Slashdot Mirror
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."
The article is wrong in the sense that it treats the photons from Sun to be in the form of heat - they are not, because their velocities are not randomised - there is a net momentum radially away from the sun. Carnot's cycle assumed the thermal energy to be in the thermodynamic form, i.e. say internal KE of gas, etc... You could simply adapt this guy's argument to a bullet hitting a plank of wood and show that wood gaining motion would break carnot's law - this is not the case.
Yes, but for a different reason... The black side heats up more than the shiny side and it recoils molecules more... - the photon effect works the other way round but is negligible in presence of air...
Sure, if you could make perfect mirrors. And if you only cared about the motion of light itself.
Gold's theory ignores one important aspect- the doppler effect.
Let's do a gedankenexperiment (thought experiment).
Hypothesize that you have such a solar sail and it's already in orbit and starting to pull away from earth (say, because an ullage rocket has initiated a short accelleration)
The incoming solar photons - IN THE FRAME OF REFERENCE OF THE SAIL - enter and leave at constant wavelength. But the sail is moving with respect to the rest of the solar system (the ullage rocket kick-started this motion)... so the wavelength measured in the frame of reference of the sail mirror is not correct.
In the external (non-accellerated) frame of reference of the solar system, the photons hit the mirror at some particular wavelength, but exit at a longer wavelengh (because the mirror is moving).
The count of photons is the same- but their energy is lower.
So, where did the energy go? Draw the Feynmann diagram: there's only one place it could have gone- and that's into the sail itself. Therefore the sail accellerates further.
A similar gedankenexperiment will show that a sail moving _toward_ the sun pumps energy into the photons, and so decellerates.
Didn't Voyager and Galileo take advantage of the solar wind to get way out there in a short time?
No (human) spacecraft to date has used the solar wind for propulsion - the solar sail is the only realistic mechanism for doing so, and that's never actually been tried (there was to have been a test of the Cosmos 1 couple of years ago but it suffered a launch failure).
Nae bother
The solar sail and the mirror
Thomas Gold (Center for Radiophysics and Space Research, Cornell University)
The radiation pressure exerted by incoherent light on diverse surfaces is examined. The thermodynamic rule, first given by Carnot in 1824, describes the limitation to the amount of free energy that can be obtained from a source of thermal energy, and he gave the compelling reason for this rule, that if more free energy than he had prescribed could ever be extracted, then a heat pump could use that free energy and re-create all the heat energy that had been consumed. A perpetual motion machine could then be constructed. Now, 179 years later, it is proposed to fly a spacecraft that is expected to gain velocity from the radiation pressure the sunlight is expected to exert on solar sails, panels of thin plastic sheets, mirror surfaced on the side facing the sun. However a detailed examination of this proposal shows it to be in direct conflict with Carnot's rule, and no such pressure can be expected. Either Carnot's accepted rule is in error, or the solar sail proposal will not work at all.
Carnot, a French engineer had described in 1824 a basic law of thermodynamics: heat energy can be converted into "free" energy, such as mechanical energy of motion, but only in an engine which must have certain properties. Heat must enter it at a temperature which we will call T1, and it must then be degraded in the engine to a lower temperature, T2. A certain fraction of this flow of heat energy can then be converted into free energy. The maximum fraction that can be so converted is given by (T1 - T2)/T1. He had shown that the cycle is reversible, so that a heat pump can be constructed that would use free energy to deliver heat; moreover that it would be able to reverse the heat flow from T1 to T2 precisely, if given the maximum free energy obtainable from the heat engine. Thus he showed that a perpetual motion machine could be constructed if either the heat engine or the heat pump could achieve a higher efficiency than that which he had stipulated. Any device that can obtain free energy from a supply of heat, by whatever means, is thus covered by Carnot's rule.
179 years have gone by during which all the heat engines we now employ for every aspect of our civilization have been designed, and all their designers have recognized Carnot's rule as the ultimate aim of their designs. The physicists of that long period have all agreed with that rule. Lord Kelvin based his deduction of the absolute zero of temperature on Carnot's considerations. The absence of perpetual motion machines seems to show that no one has succeeded in overcoming the limitations prescribed by Carnot.
Yet now, we have a proposal on the table that runs counter to the rule of Carnot. It is proposed that the radiation pressure on a mirror from a hot body, the Sun, could be used to supply propulsion energy and momentum to a spacecraft, and thus facilitate interplanetary travel of vehicles, without the need for any other means of propulsion. What a desirable solution this would be! The Sun would pour out its energy whatever we do to it, and the momentum associated with that, calculated by Maxwell and confirmed later by Einstein, would be E/c, where E is the amount of energy emitted in a given interval of time, and c is the velocity of light. If a perfect mirror is used to receive the sunlight and its momentum, the re-emission of that light would gain the same momentum once more, and thus the force exerted on a perfect mirror would be doubled. The best mirrors are not completely perfect, but this would cause only a small loss of efficiency. It is proposed to use thin plastic sheet with aluminized mirror surfaces for these "solar sails". The speeds were calculated for a certain speeds of interplanetary travel to be obtained. A fund of several million dollars was assembled for the first space experiment of the new technology is proposed to be launched within a few months of writing this.
But what will be the performance of the
When you can bandy about cool names like "the Carnot cycle", why let actual facts get in the way?
This guy is wrong. Period. The solar sail would not be a heat engine -- it's not an engine of any kind -- so Carnot's analysis does not apply. Radiation pressure does exist and can be used to blow objects away from the Sun. Don't believe me? Too bad, because we have photographic evidence: The tails of comets always point away from the Sun (during the approach and departure of the comet), in part due to the radiation pressure on the dust that makes up the tail.
The Mongrel Dogs Who Teach
http://science.slashdot.org/article.pl?sid=01/07/2 0/1246254 2 2/0321239
http://science.slashdot.org/article.pl?sid=01/07/
There was another Slashdot article about solar sails (from 2002-04-29) here:
http://science.slashdot.org/article.pl?sid=02/04/2 9/1246221
It mentions a test to be launched "sometime after September". I don't know if that test ever launched.
Nope. They could maneuver in a way similar to that sailing ships use to go upwind. By angling the sails correctly and using the sun's gravity field, a solar sail vessel can fly "upwind" toward the sun. See this NASA reference for a basic primer.
light-mill or radiometer do not rotate due to "radiation pressure" as I was once taught and is even the explanation provided in the Encyclopaedia Britanica today. Check out link above for an interesting description.
The Planetary Society already has a solar sail project going and is preping to launch soon.
http://www.planetary.org/solarsail/
There is a very introductory book about solar sailing by Louis Friedman, Executive Directory of the Planetary Society: it's Starsailing: Solar Sails and Interstellar Travel (yeah, yeah, it's amazon, thbbbppp) and here are some selected chapters. It rocks because it walks you through the equations and such. It's really NOT hard to understand. I found it in my high school when I was a kid and really fell in love with the whole concept. It really rocks.
In the book it points out that the concept was tested with the Mercury probe that NASA sent way back (Mariner-10) in that they used the solar panels to get a spin from light pressure (iirc, it's in the book and I haven't read it in 10 years...)
Do you know why the road less traveled by is littered with the bones of the unwary?
The black vanes on the spinning apparatus absorb light more efficently than the white ones.
This absorption of energy causes the black side of the vane to be hotter (by a very small amount, i'm sure) than the white side.
The heat radiating off of the dark side of the vanes works much in the same manner as a jet engine (without the need for a compression chamber). Note that the reason it's in a glass bulb is to impose a partial vacuum is to reduce air friction which would keep the vanes from moving. Also the use of a needle point piviot further reduces the friction.
So, in a word no. The solar sail intends to gather energy by photons bouncing off of a sheet, while your example is really just a simple heat engine.
You can verify this independantly with a little obseration and thought. After all wouldn't the white side of the vanes be providing the thrust if the energy was harnessed from potons bouncing off of it?
Mariner 10 did in 1974, although not as a primary means of propulsion. (I assume that by "solar wind" you mean radiation pressure, not actual solar wind?)
There's no conflicts here with comet tails. Comet tails form because the solar wind (which no-one doubts exists) causes a *temperature* change in the frozen particles, just like the black side of the blade in a radiometer. If the comets were hot enough not to be heated by the radiation, or were 100% reflective, the tail wouldn't exist.
As for the tail pointing from the comet, that's true for one of the tails. There's at least three for each comet:
1: The radial tail, which points away from the sun.
2: The debris tail, pointing in the wake of the comet.
3: The plasma/ion tail, which is caused by heavier particles sent out by the sun.
(4): Some even claim a fourth tail, pointing towards the sun, which is interesting, because the release of that matter would be a newtonian force pushing the comet further away.
And no, there is no
5: Profit!
Anyhow, there's been no tests showing that a solar sail *could* work, unless it can bleed off material and retain a cold temperature like a comet does.
Comet tails.
Comet tails *always* point away from the sun.
Must be something pushing the tail particles away.
QED
www.christopherlewis.com
You are correct that the Carnot efficiency cannot be calculated for this case, as it is not returning to it's original position. However, in the postulated case of a perfect mirror, there is no heat sink, as the photons will be reflected back towards the sun, and not radiated from the back of the sail. If the photons are absorbed by the sail, then some are radiated to the heat sink, and the expansion is no longer adiabatic (which is consistent with the lower momentum transfer). Note that if the mirror ~was~ returned to its original position, and the sun was insulated against losses in directions other than towards the sail, you would have an adiabatic expansion and then compression of the photons, leaving you with no net change in energy.
Gold complains in his paper of physicists not treating photons thermodynamically. That's funny - I seem to remember working out a problem using an adiabatic expansion of a photon "gas" in my undergraduate days. I don't remember the equation of state off the top of my head, but I guarantee you can find it in Callen or any other thourough thermo book.
Yes, IAAP. I also think that slashdotters should note that this was published in the Arxiv, which is NOT a peer reviewed publication (although I must say that the Arxiv rocks!). The Arxiv is sort of like an open source scientific journal, or a BBS for scientific papers. I highly recommend wandering around in it for a few hours, but remember to take everything in there with a grain of salt.
Darn. I was going to try NOT posting to slashdot today. Oh, well. Feed the addiction (sticks needle labeled "/." in arm).
Um, sorry, no. Photons have no mass. You need the full form of the equation: E^2=(pc)^2+(mc^2)^2 which for a massless photon (m=0) would become E=pc. p in this case is momentum, which photons do have.
When the photon strikes the sail it imparts momentum to it. LOTS of photons=lots of momentum and near the Sun there are a LOT of photons. Bear in mind the sail will need to be HUGE. Also momentum has direction (vector not scalar).
Oh, and the effect of the odd hydrogen atom should be very small in comparison to the LOTS of photons constantly (alright, discretely if you're delving into duality) striking the surface of the sail. It will be slowed down slightly by atoms floating through space but if the sail is large enough and light enough then theory says it should be able to break free of the Sun's gravity (of course that's what the original argument is about). Do you really think one or two little hydrogen atoms will be much of a problem?
No numbers were harmed during the course of this diatribe, but a few ego's were bruised...
If you can't think of something nice to say then don't say anything at all. No, REALLY.
Well, I am a physicist (PhD MIT '97) and unless I'm missing something *really* subtle, Gold's argument is just plain wrong. As earlier posters pointed out, this is not a system in equilibrium, nor is it closed. Conservation of momentum (which applies perfectly well to photons with the relations
energy=speed of light * momentum or e=c*p or p=e/c
and
energy=planck's constant*frequency=h*nu=h*c/lambda
p=e/c=h*c/lambda/c=h/lambda
When I reflect a photon from an object the total momentum of the system = photon + object must remain constant (if we're scattering elastically)...
Inelastic scattering effects (doppler, etc.) don't change the basic story here. SOME momentum will get transferred to the mirror with each photon bounce...
Also, there's no reason I can't reflect the photons from the solar sail at a slight angle to the sun (just like tacking into the wind) at the cost of losing some of the momentum kick from the photons, the photons now travel off into empty space (as opposed to the sun) making the system clearly open... It will continue to be accelerated so long as photons hit it (albeit with decreasing numbers of photons/sec as it gets farther out...)
Yes, but for a different reason
The author of the original article seems to be unaware of this:
Crookes' radiometer has invariably rotated in the opposite sense to the expected one. The black side of the paddles invariably recedes from the light, and many explanations have been offered, but not including that which would seem the most obvious: the absence of radiation pressure on the bright side
Sure, the temperatur change is necessary for the comet's body to evaporate partially, so that the tail can form. But that doesn't have anything to do with the direction in which the tail points.
The illegal we do immediately. The unconstitutional takes a little longer.
--Henry Kissinger
In the way of background, note that Gold is the same Gold of the Bondi-Holye-Gold steady state cosmological model, proposed in the 1940s and 1950s as an attempt to "fix problems" with the big bang model, and has long held non-conventional views on light. Gold and others invoked "tired light" -- photons which became redder from their point of emission, even though doing so contradicted momentum-energy conservation. It's a archetypical example of a theory trashing a fundamental principle in order to exaplain last week's cosmological observations. We should always be wary of our assumptions, but all too often, cosmological theorists will attempt to make a splash by abandoning them in favor of explaining very tenuous and often incorrect observations.
Gold has always been an outsider in the astrophysics community, but has done some very good work over the years; including some seminal work on pulsars. He was Peter Goldreich's (major figure in theoretical astrophysics, for those not familiar) Ph.D. advisor.
Those interested in the history of the steady-state model, including attempts to resurrect it, and the many errors it commits, can check out this page.
Science, like Nature, must also be tamed, with a view turned towards its preservation.
Glacial? Hardly.
Solar sails should be able to pull 1/1000th of a G. That's Pluto and back in under six months.
In the mid-late 1800's, traders got rich running clipper ships to Asia and back using the same time frame.
Heinlein has an essay on it in his Expanded Universe collection. I seem unable to find a web version of it, though.
I'm not crazy,I'm actively irresponsible.
I see two problems with the reasoning.
1) if the mirror is moving (fast) away from the sun, the reflected light will have lower frequencies (doppler effect) than the incomming light, so that the reflected light is of `lower temperature'.
2) When defining the two Carnot's temperatures, T1 and T2, I think T1 is rightly set to the sun's temperature; but T2 should be that of the `outer universe', the 3K background radiation. Then one sees that the max efficiency of the aparatus is 6000K/3K, a rather high efficiency indeed. The 3K would be the push back on the mirror on the other side.
Yes, the mirror may have a black back side, but that doesn't matter. After some time, the mirror would become 3K (equilibrium, if the black side is turned to the background radiation), and it will radiate 3K ratiation.
(Like it would do if the back side of the mirror would be a mirror).
Summary: on the sun-side of the mirror, there is a push of 6000K radiation (sun temperature), on the back-side is a push of 3K radiation. There you have your efficiency.
Can you expand on this? I've never heard of this, and I can't think of anything in my 40+ years of layman's reading on physics that could be expressed this way.
Here's a few links. Google for "gravitational redshift" and you'll get lots more.
Link.
Link.
To summarize, the gravitational redshift (or blueshift, for light falling into a gravitational potential well) is a real effect. It was demonstrated by Pound and Rebka at Harvard University in 1960. They used essentially monochromatic gamma ray photon sources at the top and bottom of an elevator shaft, and measured the shift in frequency for photons traversing the shaft each direction. Kudos to Einstein--General Relativity gets another check mark.
~Idarubicin
Disagree. IWAP, when I left University. Thermally, the sail will heat up and will imet equal numbers of photons either side. There will therefore be, as the paper concedes, a short period while acceleration is higher as it absorbs photons until it reaches equilibrium. However, the disagreement is whther the final situation has residual thrust.
It doeds, because the important photons are those which are reflected. Hypothesize that the mirrore is perfect: 100% of all photons are reflected, none absorbed, and the mirror does not heat up at all. The paper would assert that the mirror therefore feels no force at all. And yet, on one side it is being battered by photons, whcih come in with a momentum vector one way and leave with the vector reversed. ISTM that this purely theoretical mirror must feel some force. The question is whether a real mirror can come close enough to the theoretical to get perceptible thrust. The thermal mechanism described is in fact a leakage mode of the system - a defect to be minimoised.
The fallacy is that a sail is a heat engine. It is not. Heat is disorganised energy - atoms flying around in random directions. Heat engines convert disorganised energy - the random motion of atoms - into organised energy. One example of such an engine is the Stirling engine, which converts hot gas into rotary motion - and obeys Carnot's law. Solar radiation is organised energy - and the sail converts one form of organised energy into another; it is a form of gearbox, if you will, and can approach 100% efficiency. The analog is to the sail, not the Stirling engine. A sailing boat can convert wind into movement as long as the wind lasts. (The fact that the wind is created by a heat engine is irrelevant - the wind is organised and they thermodynamic price has been paid before it reaches the sail.)
Consciousness is an illusion caused by an excess of self consciousness.
Gold says that as the sail is a perfect mirror the light is reflected at the same temperature and so Carnot's law applies. But of course, if the light were reflected at the same temperature it wouldn't be losing any energy so a working solar sail would violate conservation of energy and the laws of thermodynamics would be the least of our problems.
Gold's problem is that he forgets both conservation of momentum and conservation of energy. What happens is that as the sail is kicked forward (gaining both energy and momentum) the photon is reflected with slightly less energy than it arrived with (ie is cooler) and conservation of momentum, conservation of energy and the laws of thermodynamics still apply.
My back of an envelope calculations tell the fractional energy loss is E/(M*c*c) where E is the photon energy and M the sail mass.
You will notice that Gold's article doesn't appear in a refereed journal: this is because most referees have heard of conservation of energy.
Dr John Rowe
School of Physics
Exeter
UK
A supernova is really just a switchover phase for the star from being supported by electron to neutron pressure. Stars supported by photon pressure do not exist so far as I know. I would guess that any body sufficiently light that photons could compensate for gravity are too light to radiate in the first place. Electron pressure and neutron pressure work so well because they're both spin 1/2 particles, which resist compression much more strongly than bosons like photons (photons actually like being in the same state with other bosons, which is how Bose-Einstein condensates are formed. In contrast fermions obey the Pauli exclusion principle and can thus exert a degeneracy pressure) The photon pressure talked about with solar sails is different than degeneracy pressure, which is really a response to being compressed.
Stars up to 1.44 times the mass of the sun (calculated first by Chandrasekar, a really nifty guy who wasn't believed at all at the time) are supported by electron pressure. When that star gets too big, a big supernova results, electrons and protons in the star fuse into neutrons in the biggest atom ever, a neutron star, which I think has a mass-limit of three times the mass of the sun before it becomes a black hole (in theory).
though intellectual honesty impels me to concede it also says
The Mongrel Dogs Who Teach
> Not negligible, but solar intensity does fall off as 1/r^3.
I should think the photon flux would fall off as 1/r^2.
(surface of sphere, not volume of sphere).
The light can only reflect losslessly when the sails are still (in a particular frame of reference). In the frame of reference of the bomb, as the sails accelerate they begin to redshift the photons a little bit at each bounce because the sail is moving, so the sail becomes less and less efficient. In the frame of the reference of the sail, it is the flash from the bomb which is becoming redder and redder and less and less efficient as it recedes faster and faster. Since the bomb produces a finite supply of photons, this setup gives a finit acceleration. The sun, however, produces an "infinite" supply of photons which are discarded (slightly reddened) after one use.
Consciousness is an illusion caused by an excess of self consciousness.
Not an inert gast - it doesn't depend on the chemistry of the gas, only its physics. You have to use no gas - a hard vacuum, not the soft one these things usually have.
Consciousness is an illusion caused by an excess of self consciousness.
I recommend a good simple relativity textbook or a class. I'm not really sure anything that anyone can say on slashdot will sufficiently convince anyone else that the E=mc^2 that is thrown around so liberally is an incomplete picture. All I can say is the post above quoting the equation E^2=p^2c^2 + m^2c^4 is the correct one.
I'd highly recommend Helliwell's Relativity, though I think it's only sold through the Claremont colleges bookstore, so I doubt that's very helpfull. Beyond that, I Feynman's lectures have a good section on it, and can usually be picked up used. Other than that, the Schaum's outline series is really good for more advanced stuff, so I bet their intro books are very readable, too, though not as detailed as full texts on the subject. Looks like they don't have a special volume devoted to it, but their College Physics volume has one chapter on it.
Actually, with the gravity of the sun, the angle of the sail and the direction of motion relative to the orbit the vessel is in, it is perfectly possible to tack. If you direct the sail so that your thrust is oppossing your orbital velocity or adding to it, you can alter the radius of your orbit.
A Crookes' radiometer has four vanes suspended inside a glass bulb as you've described (see this picture). Inside the bulb, there is a good vacuum. When you shine a light on the vanes in the radiometer, they spin -- in bright sunlight, they can spin at several thousand rotations per minute!
The vacuum is important to the radiometer's success. If there is no vacuum (that is, if the bulb is full of air), the vanes do not spin because there is too much drag. If there is a near-perfect vacuum, the vanes do not spin unless they are held in a frictionless way. If the vanes have a frictionless support and the vacuum is complete, then photons bouncing off the silver side of the vanes push the vanes, causing them to rotate. However, this force is exceedingly small.
If there is a good but incomplete vacuum, then a different effect called thermal transpiration occurs along the edges of the vanes, as described on this page. The effect looks as though the light is pushing against the black faces. The black side of the vane moves away from the light.
If you're bothered by photons having momentum and kinetic energy which is harnessed in the sail, consider the electromagnetic waves comeing from the sun instead. There is an energy flux over an area, called the Poynting vector (N = E x H), which allows you to calculate the pressure an electromagnetic wave exerts on an object (which varies depending on its reflectivity).
You make some good points but you seem to be confusing details.
Comet tails form because solar winds force the commet's halo (the commet's own evaporated material) away from the sun. The gas is formed because the commet, being a very cold frozen thing is heated by the sun, which is a very hot thing. This heating happens in the usual way: the sun gives off radiation, the commet absorbs it. The commet is also being pushed by the momentum of the photons BUT not in any observable way, only the extremely light gas particles can be displaced by any observable amount and, as is evident from commet observations, they are.
The article also refers to Crookes radiometer in an irritatingly inacurate way. A Crookes radiometer works in a very simple way, the black side absorbs photons and heats up while the mirrored side (more often simply white) reflects them. The hot black side heats the air adjascent to it. Now, PV = NRT as any student of Thermo will know, in a Crookes Radiometer V,N,R are contant so Pressure is preportional to Tempurature. Since the air on the black side heats up there is a slight pressure change which causes the blades to move in the dirrection of lower pressure. This has nothing to do with Photon's momentums or other such high physics and is a simple phenomena.
>The author is just an idiot. Its like he picked of a tv show "physics for dummys", remembered a few complicated words and wrote some superduper article.
Ph.D. from Cambridge, faculty at Cornell. The best explanation I can think of is today's fortune on the Slashdot home page, "An expert is someone who avoids the small errors while sweeping toward the grand fallacy".
You can check this: just ask where aT^4/3 (radiation pressure) is equal to the product of density*N_A*k*T/mu (gas pressure), with mu the mean molecular weight, Na and k atomic constants, and T the temperature. You'll get
density = 1.5 x 10^-23 T^3 g cm^-3
meaning that radiation pressure dominates gas pressure only for very high temperatures and low densities. This is the case in some outflows, for instance, but not in stellar interiors.
The article, though, is still bollocks. :-)
Light does indeed do both. As it drops deeper into a gravitational potential, it blueshifts; as it emerges, it redshifts. Its path will also appear to be bent by the gravitational field. (I could introduce you to a number of physicists who will adamantly insist that the light follows a straight path along curved space, but that's getting awfully picky.)
Light cannot emerge from a blackhole because any photons emitted inside the event horizon get redshifted down to nothing on their journey out. They may indeed follow a curved path while they do so, but this is not essential to the process. Some physicists speculate that this bending has an interesting consequence right near the event horizon. In principle, photons on the right trajectories could orbit the black hole--kind of a neat thought experiment. Of course, any speculation about what goes on inside a black hole is merely that, since actual observations are rather difficult to come by.
~Idarubicin
I know you claim to know what you're talking about but you might want to go back and look at those plans again because voyager had absolutly no ion engines on it what so ever, it used hydrazine thrusters. And it also uses gyroscopes to change the directions it points. And at least for voyager 2 its generator is still moderately powerful, about 300 watts extrapolating from 330 watts in 1997. And while many of the instruments are turned off they still won't all be turned off totally until 2020. When it really won't be able to power its instruments.
A Bugg
You are incorrect - the tubes have only a partial vacuum. In a hard vacuum, the motion stops. Only with a very delicate instrument can you get a radiometer to function in a hard vacuum, because radiation pressure is a much smaller effect than the complicated gas phenomenon really behind it, and in fact you'll notice that a Crooke's Radiometer spins the wrong way - it spins away from black, where a radiation pressure effect would spin away from white.
. php?mo de=1&faqID=42
For a complete explanation of Crooke's radiometer, see:
http://www.stillmoving.ca/physics/usenetFAQ
My amazing wife - Artist, Author, Philosopher - Laurie M
It is obvious that if he is at the Cornell Center for Radiophysics and Space Research, then he is not an actual physicist there. I actually suspect some ulterior motive behind this.
There is no thermodynamic problem with radiation pressure and solar sails, Carnot, Maxwell and Einstein are correct, and Thomas Gold is wrong.
The photons that leave from the Sun are at a Maxwellian distribution at the temperature of the Solar photosphere, many thousands of degrees.
The photons which are re-emitted by the heat of the sail are at the temperature of the sail, which is obviously less than the temperature of the Sun.
There is a temperature gradient, hence work is done.
Gold is also obviously uneducated in physics because of his assertions about how momentum is 'E/c' and how 'E' is a scalar but momentum is a vector, blah blah blah.
The correct way to do an electromagnetic problem like this in purely classical E&M (which is all that is necessary to solve this problem) is to use the stress energy tensor, which is certainly non-zero for propagating electromagnetic waves.
You then use this to find the forces on material bodies, e.g. dielectrics or conductors. You have to think about the stress-energy tensor on both sides of the material body and add up forces.
(interesting note, you can also get angular momentum out of this too, appropriately polarized electromagnetic waves can exert a torque).
I learned this in my sophomore year physics class.
Actually, this sounds like a good problem set problem. "Hmmm....now that you ask about it--------yes, it WILL be on the final exam! muahahha."
Given a plane wave source of incoming radiation with flux F_0, with a certain amount of it R1 reflected and the remainder absorbed, what would be the net acceleration in the direction of the radiation of the solar sail with emissivity E once it has reached thermal equilibrium, accounting for all radiation forces. Assume space is at 0 degrees.
What you would do is to put in on the incoming side, the stress energy tensor for the incoming radiation, the outgoing radiation, plus, on both sides, radiation emitted from a black body.
You first get the energy balance right, and then solve for the acceleration.
If you want to be tricky, you would go into the accelerating frame of the sail, and then appropriately red-shift down the incoming frequency of the radiation.
Here is my suspected ulterior motive. This thermodynamic fallacy is exactly the same one that creationists use to deny the obvious explanation for why plants live and complex life evolved.
Namely that the Sun has been providing input energy at 10,000 degrees and the Earth has been re-emitting energy at 300 degrees K and the temperature difference over 4 billion years can do a fair amount of work.
Has essentially been done in 1901. See P.N. Lebedev, Ann Phys. (Leipzig) 6:433 (1901).
Yeah, he probably doesn't know shit
Actually, a solar sail makes use of solar radiation pressure (i.e. pressure exerted on a surface due to momentum transfer from reflected photons). The "solar wind" is a stream of energetic particles (not light) emitted from the sun. It would produces several orders of magnitude less force on a solar sail than would solar radiation pressure.
That said, you are correct that it is possible to move in a sunward direction by orienting the sail in a way that generates a force opposite to the velocity vector of the sail. Fundamental orbit mechanics tells us that reducing the energy of an orbit will cause the size of the orbit to decrease.
Professor Gold is not only the progenitor of the primordial mantle methane, errrr, hypothesis, he is also the progenitor of the idea that the Lunar landings would not have been feasible because the surface dust was not solid enough to support the weight of a lander.
This idea is rumored to have prompted the entire "Ranger" series of spacecraft (at umpteen million dollars) primarily to demonstrate that the Lunar surface was solid.
Oh, and the supporting statements by Steven Soter in the New Scientist article regarding the thermodynamics of photonic momentum-transfer? Three guesses who was Steven Soter's Ph.D. adviser...
Just repeat after me: "Tenure is *good* for science. Tenure is *good* for science. Tenure is *good* for science."
The debris trail "falls behind" because it's pushed that way by the solar wind. Being much less massive than the comet itself, the solar wind affects it more. On the outbound leg, they'd precede the comet, not follow it, because the solar wind would push them along faster. It's pretty much elementary mechanics, right?
The Planetary Society as mentioned in the article has their own solar sail which is going to be launched in a few months.
They are the ones actually doing something about this, NASA and the ESA have yet to get off their butts and actually do any real tests.
You can join them and give support to these and other space projects. Much better than complaining about NASA.
This guys argument is based on the fact that after a certain time, the light is no longer absorbed because the mirror is at it's equilibrium temperature. This proves that he does not grasp simple physics. The equilibrium means that the material emits as much light as it absorbs. The reason the sail works is that the light is emitted equally in the forward and backward direction, but is only absorbed from the back. This means there is a net momentum transfer.
Now, one obvious remark would be that there is no energy transfer, because as many photons are emitted as there are photons absorbed. This means that the energy of the mirror cannot change and therefore we have a contradiction (mirror gains more momentum, but not more energy). However, this all works out if you take into account the Doppler shifts of the emitted radiation with respect to the absorbed radiation.
This stuff is all understood, I cannot understand where this guy gets his ideas. His arguments can equally well be applied to laser cooling and we know that that works!
-- Please put this in your sig if you think
>causes a *temperature* change in the frozen particles
k no w.pdf ...as I could in 30 seconds.
/ 51 090979.pdf ... contains a detailed study of the radiation pressure effects on a satellite going to the moon -- even though the effect is very small, it is required in this case to accurately calculate the orbit.
p re ssure.html
That does not provide propulsion, only the ability to leave the nucleus. The _dust_tail_ of a comet is still generally accepted to be due to radiation pressure.
> Anyhow, there's been no tests showing that a solar sail *could* work
Type "laser levitation" into Google. You'll get just under 10,000 hits, the first few pages of which are almost all examples of real-world experiments. People are doing this as I write this.
Your statement above is ill-informed. Then again, apparently Gold is too, because he makes direct claims that this everyday practical system does not exist, even though he could just as easily find this:
http://astp.msfc.nasa.gov/ast/presentations/3g_
More to the point, if you type in "radiation pressure" you'll get an equally useful list of examples of space-related examples. The very first hit:
http://www.terrapub.co.jp/journals/EPS/pdf/5109
Or you could read this short little blurb, which is quite useful:
http://yarchive.net/space/spacecraft/radiation_
Solar sails are not heat engines. Period. Neither are electric motors, balls rolling down hills, springs, or lots of other things. All of these "other things" have effeciencies much higher than the ideal heat engine.
Consider: what is the temperature of the electricity after it comes out of a motor? Does that explain the power you're getting from it?
Heat engines are only one way of many to extract energy, one our culture has become dependant on because we can dig up fuel cheaply. It is not a particularily good one however, and our widespread use of heat engines is likely to change in the next 20 years or so.
For instance, take an existing car that gets, say 30mpg. Now take out the normal engine, and replace it with an electric motor, fuel cell, and a reformer that extracts the hydrogen from the gas. This will improve milage to about 40mpg, even though reformers are rather ineffecient. This is because the energy in the fuel can be still extracted with higher effeciency as electricity than as heat, notably in the motor which is close to 90%.
Gold's argument is just plain dumb. I can't imgine how anyone could seriously suggest it. I wonder if even he believes it. In fact, a radiometer DOES spin towards the black side in a high vacume, a fact he should be familiar with.