space@mit-mc (02/21/85)
From: John Heimann <jheimann@BBNCCY.ARPA>
I sat down and tried to sketch out just how fast a solar sail could
reasonably be expected to go, assuming a point source of illumination (the
sun), the usual inverse square law for drop-off of photon intensity, a bit of
handwaving about the mean photon momentum, and some reasonable figures for the
density of the sail material. I realized then that I wasn't really sure how
the thing was to operate after all. Are solar sails designed to get their
momentum from elastic collisions with solar photons (reflecting sunlight), or
do they get most of it from collisions with other particles that are constantly
emitted from the sum (e.g. neutrons)? A typical neutron from a nuclear
reaction packs a lot more momentum than a photon from the same reaction, so
this is an important issue. Also, what's a good assumption as to the
composition of the sail? Would a large expanse of metallized mylar be
reasonable, or would this rip to shreds under meteor impact? Not having read
anything about the subject in years, I really am not up on the latest ideas -
although the old ideas were certainly interesting. Any technical info. would be
appreciated.
John
("Lux et Veritas")space@mit-mc (02/21/85)
From: Rick McGeer (on an aaa-60-s) <mcgeer%ucbkim@Berkeley> I'm pretty sure that a heavy particle would go right through the sail, with little net momentum transfer -- unless, that is, you made your sail of lead or gold (better -- highly reflective). Rick.
derek@uwvax.UUCP (Derek Zahn) (02/22/85)
I is only an ameteur, so bear with me.
I am under the impression that a Light Sail would get its propulsion from
the solar wind, a stream of protons and neutrons that are expelled from
the sun constantly (and cause the Aurora Borealis, I think). It seems to
me that the "light pressure" from photons would be insufficient for any
substantial thrust.
What we need to know (from some more knowledgable source), is the velocity
and density of the solar wind. If someone could post this and similar
information, I would appreciate it, since I have been thinking about it
lately. Anyway, from this, it should not be too hard to compute what
velocity could be attained by a light sail if we make some assumptions about
area/mass of the sail material.
I think that when launching a light sail, we would want to launch it directly
toward the sun. The reason for this is that closer to the sun the solar wind
should be denser and we can get more thrust there -- in fact, I suspect that
most of our acceleration will be gotten quite close to the sun.
Any comments?
derek
--
Derek Zahn @ wisconsin
...!{allegra,heurikon,ihnp4,seismo,sfwin,ucbvax,uwm-evax}!uwvax!derek
derek@wisc-rsch.arpaethan@utastro.UUCP (Ethan Vishniac) (02/22/85)
> From: John Heimann <jheimann@BBNCCY.ARPA> > I sat down and tried to sketch out just how fast a solar sail could > reasonably be expected to go, assuming a point source of illumination (the > sun), the usual inverse square law for drop-off of photon intensity, a bit of > handwaving about the mean photon momentum, and some reasonable figures for the > density of the sail material. I realized then that I wasn't really sure how > the thing was to operate after all. Are solar sails designed to get their > momentum from elastic collisions with solar photons (reflecting sunlight), or > do they get most of it from collisions with other particles that are > constantly emitted from the sum (e.g. neutrons)? A typical neutron from a > nuclear reaction packs a lot more momentum than a photon from the same > reaction, so this is an important issue. *** REPLACE THIS ROCKET WITH YOUR SAIL *** However, a typical neutron from a nuclear reaction in the core of the sun will not emerge from the surface. Neither will the typical photon. Both are subject to scattering. The following numbers are taken from Allen's "Astrophysical Quantities" which is a standard astronomical reference book. The total energy flux from the sun is 6.27x10^10 ergs/cm^2/sec. If the sail reflects incident photons elastically then the pressure on the sail is 2F/c (solar radius/distance)^2, where c is the speed of light. Normalizing to Earth orbital radius this becomes about 9x10^-5 dynes/cm^2. Now suppose the sail also suffers inelastic collisions with particles in the solar wind. Allen gives the density of the solar wind as being about 5 protons per cubic centimeter near the Earth. The typical velocity is 450 km/sec. This gives us a wind pressure of (proton mass)x5x(Velocity of wind - velocity of sail)^2 normalized to the radius of the Earth's orbit. This gives us a pressure of about 1.7x10^-8 (1-velocity of sail/450km/sec)^2. However, Allen also mentions that the number density is inversely proportional to the velocity up to some (unspecified) limit. This suggests that the above estimate is an underestimate since the occasional high-momentum particles will be more important than the more common low-momentum ones. Nevertheless the above estimate makes it clear that photons are probably more helpful, if only by a little bit. A more important point is that if you just hold up your sail against the sun you'll have problems unless your Area/mass ratio is quite large. Below a critical limit you will simply reduce the sun's gravity by a constant fraction. The angular momentum you posess from the Earth's orbit will carry you out to some distance from the sun into a new, and larger orbit, and there you will stay. You need to tilt your sail to gain angular momentum as you go. I hope this helps. No comments on the mylar. That's beyond me. I'm sure people have written letters about this in Nature or some such place. "Don't argue with a fool. Ethan Vishniac Borrow his money." {charm,ut-sally,ut-ngp,noao}!utastro!ethan Department of Astronomy University of Texas Austin, Texas 78712 *Anyone who wants to claim these opinions is welcome to them*
henry@utzoo.UUCP (Henry Spencer) (02/24/85)
> ... Are solar sails designed to get their > momentum from ... reflecting sunlight ..., or > do they get most of it from collisions with other particles that are > constantly emitted from the sum (e.g. neutrons)? ... Sails are optical reflectors. Neutrons and the like are much rarer than photons, and are hard to stop as well. > ... Also, what's a good assumption as to the > composition of the sail? Would a large expanse of metallized mylar be > reasonable... Low-performance sails are typically assumed to be aluminized mylar. High-performance sail designs are all (as far as I know) derived from Eric Drexler's designs, which are aluminized *nothing*, just a layer of aluminum about the thickness of a virus. (Sorry for lack of numbers, my references aren't handy.) They are just as reflective and far, far lighter than aluminized-mylar sails. A hexagonal sail 10 km across weighs only about 20 tons. The major limitation of aluminum sails is that they *must* be assembled in space; they are basically rigid structures and cannot be folded up into a small package for launch. If you really want high performance, especially in low Earth orbit, one intriguing notion is to take a Drexler aluminum sail and punch it full of very tiny (smaller than a wavelength of light) holes. Holes which are significantly smaller than a wavelength of light will not affect the reflectivity of the sail, but they will lighten it considerably. Just as interesting, air molecules at orbital altitudes are independent of each other and will go through tiny holes just as easily as through big ones. Not only does one get (say) a 75% weight reduction, one also gets a 75% air-drag reduction. The snag here is that nobody knows how to make perforated-sail material yet. Ultra-thin aluminum is easy, but the holes are hard. (Last I heard, anyway.) > ... or would this rip to shreds under meteor impact? ... The density of dust and debris is too low to significantly damage a sail, except in anomalous areas like planetary rings. -- Henry Spencer @ U of Toronto Zoology {allegra,ihnp4,linus,decvax}!utzoo!henry
space@mit-mc (02/26/85)
From: Lynn.es@XEROX.ARPA In the talks I have heard on solar sails by the experts, it was stated that the pressure of sunlight is in the range of a thousand to a million times that of solar wind particles. They speak of mylar lasting a few months, and something more resistant to space (Kapton, or something like that) lasting for a few years. They also state that it is impractical to sail beyond Mars or Jupiter because of diminished sunlight. There are thoughts on beaming light at a sail to overcome this, but that is another story. /Don Lynn
derek@uwvax.UUCP (Derek Zahn) (02/27/85)
> In the talks I have heard on solar sails by the experts, it was stated > that the pressure of sunlight is in the range of a thousand to a million > times that of solar wind particles. Got time for a dumb question? Too bad. I can't figure how the energy of photons from the sun is going to be converted into motion of the light sail. I jus' don't get it. If the photon is reflected, there can be no change in momentum of the sail (momentum being conserved), unless the photon loses energy. In this case, what is the mechanism that causes the transfer of momentum, and how efficient could it possibly be? If the photon is absorbed, it seems much more likely that the energy would be converted to heat. Somebody help, for I is baffled. derek -- Derek Zahn @ wisconsin ...!{allegra,heurikon,ihnp4,seismo,sfwin,ucbvax,uwm-evax}!uwvax!derek derek@wisc-rsch.arpa
eder@ssc-vax.UUCP (Dani Eder) (02/28/85)
> > From: John Heimann <jheimann@BBNCCY.ARPA> > > I sat down and tried to sketch out just how fast a solar sail could > > reasonably be expected to go, assuming a point source of illumination (the > more helpful, if only by a little bit. A more important point is that > if you just hold up your sail against the sun you'll have problems unless > your Area/mass ratio is quite large. Below a critical limit you will simply > reduce the sun's gravity by a constant fraction. The angular momentum you > posess from the Earth's orbit will carry you out to some distance from > the sun into a new, and larger orbit, and there you will stay. You need to > tilt your sail to gain angular momentum as you go. > "Don't argue with a fool. Ethan Vishniac > Borrow his money." {charm,ut-sally,ut-ngp,noao}!utastro!ethan Congratulations! Ethan has rediscovered the 'lightness ratio'. This is a performance measure for solar sails. It is the ratio of light pressure to gravitational attraction for a given sail. Since both fall off as inverse square of distance, the figure is a constant for that sail. Light pressure is F=2P/c, where P is the light falling on the sail (watts), c is the speed of light, and 2 is for a perfect reflector. A typical real sail might be 1.8, meaning 80% reflected light. The gravitational attraction is GMm/r^2, where G is the gravitational constant, M is the mass of the Sun, m is the mass of the sail, and r is the distance between them. A lightness ratio of 1 means the sail can hang motionless, balanced between gravity and light. If the ratio is greater than 1, then on a radial escape mission, at every point on the trajectory, net outward force is (lightness ratio - 1)x gravity. The final velocity is then (L.R. - 1)^.5 x escape velocity. Since escape velocity depends on where you start, there is no single answer. For the more complicated case of a spiral out mission, I don't know what the answer would be. As for what you make your solar sail out of, you use VERY thin aluminum foil, preferably less than one micron thick. Typical plans call for vapor depositing the sail material in orbit, then somehow getting it off the substrate. Use graphite fibers to hang the aluminum off of. Spin the whole structure slowly, thus all the structure is in tension, and tends to stay flat. Don't get too near the Earth. Below about 1000 km, air drag exceeds light pressure, and you fall out of the sky very fast. Eric Drexler of L5 fame, and Robert L Forward, at Hughes Research Laboratories (and science fiction writer) are two names you can look up in abstracts for articles. I've been a fan of lightsails (any non rocket transportation, in fact) for quaite a while, so I can try to answer any more questions you might have. Dani Eder / ssc-vax!eder / Boeing / Advanced Space Transportation
ethan@utastro.UUCP (Ethan Vishniac) (02/28/85)
> Got time for a dumb question? Too bad. I can't figure how the energy of > photons from the sun is going to be converted into motion of the light sail. > I jus' don't get it. If the photon is reflected, there can be no change in > momentum of the sail (momentum being conserved), unless the photon loses > energy. In this case, what is the mechanism that causes the transfer of > momentum, and how efficient could it possibly be? If the photon is absorbed, > it seems much more likely that the energy would be converted to heat. > Somebody help, for I is baffled. > > derek *** REPLACE THIS LIGHTSAIL WITH YOUR WARPDRIVE *** Momentum is a *vector* quantity. When a photon is reflected it has a momentum which is the negative of its original momentum. The momentum transmitted to the sail is twice the original momentum of the photon. Of course, for a sail of finite mass this implies that there is actually a small loss of energy for the photon and its recoil momentum is not exactly equal in magnitude to its original momentum. [From conservation of energy]. "Don't argue with a fool. Ethan Vishniac Borrow his money." {charm,ut-sally,ut-ngp,noao}!utastro!ethan Department of Astronomy University of Texas Austin, Texas 78712 *Anyone who wants to claim these opinions is welcome to them*
nemo@rochester.UUCP (Wolfe) (02/28/85)
> Got time for a dumb question? Too bad. I can't figure how the energy of > photons from the sun is going to be converted into motion of the light sail. > I jus' don't get it. If the photon is reflected, there can be no change in > momentum of the sail (momentum being conserved), unless the photon loses > derek Remember that the momentum is directed. The mv of the photon becomes -mv when it is reflected, so there is a net momentum change of 2mv, which is imparted to the reflecting surface. There are these little globes with vacuum and a paddle/pivot mechanism inside which work on this principle. The four paddles are arranged so that the intersection of the two planes is the axis of rotation of the pivot. Each paddle has a black side and a silver side with radial symmetry. Placed in light, the paddle(s) on one side of the pivot will reflect the light, the one(s) on the other will absorb the light. The absorbing surface gets only the inelastic momentum change, or mv, while the reflecting surface gets the 2mv momentum change. This creates a torque on the paddle structure, and the little bugger rotates. Available at novelty stores near you! Nemo
al@ames.UUCP (Al Globus) (02/28/85)
> I is only an ameteur, so bear with me. > > I am under the impression that a Light Sail would get its propulsion from > the solar wind, a stream of protons and neutrons that are expelled from > the sun constantly (and cause the Aurora Borealis, I think). It seems to > me that the "light pressure" from photons would be insufficient for any > substantial thrust. Yer wrong. Thrust is from reflecting photons.
judah@uwvax.UUCP (Judah Greenblatt) (03/01/85)
> ... Each paddle has a black side and a > silver side with radial symmetry. Placed in light, the paddle(s) on one > side of the pivot will reflect the light, the one(s) on the other will absorb > the light. The absorbing surface gets only the inelastic momentum change, > or mv, while the reflecting surface gets the 2mv momentum change. This > creates a torque on the paddle structure, and the little bugger rotates. > Available at novelty stores near you! > Nemo The principle of light pressure is correct, but unfortunatly, it is not what drives a radiometer (as these little toys are called). If you look closely at a radiometer spinning in the sunlight, you will see that the vanes spin with the SILVER face leading and the BLACK face trailing. If light pressure drove the spinner, the BLACK face should lead and the SILVER face (which would be pushed twice as hard by the light pressure) would follow. The radiometer is actually a simple heat engine: the BLACK side of the vane heats up slightly more than the SILVER side and the vanes are driven by the difference in momemtum of gas molicules bouncing off the warmer and cooler sides. Light pressure is actually much weaker than the forces that drive a radiometer. Judah Greenblatt ARPA: judah@wisc-rsch.arpa U. of Wisconsin C.S. Dept. UUCP: {seismo ihnp4 lbl-csam}!uwvax!judah
peter@micomvax.UUCP (03/01/85)
On the question of how photons can impart energy to a light sail, I imagine a reflected photon would lose energy to the sail, so it (the photon) would go to a lower frequency, longer wavelength, etc. You shine blue light at a receding light sail, and get red light back. For a sail starting from rest, the photon wavelength shift would probably be immeasurably small, but still non-zero.
space@mit-mc (03/02/85)
From: Mike Caplinger <mike@rice.ARPA>
The original question was more about conservation of ENERGY than
conservation of momentum. The momentum of a reflected photon is
reversed, but its energy, a scalar, is the same (assuming perfect
reflection = no wavelength shift. The velocity, hence kinetic energy,
of a photon can't change.)
So if the sail starts moving from the impulse, where did the kinetic
energy of its motion come from? Remember that in a collision both
momentum AND energy are conserved.
I would really like to know the answer. My physics seems to be too
rusty to generate it, but I know there's something funny...
- Mike
ps. Those bulbs with the vanes ("radiometers") invariably spin in the
WRONG direction. That effect is caused by bad vacuum in the bulb
causing convection currents off the black surface. If light pressure
were doing it, they would rotate black surface first, as the white
surfaces are reflecting, not absorbing, and get twice the momentum
exchange.space@mit-mc (03/03/85)
From: Rick McGeer (on an z29-e) <mcgeer%ucbkim@Berkeley> I've said so many contradictory things about this issue that most of them must be wrong. As a result, I leave this to someone who knows more about physics than I do. However, Tipler, pp 958-960 seems to claim that the increased kinetic energy of a particle struck by a photon is due to the Compton effect, which I gave in my last note. I can't figure out what's wrong with the momentum calculations right at present, though... Rick.
sher@rochester.UUCP (David Sher) (03/03/85)
To you net.physics people this article is a result of a discussion of light sails and how they work. The question I am addressing is does being reflected by a light sail change the wavelength of the reflected light. This is a tricky problem because it is not well defined. (I am taking this from rememberances of a modern physics class I took 3 years ago so I am not authoritative). Light only has a wavelength relative to an observer. (or a frame I guess). There was an interesting problem I was given in the afore mentioned physics class which was given an observer for which a beam of light with wave length (relative to the observer) lambda is reflected from a mirror moving with relativistic velocity v what is the wave length of the reflected light. I believe it is not the same as the original light except when v is a small fraction of c (whats an epsilon between friends). Try throwing mirors around and see for your self :-). -David Sher
geoff@desint.UUCP (Geoff Kuenning) (03/03/85)
In article <133@uwvax.UUCP> derek@uwvax.UUCP (Derek Zahn) writes: >Got time for a dumb question? Too bad. I can't figure how the energy of >photons from the sun is going to be converted into motion of the light sail. >I jus' don't get it. If the photon is reflected, there can be no change in >momentum of the sail (momentum being conserved), unless the photon loses >energy. In this case, what is the mechanism that causes the transfer of >momentum, and how efficient could it possibly be? If the photon is absorbed, >it seems much more likely that the energy would be converted to heat. >Somebody help, for I is baffled. Wahl, Ah cain't resist demonstrating that I still remember my physics, despite senility... (a) Momentum is a vector. If the photon is reflected, it's momentum is reversed. To compensate for this, the sail must receive *twice* the photon's momentum in the opposite direction. In other words, before the collision the system has a momentum of 1 frobozz away from the sun. After the collision, the system must have the same momentum; since the photon has 1 frobozz of momentum directed *towards* the sun, the sail must pick up 2 frobozzes away from the sun to produce a net of 1 frobozz away. This is why reflective collisions are preferable. (b) Momentum and energy are two different things. Momentum must still be conserved in this collision; in this case the system (including the absorbed photon) must retain 1 frobozz away from the sun. This means that the photon will be slowed dramatically. Think of two frictionless clay lumps sliding on a surface, equal in weight. After the collision, the mass is doubled, so the velocity must be halved to conserve momentum. In the light sail case, things are a lot less equal, but it still works. The energy of the photon goes two places: a tiny bit becomes kinetic energy of the light sail, and the rest becomes heat. The photon was high-energy because energy is equal to (m*v**2)/2; the light sail only needs a little of this because its velocity is so low. -- Geoff Kuenning Unix Consultant (213) 545-4413 ...!ihnp4!trwrb!desint!geoff
karn@petrus.UUCP (03/04/85)
It would seem to me that photons reflected off a sail would indeed appear to be shifted in wavelength to an observer watching both the outgoing solar radiation and the reflected radiation. This would be due to the doppler shift caused by the relative motion of the sail with respect to the sun. The effect is equivalent to that which makes a police radar function. Energy is still conserved. Phil
ems@amdahl.UUCP (ems) (03/05/85)
> > (b) Momentum and energy are two different things. Momentum must still be > conserved in this collision; in this case the system (including the > absorbed photon) must retain 1 frobozz away from the sun. This means that > the photon will be slowed dramatically. Ah, I think you may have meant something else here. Like maybe the wavelength would change or some such. I'm not real up on my physics, but I thought that C was a constant... (Until the ANSI committies get done with it ? :-) E. Michael Smith ...!{hplabs,ihnp4,amd,nsc}!amdahl!ems Computo ergo sum The opinions expressed by me are not representative of those of any other person - natural, unnatural, or fictional - and only marginally reflect my opinions as strained by the language.
space@mit-mc (03/05/85)
From: Paul Schauble <Schauble@MIT-MULTICS.ARPA> rochester!nemo writes: > There are these little globes with vacuum and a paddle/pivot mechanism > inside which work on this principle. The four paddles are arranged so > that the intersection of the two planes is the axis of rotation of the > pivot. Each paddle has a black side and a silver side with radial > symmetry. Placed in light, the paddle(s) on one side of the pivot > will reflect the light, the one(s) on the other will absorb the light. > The absorbing surface gets only the inelastic momentum change, or mv, > while the reflecting surface gets the 2mv momentum change. This > creates a torque on the paddle structure, and the little bugger > rotates. Available at novelty stores near you! > Nemo This says that the beast should rotate witht he white (reflective) side retreating. Take a close look at one. They rotate in the other direction! Now, would anyone care to provide an explanation?? Paul
space@mit-mc (03/05/85)
From: jrv@Mitre-Bedford >> There are these little globes with vacuum and a paddle/pivot mechanism >> inside which work on this principle. The four paddles are arranged so >> that the intersection of the two planes is the axis of rotation of the >> pivot. Each paddle has a black side and a silver side with radial >> symmetry. > This says that the beast should rotate with the white (reflective) side > retreating. Take a close look at one. They rotate in the other > direction! Those globes really have a small amount of air in them. The black side of each paddle gets hotter than the white side, so the air molecules that strike the black side recoil with a higher velocity than those striking the white side. Those air molecules push the paddles, not the photons. - Jim Van Zandt