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# Re: [erm] space mirrors

From: Thomas Anderson <thomas.anderson@u...>
Date: Mon, 2 Nov 1998 20:11:37 +0000 (GMT)
Subject: Re: [erm] space mirrors
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On Mon, 2 Nov 1998, Andrew & Alex wrote:
> Thomas Anderson <thomas.anderson@university-college.oxford.ac.uk>
wrote:
> >I'm
> >skeptical of this: warming up Archangelsk is going to take a lot of
> >reflected sunlight ...
>     The full system was intended, if I recall correctly, to be several
> kilometers in diameter and there to be several.

still skeptical, for totally unscientic reasons - archangelsk is big and
cold. surely space mirros can't heat the whole thing up much? still, if
the russians reckon it will, chances are those lads have done their

> >a sun shield is a whole different ball game - it would have to be
> >absolutely immense.
>     Use a solar sail (not the DS9 version, though!). Fairly easy to
get
> several kilometers in diameter. Again though it would be vulnerable to
space
> tugs.

still not all that much good; your thing in orbit has to have an angular
size as seen from the surface quite a bit bigger than the sun, to cover
a
large area. if i've got this right, then:

s radius of patch of ground you want to darken
d distance from ground to block
D distance from ground to sun

r = s + (R - s)d/D

R - s pretty much equals R:

r = s + Rd/D

if it's in geosync (as it would have to be to darken a fixed point), d =
40e6 metres. for earth, D = 150e9 metres, R is ... erm ... lots. hang on
-
the web says 700e6 m. let's darken a 10e3 m patch.

r = 10e3 + 700e6 * 40e6 / 150e9
= 210e3 m

right, it would have to be 210 km across. the size of the sun is the
dominant term, so you can cover a larger area without much more size.
remember that area is proportional to r squared, so cost is probably
protportional to (r^2 log r).

of course, we're all well aware of how wonderful my maths is, so ymmv.

Tom
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