[extropy-chat] Forbes Magazine on Robotics

[spike]

> bounces at lists.extropy.org] On Behalf Of Keith Henson
...
> 
> This is a group that is not into heavy engineering, but I was still
> surprised nobody commented on the mechanically powered space elevator I
> posted about.  Massive investment, the cable is borderline nanotechnology,
> but it would solve the energy problem... Keith Henson


Keith, the mechanical aspects of the space cable problem might someday be
solved by the appropriate materials technology, but that does not answer an
issue with such a structure that I wrote about in a technical paper in 1991.
Every satellite must cross the equatorial plane twice each orbit.  I
estimated the cumulative cross section of all the satellites below GEO and
calculated that there is about a 45% chance of a catastrophic collision per
year.  The space station alone contributes about 20% of that risk.  

This is an anti-intuitive result for space guys because orbital collisions
never happen.  But a vertical cable is a whole nuther case: satellites are
points of mass mostly traveling in the same direction, whereas a cable is a
stationary line.  AC Clarke *almost* dealt with this problem in Songs of
Distant Earth, but his solution doesn't work.  Vibrating the entire cable in
a first-mode doesn't actually reduce its chance of collision.  I thought of
a slight improvement on Clarke's solution: twirling the entire cable around
an axis so that the endpoints stay stationary.  (Imagine a parenthesis
twirling about a vertical axis.  Endpoints rotate but do not move.)  

The twirling scenario solves the problem of cable flexing and the endpoint
moving up and down, but even that doesn't change the cumulative probability
of sub-GEO satellite collisions.  It does kinda work for sci-fi stories
where writers must offer *some* solution to these kinds of issues, even if
they do not actually work in the real universe.

So nanotech or otherwise, we cannot have a GEO cable or the elevator you
proposed until we take down all the sub-GEOs.  If that were not bad enough,
we have no feasible means of taking down all the sub-GEOs, even if we wanted
to, even assuming advanced nanotech.  {8-[  Dammit.  {8-[

spike