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I thought this whole controversy was interesting, and
it made me think about something that hadn't occurred to me before -- that some
of the radiative heat from an imperfectly-reflective solar sail will, to
some extent, oppose the direction the sail is being flown in. (However,
the heat will scatter in every direction and the forces will cancel, whereas
sunlight will be coming only from one direction -- or, pedantically, from a very
narrow range of angles.)
But it also made me wonder about something.
Back in the 80s sometime, I read about a proposed solar sail structure using a
sail with very small holes -- holes a little less wide than the average
wavelength of sunlight. It would be more like a photon-sieve than a
photon-reflector. It would be .... nanoporous?
The argument was that you'd get a much more efficient
solar sail. Being mostly holes it would weigh far less, but it would
still use most available momentum from incident photons (50%-90%, depending
on hole size). The net thrust might therefore be several times greater (5x
or so?) than that predicted for non-nanoporous sail fabric
candidates.
Someone pointed out that you'd probably need a very
tightly focused X-ray laser to punch holes this size in any material, though my
feeling was you could do something similar with E-beam lithography. After
all, your average DRAM bit-cell array featured inter-wire spacings lower than
the wavelengths of visible light as far back as the 80s. And if such a
fabric were fabricated using standard IC fab techniques, it wouldn't have to be
nearly as close to perfect as we expect of circuitry, but just good enough to
hold together.
In any case, the fabrication cost per unit area would
be dramatically higher -- it's probably yet another one of those "sit back and
wait for nanotech" propositions. But it's still interesting, and
the materials science and microfab issues might not be such a big deal
these days. (And breakthroughs happen. Who predicted
aerogels?) It might help make solar sailing practical as a propulsion
technology at outer-planet distances, if solar sailing is practical at
all. Of course, the inverse-square law means that a 5x efficiency
improvement translates to much less increase in navigability with distance from
the Sun, but it's something, anyway. The main initial application might be
in getting range increases for outer planetary missions currently slingshotted
through inner planet orbits -- to steer spacecraft onto slingshot trajectories
that would otherwise defy orbital mechanics, or that at least would currently
require a prohibitive amount of kick-motor work.
-michael turner
----- Original Message -----
Sent: Monday, July 14, 2003 2:01
AM
Subject: Solar sailing does not
violate the laws of physics
"Solar Sailing Violates the
Laws of Physics" - Not So
by Louis
Friedman
July 9, 2003: Prof. Thomas Gold, a noted Cornell
University astrophysicist, recently has published an article on the World
Wide Web claiming that solar sailing "violates the laws of physics.".
Specifically he says of solar sailing, "…no such pressure can be expected,"
and of our Cosmos 1 project, "The proposed solar sail cannot be accelerated by
sunlight."
I know Prof. Gold well - he was a principal
professor of mine at Cornell University, where I minored in astrophysics. He
is famous for provocative theories that have sometimes proved wrong: two of
the most notable being a pre-Apollo, pre-Surveyor conjecture of deep lunar
dust that might swallow up spacecraft or people, and a more recent speculation
about capturing huge pockets of methane below the Earth's surface as an energy
source. But he is also famous for seminal work on the steady-state theory of
the universe, and on devising experimental tests of theories in astrophysics.
Additionally he contributed important theory to the studies of the
magnetosphere and interactions with the solar wind. At Cornell, he taught me
much about thinking and challenging paradigms in science.
A number of
colleagues have contacted me since the web posting (on a rather obscure
British web site of "e-print physics archives", http://uk.arXiv.org.) The British publication,
"New Scientist," ran an article about the posting, giving it more notoriety. .
They ask if I am upset, somehow perceiving that a theoretical challenge to
solar sailing might undermine our effort to fly the first solar sail
spacecraft. Quite the opposite - I am delighted. As Prof. Gold taught me,
theories that can be tested experimentally are extremely valuable. If our
solar sail mission were a test of physics theory it would be terrific.
Alas, things are not quite like that. The theory of light pressure has
already been tested and proven. The force from sunlight pressure has been
observed and measured. It was observed in comets long before the beginning of
the space program, and has been precisely predicted and measured on a number
of space missions including many Earth-orbiting satellites, the small
communication reflectors put in the ionosphere in the 1960s, and even for the
attitude control of the Mariner Venus Mercury spacecraft in 1973. The force is
now measured and accounted for on every space mission.
Furthermore the
theoretical basis of the sunlight pressure force was very well established by
Maxwell in the 1860s, and recognized by Einstein in his fundamental work on
the nature of light, and in the basics of quantum theory which derives the
energy from light particles in terms of the frequency of the light. That light
has energy and momentum is firmly understood in theory and experiment. When
light hits the sail, momentum is transferred, and that is what causes the sail
to be accelerated (either in the direction of motion or opposed to it,
depending on which way the sail is tilted).
Gold's argument seems to
neglect the quantum mechanics particle nature of light. He argues based solely
on 19th century thermodynamics of heat engines. This works for large systems,
but not at the quantum level, and not for photons (the massless particles of
light). Another mistake by Gold (perhaps forgivable for a theoretician) is to
argue on the basis of perfection. He assumes the sail is a perfect mirror and
neglects absorbed photons. But the sail is not perfect and both reflected and
absorbed photons contribute to the resultant force. He also claims to have
performed experiments with the Crooke's radiometer (that little toy that
purports to twirl blades around from light pressure). In fact it is well known
that the radiometer blades turn due to the heating of air molecules by the
light, causing a differential thrust on the two sides of the blades. Only in a
perfect vacuum would light pressure be a significant factor. Gold created a
vacuum in a laboratory experiment - but his vacuum wasn't perfect, or even
good enough, and he got the wrong answer from his experiment. (The same reason
we are insisting that the Cosmos 1 spacecraft fly in a high enough orbit so
that air molecules don't interfere with solar sailing.)
A colleague of
mine, Steve Soter an astrophysicist at the Hayden Planetarium, first informed
me of Gold's argument more than a year ago. At his urging I called Prof. Gold
and discussed it with him. He has forgotten more physics than I ever knew, so
I was loathe to argue - but I urged him to write up his theory where it could
be reviewed in the scientific literature. This web posting is all that has
resulted. I have circulated it to many colleagues to see if I am missing
something.
I thank Henry Harris, Chris Russell, Colin McInness, Jim
Burke, Chauncey Uphoff, Jerome Wright and others for helping me with the
physics arguments. After all, I am "only" an engineer. I also thank Prof. Gold
for the challenge and stimulation.
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