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Path: utzoo!henry
From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: net.space
Subject: Re: Light Sails
Message-ID: <5095@utzoo.UUCP>
Date: Sat, 23-Feb-85 19:58:19 EST
Article-I.D.: utzoo.5095
Posted: Sat Feb 23 19:58:19 1985
Date-Received: Sat, 23-Feb-85 19:58:19 EST
References: <595@mordor.UUCP>
Organization: U of Toronto Zoology
Lines: 41

> ...  Are solar sails designed to get their
> momentum from ... reflecting sunlight ..., or
> do they get most of it from collisions with other particles that are
> constantly emitted from the sum (e.g. neutrons)?  ...

Sails are optical reflectors.  Neutrons and the like are much rarer than
photons, and are hard to stop as well.

> ...  Also, what's a good assumption as to the
> composition of the sail?  Would a large expanse of metallized mylar be
> reasonable...

Low-performance sails are typically assumed to be aluminized mylar.
High-performance sail designs are all (as far as I know) derived from
Eric Drexler's designs, which are aluminized *nothing*, just a layer of
aluminum about the thickness of a virus.  (Sorry for lack of numbers,
my references aren't handy.)  They are just as reflective and far, far
lighter than aluminized-mylar sails.  A hexagonal sail 10 km across
weighs only about 20 tons.  The major limitation of aluminum sails is that
they *must* be assembled in space; they are basically rigid structures
and cannot be folded up into a small package for launch.

If you really want high performance, especially in low Earth orbit, one
intriguing notion is to take a Drexler aluminum sail and punch it full
of very tiny (smaller than a wavelength of light) holes.  Holes which are
significantly smaller than a wavelength of light will not affect the
reflectivity of the sail, but they will lighten it considerably.  Just
as interesting, air molecules at orbital altitudes are independent of
each other and will go through tiny holes just as easily as through big
ones.  Not only does one get (say) a 75% weight reduction, one also gets
a 75% air-drag reduction.  The snag here is that nobody knows how to
make perforated-sail material yet.  Ultra-thin aluminum is easy, but the
holes are hard.  (Last I heard, anyway.)

> ... or would this rip to shreds under meteor impact?  ...

The density of dust and debris is too low to significantly damage a
sail, except in anomalous areas like planetary rings.
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry