Re: Global Energy etc.

[David Bowman <David_Bowman@GEORGETOWNCOLLEGE.EDU>]

Regarding Leigh's comment:

> Such a tiny reflector at the L1 point* doesn't shade anyone. The Sun has
> an apparent diameter of half a degree. The reflector Teller proposes
> would merely be seen to cover two percent of the Sun's apparent surface
> from any point on Earth. It's an old idea; many had it before Teller.
> ...

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. This is nearly the diameter of Pluto.  (I neglected
elliptical orbit effects, non-Lambertian limb darkening effects on the
solar disk, etc., etc.)

>                           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.
Of course if one was to build an opaque shield with nearly the area
of cross section of the planet Pluto, I expect it would *have* to be
pretty thin in the first place for it to be built from a doable amount
of resources.

David Bowman
David_Bowman@georgetowncollege.edu