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From: lsk91@campus.swarthmore.edu
Newsgroups: sci.space
Subject: Re: Solar sails and Belt mining
Message-ID: <7ZF4PPZ@cs.swarthmore.edu>
Date: 16 Jun 91 23:40:18 GMT
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In article <1991Jun16.125917.7326@demon.co.uk>, printf@cix.compulink.co.uk (Ian Stirling) writes...
>If you can make a workable solar sail how much more difficult is it
>to make a large parabolic mirror for melting large boulder size
>chunks of rock?

It was pointed out to me last year that heating by solar flux doesn't
work so well in space since there are no convection currents in the molten
stuff (no gravity).  I don't recall the specifics of this.

If it could work some other way, you wouldn't want to use a parabolic
mirror to focus an image of the sun on the rock, you'd want to use a light
funnel.  See _Scientific_American_ of a few months back for a good
article on light funnels.  They concentrate *all* the incident solar flux
on a spot, a lens or mirror just focuses an image, and thus much flux
does not help heating.

>you get higher temperatures than the surface of the sun by filtering
>the incoming light at the mirror(diffraction grating ?)to leave only the
>higher energy photons,this does not seem to violate any laws as you
>are only able to use a small fraction of the incoming light to heat
>the object but most of the light goes past at lower overall energy.

Huh?  Even the low energy photons should help to heat up the rock,
so no point in filtering them out.  Leaving only high energy photons
does not necessarily yield higher than surface of the sun temperatures,
although you may change the energy distribution profile of your solar
flux, you won't change its "temperature".