From: spike66 (spike66@attbi.com)
Date: Tue Feb 11 2003 - 22:47:25 MST
Damien Broderick has written story which has a
skyhook being grown by a nanoreplicator. As I
read the story, one of the many difficulties with
making an orbit-synchronous cable occurred to
me: that the cable would need to get an enormous
amount of angular momentum from somewhere in order
to keep one end pointed at the planet.
Imagine a planet with a high-carbon satellite
conveniently located in synchronous orbit. This
satellite is tidelocked to the planet, that is
the same face always points toward the planet.
So the sidereal rotation rate of the satellite
is one rev per "day".
Imagine nanobes on the satellite decide to build a
diamondoid cable down to the surface. As they
do so, they manufacture and pay out cable into
space to maintain the CG at synchronous altitude.
By the law of conservation of angular momentum,
the sidereal rotation rate of the satellite will
slow down as the moment of inertia of the satellite
goes up. One can imagine that a completed cable/
satellite system would have a jillion times the
moment of inertia of the satellite without the
cable, so the sidereal rotation would have slowed
a jillionfold, so the cable is no longer synchronous.
The satellite still hangs in one spot over the
planet, but appears to be rotating about its
axis at about one revolution per day.
One way to deal with this is to spin the satellite
at a jillion revs per day before the construction
begins, which presents its own practical difficulties.
I wrote a paper once before about the problems
with making a geosynchronous cable, but I just
thought of this new one yesterday.
Has anyone seen a study which deals with this
angular momentum problem? Anders, Robert, have any
of you space cable fans recognized this problem?
spike
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