h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

When you hear about plans for solar sails, they always talk about them as large reflective surfaces. It seems an odd idea to me that you would make it a reflective surface rather than a black one. Surely the black one will absorb all the energy, whereas the reflective one will only get a small fraction and the rest of the energy is sent back.

There are those little lightbulbs with a small rotor inside, which rotates when sunlight hits it. In these they also rotate away from the black surface as I recall. So the black side must get a bigger push than the silver side. I was at a university open day recently where I talked to a physics professor, and he wasn't able to explain it to me. So could you people perhaps tell me what would make for better solar sails, reflective or black surfaces?

Hi Xanatic,

The answer is that black surfaces would make for better sails even though they always seem to be portrayed as shiny in diagrams.

The solar wind is made up of energetic particles which would provide the push as they strike the sail and give up their momentum. Apparently photons also have momentum and would provide some of the thrust but they need to be absorbed. Reflected photons will not transfer any momentum to the sail.

Personally I have doubts about the viability of the solar sail idea although I do know there is a group at Caltech - who are the ones who would know - that are very enthusiastic about it.

F

Erm, I'd have thought it was the other way round...

Surely, thinking about Newton's for every action there must be an equal and opposite reaction... the fact that the photons were being bounced off a surface would push that surface away from the photon itself... If the photon were absorbed, all that would happen is that the sail itself would be heated up - not moved.

And haven't they already tried a small solar sail experiment launched from one of the shuttle missions?

Accepting for one moment that solar sails are a viable form of transport, how would you steer the craft? A wind powered ship has a rudder to act against the water (or air) that the craft is in. Would the craft have to be fitted with small rockets to guide it?

Whisky,

Everyone thinks that at first but photons always travel (in space) at the speed of light. If the bounced photon added momentum to the sail it would also add kinetic energy to it but in so doing it would not have lost any energy of its own; because it would depart at exactly the same speed it arrived. This violates the first law of thermodynamics. The momentum is only transferred if the photon is absorbed (which I don't understand but apparently is true).

Steering the sail is another matter entirely. With a yacht you have a rudder and a daggerboard to take advantage of the resistance of the water. This allows you to divert the push from the sail at an angle to the wind direction. In space there is nothing to provide such resistance but you would almost certainly be orbiting some object or the sun itself. The boys (and girls) at Caltech claim you can use this instead of the daggerboard and rudder by steering the sail and vectoring the thrust to one side. Personally I don't believe this will work but who am I to argue...

F

Fathom... I'll take your word for it... I'm still working on Newtonian physics... einstein looses me every time

Actually in sailing with wind he practice of balancing sails with the wind is an important factor. If the sails are not all trimmed equally ( this does not mean the same, only the same in effect on the course )excessive rudder must be used, which wastes energy. Solar sails that could be turned to be more efficient on one direction would causing an opposite turn and in effect steer the vessel without any rudder, which would have no effect anyway without some sort of resistance. In old square rigged ships steering by the sails was a common exercise to train officer should the rudder become disabled. On modern sail boats this is not possible as the sails are all on one side. I have tried when my rudder broke on more than one occasion.

F S

Caltech itself thinks that the reflective material would be better - producing twice the force of an abosorbant material. See http://www.ugcs.caltech.edu/~diedrich/solarsails/intro/intro.html for info Until later....
BCNU - Crescent

I still think they won't work, how can you have momentum with mass, howwwww?

Do you mean momentum without mass?

Presumably you mean "how can you have momentum without mass?"

I know it puzzles me too. But photons do, although not in the same way that other particles like electrons and neutrons do.

Steering with the sails without the rudder still relies on the resistance of the water against the hull of the boat. Sails also operate in an aerodynamic environment and can use the wind in the same way an aircraft wing does. Despite the existence of the solar wind I don't think space represents this kind of environment. In aerodynamic terms I think the Reynolds numbers are too small. A solar sail will almost certainly have to be rigid becuase there is not enough solar wind to enable it to 'fill'. Especially if you want it to be steerable.

When I get home I will look up the Caltech link.

F

Sorry:

"But photons do, although not in the same way that other particles like electrons and neutrons do."

should have read:

"But photons do, although not in the same way that other particles like electrons and neutrons which do have mass have momentum.

Momentum is 'traditionally' mass x velocity but for photons the mass bit is represented by their frequency or 'energy'.

F

E=mc2

mass is energy, energy is mass, hence photons do (and have indeed been measured to) have momentum (with no mass).

This is one of those areas of the quantum world where it is a fundamental mistake to apply rules from the macroscopic world to the microscopic.

Personally I also believe solar sails to be a pipe dream but this doesn't mean it shouldn't be tried. UNfortunatlely I suspect that interstellar travel will never be a realistic possibility despite all those lovely space operas.

Forgot to add my to the original question.

With the solar wind being made of some really quite destructive particle (gamma and X-rays for starts) is it not actually quite a bad idea to have black solar sails? If you start absorbing gamma radiation you're not likely to have much of a sail left after a while...

This I think is the main problem with solar sails. If a passing piece of space debiris (rock, asteroid, piece of cosmic dust...) tears a hole in one then it's stuffed.

Thanks Orcus,

The sail would undoubtedly heat up unless you chose to make it shiny in order to reflect (and thereby lose out on the momentum of) photons. Since Gamma and X-rays are also electromagnetic radiation they are photons too. There are however plenty of particles with mass in the solar plasma carried in the solar wind - electrons, neutrons, alpha particles, protons etc. so you could simply use this.

I don't think a few punctures would matter but I do think the sail needs to be rigid. Rotation would do, or inflatable tubes rather than,say, metal spars which might be awkward to transport into space.

F

D**n you people!
Your post has been gnawing at me for over a hundred mile drive home from a job site!

I think I have solved all the problems - except if it works - and I have chosen shiny sails for reasons that will become obvious.

I will post this as an entry as soon I can type it all in and will give the link here. Anyone who wishes to contribute will be most welcome, and will be credited if desired. I especially want a few good Si-Fi references as it has been a long time since I have kept up with them.

I shall return

F D

SF fact(ish) no.1:

Solar sails first appeared in the writings of Arthur C. Clarke.

Whereabouts? No idea.

Here is a start;

A2227268

If you like let me know what you think in this tread for now, that way others who may have ideas can see your thoughts.

F S

Hmmm, but one way mirrors do rely on one side being a dark room. If you had a one way mirror in between to an equally lit room, you would be able to see through pretty easily from both sides. And the normal particles in the solar wind would still affect them.

Perhaps a large magnetic field would be a better option. One that would be pushed forward by charged particles like electrons. It could then be turned off when you reached another star, and it wouldn't get destroyed by space dust.