Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/18/84; site utastro.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!allegra!bellcore!decvax!genrad!mit-eddie!godot!harvard!seismo!ut-sally!utastro!ethan
From: ethan@utastro.UUCP (Ethan Vishniac)
Newsgroups: net.space
Subject: Re: Light Sails
Message-ID: <1031@utastro.UUCP>
Date: Thu, 21-Feb-85 18:29:23 EST
Article-I.D.: utastro.1031
Posted: Thu Feb 21 18:29:23 1985
Date-Received: Tue, 26-Feb-85 20:33:29 EST
References: <595@mordor.UUCP>
Organization: U. Texas, Astronomy, Austin, TX
Lines: 56

> From: John Heimann <jheimann@BBNCCY.ARPA>
> 	I sat down and tried to sketch out just how fast a solar sail could
> reasonably be expected to go, assuming a point source of illumination (the
> sun), the usual inverse square law for drop-off of photon intensity, a bit of
> handwaving about the mean photon momentum, and some reasonable figures for the
> density of the sail material.  I realized then that I wasn't really sure how
> the thing was to operate after all.  Are solar sails designed to get their
> momentum from elastic collisions with solar photons (reflecting sunlight), or
> do they get most of it from collisions with other particles that are 
> constantly emitted from the sum (e.g. neutrons)?  A typical neutron from a 
> nuclear reaction packs a lot more momentum than a photon from the same 
> reaction, so this is an important issue.

*** REPLACE THIS ROCKET WITH YOUR SAIL ***

However, a typical neutron from a nuclear reaction in the core of the sun
will not emerge from the surface.  Neither will the typical photon.  Both
are subject to scattering.

The following numbers are taken from Allen's "Astrophysical Quantities" which
is a standard astronomical reference book.  The total energy flux from the
sun is 6.27x10^10 ergs/cm^2/sec.  If the sail reflects incident photons
elastically then the pressure on the sail is 2F/c (solar radius/distance)^2,
where c is the speed of light.  Normalizing to Earth orbital radius this becomes
about 9x10^-5 dynes/cm^2. 

Now suppose the sail also suffers inelastic collisions with particles in the
solar wind.  Allen gives the density of the solar wind as being about 5 protons
per cubic centimeter near the Earth.  The typical velocity is 450 km/sec.
This gives us a wind pressure of
   (proton mass)x5x(Velocity of wind - velocity of sail)^2
normalized to the radius of the Earth's orbit.  This gives us a pressure of
about 1.7x10^-8 (1-velocity of sail/450km/sec)^2.  However, Allen also mentions
that the number density is inversely proportional to the velocity up to some
(unspecified) limit.  This suggests that the above estimate is an underestimate
since the occasional high-momentum particles will be more important than the
more common low-momentum ones.
  Nevertheless the above estimate makes it clear that photons are probably
more helpful, if only by a little bit.  A more important point is that
if you just hold up your sail against the sun you'll have problems unless
your Area/mass ratio is quite large.  Below a critical limit you will simply
reduce the sun's gravity by a constant fraction.  The angular momentum you
posess from the Earth's orbit will carry you out to some distance from
the sun into a new, and larger orbit, and there you will stay.  You need to 
tilt your sail to gain angular momentum as you go.
  I hope this helps.  No comments on the mylar.  That's beyond me.  I'm
sure people have written letters about this in Nature or some such place.


"Don't argue with a fool.      Ethan Vishniac
 Borrow his money."            {charm,ut-sally,ut-ngp,noao}!utastro!ethan
                               Department of Astronomy
                               University of Texas
                               Austin, Texas 78712

*Anyone who wants to claim these opinions is welcome to them*