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Re: Global Energy etc.



At 5:48 PM -0500 1/19/01, David Bowman wrote:
Regarding Leigh's comment:

>Such a tiny reflector at the L1 point* doesn't shade anyone.
> ...

According to my calculations the Earth's L1 point is about 0.00997 AU
from the Earth toward the Sun. A circular shield blocking 2% of the
sunlight reaching the Earth from that position would have to have a
diameter of 1963 km.
...

I guess you don't consider that "tiny", eh?

> Because that point is not one of stability in
the Lagrange scheme, and because the real Earth's orbit is eccentric, the
reflector's position would have to be controlled by rockets. Fuel
consumption could be reduced by making a larger reflector, but there is
no way known to eliminate the need for these rockets completely.

Don't forget that the instability is only along the radial direction
between the Sun and the Earth (at least for a sufficiently circular
orbit). The L1 point is stable in the other 2 dimensions of space.
Essentially only radial stationkeeping nudges would be needed (assuming
that the effects of the eccentricity of the Earth's orbit, orbital
angular velocity changes, etc. would not mess up the stability in the
tangential direction to the orbit). It seems such radial stationkeeping
might be able to be done without using any rocket fuel at all. How about
controlling the albedo of the illuminated side of the shield (maybe by
rotating darkened and light colored louvers, or some other mechanism)?
Changing the reflectivity of a thin foil shield will change the radial
momentum imparted to it by the sunlight. Reflected photons give twice
the impulse of absorbed photons. Of course I haven't worked out the
range of impulses that would be needed for a realizably sized shield
to see if controlling the radiation pressure would be sufficient to
accomodate the need for station keeping for a suitably thin shield.

That, David Bowman, is a very clever idea which should be applicable
to other astrodynamic applications (since I don't expect this one
will ever get off the ground). You now have one year to patent it.
I don't know what space mission might require station keeping at the
L1 point, but I'm sure there's something useful that could be found
to be practical if such an active, fuel free station keeping method
were known. Solar sailing can provide both radial and tangential
forces so long as the craft remains on the sunward side of L1. The
structure could consist of many long vanes, like a Venetian blind,
and the albedo could be varied between zero and one by turning them.
lateral forces could be regulated by the fraction that are cocked
one way, with arrays of vanes oriented in both nonradial directions.
Hey, you want to join me in a patent application?

Leigh