Re: [erm] space mirrors

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
homework. i'll buy it.

> >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
r radius of block
R radius of sun
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