From: hal@finney.org
Date: Wed Nov 29 2000 - 10:18:45 MST
Ross writes:
> Can tethers have a complete orbit? For example, from an orbit could a
> satellite have a cable that reached all the way around the planet back to
> itself? That might be a first stage about putting a structure around the
> entire planet. If a couple cables were in orbit, they could generate
> deionizing fields around the planet outside of it's magnetosphere.
I think there are problems with stability in such a structure.
I've read that if you had a *rigid* ring, it could not be in orbit.
One side or the other would tend to drift downwards. It wouldn't make
any difference if it were rotating or not. Of course this is only for
an idealized, perfectly rigid ring, and no solid materials could be
particularly rigid at the size scale we are talking about.
For a flexible ring, I think the problem is that if the orbit is slightly
elliptical, as any true orbit will be, the particles of the ring want to
move faster during the lower part of the orbit. This will compress the
ring in some portions and stretch it elsewhere. I suspect that this
energy transfer will cause the ring to become deformed and to collapse
into the atmosphere eventually.
You could possibly fix these problems with active elements, little rockets
or perhaps solar mirrors to monitor the position of each ring element
and herd it back into the desired configuration. But this would add
greatly to the complexity of an already difficult project. 25000 miles
of cable of any practical thickness is going to be a mighty big spool.
Also I'm skeptical of this "deionizing fields" concept, because a cable
moving parallel to it own length won't cut any magnetic field lines and
so won't generate any significant currents.
Hal
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