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New Scientist blows it

A friend brought this article to my attention:


This is not the first time that Prof. Gold has blown it; he was the guy 
who claimed the Apollo landers would be swallowed in a sea of dust. But 
that time he could at least plead ignorance, since nothing had yet tried 
to soft-land on the moon. No such case here. Here is the response I sent 
the editors. --Phil

Tommy Gold and others quoted in the article about solar sails really
should consult some real spacecraft engineers. For us, solar
radiation pressure is an everyday reality. Solar radiation pressure
is a major perturbing force on GPS satellite orbits, for example.

AMSAT, a group of radio hams that builds its own satellites, has for
decades used radiation pressure to impart slow spins to its satellites
with "blade turnstile" antennas. Paint one side of each blade black and 
the other white, and the spacecraft slowly spins like a Crooke 
radiometer -- but in the opposite direction, away from the white surface.

A Crooke radiometer is a very different beast.  The glass bulb is not
evactuated, so thermal heating on the black side of the vanes heats
and expands air, pushing the vane away from the black surface. This
force overwhelms the much smaller photon pressure, but in the vacuum
of space only the radiation pressure exists.

Gold's thermodynamic argument is silly and wrong. A solar sail is not
a heat engine, so the second law doesn't apply. The first law (energy
conservation) does apply in a very simple way: the photons reflecting
off the sail are red-shifted by the sail's motion, removing energy
from the photons and imparting it to the sail by accelerating it.

Phil Karn
San Diego, California

Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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