Aye, penumbra; solar light pressu(was Correcting my own math -- CORRECTED)

On Feb 3, 2012, at 7:45 PM, hitssquad wrote:

>
> --- In spacesettlers@yahoogroups.com, Al Globus wrote:
> > I do not think the spacing between sats needs to be nearly as
> large as proposed. I don't know where the numbers came from, perhaps
> comsats.
>
> The 109x diameter spacing I cited is the maximum length of the umbra:
> http://en.wikipedia.org/wiki/Umbra
>
> "The umbra (Latin for "shadow") is the innermost & darkest part of
> the shadow, where the light source is completely blocked by the
> occluding body. An observer in the umbra experiences a total eclipse."
>
Are you concerned that the sats will eclipse each other during certain
parts of the orbit at the equinoxes? Seems there should be a way
around that.

>
> This light-pressure problem was mentioned by Keith Henson almost
> three years ago, by the way. See his posts on light pressure, here:
>
> +++---+++
> In GEO the earth is the object being orbited around and the light
> pressure is not radial from the earth.
>
> Consider a very light sail in GEO. If it is anything but edge on to
> the sun, it would be blown out of GEO and away from the earth like
> dandelion fluff.
>
I've runs sims on that. Most of the acceleration cancels out over a
24 hour period, but not quite all. At 100 g/m^2 after a year you have
unacceptable movement so some station keeping is needed. However,
it's not all that bad. Just an engineering problem to solve.

To be specific:

-- The force along the orbital direction cancels out in 24 hours

-- The force normal to the orbital plane cancels out in 24 hours

-- The only component that doesn't cancel is normal to the Sun, but
due to orbital mechanics this mostly affects the eccentricity -- which
you can live with. You might even be able to use it to avoid the
shadowing problem above.

You just need to be clever, and not give up at the first little
problem that rolls along.