Jorge R. Frank wrote:
wrote in
oups.com:

Why not insist on 1 gee at 1 RPM? Not many (if any) tests of
spin-generated 'gravity' have been done but the consensus seems to
be
that 1 RPM won't induce nausea in most people while 3RPM might.

The astronauts who will fly the early Mars missions will be a tightly

selected lot. I figure that adding RPM tolerance to the list of
selection
criteria will not narrow the field down too far.

Maybe so but another factor to consider is a space version of
'sea-legs'; if astronauts become adapted to the severe Coriolus effects
of a rapidly spinning ship, it may dangerously interfere with their
responses/movements once they land at Mars, at least during the
critical times of landing and setting up a base.

Also,
if we give the crew 1gee for the transit to/from Mars, they'll only
be
exposed to 0.38gee for the time they're on the surface (a few
months?).
We know people can take zero-gee for a year. Why waste a year or
two
testing people in LEO at Mars-gee?

I see the early Mars missions as precursors to a permanent presence,
so we
might as well find out now whether people can handle 0.38 g for
extended
periods.

The best place to do that is on Mars. Our (US and Soviet) experience
with zero-gee is enough to let us take that chance.

All the other long-term obstacles to Mars settlement - atmosphere,
water, radiation - can be solved, but we can't do anything about the
surface gravity.

Well, we could on a local scale but it would be expensive and a pain: a
mile-diameter toroidal city riding on a magnetic levitation track and
spinning at 300 km/hr would provide 1 gee. If you mean to settle the
entire surface then, yes, we might need a medical solution to the
problem.

If 0.38 g turns out not to be tolerable for long periods,
manned Mars missions will never be more than "flags and footprints",
and
therefore probably aren't worth doing at all.

I disagree: the data collection a crew could do in one year is much
more than "flags and footprints".

The cost to design, build, and test a 1 mile tether system won't be
significantly greater than the costs for a 76m system.

I agree, but a shorter tether gives you more mission flexibility. For

example, midcourse correction burns after TMI will almost certainly
be
necessary. The dynamics of performing the burn with the tether
extended
will be worse with a longer tether, requiring more sophistication in
the
control system. Also, retracting/re-deploying the tether becomes much
less
of an ordeal.

Anyway, we're both arguing nits here, relatively speaking -
regardless of
the details of tether length, this is going to require far fewer
breakthroughs than relying on nanotech.

For that matter, using faster propulsion to shorten the trip time
would
also probably be easier than nanotech.

Which brings to mind another use for a tethered ship: using
electrodynamics, current running through the tether to interact with
the Sun's magnetic field. This might be useful to propel or at least
course-correct the ship. If possible, then a longer tether would be an
advantage.