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Solar Sail Will Work, says Planetary Society
degauss writes "In response to Cornell Physicist Thomas Gold's paper declaring the theroy behind solar sails flawed (previously mentioned in this Slashdot article), Louis Freedman, executive director of the Planetary Society (the organization behind the COSMOS project), has written a brief rebuttal to the claims in Dr. Gold's paper regarding the feasibility of solar sails for use as a method of transportation in space. He does not go in to detail with equations and such, but does give an overview of the reasons he believes Gold's hypothesis is incorrect."
What, like this one?
Read the artical first next time mate.
-Alex
Any object that orbits another body is using centrifugal force (pushing it away from the center) to ballance the gravitational force (pulling it toward the center). By giving any object a small push faster around in its orbit you increase its centrifugal and cause the object to move to out to a farther orbit.
So yes, adding solar sails to a NEO would help push it away into a farther orbit. On the othe hand solar sails generate only a tine ammount of force (not even enough move it through air). That said, I doubt that solar sails would help move an object the size (pronounced mass) of mount everest away for an Earth bound collision in under 10 billion years of constant work.
Although, I small space probe might be moved quite a bit. Not at first, but over time it could get going quite fast.
Though no idea should be dismissed a priori in science, this alone calls into question Dr. Gold's ability. Unless they're using "steady state" in a manner unconnected to its traditional usage, Dr. Gold is on the side of a theory that has pretty much fallen by the wayside. Excluding the increasingly, um, eccentric Fred Hoyle, there are no real leaders among the handful of proponents of steady state.
The Mongrel Dogs Who Teach
Ever heard of Solar Wind?
It was obvious from the original story that the guy was wrong. He made fundamental errors that you could spot after a freshman level course in physics 101. The rebuttal doesn't really address the specific flaws in the original paper- it has the character of "ahh this is nonsense" which it is. But the original paper has some obvious conceptual errors:
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.
The two temperatures are NOT the same. They are slightly different. The mirror is not infinitely massive, so in the mirror's own reference frame the photons reflecting from the mirror have a lower energy / longer wavelength after their elastic collision with it- the mirror receives a small bit of momentum from each photon in the collision. And in the sun's frame, the mirror is receding and the reflected photons are doppler shifted. He can't assume that the incident and reflected energy are the same and run off making derivations from that. They are extremely close, but the difference between them is not zero like he assumes.
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.
Does he even realize the sun is a point source? The sun shines on one side of the sail, not both sides! One side is exposed to radiation with a temperature of 300K. The other side sees only 3K radiation. The sail temperature will rise to some intermediate temperature between 3 and 300K and reach thermal equilibrium with all available radiation. So what? This means nothing for momentum transfer! Once it reaches thermal equilibrium, the sail is receiving X watts of radiation coming from one direction, and radiating X watts thermally in all directions! While the wattages are the same for both, the radiated energy has no overall momentum, while the incoming energy has a very definite momentum. The point isn't to heat the sail, it's to move it. He seems to be confusing the sail's kinetic energy of motion with its internal thermal energy.
The rebuttal is very sparing. I think it would probably have been more vicious if its author didn't "know Prof. Gold well" and didn't have any reservations about embarrassing him.
Um - maybe because air molecules are rather scarce where they want to actually use solar sails? The point of the experiment is to test the practice of solar sailing in something approximating the target environment... or were you being sarcastic?
A Rebuttal by Geoffery Landis.
Do you know why the road less traveled by is littered with the bones of the unwary?
F=dP/dt. What more do you need.
1. Observation and description of a phenomenon or group of phenomena. - In this case, all sorts of different light experiments
2. Formulation of an hypothesis to explain the phenomena. In physics, the hypothesis often takes the form of a causal mechanism or a mathematical relation. - In this case, Maxwell's theory, and then the Quantum Mechanical changes it suffered
3. Use of the hypothesis to predict the existence of other phenomena, or to predict quantitatively the results of new observations. - In this case, there are 2 predictions in consideration: Solar Sails work because of [read the article for info], Solar Sails don't work because of [read the older article for info].
4. Performance of experimental tests of the predictions by several independent experimenters and properly performed experiments. - In this case, build it and try it.
You are suppose to argue beforehand,that is step 3. That is the way you really understand your theory and its implications. The arguing beforehand is a very important step to clarify what someone is really saying with his/her theory. After observing a phenomena it is very easy to come up with hundreds of explainaitions, but the only good ones are the ones that predict new stuff, and clarifying the theory allows us to make more precise experiments that really show light into the important issues. Arguing the different theories is what makes them more specific that just betting for the outcome of an experiment.
"There is no teacher but the enemy."-Mazer Rackham
You are making a personal attack, not attacking his ideas.
:
Here are two cases
(a) Another BIG proponent of the Steady State universe is Fred Hoyle.(While we are at it, let's throw in that Hoyle also supported life from space rocks theory). Is he a quack? No. He has good arguments. In fact, Fred Hoyle is sadly forgotten for his VERY seminal work on figuring out how the Sun nuclear engine works. Sadly the Nobel committee only awarded Willie Fowler the Nobel though Hoyle arguably did as much to solve the problem : an scandalous injustice that many astrophysicists now rued.
(b) You can also attack Friedman's comment about the "obscure british "preprint physics archives".
A number of colleagues have contacted me since the web posting (on a rather obscure British web site of "e-print physics archives", http://uk.arXiv.org.)
The arXiv is the main distribution of physics and maths papers nowadays. Everybody in the field reads the archives. In fact the uk.arXiv.org is just one of the many mirrors of the main site arXiv.org hosted, ironically by Cornell.
So Friedmann did a "personal attack" on Gold (implying that Gold only published his findings in some no-name website). THat's not a good thing. A good scientist names his source without judgement : the reader can decide for himself whetehre it is good or not by its contents.
Mode (3) smart-aleck mode. Press * to return to main menu.
He is off his rocker.
Applying the laws of thermodynamics that govern a heat engine to a sail is just not applicable. You might as well apply Kirchoffs current law to a water balloon.
This is a straight forward conservation of momentum problem. Give a sail of size A, with reflectivity R and a photon flux F(photons/sec) impinging on the sail with average frequency f you will have have momentum imparted to the sail of M=2*(F*R*h*f)+(1-r)*h*f. h is plancks constant. QED and very straightforward.
Well I was dissapointed by pons and fleischman, Golds theory about the inorganic, I now see how and why you can have otherwise respectable people make completely foolish statements
Looks to perhaps accelerate a bit faster than you would suggest.
There is no such thing as centfugal force. The two forces in effect with orbit are momentum, and gravity.
Momentum wants to keep the earth going in a straight line out into deep space.(tangent to the orbit we are currently in) Gravity pulls us away from that line. Closer to the sun. The spot closer to the sun happens to be right on the orbit line. Repeat.
Tommy Gold and others quoted in the article about solar sails really should consult some real spacecraft engineers. For us, solar radiation pressure is an everyday reality. Solar radiation pressure is a major perturbing force on GPS satellite orbits, for example.
AMSAT, a group of radio hams that builds its own satellites, has for decades used radiation pressure to impart slow spins to its satellites with "blade turnstile" antennas. Paint one side of each blade black and the other white, and the spacecraft slowly spins like a Crooke radiometer -- but in the opposite direction, away from the white surface.
A Crooke radiometer is a very different beast. The glass bulb is not evactuated, so thermal heating on the black side of the vanes heats and expands air, pushing the vane away from the black surface. This force overwhelms the much smaller photon pressure, but in the vacuum of space only the radiation pressure exists.
Gold's thermodynamic argument is silly and wrong. A solar sail is not a heat engine, so the second law doesn't apply. The first law (energy conservation) does apply in a very simple way: the photons reflecting off the sail are red-shifted by the sail's motion, removing energy from the photons and imparting it to the sail by accelerating it.
I don't know about the solar panel thing you're mentioning, I've never heard that before. However you are wrong on one point. NASA has never orbited Mercury. It sent Mariner Ten to Venus and then Mercury... It flew by Mercury twice while orbiting the sun, photographing about 40% of the surface.
This space available.
There are 3 sources of momentum transfer as I see it.
1) Light impinging on the sail and reflecting back towards the sun. The photons have momentum due to relativity.
2) Photons being absorbed by the sail (increasing its temperature and transferring momentum to the sail)
2) Photons being radiated in all directions by the sail (radiant heat).
Conservation of momentum shows that the sail has to accellerate, but he's right that it will start out increasing rapidly, and as it heats up it will slow down its accelleration (because it starts to radiate infra-red on the side away from the sun)
I think this is the sense in which it is a heat engine.
The conservation of energy and the fact that the sail accellerates away from the sun does imply red shifts of the reflected photons (ie. reduction in their energy). This doesn't seem to bother me at all. It seems to be what bothers Gold.
((lambda (x) (x x)) (lambda (x) (x x))) http://www.endpointcomputing.com a scientific approach to custom computing.
Didn't we have a big reflective object in high orbit already? Do we not have orbital data from them that tells us if there is a solar wind pressure?
a ry /Echo/DI55.htm
http://www.centennialofflight.gov/essay/Diction
Improving on the idea of sending and receiving
signals from the moon, in 1960, NASA launched a
balloon satellite that would reflect communications
signals. Echo I was a balloon made of
aluminum-coated Mylar that was launched by a
rocket into space. When it reached orbit 1,000
miles (1,609 kilometers) above the Earth, it
inflated from inside a 26.5-inch (67.3-centimeter)
magnesium sphere to 100 feet (30.48 meters) in
diameter. Circling the globe every two hours, it
shone more brightly than the North Star in the
evening. The balloon captured the imagination of
people who had watched the first man-made object
in space, the Russian satellite Sputnik 1, orbit the Earth in 1957.
If Slashdot were chemistry it would look like this:Cadaverine
So the rebuttal says the article was flawed because it was written assuming a perfect mirror, and not assuming a perfect vacuum. While the part about not assuming a perfect mirror is very true and valid, the part about space being a perfect vacuum seems a little suspect. I mean, it might be close, but you have to consider the size that a solar sail would be. Especially if operating in a cloud of interstellar dust, etc. friction would be noticable.
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Crudely Drawn Games
Let me sum up quickly.
1) Photons bouncing off a mirror will impart momentum to the mirror. That's Newton's laws right there, as fundamentally a part of the fabric of the nature of reality as we know it as damned near anything else we have ever studied. If Gold were right and a Solar Sail wouldn't work, then it would mean Newton's 3rd law would be violated, and that's super bad mojo, mojo of a scale that Gold was unable to back up with sufficient evidence and argumentation in his penny-ante paper.
2) The laws of thermodynamics are not violated by the operation of Solar Sails. In any given inertial reference frame the reflected photons will be "red shifted" and have a slightly lower energy. This is how energy is conserved (since the movement of the sail represents work, and thus energy). This is also how the 2nd law of thermodynamics (non-decreasing entropy) is followed, since the redshifted photons are higher in entropy (for slightly complicated reasons) and balance the work done.
3) Light pressure is not theoretical, it has been detected, measured, and, indeed, used many times in many circumstances. Its properties have corresponded very closely (to about as many 9s as you'd like) to what has been theorized.
In short, Gold is full of crap and the New Scientist ought to be ashamed at printing his stupidity.