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Solar Sailing and Physics
Roland Piquepaille writes "In this article, the New Scientist writes that the next generation of spacecraft might be propelled with the help of the sun. "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." 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." And this is where it's becoming interesting. Gold's paper, "The solar sail and the mirror," states that "either Carnot's accepted rule is in error, or the solar sail proposal will not work at all." So, as this illustration from New Scientist shows, the real question is: "Can it really sail away?" We'll know it in September when the first tests are done. In the mean time, read this summary for more details and read the original stories for far more information."
Here's the crux of his argument: But what will be the performance of the mirror as a heat engine? If the mirror receives heat energy from the Sun and converts some of this into free energy, namely the kinetic energy of its motion, it falls into the strict definition of a heat engine, and Carnot's rule defining the maximum efficiency for this energy conversion must apply. We can determine the incoming temperature of the radiation by measuring the temperature an absorbing (black) body would reach when exposed to the radiation being sent to the mirror, and the temperature a black body would reach exposed to the outgoing radiation from the mirror, both measurements carried out in common motion with the mirror. Carnot's rule would then give the maximum efficiency as that fraction of the heat flow trough the mirror, given by the difference of the two temperatures, divided by the input temperature. It would be that fraction of the heat flow that could maximally appear as kinetic energy gained by the mass of the mirror. If this was a perfect mirror, the two temperatures will be the same, and it follows that the mirror cannot act as a heat engine at all: no free energy can be obtained from the light. The proposed solar sail cannot be accelerated by sunlight.
Carnot only applies to closed systems. In textbook examples of heat engines, the engine, the heat source and the heat sink are all included in the analysis. Gold has included the engine (the sail) and heat source (the sun), but he's neglected the heat sink (the almost-perfect blackbody of intergalactic space). It isn't the temperature difference between absorption and emittance that matters, it's the temperature difference between source and sink, and that difference is huge here.
Won't it only be useful for travel away from the sun? So, it might be used in say space probes, but nothing like a Mars mission or at least only one way in a Mars mission.
'He was a dreamer, a thinker, a speculative philosopher... or, as his wife would have it, an idiot.' - Douglas Adams
Actually, the number of misconceptions and errors in this "article" boggle the mind... For example,
Except, of course, that that expression is for the magnitude of the momentum. Duh. The momentum carried by the photons emitted by the Sun lies in the direction those photons take; for any given photon, the momentum is radially away from the Sun. For all of them together, the momentum is zero because they all cancel -- but that happens only when you integrate over the entire sphere. For the tiny portion hitting a sail, there would be net momentum.
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Also, its not a perfect vacume, and look real close,,,no reflectors..black side and white side..and look which way it spins. What happens is the light heats the black side and the few molecules floating around in there hit the hot side and bounce off faster, having grabbed heat, imparting motion, or something to that general effect.
"The sun provides negligable energy out past the orbit of Mars"
I doubt this approach uses light as a form of energy. The idea here is to think of the light photons as 'mass' rather than 'energy'. Since E=mc^2, it follows:
m=E/c^2. since c=velocity of light (10 power 10) and E could be 10 power -24, the mass of a photon could be infinitesmally small, and negligible.
My chief concern here would be, if a satellite can be propelled by reflecting photons, then the 'deflection' caused by a single hydrogen atom (of which there could be lots in space, besides dust and gases) could cause deviations, millions of times greater in magnitude compared to the desired motion.
Simply stated - unworkable, but then, try telling that to 'scientists', specially those reading Slashdot!
If you keep throwing chairs, one day you'll break windows....
The trouble with that analogy is that you can only tack a sail boat because it has a centreboard to stop it from going sideways, which is difficult to arrange for in space. Try it without sometime.
The other comments about gravity doing the moving-towards-the-sun bit sound right, though.
There is a solar wind consisting of particles with rest mass. That can do the pushing.
Finally! A year of moderation! Ready for 2019?
His Crookes radiometer description was incomplete The good ones have 2 TWO globes that look the same BUT spin in the opposite directions. One is powered by the black heating up the air. The second is in a VACUUM and is powered by the photons bouncing off of the mirror. Please read before mod.
The article is cluttered with flaws and unfortunate misinterpretations of laws of physics. These flaws do not turn better if they are part of a complex theoretical explanation.
1.) Electromagnetic radiation has momentum, otherwise, there would be no electromagnetic forces. Period.
2.) Light is not heat - it is a directed stream of photons and a solar sail is by no means a heat engine being limited by the Carnot principle.
3.) Energy conversion holds due to doppler effect - reflected photons get their frequency shifted to red.
4.) Sailing boats have been working perfectly for thousands of years, using a similar principle with air carrying the momentum. And the air was not absorbed by the sails!
I will pick on this one scientifically ignorant claim:
"Would it be better to place a black sheet there instead of a mirror-faced one? Unlike the mirror, this could absorb energy and the momentum associated with that. But it would do this only from the moment of its exposure until it reached thermal equilibrium with the available radiation. Then energy absorption would cease, and with that the delivery of momentum to the sheet would also cease. For any lightweight sheet, this time would be only seconds."
Energy absorption does not cease in equilibrium! Rather, the amount of energy absorbed by the sail from the sun is equal to the amount of energy emitted by the sail into space.
Allow me to make an analogy: The earth is in thermal equlibrium with the sun (close enough). Would you like to go outside on a sunny day in Las Vegas and tell me that energy absorption has ceased? Why is your skin turning red?
Well, I don't think you can use these forms of the equasions, since photons have no mass. They do have kenetic energy, IIRC.
autopr0n is like, down and stuff.
The same way sailing ships move against the wind... A zig-zag course, called tacking. Umm, tacking only works because there are two media; the water and the wind. A aerodynamic shape (sail) uses the wind to create a thrust vector in the direction the wind is blowing. A hydrodynamic shape (the hull) uses the water to create a thrust vector perpendicular to the keel pointing in the general direction the wind is coming from. Add these together and you end up with a vector that points parallel to the keel, either forward or backward. In free space you only have one medium, the solar wind. This is like a balloon rather than a ship, and the solar sail (like a balloon) will go the direction the wind is blowing no matter how it is aligned to it. It is possible to get a perpendicular (to the wind) vector component based on what direction you deflect the solar wind, but without a second medium to interact with you cannot get a negative parallel component.
The sun provides negligable energy out past the orbit of Mars.
Not negligible, but solar intensity does fall off as 1/r^3. In Mars orbit, the solar radiation on a surface normal to the incipient light is about 60% of that in Earth orbit. This represents the fact that the photon are spreading out in a sphere from their source--though there are just as many as there were on the surface of the Sun, there's now a whole lot more space in between them.
I'm most familiar with this in the context of solar powered spacecraft. To operate a solar S/C near Mars, you need massive unwieldy solar arrays that are expensive to launch. The only other viable power source for space, currently, is thermoelectric conversion from the heat generated by nuclear decay (not a live reaction), and is only 6-7% efficient.
We still need someting like Prometheus in order get around and about in places where the sun doesn't shine brightly.
Agreed. The Nuclear Space Initiative is the only way we will ever get something sizeable to the outer planets--and back. Spaceflight requires too much power to be generated any other way, at least with the science we can do now.
-Carolyn
Like Daddy always said: if you can't dazzle 'em with brilliance, baffle 'em with bullshit.
Is that like saying that sailing on an ocean into the wind is impossible? Hate to tell you, but it can be done. The only problem with doing this on a solar sail is figuring out exactally how one 'tacks' into the light.
Of course, this also discounts that one can use other gravitational bodies in order to change trajectory, and if you're going to Jupiter and comming back in one nonstop trip, you can always fold/destroy the sail and coast on the initial momentum from the trip out.
Fortunately it is also irrelevant. When dealing with photons, the kinetic energy equation is E=cp as parent stated. And no, you don't substitute mass into p - De Broglie's relationship makes this E=hc/(lambda), where h is Planck's constant and lambda is the wavelength of the light.
-Looking for a job as a materials chemist or multivariat
Reflected back, and red-shifted if the mirror's moving away. The physics here is elastic scattering, not thermodynamics. A photon with given energy and momentum hits the mirror which has given energy and momentum, photon reflects, and when you solve equations for conservation of momentum and conservation of energy you find that the mirror is moving a little faster in the original direction of the photon.
Thermodynamics would only come up if someone claimed to be sailing on microwave photons from the 3-degree blackbody radiation. *That* would be a perpetual motion machine.
Gold's article was painful to read and I wonder if it was a troll of some kind, perhaps the white-hat kind designed to make people think in depth and understand a subject better.
A few other things that bothered me: radiation pressure is an observed phenomenon, nuclear weapons wouldn't work without it. The reason Crooke's radiometer spins the wrong way is known -- the black side is a little hotter and causes convective currents in the traces of remaining gas. The evidence for this is that if you make a Crooke radiometer with a complete vacuum it doesn't spin.
Gold, by the way, has an interesting track record which is worth looking up.
The solar sail is a heat engine. But he's still wrong.
The basic claim is that the photon doesn't lose energy to a perfect mirror. But that's wrong. It neglects both the ACCELLERATION of the mirror due to the impact of the photon, and the red/blue shift of the photon when reflected from a mirror in motion relative to the observer.
It's easy to understand the lightsail/sun/photons system as a heat engine: The lightsail is the piston and the photons are the working fluid.
Just as with a piston, if the lightsail were held still (and the mirror were perfect or imperfect but at solar temperature) the photons would rebound without loss of energy. But the high photon-gas "pressure" on one side of the "piston" versus the near-vacuum (dark sky) on the other side means there is a force on the mirror. If not held it will accellerate.
Just as with a piston, no work is done on it until it starts to move - and the faster it moves the more work is done on it. But the faster it moves the more the light is red-shifted, i.e. the "gas is cooled", so the more rapidly work is done. Exactly what you expect in a piston engine.
You could also push the light sail toward the sun (as when decellerating at the far end of the trip). In this case the photons would be blue-shifted and the work from pushing the sail against light pressure would thus go into "heating" the photons - and the sun, if the sail was pointed properly so the "photon gas" hit the far end of the "cylinder" rather than escaping.
His analysis assumed the sail was unmoving and unaccellerating, which is just plain confused.
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On a more technical level, the incoming photons do not obey Maxwell-Boltzmann statistics (not even vaguely approximately), so it is not semantically valid to make thermodynamic statements about them.
As to energy conservation, photon reflection is physically an absorption followed by a reemission. Since the mirror is accelerated by the process, an observer in the rest frame sees a doppler redshift of the reflected photons, and thus energy balance is maintained.
Finally, even if you wanted to sprinkle goat blood on the photon spectrum and call it a thermodynamic quantity, the redshifting preserves the blackbody spectrum (one of physic's remarkable results) while making it "colder", and thus the "temperature" decreases appropriately.
Show me the Planckian radiators, Gold! And then we can talk thermodynamics.
-- ;-)
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OK, the bit about the photon gas being the working fluid is clever. But Gold's point is, solar sails would violate the Carnot condition. Carnot's analysis applies only to closed, cyclic engines. No one is proposing this as a closed, cyclic engine.
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