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Solar sailing DOESN"T break laws of physics'



 
 
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  #1  
Old July 3rd 03, 07:10 PM
Geoffrey A. Landis
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Posts: n/a
Default Solar sailing DOESN"T break laws of physics'

Despite a recent article in New Scientist, a solar sail does not break
the laws of physics.

In an article in New Scientist recently, maverick astronomer Thomas
Gold cast doubt about solar sails:
"Thomas Gold from Cornell University in New York says the proponents
of solar sailing have forgotten about thermodynamics, the branch of
physics governing heat transfer. Solar sails are designed to be
perfect mirrors, meaning that they reflect all the photons that strike
them. Gold argues that when photons are reflected by a perfect mirror,
they do not suffer a drop in temperature. "

Unfortunately, Gold has apparently forgotten to account for a
well-known physical effect: the Doppler shift.

It's worth saying that the photon pressure on a spacecraft is not
theoretical; its effect on spacecraft is measurable, and it has been
observed and measured to great precision routinely in space. Photon
pressure-- the solar sail effect-- has already been used for an
operational space mission; it was for spacecraft attitude control on
the Pioneer Venus-Mercury mission.

The Crookes radiometer does not operate on photon pressure, and the
explanation for how it operates has been known for over a century.

The energy transfer to a solar sail can be accounted for from the
Doppler shift of reflected photons; even when the reflectivity is
100%, a photon looses energy when reflecting from a moving sail. This
effect exactly corresponds to the energy increase of the sail. No
sophisticated physics is needed to analyze this effect, it is a
problem suitable for a homework assignment for a college
undergraduate.

When the sail is moving, then the reflected photons are Doppler
shifted, and leave the sail with lower energy than they arrived. This
loss of energy exactly equals the energy imparted to the sail, a fact
which can be trivially verified by using Newton's laws, the Doppler
formula, and the Einstein equation for photon momentum p=E/c

If the sail is not moving, there is no Doppler shift. However, note
that since energy is proportional to momentum squared, the derivative
of energy with respect to momentum is zero for a non-moving sail.
Thus, when the sail is stationary, it can reflect photons with perfect
efficiency and still gain momentum at no energy cost.

For completeness, note that if the sail is moving *toward* the light
source, then the phtons are Doppler shifted to *higher* energy by the
reflection. This implies that the sail must lose energy-- which is
correct; when the sail moves toward the light source, it slows down.

--
Geoffrey A. Landis
http://www.sff.net/people/geoffrey.landis
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  #2  
Old July 3rd 03, 07:20 PM
Gregory L. Hansen
external usenet poster
 
Posts: n/a
Default Solar sailing DOESN"T break laws of physics'

In article ,
Geoffrey A. Landis wrote:
Despite a recent article in New Scientist, a solar sail does not break
the laws of physics.

In an article in New Scientist recently, maverick astronomer Thomas
Gold cast doubt about solar sails:
"Thomas Gold from Cornell University in New York says the proponents
of solar sailing have forgotten about thermodynamics, the branch of
physics governing heat transfer. Solar sails are designed to be
perfect mirrors, meaning that they reflect all the photons that strike
them. Gold argues that when photons are reflected by a perfect mirror,
they do not suffer a drop in temperature. "


http://www.newscientist.com/news/news.jsp?id=ns99993895

Dang, I don't know what to say. Gold is a putz, and I thought New
Scientist was better than that. I didn't realize a solar sail was a heat
engine.
--
"Is that plutonium on your gums?"
"Shut up and kiss me!"
-- Marge and Homer Simpson

  #3  
Old July 3rd 03, 08:10 PM
Uncle Al
external usenet poster
 
Posts: n/a
Default Solar sailing DOESN"T break laws of physics'

"Geoffrey A. Landis" wrote:

Despite a recent article in New Scientist, a solar sail does not break
the laws of physics.

[snip]

Actually, it does as proposed. The sail will come into thermal
equilibrium with the radiation field and emit photons from its other
side, counter-thrusting. Solar wind will mostly stick rather than
bounce, incrementally increasing sail mass. The claimed efficencies
will be, surprise!, *much* higher than anything obtained. More
studies will be needed. Damn! It didn't scale linearly like it was
supposed to.

If the sail is a dielectric mirror, I don't know how it could be
fabricated to perform given engineering, cost, and weight constraints
including dense folding for launch. (This includes 3M multimicrolayer
plastic supermirrors). Dielectric sails will accumulate patches of
static charge from the solar wind. Judging from meter-scale
satellites, a square kilometer of arcing HV capacitor plate will be...
memorable. Hard UV will disintegrate organics.

If the sail is metallic, there will be god's own Hell of field
interactions. Space is rich with large scale electromagnetic this and
that. If you think solar flares are tough on power lines (EMP pulse),
imagine what they will do to a square kilometer of conductor. The
thing might buck like a bronco. Mechanical control lines must be
maximally thin or their mass is punitive. I don't care if you use
Kevlar - a klick of thin will stretch like taffy and suffer a low
speed of tensile conductance. There will be no real time control.

Uncle Al says, "Baikonur sleighride!"

Lastly, the payload or at least the control pod must be on the same
side as the sun since the solar sail can only bulge away from the
light. Any attempt at a building a rigid framework will negate the
payload. If the focus washes across the pod, hasta la vista baby.
Non-imaging caustics will also incinerate the pod.

I get 10^5 cm/km and 10^10 cm^2/km^2. At 10 mg/cm^2 the solar sail
alone weighs 100 metric tonnes. The thinnest capacitor aluminum foil
is 0.0015 inch for an areal density of 10.3 mg/cm^2. 0.0005"
aluminized Mylar is commercial, but it doesn't like getting warm or
irradiated and it is *fragile.* Hey... aluminum metal doesn't
tolerate heat either. 5-micron aluminized Kapton is about the best of
all worlds - unless you have to pay for a km^2 of the stuff as a
throwaway. Use of Parylene-C ultrathin membrane as in ornithopters
would bust even NASA's budget.

If you push optimistic numbers,
http://solarsails.jpl.nasa.gov/intro...struction.html
you get a bowl of moldy farina.

Oh yeah... even chemically tough Kapton in orbit gets chewed -
especially its reflectance,
http://setas-www.larc.nasa.gov/esem/..._append_b.html

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
"Quis custodiet ipsos custodes?" The Net!
  #4  
Old July 3rd 03, 09:25 PM
Ed Keane III
external usenet poster
 
Posts: n/a
Default Solar sailing DOESN"T break laws of physics'


Geoffrey A. Landis wrote in message
om...

When the sail is moving, then the reflected photons are Doppler
shifted, and leave the sail with lower energy than they arrived. This
loss of energy exactly equals the energy imparted to the sail, a fact
which can be trivially verified by using Newton's laws, the Doppler
formula, and the Einstein equation for photon momentum p=E/c


Doppler shift is caused by relative velocity and has nothing
to do with acceleration or deceleration. Any energy imparted
to the motion of the sail will cause a change in velocity and
will be indicated by a change in Doppler shift.

If the sail is not moving, there is no Doppler shift. However, note
that since energy is proportional to momentum squared, the derivative
of energy with respect to momentum is zero for a non-moving sail.
Thus, when the sail is stationary, it can reflect photons with perfect
efficiency and still gain momentum at no energy cost.


This is exactly what Gold argues. Except he points out, correctly,
that if there is no energy lost from the photons it violates the laws
of thermodynamics, or more simply conservation of energy, by
getting something for nothing if the sail gains momentum. Remember
that the sail is floating freely. It is stationary but elastic. It will not
be
stationary after the light hits it and the light is reflected after it hits
it.

-Ed




  #5  
Old July 3rd 03, 09:28 PM
Geoffrey A. Landis
external usenet poster
 
Posts: n/a
Default Solar sailing DOESN"T break laws of physics'

In Uncle Al wrote:
"Geoffrey A. Landis" wrote:
Despite a recent article in New Scientist, a solar sail does not
break the laws of physics.

[snip]

Actually, it does as proposed. The sail will come into thermal
equilibrium with the radiation field and emit photons from its other
side, counter-thrusting.


Yes, but if reflectivity is R, the amount of photons absorbed is A=(1-R).
These are then radiated from both sides. The re-radiation contributes
no force. For a sail perpendicular to the sun, therefore, the force (
assuming thermal equilibrium) is
F = I (2R+A)/c

accounting for the fact that the reflected photons contribute momentum
once on arrival and once on departure, for a factor of 2, and the
absorbed ones contribute momentum only on arrival, not on departure.

...


--
Geoffrey A. Landis
http://www.sff.net/people/geoffrey.landis
  #6  
Old July 3rd 03, 10:30 PM
Steve Harris
external usenet poster
 
Posts: n/a
Default Solar sailing DOESN"T break laws of physics'

Yes. If you need ultimate proof, consider not just the
Doppler effect, but the Compton effect. Electrons are
perfect mirrors and have no temperature. But thermodynamics
is upheld anyway when they scatter photons.


"Geoffrey A. Landis" wrote in
message
om...
Despite a recent article in New Scientist, a solar sail

does not break
the laws of physics.

In an article in New Scientist recently, maverick

astronomer Thomas
Gold cast doubt about solar sails:
"Thomas Gold from Cornell University in New York says the

proponents
of solar sailing have forgotten about thermodynamics, the

branch of
physics governing heat transfer. Solar sails are designed

to be
perfect mirrors, meaning that they reflect all the photons

that strike
them. Gold argues that when photons are reflected by a

perfect mirror,
they do not suffer a drop in temperature. "

Unfortunately, Gold has apparently forgotten to account

for a
well-known physical effect: the Doppler shift.

It's worth saying that the photon pressure on a spacecraft

is not
theoretical; its effect on spacecraft is measurable, and

it has been
observed and measured to great precision routinely in

space. Photon
pressure-- the solar sail effect-- has already been used

for an
operational space mission; it was for spacecraft attitude

control on
the Pioneer Venus-Mercury mission.

The Crookes radiometer does not operate on photon

pressure, and the
explanation for how it operates has been known for over a

century.

The energy transfer to a solar sail can be accounted for

from the
Doppler shift of reflected photons; even when the

reflectivity is
100%, a photon looses energy when reflecting from a moving

sail. This
effect exactly corresponds to the energy increase of the

sail. No
sophisticated physics is needed to analyze this effect, it

is a
problem suitable for a homework assignment for a college
undergraduate.

When the sail is moving, then the reflected photons are

Doppler
shifted, and leave the sail with lower energy than they

arrived. This
loss of energy exactly equals the energy imparted to the

sail, a fact
which can be trivially verified by using Newton's laws,

the Doppler
formula, and the Einstein equation for photon momentum

p=E/c

If the sail is not moving, there is no Doppler shift.

However, note
that since energy is proportional to momentum squared, the

derivative
of energy with respect to momentum is zero for a

non-moving sail.
Thus, when the sail is stationary, it can reflect photons

with perfect
efficiency and still gain momentum at no energy cost.

For completeness, note that if the sail is moving *toward*

the light
source, then the phtons are Doppler shifted to *higher*

energy by the
reflection. This implies that the sail must lose energy--

which is
correct; when the sail moves toward the light source, it

slows down.

--
Geoffrey A. Landis
http://www.sff.net/people/geoffrey.landis



  #7  
Old July 3rd 03, 10:48 PM
Henry Spencer
external usenet poster
 
Posts: n/a
Default Solar sailing DOESN"T break laws of physics'

In article ,
Uncle Al wrote:
Despite a recent article in New Scientist, a solar sail does not break
the laws of physics.


Actually, it does as proposed. The sail will come into thermal
equilibrium with the radiation field and emit photons from its other
side, counter-thrusting.


If the sail material has the same radiative properties on both sides,
there is no net counter-thrust at all -- the sail will emit equally from
*both* sides. In any case, for a good reflector, this is quite a small
effect compared to the thrust of the reflected light.

Guys, this stuff is not new. The basic physics have been understood quite
well for decades. Do a bit of research before sounding off, please. See,
for example, Appendix A of Jerome Wright's "Space Sailing".

Solar wind will mostly stick rather than
bounce, incrementally increasing sail mass.


Solar-wind atoms will stick *momentarily*, and then wander off again.
There may be a very slight initial increase in sail mass, but it will
reach equilibrium quickly. And in any case, the solar wind carries
several orders of magnitude less momentum than solar light pressure,
so its effects are slight.

The claimed efficencies
will be, surprise!, *much* higher than anything obtained. More
studies will be needed. Damn! It didn't scale linearly like it was
supposed to.


Sure it does. This stuff is well understood and is routinely allowed for
in precision tracking of deep-space missions. Moreover, unbalanced light
pressure is the biggest source of attitude disturbance for GSO comsats.
There is very little that isn't known about the subject by now. The major
unknowns of solar sails are the engineering hassles of deploying and
controlling large areas of very lightweight material.

If the sail is metallic, there will be god's own Hell of field
interactions.


Hardly. Almost all current sail designs are simply aluminized plastic...
quite like materials routinely used in the construction of spacecraft,
including some quite large ones.

Space is rich with large scale electromagnetic this and that.


Very feeble this and that.

If you think solar flares are tough on power lines...


Solar flares, by themselves, wouldn't do anything at all to power lines.
The trouble comes not from the flares themselves (more precisely, from
the associated particle events), but from the way they punch and pummel
Earth's magnetosphere. Outside the magnetosphere, nothing happens.

There have been deep-space spacecraft with very long wire antennas, for
low-frequency radio astronomy. They've had no trouble.

Lastly, the payload or at least the control pod must be on the same
side as the sun since the solar sail can only bulge away from the
light. Any attempt at a building a rigid framework will negate the
payload. If the focus washes across the pod, hasta la vista baby.
Non-imaging caustics will also incinerate the pod.


You seem to be under the impression that solar sails are like parachutes.
They're not; they can't be, because light doesn't act like air. It won't
keep a flexible canopy inflated. Sails have to be *rigid*, either from
centrifugal force or from structural members. The structural mass is
annoying but not prohibitive. Again, please do some research before
sounding off; this problem has been explored in considerable depth and
reasonable solutions do exist.

I get 10^5 cm/km and 10^10 cm^2/km^2. At 10 mg/cm^2 the solar sail
alone weighs 100 metric tonnes.


10mg/cm^2 is 100g/m^2, which may sound light to you, but is a load of lead
bricks by solar-sail standards. JPL's late-1970s design for a solar-sail
Halley-rendezvous mission had a total mass (film, reflector, backside
emitter, joints) of 3.2-3.5g/m^2. They thought 1g/m^2 was a reasonable
near-term lower limit. Highly advanced sail materials could reach
0.1g/m^2 or perhaps a bit less, although not soon.

The thinnest capacitor aluminum foil
is 0.0015 inch for an areal density of 10.3 mg/cm^2.


Massive, horrible, hideous. That's 38um thick. The JPL Halley design was
2um of Kapton, topped by 100nm of aluminum, with 12.5nm of chromium on the
back. 2-3um Kapton is commercially available.

0.0005"
aluminized Mylar is commercial, but it doesn't like getting warm or
irradiated and it is *fragile.*


Sails will be fragile. But they won't be exposed to much stress.

Hey... aluminum metal doesn't tolerate heat either.


It doesn't get particularly hot.

...Use of Parylene-C ultrathin membrane as in ornithopters
would bust even NASA's budget.


Hardly. Parylene N was considered for the Halley mission, but its space
compatibility wasn't certain then (not sure if it is now), and it wasn't
available in large sheets.

Oh yeah... even chemically tough Kapton in orbit gets chewed -
especially its reflectance,
http://setas-www.larc.nasa.gov/esem/..._append_b.html


Bare Kapton is quite vulnerable in low Earth orbit. Sails cannot operate
in low Earth orbit anyway, because of air drag. Atomic-oxygen erosion is
insignificant long before drag falls low enough for practical sail
operation.
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |
  #8  
Old July 3rd 03, 11:48 PM
MSu1049321
external usenet poster
 
Posts: n/a
Default Solar sailing DOESN"T break laws of physics'

If you spin the sail and put the payload pod at its center, you won't
incinerate it. Yes?
  #9  
Old July 4th 03, 12:22 AM
[email protected]
external usenet poster
 
Posts: n/a
Default Solar sailing DOESN"T break laws of physics'

In article , Uncle Al writes:
"Geoffrey A. Landis" wrote:

Despite a recent article in New Scientist, a solar sail does not break
the laws of physics.

[snip]

Actually, it does as proposed. The sail will come into thermal
equilibrium with the radiation field and emit photons from its other
side, counter-thrusting.


Nope. It radiates from both sides, thus its own emission results in
no momentum change (thus no "counter-thrust). As long as it is
illuminated unisotropically, it'll accelerate. Mind you, this does
not mean that it is practical for any serious purpose, but that's
another matter.

Mati Meron | "When you argue with a fool,
| chances are he is doing just the same"
  #10  
Old July 4th 03, 12:27 AM
[email protected]
external usenet poster
 
Posts: n/a
Default Solar sailing DOESN"T break laws of physics'

In article , "Ed Keane III" writes:

Geoffrey A. Landis wrote in message
. com...

When the sail is moving, then the reflected photons are Doppler
shifted, and leave the sail with lower energy than they arrived. This
loss of energy exactly equals the energy imparted to the sail, a fact
which can be trivially verified by using Newton's laws, the Doppler
formula, and the Einstein equation for photon momentum p=E/c


Doppler shift is caused by relative velocity and has nothing
to do with acceleration or deceleration. Any energy imparted
to the motion of the sail will cause a change in velocity and
will be indicated by a change in Doppler shift.

If the sail is not moving, there is no Doppler shift. However, note
that since energy is proportional to momentum squared, the derivative
of energy with respect to momentum is zero for a non-moving sail.
Thus, when the sail is stationary, it can reflect photons with perfect
efficiency and still gain momentum at no energy cost.


This is exactly what Gold argues. Except he points out, correctly,
that if there is no energy lost from the photons it violates the laws
of thermodynamics, or more simply conservation of energy, by
getting something for nothing if the sail gains momentum.


But there is energy lost from the photons. Do the math. That's where
the Doppler shift matters.

Mati Meron | "When you argue with a fool,
| chances are he is doing just the same"
 




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