top. The top is the most vulnerable radiation leak in this
colony, I believe.

In future drawings I plan to make 2 habs spinning in opposite directions
- one on the north and the other on the south pole. The net angular
momentum is zero and so the entire mass would be more manueverable. (I
stole this idea from Mike combs who has imagined two linked Bernal
spheres spinning in opposite directions. His structure would also have
zero net angular momentum)

Also in my colony the atmosphere doesn't fill the entire cylinder - just
a cylindrical shell. Less nitrogen would be needed.

here the shielding doesn't rotate - in orbit about Venus
might have access to a good source of biomass feedstock. (Metal and
rock would be sent from the Moon.)

Asteroids could also be a source of metal and rock.


Since the gas giants have very deep gravity wells, comets and Pluto
seem to be the other potential non-terrestrial sources of nitrogen in
the Solar System.

We still know very little about the composition of most asteroids.
Perhaps some have ammonia or other nitrogren compounds. Especially the
outer main belt and the Trojans.


John Savard
http://home.ecn.ab.ca/~jsavard/index.html


--
Hop David
http://clowder.net/hop/index.html