Solar Sails-Are They Possible?

This scientist claims that solar sails won't work. Check out the link

What do you think?

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The basic idea is right (solar sails can nt be considered to be perfect mirrors, and thermodynamic effects should be considered), but I think the conclusions are flawed.

Yes, the sail will absorb some photons, and heat up. The sail will also re-radiate heat according to its temperature. The sail will quickly reach equilibrium with the heat energy absorbed equalling the heat energy radiated. The sail will gain the momentum of each photon absorbed, it will gain twice the momentum of each photon reflected, and assuming re-radiated photons are radiated equally in all directions, they contribute nothing as far as momentum goes.



The refutation mentioning Crookes' radiometer is also flawed. It is true that your regular toy light-mill works by thermal creep around the edges of the vanes. This is because they don't have a great vaccuum. However, it was demonstrated as early as 1901 that a Crookes Radiometer with a very good vaccuum, virtually frictionless bearings, and coated vanes (to prevent outgassing) will spin as predicted by transfer of momentum from photons.

 

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I must have advised this sight 20-30 times now!

Go here: http://www.islandone.org/APC/Sails/00.html

Solar sails have been proven to work, solar light pressure has move satellites and has been used to control rotation on Mariner 10. laser experiments have lifted solar foils in a vacuum proving that light can move matter.

 

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Unless I'm not reading the article right, Chuckster, what
Gold is saying is that solar-sails don't work the way some
have hypothesized (via reflection), not that they don't work.

WellCookedFetus ... They have not been proven to work.

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I'm sorry but the concept has been proven.

 

The 'concept' =/= A 'solar sail'.

As far as I know, the only attempt to deploy one failed.

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No the concept: propulsion via light direct… proven.

 

Either you are confusing 'solar wind' with sunlight, or
you have yet to post anything other than conjecture.

See: http://web.mit.edu/space/www/

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If one reads the original article one notices that it is full of flaws. I wouldn't really be suprised if someone told me this is a joke, but it would be a really bad one. There is a lot of comments about this in today's slashdot here.

 

Chagur,

http://www.space.com/news/lasersail_000301.html

http://highway2space.com/ast/presentations/3g_know.pdf

 

Having read the NewScientist article and now the 'original
article', I fail to see how you can say 'it is full of flaws'. Do
you have any specific 'flaw' in mind, myhr?

Are you referring to the JPL microwave-beamed experiment?
If so, actual solar sails are far from 'proven', WellCookedFetus.
Note: The JPL bit was done three years ago and nothing has
..........come of it.

See: http://www.jpl.nasa.gov/releases/2000/lasersail.html

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WellCookedFetus ...

You're grasping for straws!

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and how is that so Chagur? I provide proof and you say it not so?
You fail to make a legitimist counter argument by providing nothing against my evidence, making you the one suffering form a straw man fallacy.

 

Actually, Chagur, in space the impulse imparted by the solar wind on satellites is much less than that of photon pressure from the sun.

 

The referred to JPL experiment used microwaves to 'lift'
the 'sail' and a high power laser to move it slightly ... That
is not 'proof of concept' by far.

You're still grasping, WellCookedFetus.

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No it proof entirely, light provided pressure, in space even the smallest pressure is propulsive. That’s total and perfect proof: if a light can push a piece of solar sail material in a lab, then the same thing can be replicated anyway, such as outer space with sunlight.

 

WellCookedFetus ...

Hmm ... "first known measurements of laser photon thrust
performance" using "laser powers from 7.9 to 13.9 kilowatts"

Not exactly non-coherent 'sunlight', wouldn't you say?

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AD1 ...

I take it then that measurement has been refined enough to
differentiate between the two. Earth shadow effect?

Last I knew, '98, they still weren't sure how much of the total
interaction (Earth albedo, IR distribution, molecular and photon
interaction) could be attributed solely to photon action.

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a square meter here at one AU get 1.4 Kwatts of sunlight, estimates are that at one Au a square kilometer of solar sail could provide 9 Newtons for thrust: it’s not much but in space with virtually no frictions you can get anything to accelerate with that much thrust.

 

Flaws in the article

This is actually the first time I have read anything that pretends to be serious physics, but which I have been able to spot at first sight that is wrong (normally it is more hidden, and people as me don't find them easily).

The first thing that I noticed is that he is using a concept of the maximum efficiency of a cyclic thermodynamic process (the Caront cycle) on a process which is obviously not cyclic.

Later asuming that the total energy of radiation irradiated on a black body is converted into kinetical energy is something no physicist would do. Most of the energy will of course heat the body.

But the best is still to come:

The magnitude of the momentum of the radiation is E/c, but it do indeed have a direction. This is basic electromagnetism, which I every physicist should know (and if there is some new revolutionary idea that known electromagnetism, quantum mechanics, etc. is wrong, he should state that clearly).

And in the and he asumes that one can define a temperature in the radiation field of the sun as the temperature a black body will reach in that point. This is only possible if the radiation is isotropic (that is: is equal in all directions). If you place to equal black plates in the radiation field, one with one face towards the sun and the other with the face parallell to the radiation, the latter one will get a lower temperature than the first, and the temperature will be impossible to define.

You can find more comments by people more competent than I among the slashdot comments. (And even more flawed comments)

I don't claim that I could not be wrong or misunderstand something, but when it is this much and this obvious I think the probability for that is low.