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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."
This reminds me of some of the most common last words:
"Check this out! - It's gonna work!"
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.
The interesting thing is that the Mercury orbiter that NASA launched (one of the pioneer series) used the pressure of sunlight on its solar panels, just like a solar sail would on the sail material, to give it a spin. That, IMO, gives the theory supporting solar sails working a whole lot more credibility.
Do you know why the road less traveled by is littered with the bones of the unwary?
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?
I have learned to use the word 'impossible' with the greatest caution.
To make laws that man cannot, and will not obey, serves to bring all law into contempt.
--E.C. Stanton
A Rebuttal by Geoffery Landis.
Do you know why the road less traveled by is littered with the bones of the unwary?
this alone calls into question Dr. Gold's ability
Depends on when he did this "seminal work on the steady-state theory". I'm in my 40s, and I remember when it was still considered acceptable to have some reasonable doubt about the big bang. If he's in his 70s, and did this work, say, 50 years ago, it's possible that the work was considered completely solid at the time. And the word "seminal" does imply that it was a while back.
I'm reminded of Stephen J. Gould's defense of the Bishop of Usher (the one who determined that the universe was created in 4004BC). Looking at that date based on what we know now, it's easy to assume that he was a religious fanatic, but if you look at what was known at the time he did the work, it turns out that he actually did some pretty solid scientific research to come up with that date. The fact that he was working from a set of bad assumptions was not really his fault.
So, back to Gold, if he's still a proponent of the steady-state, then he can probably be dismissed as a quack, but the fact that he once worked on the theory doesn't really say anything one way or the other.
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
Sweet!
My career as a Solar Pirate is looking more promising everyday
Yarrr!
On Wall Street they say "buy low, sell high" On the pad we say, "buy high, sell high" Isn't that somehow better?
In this country we follow the laws of THERMODYNAMICS. This type of hacking is ILLEGAL. If you want to hack Solar Sails YOU MUST PURCHASE SAID ENERGY. We are going to see that this website is taken down immediately. We will log IP addresses of anyone who visits this site and we WILL find you and prosecute you to the maximum extent permissible under the LAW.
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.
a presigious US astronomer, wrote a paper in 1902 in which he concluded:
p 16 7y1977-78.pdf
"Flight by machines heavier than air," Simon Newcomb declared one day in 1902, "is unpractical and insignificant, if not utterly impossible."
His arguments were quite reasonable on the surface - Imagine a bird as a model. If you increase the size of the bird, the mass increases proportionally to the third power of its wingspan. But the surface area of the wings only increases proportionally to the square of its wingspan. Thus something much larger than a bird would never be able to fly, and all attempts to build heavier than air flying machines capable of carrying a human would prove futile.
Fortunately, the Wright brothers never read his paper, or at least never took him seriously.
About 40 years later it was argued by learned men that manned supersonic flight would never be achievable.
http://www.garfield.library.upenn.edu/essays/v3
Marconi wasn't formally educated, and he was laughed at for spending vast sums of money to send a radio signal across the Atlantic ocean. Any fool knows that radio waves couldn't penetrate the earth, and was limited to line of sight communication! Yet despite all logic, the damned fool contraption eventually worked. It was only later that they discovered the ionosphere could reflect certain frequencies back to earth.
Even great men of science make mistakes sometimes.
My rights don't need management.
in more speed then any conventional propelled rocket that you send in space.
The main practical difficulty I see is stopping. You can't slap propelled rockets on the ship to do the job; if you did, I would want to know you didn't propell you ship with that to begin with.
Maybe some fancy gravity trickery... deaccelerate as a star's gravity starts to whip you around. Other than that, I don't see how you could do it. I still don't see a use for these other than minute corrections in satellite trajectories. They're not directional, and the methods by which, say, sailboats can sail against the wind won't work here. Only way to slow yourself down is to stop sailing and let gravity do it's thing... a big problem when you're somewhere in space in which gravity isn't acting against you. Moreover, we wouldn't get very far away, because the force provided by sunlight diminishes exponentially as you move further away. And going towards another star wouldn't help, because you can't sail against the "wind" in this case (ship sails can because of how the wind will curve and press on the sail in a different direction than what it was originally travelling, which won't happen with light).
We're getting to the point where it will just take too long to go where we want to go, and eventually it's going to make us ask if we really can go there. I mean, hundreds of years later, who's going to care that a probe, unable to communicate with us, is careening somewhere past Neptune? As for people, don't hold your breath on this transporting us; it just takes too long. I don't know about you, but going to another planet wouldn't be worth most of my life, if not the whole thing and part of my children's. And I don't even want to hear this whole "Once we figure out how to go faster than light" garbage. You've all been taking Star Trek too seriously. Granted, people didn't believe we could fly, or the earth was round, blah blah blah. As we progress we do realize that things were wrong, but some things become more compellingly right as well. The speed of light, the fact that we can't exceed it and its correlation with time are what defines our reality. Sending something faster than light, AND slowing it back down without the obvious logical and physical laws getting in the way is impossible. Sending something as complicated and sensetive as an organism is absurd. Sending an organism as intricate as a person should be grounds for insanity.
Sorry, that just bothers me when geeks worldwide sit back after watching something on the Sci-Fi channel and think "Man, once we learn to warp... that'll be good times my friend", to which his friend replies "Affirmative" and starts taking readings on his platic light-up box... er, tricorder.
It's always the turtle that wins...
Unless there's a dog running after you. I got 5 bucks on the hare.
I'm not sure what, but I'm pretty "profit" needs to go in there somewhere.
- None can love freedom heartily, but good men; the rest love not freedom, but license. -- John Milton
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.
There is no such thing as centfugal force. The two forces in effect with orbit are momentum, and gravity.
If you want to get pedantic about it, there's no such thing as gravity. It's just inertia/momentum acting on a curved space-time.
(at least if you buy general relativity)
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.