🚀 go-pugleaf

In article <51016@seismo.CSS.GOV>, black@seismo.CSS.GOV (Mike Black) writes:
|>NASA claims they intend to generate electricity with the tether 
|>experiment coming up Monday.  Seeing as how a magnetic field has
|>no energy, wouldn't any energy gained come the movement of the
|>shuttle & tether?  If so, you would lose orbital velocity which
|>would have to be made up by additional thrust thereby negating
|>any energy gained.  Or am I missing something?

You're missing something. There is a potential drop across the distance.
So you are connecting a junction at two points and extracting electrical
energy from it. It is a free lunch scenario. The energy is extracted
from the stored energy in the magnetic field structure of the earth.
The idea is that magnetic field lines tend to run from pole to pole.
electrostatic potential drops can exist perpendicular to the magnetic
field since the magnetic field itself can be an electric insulator
when the charge carriers are ionized particles. This is all a result of
a very fundamental property of plasma containment. The zero-th order
orbit of a particle in a magnetic field is a helix with axis parallel to
the magnetic field. Now there are 1st order corrections, etc, but this
is the idea.

If this is still counterintuitive, think back to the common sophomore
physics lecture demonstration of a homopolar generator. That is, take
a copper disk and spin it in a magnetic field. You will find a potential
drop between  the center and the edge. Now the electric and magnetic field
structure in the magnetosphere is much more complex, and to the best of
my knowledge is not a rotation induced phenomenon, but you can get the
idea of how you potentials can exist across field lines.

Now the tricky part is how are you going to control the tether. The only
thing you can do is let out slack or pull it in. Have you ever tried to
push a chain? Moreover, the tether will be conducting a current and will
thus experience a body force (J X B), so its dynamics are not at all
apparent to me. It may be quite interesting. How much as NASA done at
trying to predict this? I conference I went to in the last year spent
a good deal of time during one of the sessions talking about this, but
I confess that I spent most of the time working on some other problems
with a colleague and missed most of it.

Finally, the tether works both ways. I would be more interested in it
in what it can tell us about how the magnetosphere behaves. It can provide
excellent two-point diagnostics. I would love to do some higher order
spectral analysis on the time history of the signal. There's great opportunity
for some rally neat linear and nonlinear results. Moreover, it might even
have some forseeable near-term utility. It is nice because you
could conceivably leave it in place for long periods of time (in other
missions). Talk about some fun physics, sign me up. :)

john w. cobb
jwc@fusion.ph.utexas.edu