Solar Sails - may not work
Solar Sails - may not work
(OP)
Ripped off New Scientist, I agree with the objection, the sails need to be black, not mirrors.
The World's No.1 Science & Technology News Service
Solar sailing 'breaks laws of physics'
12:38 04 July 03
Exclusive from New Scientist Print Edition. Subscribe and get 4 free issues.
The next generation of spacecraft propulsion systems could be dead in the water before they are even launched. A physicist is claiming that solar sailing - the idea of using sunlight to blow spacecraft across the solar system - is at odds with the laws of thermal physics.
Both NASA and the European Space Agency are developing solar sails and, although never tested, the concept is quite simple. A solar sail is essentially a giant mirror that reflects photons of sunlight back in the direction they came from.
Although photons do not have mass, they are considered to have momentum, so according to the law of conservation of momentum, the photon loses some of its energy to the sail as it bounces off, giving the sail a shove in the opposite direction.
Can it really sail away?
But 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.
Perfect mirrors
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.
That brings in a thermodynamic law called the Carnot rule, which basically states that you never get something for nothing: if there is no temperature change when the photons are reflected, it is impossible to extract any free energy from them to push the sail along.
"Carnot's rule says there must be a degradation of energy in any machine that turns out free energy," Gold says. "A mirror does not have any degradation."
This does not mean sunlight cannot exert a force - comet tails point away from the Sun, and are often cited as evidence in favour of solar sails. But Gold says this is because a comet tail is not a perfect mirror: it absorbs some of the light. In this scenario Carnot's rule says some energy can be extracted, so long as the object absorbing the light remains cooler than the radiation itself.
Heating up
A solar sail that absorbed photons would heat up within seconds, Gold argues. The claim has been greeted with scepticism. "There may be limits on how much solar radiation can be turned into work, but I do not think these are thermodynamic limits," says Jeffrey Lewins, a thermodynamics expert at the University of Cambridge.
But Gold insists that thermodynamics does have to be taken into account. "It's no good saying, 'I cannot turn heat into free energy, but I can if I turn the heat into radiation first'," he says. "That's obviously nonsense."
Steven Soter, an astronomer at the Hayden Planetarium in New York, is open to Gold's idea. He says applying conservation of momentum to photons could be a mistake. "Light is very different from matter, and one may wonder if the momentum rules are also different."
There may also be evidence to support Gold's theory, in the form of a quirky device called a Crookes radiometer. It consists of four paddles attached to the arms of a rotor, inside a vacuum jar. Each paddle is silvered on one side and coated with a black absorber on the other.
When placed in sunlight, the rotor spins. If the theory of solar sailing is right, the rotor should spin with the reflecting silver surfaces moving away from the light. But it actually spins the other way, just as Gold predicts.
The dispute could be settled in September, when the Pasadena-based Planetary Society hopes to launch Cosmos 1, the world's first solar sail. The 100-kilogram craft will be sent into orbit around the Earth, before unfurling a set of reflecting blades in an attempt to boost its altitude. Louis Friedman, the project's director, is undaunted by Gold's criticism. "Solar sailing is possible," he insists.
The World's No.1 Science & Technology News Service
Solar sailing 'breaks laws of physics'
12:38 04 July 03
Exclusive from New Scientist Print Edition. Subscribe and get 4 free issues.
The next generation of spacecraft propulsion systems could be dead in the water before they are even launched. A physicist is claiming that solar sailing - the idea of using sunlight to blow spacecraft across the solar system - is at odds with the laws of thermal physics.
Both NASA and the European Space Agency are developing solar sails and, although never tested, the concept is quite simple. A solar sail is essentially a giant mirror that reflects photons of sunlight back in the direction they came from.
Although photons do not have mass, they are considered to have momentum, so according to the law of conservation of momentum, the photon loses some of its energy to the sail as it bounces off, giving the sail a shove in the opposite direction.
Can it really sail away?
But 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.
Perfect mirrors
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.
That brings in a thermodynamic law called the Carnot rule, which basically states that you never get something for nothing: if there is no temperature change when the photons are reflected, it is impossible to extract any free energy from them to push the sail along.
"Carnot's rule says there must be a degradation of energy in any machine that turns out free energy," Gold says. "A mirror does not have any degradation."
This does not mean sunlight cannot exert a force - comet tails point away from the Sun, and are often cited as evidence in favour of solar sails. But Gold says this is because a comet tail is not a perfect mirror: it absorbs some of the light. In this scenario Carnot's rule says some energy can be extracted, so long as the object absorbing the light remains cooler than the radiation itself.
Heating up
A solar sail that absorbed photons would heat up within seconds, Gold argues. The claim has been greeted with scepticism. "There may be limits on how much solar radiation can be turned into work, but I do not think these are thermodynamic limits," says Jeffrey Lewins, a thermodynamics expert at the University of Cambridge.
But Gold insists that thermodynamics does have to be taken into account. "It's no good saying, 'I cannot turn heat into free energy, but I can if I turn the heat into radiation first'," he says. "That's obviously nonsense."
Steven Soter, an astronomer at the Hayden Planetarium in New York, is open to Gold's idea. He says applying conservation of momentum to photons could be a mistake. "Light is very different from matter, and one may wonder if the momentum rules are also different."
There may also be evidence to support Gold's theory, in the form of a quirky device called a Crookes radiometer. It consists of four paddles attached to the arms of a rotor, inside a vacuum jar. Each paddle is silvered on one side and coated with a black absorber on the other.
When placed in sunlight, the rotor spins. If the theory of solar sailing is right, the rotor should spin with the reflecting silver surfaces moving away from the light. But it actually spins the other way, just as Gold predicts.
The dispute could be settled in September, when the Pasadena-based Planetary Society hopes to launch Cosmos 1, the world's first solar sail. The 100-kilogram craft will be sent into orbit around the Earth, before unfurling a set of reflecting blades in an attempt to boost its altitude. Louis Friedman, the project's director, is undaunted by Gold's criticism. "Solar sailing is possible," he insists.
Cheers
Greg Locock





RE: Solar Sails - may not work
http://arxiv.org/html/physics/0306050
I'm betting the problem here is he's treating the incoming sunlight as heat. This is wrong, I think because the sunlight is all headed in the same(general) direction. It does not contain the same amount of entropy that normal "heat" energy does. It is ordered radiation, not the random movements of molecules.
The effect used to explain Crookes' radiometer that he scoffs at has been observed in systems of comlpetely different types, as well as in Crookes' radiometer. As well, a radiometer capable of observing the radiation pressure effects has been constucted on multiple occasions and shown Maxwell to be correct.
http://math.ucr.edu/home/baez/physics/General/LightMill/light-mill.html
IIRC the momentum of light arises from the Poynting vector anyhow and predates the p = E/c.
RE: Solar Sails - may not work
However, if the wall can move then it absorbs energy and the ball rebounds with less KE than before.
Now, a mirror offers no mechanism by which a photon can reduce its energy, therefore the photon cannot give energy to the mirror.
It's not Carnot, it is conservation of energy that is the basis of the argument.
Cheers
Greg Locock
RE: Solar Sails - may not work
Keep in mind that by absorbtion or reflection, the momentum exchange per photon is incredibly small.
I doubt they have a perfect mirror either.
I'd trust him more if he'd done a relativistic dynamics analysis in his paper and cited these explained away differences in the Crookes' radiometer.
What he implies inhis paper would have implications far beyond reflections of mirrors
RE: Solar Sails - may not work
It occurs to me that a black surface would still emit photons due to Black Body radiation. Like the greenhouse phenomenon they would be much longer in wavelength than those absorbed. There is no need even for the same number of photons to be emitted as absorbed. A black solar sail would just end up hotter, but it would definately not produce more thrust.
The Crookes radiometer phenomenon is just a red herring. In perfect vacuum the silver side does push the black side. A non perfect vacuum produces a phenomenon known as thermal transpiration, where expanding air causes a vortex around the black side edge - apparently producing thrust.
Check out:
http://www.howstuffworks.com/question239.htm
This is the danger of trying to reason out a relativistic phenomenon using Newtonian physics - it just don't work!
Mart.
RE: Solar Sails - may not work
http://www.planetary.org/solarsail/ss_and_physics.html
I'll just add, the most compelling argument that Dr Gold is wrong, is that every s/c is designed with solar radiation pressure in mind...
RE: Solar Sails - may not work
Photons have velocity of c, and enough of them exchanging kinetic energy can move the sail.
Peter V
RE: Solar Sails - may not work
Cheers
Greg Locock
RE: Solar Sails - may not work
Photon_Energy=Planck_Constant*Photon_Frequency
The red shift will be very small, but then the force that accelerates the mirror is very small.
You could visualise this as a piston being pushed by a train of pressure waves (i.e. sinusoidal about pressure higher than atmospheric). The waves are reflected of the piston, but if the piston moves fast enough (relative to the rest frame) the wavelength gets longer (ie frequency drops).
Ok in the real world we both know the higher than atmospheric pressure is really pushing the piston, but that's my point. Using the thermodynamics derived from Newtonian physics just isn't relevant in this instance, unless you are trying to warm up the mirror as well as push it. You need to have an understanding of relativity, and (to some extent) quantum physics to see this one.
I did my (aero) engineering degree at Loughborough, physics degree with the Open University so I'm not trying to hoodwink you!
Mart
RE: Solar Sails - may not work
The red shift mechasnism would give maximum thrust at v=0.5c (I think) whereas the momentum argument will give max thrust at v=0
The great thing is that this is going to be tested soon.
Cheers
Greg Locock
RE: Solar Sails - may not work
I see the source of the confusion now. In the same way that the beauty of a good engine lies in it's details, the beauty of physics lies in the details.
Consider an initially stationary mirror. The photon stravelling at speed c, strikes the mirror then bounces back off at speed c. The mirror recoils at the instant of impact, so that the reflected photon is produced from a moving mirror. Now relativistic doppler shift is not the same as acoustic doppler shift (with which you are no doubt familiar), but ends up demonstrating a similar effect. Since the mirror is moving away from the source, the reflected photon will now be redshifted, as it is re-emited from the mirror.
As was said before, the redshifted photon will possess less energy than the original photon. Since E=MC^2 or M=E/(C^2), then the reflected photon will have less mass, therefore momentum can be said to have been transferred from the photon to the mirror.
I could go all relativistic mathematical on you, but there is no point since I'm sure you see the reasoning. As for the quantum physicists? You don't want to know what they think - trust me on that!
Mart
RE: Solar Sails - may not work
But this shoud be a general result. I don't notice that mylar (or any other eg mercury) mirrors red shift particularly.
Cheers
Greg Locock
RE: Solar Sails - may not work
A result of General relativity tells us that radio transmissions from stationary (relative to earth C og G) will be blue shifted as they travel down the our gravitational field. I'll bet that the radio signals from QuinetiQ 1 aren't even vaguely perceptably shifted though.
I always thought it was a shame that non of the appollo missions carried atomic clocks, since that would have been the ideal demonstation of the twin phenomenon! I don't like using the word paradox, since once you understand about proper time (time measured with particle/spaceship/galaxy of interest) you see there really is no paradox.
BTW I take it you're a Dunton lad? I've been struggling to get back into mainstream engineering since BMW/Rover fell to bits - hard times...
Mart
RE: Solar Sails - may not work
Not Dunton, me, not by 10000 miles.
Cheers
Greg Locock
RE: Solar Sails - may not work
Gold's arguments ignore laser tweezers, where laser beams are used to constrain atomic particles. I think that this is a nearly perfect example of strict photon pressure mechanics, since it turns out that the particles are able to be cooled to near absolute zero strictly by the action of the lasers.
TTFN
RE: Solar Sails - may not work
In a conference about two years ago (you can see the presentations on the site of glasgow university) Astrium admit to use soem solar sail for AOCS ,i don't remember from the presentation if it was a mirror.But it works.
RE: Solar Sails - may not work
He's first assuming that a mirror does not allow redshifts. That makes no sense.
Then he talks about an aborbing object ceasing to absorb energy once its heated up. That's not true. An object in thermal equilibrium emits and absorbs an equal but nonzero amount of energy. If the input energy had a directed momentum (and the output radiation didn't), then momentum could still be transferred.
Conservation of momentum for radiation (photons) is observed everyday in particle physics experiments. Its not voodoo.
His argument about energy and momentum conservation giving contradictory results with radiation is also meaningless. Its well known that a free particle cannot absorb a photon. This is one of several reasons. Instead, both particles scatter. Atomic and nuclear physics would be completely different if this weren't true.
Then saying E/c is not a vector? Well... yeah. That's because the vector component was left off!
RE: Solar Sails - may not work
If the sail is a perfect mirror, then all of the impact momentum exchange of the photon should be seen.
RE: Solar Sails - may not work
Cheers
Greg Locock
RE: Solar Sails - may not work
RE: Solar Sails - may not work
Indeed.
RE: Solar Sails - may not work
Cheers
Greg Locock
RE: Solar Sails - may not work
I reread your post, and I guess I answered something you didn't ask :) Anyways, the photons do not have exactly the same frequency before and after. Reflection is an absorbtion/reemission process, so the the emitted photons will be Doppler shifted.
RE: Solar Sails - may not work
Cheers
Greg Locock
RE: Solar Sails - may not work
The laser photon pressure contains the atom and forces it into nearly zero thermal motion, thus reducing its temperature to near-absolute zero..
TTFN
RE: Solar Sails - may not work
Another way of looking at this is that all of a photon's momentum can be an absorbed by a massive object. Its energy, however, cannot all be turned into the kinetic energy. The energy-momentum dispersion relations are fundamentally different for massive and massless (photons) particles. The excess energy is put into atomic orbitals and such. These excited orbitals are unstable, so they will tend to radiate (this is reflection).
Of course the emitted photons must carry momentum as well as energy, so the emitting atom must be further accelerated in response to this momentum "creation." This takes energy though. Not all of the orbital energy goes into a photon. Some goes into the photon, and the rest goes into the atom's kinetic energy. All these processes nicely balance.
If you look at things in the reference frame of the atom, the exchanges are a bit different, but you can convert using the Doppler effect. The external inertial frame is the one you really care about anyways.
RE: Solar Sails - may not work
And Gold IS senile -- but brilliant. Note his claim that ALL oil wells are in Mountains, which he forced Scientific American to treat opponents badly for.
Re the Light/heat you will find that heat energy from the Sun is about 3000 times the momentum transfer used for Sails -- thus every loss through Redshift barely affects the total energy. Sorry for the vagueness, but you can easliy see it when you look at the Halley Comet Solar Sail & Solar-electric versions: the Sail makes the Solar wing look TINY.
RE: Solar Sails - may not work
Anywho, are there any estimates on acceleration?
Probability of tears or fragmenting that would make it less effective?
RE: Solar Sails - may not work
Interesting, if impractical.
Mart)
RE: Solar Sails - may not work
The more accurate orbit propagators include radiation pressure in them.
Laser cooling via radiation pressure....
The Casimir effect...
Particle physics...
Arises out of maxwell, relativity, and quantum mechanics...
Certainly more...I'm sure it has been measured directly, the measurement of the Casimir force is almost a direct measurement of radiation pressure (albeit, from virtual photons).
If Dr Gold were right it would pretty much trash the last 100 yrs of physics. Or it calls into question the ultimate applicability of thermodynamics (an even older branch of science/engineering)
RE: Solar Sails - may not work
1 Proposal
A photon strikes a stationary perfectly reflecting surface, and is reflected at the same frequency.
Therefore it is emitted with the same energy as it arrived with, so no energy transfer can take place with a perfect mirror.
Observations - a mirror does not change the frequency of incident light.
2 Counterproposal
The photon is absorbed momentarily and then re-emitted, at the doppler shifted frequency of the now moving body.
3 Objection
The thrust due to this would be a maximum as the sail approaches the speed of light, since that would maximise the doppler shift
Note that Gold is not saying that sails won't work (depsite my title for this thread), he is saying (roughly) they should be black, not reflective.
That mission should have flown by now, how closely did the acceleration match the model? Was it possible to break out the proportion of the thrust obtained by photon reflection rather than absorption of photons and other particles?
Cheers
Greg Locock
RE: Solar Sails - may not work
TTFN
RE: Solar Sails - may not work
QED is the most accurately and precisely verified theory in existence(period!)....by experiments involving radiation pressure from virtual photons.(the precise spin of atomic electron orbitals, etc)
RE: Solar Sails - may not work
RE: Solar Sails - may not work
http://planetary.org/solarsail/latest_update.html
TTFN
RE: Solar Sails - may not work
"2 Counterproposal
The photon is absorbed momentarily and then re-emitted, at the doppler shifted frequency of the now moving body."
Interestingly, this is the modern view of reflection, at least per what I've read of Quantum Electrodynamics theory. Whether a "reflection" occurs or absorption depends on how long the delay is between absorbtion and re-emission of the photon. See Richard Feinman's books "QED" and "7 Not-so Easy Pieces" for a very readable, layman's-terms description.
The problem with making a sail black is that the emitted radiation (as thermal Stephan-Boltzman law radiation) now leaves from both the "front" and "back" sides of the sail, yielding no net thrust or momentum change.
No doppler shift (red or blue) occurs on a stationary mirror because no velocity change occurs of the mirror relative to the observer.
Radiation or photon pressure does exist, as IRstuff has pointed out in his posts.
Finally, the use of Carnot law is impossible with photons, because they travel at a fixed velocity. Gas kinetics theory states that temperature is related to the average kinetic energy of the gas particles, which is related to the average (or rms) velocity. Since velocity is unchanged, you can't state what the "temperature" of a photon is. You measure the energy content of a photon by its frequency or wavelenth, since its speed is constant. And that is relative, as Einstein pointed out.
Reflective sails do obey the 1st & second laws, on an energy basis, when the freq. shift of the incident radiation is considered. Losses occur within the sail material due to incomplete reflection of incident radiation and resulting heat being generated (and then radiated away). And, since the sail gets its highest energy photons when at rest relative to the source, the thrust of the sail drops as its relative velocity increases away from the source (sun).
Yeah, too bad about the sail mission. Gonna be awhile before somebody spends any more money on it.
RE: Solar Sails - may not work
Neither they nor their neighbor in Pasadena, the Planetary Society, seems to have had much success so far. Considering how much effort NASA and others have spent making spacecraft to confirm certain of Einstein's predictions, it sure would be nice to get a proof of concept launch finally done for potentially practical solar sail technology.
Advanced propulsion never seems to get much priority, unfortunately. It took decades before NASA finally flew a solar electric mission, Deep Space 1 [http://nmp.jpl.nasa.gov/ds1/] (which went quite well).
One recent development is Space Services' announcement they are working on a solar sail spacecraft to provide (from a libration point) a space weather observatory and communications relay for Antarctica [http://www.spaceservicesinc.com/intro.asp]. They apparently have a preliminary NOAA contract. This is a really good idea but they don't seem to have much information available at this point so it's not clear if this will come to much.
Solar sails are a great idea, but will have difficulty seeing any practical use until the concept is proved. NASA used to do a lot of engineering proof of concept missions (e.g. Gemini, the Application Technology Satellite series) but hasn't seemed very interested in these since the early 70s, which probably has a lot to do with why we keep spinning our wheels with not very aggressive programs. No one has enough confidence in unproven new technologies to risk billions on them.
RE: Solar Sails - may not work
TTFN
RE: Solar Sails - may not work
jmmo
RE: Solar Sails - may not work
TTFN
RE: Solar Sails - may not work
As to effectiveness, the photons will exchange momentum with the sail through the processes of absoption and emission. So it comes down to a discussion of electromagnetic interactions. This is clearly beyond the scope of thermodynamic principles. Furthermore, the pressure exerted is a kinetmaic pressure (force per unit area) rather than a measurement of fluid energy.
A quick review of Jackson (3rd Edition) problem 6.11 shows the basic ideas of what is occuring. However, the implicit assumption that the sail is a perfect absorber and furthermore emits no electromagnetic energy is in question. I also believe there is an assuption that no electromagnetic energy is converted to thermal motions in the sail.
As a final note, even if only 1% of the flux of solar EM energy is converted into motional energy you still gain an order of magnitude higher acceleration that by using proton momentum exchange.
RE: Solar Sails - may not work
Although the Cosmos 1 solar sail test suffered a launch failure last year, I was under the impression that the Soviets (or somebody) had more or less validated the basic solar sail physics just from using small sections for attitude control some years back. I didn't find anything specific about that with a quick Google search, but there is a page that notes at least some of the stuff that has flown before, including the Indian INSAT 2A & 3A:
ht
There is also Ben Diedrich's website
http://www.solarsails.info/index.html
Which includes a page on the physics of solar sailing
http://www.solarsails.info/sailing/sailing.txt
Though this doesn't really get into the light pressure vs. thermodynamics question.
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