[extropy-chat] Forbes Magazine on Robotics

[Keith Henson]

At 03:46 PM 8/20/2006 -0700, you wrote:
> > 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.

Already addressed.  Part of the project involves cleaning out all the big 
stuff, including the space station.  Ion engine tugs haul it up to 
GEO.  You need it anyway for the counterweight.  The other is that very 
early in the process you clone the elevator so that you can recover from it 
getting hit.

>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.)

One current approach is to anchor the cable on a ship.  You can predict 
early enough to steam the ship some miles and that moves the cable far 
enough to get out of the way of something.  Fixed point anchors will 
probably wait till orbit cleaning is far along.

>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-[

That's not true, there are many ways with and without nanotech to clean out 
orbits out to GEO.  The more amusing problem is to drain the Van Allen belts.

Keith Henson