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Light Sails Won't Work?



 
 
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  #1  
Old July 3rd 03, 04:45 PM
Rand Simberg
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Default Light Sails Won't Work?

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

Interesting...

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  #2  
Old July 3rd 03, 07:05 PM
Henry Spencer
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Default Light Sails Won't Work?

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.)
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  #3  
Old July 3rd 03, 07:15 PM
Henry Spencer
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Default Light Sails Won't Work?

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.
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first ground-station pass 1651, all nominal! |

  #5  
Old July 4th 03, 12:15 AM
Dr John Stockton
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Default Light Sails Won't Work?

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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  #6  
Old July 4th 03, 01:15 AM
Hop David
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Default Light Sails Won't Work?



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

  #8  
Old July 4th 03, 06:35 PM
Dr John Stockton
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Default Light Sails Won't Work?

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.

--
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Web URL:http://www.merlyn.demon.co.uk/ - FAQish topics, acronyms, & links.
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  #10  
Old July 6th 03, 04:25 AM
Christopher A. Bohn
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Default Light Sails Won't Work?

Good morning,

Something I've been wondering about is the effect of the "heavier"
particles in the solar wind -- will the impacts of protons and alpha
particles have a significant effect, when compared to the acceleration
realized by the reflection of photons?

Does anybody more knowledgable than I have any thoughts on the matter?

Thank you, and take care,
cb

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