Light Sails Won't Work?

http://www.spacedaily.com/news/rocketscience-03zg.html

Interesting...

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In article ,
Rand Simberg wrote:
http://www.spacedaily.com/news/rocketscience-03zg.html

Interesting...

Yes, as proof that Tommy Gold has reached his dotage.

A *moving* perfect mirror *does* reduce the "temperature" of photons
reflected from it -- by Doppler shift! Where does the energy lost in
Doppler shift go? Into added kinetic energy of the mirror.

(If the mirror is held stationary -- relative to the observer who is
measuring the details -- by some means, then there can be no Doppler shift.
But there is also no work done on the mirror, since work is thrust times
*distance*, and hence there is no added kinetic energy.)

Yes, Doppler shift at ordinary velocities is pretty damn small. But so is
the acceleration produced by light pressure.

Gold appears to be unaware that the physics of light pressure are well
understood and have been demonstrated many times -- in the laboratory, in
precision tracking of spacecraft, and in attitude control of spacecraft.

A particularly glaring example is Radarsat 1, which is in a dawn-dusk
sun-synchronous orbit (i.e. essentially continuous sunlight) and flies
with an essentially constant attitude. Its designers overlooked
solar-sail effects on its big solar arrays and radar antenna, which are
slightly tilted with respect to the Sun for engineering reasons. Turns
out that nearly 2/3 of Radarsat's stationkeeping fuel goes to fight
light-pressure drag -- it's trying to sail down into the atmosphere.
See "Radarsat Time Rate of Mean Semi-Major Axis Due to Drag", by Said R.
Marandi, in the AAS/GSFC 13th International Symposium on Space-Flight
Dynamics, 1998.

Note that the experts consulted for the article were a thermodynamicist
and an astronomer, neither of them a physicist. (Citing the Crookes
radiometer is just plain embarrassing -- it turns by thermal effects, not
by light pressure.)
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

In article ,
Hop David wrote:
Sure hope Gold is wrong. But the Crookes radiometer seems to support him

The Crookes radiometer is irrelevant. It spins because it is *not* in a
perfect vacuum; in fact, it would not work in a perfect vacuum. There is
gas in the bulb, although it's fairly thin. The black side of each vane
is warmer than the silvered side, and it warms the adjacent gas, which
expands and pushes on the vane -- it's a solar-heated jet engine! This
effect completely overwhelms the extremely feeble light pressure.

You can build a Crookes radiometer with *transparent* vanes and matte
black paint on one side of them. The light pressure is equal -- both
sides look black -- but only one side gets warm. It spins just fine.
(My father found an ancient one in an auction sale long ago. As far as
I know, it's still spinning merrily in the kitchen window back home.)

Guess we'll know for sure come September.

See my previous posting. We already know, and have known for many years.
Tommy Gold apparently has gone senile.
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

On 3 Jul 2003 18:05:08 GMT, in a place far, far away,
(Henry Spencer) made the phosphor on my monitor
glow in such a way as to indicate that:


Note that the experts consulted for the article were a thermodynamicist
and an astronomer, neither of them a physicist. (Citing the Crookes
radiometer is just plain embarrassing -- it turns by thermal effects, not
by light pressure.)

Well, so in that sense, it sustains Gold's point. ;-)

--
simberg.interglobal.org * 310 372-7963 (CA) 307 739-1296 (Jackson Hole)
interglobal space lines * 307 733-1715 (Fax) http://www.interglobal.org

"Extraordinary launch vehicles require extraordinary markets..."
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JRS: In article , seen in
news:sci.space.policy, Rand Simberg
posted at Thu, 3 Jul 2003 15:45:01 :-
http://www.spacedaily.com/news/rocketscience-03zg.html

Interesting...

The originator has obviously forgotten some fairly basic physics - that
you are also unaware of - assuming that we're thinking of the same
article, which, as I am off-line, is not to hand.

Like pushing a stationary car, 100% reflection by a stationary mirror
basically transfers momentum but not energy.

Like pushing a moving car, 100% reflection by a moving mirror transfers
both momentum and energy.

As many photons as are incident are reflected, but, in the case of the
moving mirror, with a Doppler shift which alters their energy.

Thus propulsion is compatible with the standard conservation laws.

Only trust the judgement those writers on technical subjects who
remember the essentials of basic physics; trust others only to the
extent that they are faithful copyists of well-known sources. I know of
no current examples of these working for the popular media.

I believe that an accurate and acceptable parliamentary term for the
article, if it is the one I think it is, is Horlicks; but perhaps that
is not something that the USA is familiar with.



Hop : the Crookes Radiometer is propelled "backwards" because residual
gas is "reflected" from the black, hotter side with a greater gain in
energy. Some space-walker should take such a radiometer, show it
running backwards, and then open the vessel to a proper vacuum (perhaps
using a small Wake Shield) as a demonstration.

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Dr John Stockton wrote:

Hop : the Crookes Radiometer is propelled "backwards" because residual
gas is "reflected" from the black, hotter side with a greater gain in
energy. Some space-walker should take such a radiometer, show it
running backwards, and then open the vessel to a proper vacuum (perhaps
using a small Wake Shield) as a demonstration.


Thanks. I feel like an idiot.

Hop
http://clowder.net/hop/index.html

Rand Simberg wrote:

http://www.spacedaily.com/news/rocketscience-03zg.html

Interesting...

For those who are interested in a few more details, the New Scientist
press release refers to Gold's recent treatise, "The solar sail and the
mirror," which can be found on the LANL preprint server at
http://xxx.lanl.gov/abs/physics/0306050

--


Alex R. Blackwell
University of Hawaii

(Allen Thomson) wrote in message . com...
(Henry Spencer) wrote

A particularly glaring example is Radarsat 1,

Also the shards of Pageos, pieces of metallized Mylar film that
flew around in the photon breeze like bats. (I exaggerate, but
the light pressure (aka SRP) effects were very large.) Ted
Molczan, who followed their orbits, might want to comment. SRP
was lately a factor in figuring out that the object previously
identified as USA 144 is likely a light-weight decoy.

Tom Gold has a long history of coming up with iconoclastic
ideas.

Some of Gold's stuff has not been 'iconoclastic' , just plain wrong!

I notice that Tommy Gold's paper is only published online at physics
ArXiv , which means it will likely not be submitted to a refereed
journal.
Tommy Gold has had a strange career in astrophysics. His early work
was good
physics and later he was one of the first to figure out that pulsars
were neutron stars.
But at other times he seems to go just loony toons.

JRS: In article , seen in
news:sci.space.policy, Rand Simberg
posted at Thu, 3 Jul 2003 23:45:06 :-
On 3 Jul 2003 23:15:01 GMT, in a place far, far away, Dr John Stockton
made the phosphor on my monitor glow in such
a way as to indicate that:

JRS: In article , seen in
news:sci.space.policy, Rand Simberg
posted at Thu, 3 Jul 2003 15:45:01 :-
http://www.spacedaily.com/news/rocketscience-03zg.html

Interesting...

The originator has obviously forgotten some fairly basic physics - that
you are also unaware of

"Interesting" doesn't necessarily mean that I agree with it.

But you did not say that you disagreed with it.


Actually, what I found most interesting was the fact that the New
Scientist ran with it...

NS used to be a good magazine.

***

I'm not sure; but I believe that it has been demonstrated, on Earth,
that the Crookes Radiometer, if deemed to be running backwards when
there is the usual amount of residual gas, does run forwards if the
pressure is reduced sufficiently and the illumination is adequate.

--
© John Stockton, Surrey, UK. / ©
Web URL:http://www.merlyn.demon.co.uk/ - FAQish topics, acronyms, & links.
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In article ,
(Henry Spencer) writes:
|
| Note that the experts consulted for the article were a thermodynamicist
| and an astronomer, neither of them a physicist. (Citing the Crookes
| radiometer is just plain embarrassing -- it turns by thermal effects, not
| by light pressure.)

Well, I always thought that thermodynamics was a branch of
physics, but evidently even the term "physics" has now started
to overspecialise :-)

This seems to be a lovely example of people knowing more and more
about less and less.


Regards,
Nick Maclaren.