Talk:Solar sail

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I'm apparently having difficulty with the wording of this bit:

The radiation pressure from the Sun against an absorbing sheet with a mass of 0.8 g/m² is equal to its weight with respect to the Sun's gravity, independent of its distance to the Sun. This mass/area ratio is doubled in the case of perfect reflection.

"doubled" in this case seems to mean that the radiation pressure from the Sun against a perfectly reflecting sheet of 1.6 g/m² is equal to its weight with respect to the Sun's gravity, whereas it should actually be 0.4 g/m² since reflecting photons gives twice the momentum that absorbing them would. So I changed it to "halved", and Patrick reverted with an explanation that seems to be in complete agreement with what led me to make the change in the first place. So I plead confusion. :) Bryan 00:45, 16 Sep 2004 (UTC)

The radiation pressure on an absorbing sheet, regardless of mass, is equal to the gravity on a mass of 0.8 g/m², so there is a balance if that is the actual mass. If the radiation pressure is doubled then it can support a larger weight.--Patrick 07:01, 2004 Sep 16 (UTC)
Ah, lightbulb. That was a very dumb mistake for me to make, but once I made it it really buried itself deeply in my head. Sorry about that. :) Bryan 15:11, 16 Sep 2004 (UTC)
No problem, that can happen to everybody.--Patrick 19:51, 2004 Sep 16 (UTC)

Contents

[edit] What happened to the Japanese prototypes?

The section on 'current progress' mentions the Japanese prototypes launched in August. Could somebody add something about what happened to them, and whether they were successful? The Singing Badger 17:10, 30 Sep 2004 (UTC)

[edit] Please note, "attitude" is correct, stop changing it

This must be the umpteenth time I have had to change "altitude" back into "attitude". Yes, "attitude" is perfectly correct, it controls the direction the spacecraft is turning. There is little need for "altitude" control in space, for one, because there isn't really a "height" in space - its all subjective, but certainly attitude is needed because direction is far more important. Solar sails aren't really meant to be manoeuvred through the atmosphere, where altitude applies, either. Can you people please stop doing this? Its VERY VERY EXASPERATING. "Attitude" IS a valid word for spacecraft. -- Natalinasmpf 14:14, 22 Jun 2005 (UTC)

How about putting some HTML comments <!-- like this --> into the text at places where "attitude" keeps getting changed, explaining that the usage is correct? That way any editor who goes in to change it will definitely see it. I've used this approach before in other articles and it seems to work pretty well. Bryan 06:34, 23 Jun 2005 (UTC)

Hmm, I'll see where I could place it. I just don't want the comments to be too invasive, but it seems worth it anyway.

[edit] photons have momentum ? proof ?

if, as the article states, a reflected photon provides thrust to a physical object; then does this not suggest that the photon has mass? In other words, since force is created from the change in momentum, then in order for a photon to create some sort of change in momentum of the solar sail, the photon must posses a non-zero mass.

J. Crocker 18:01, Jun 23, 2005 (UTC)

See Talk:Cosmos 1 for parallel discussion. Apparently the photons are absorbed and re-reflected at a lower energy (lower frequency). I'd like some references, though. - Omegatron 20:37, Jun 23, 2005 (UTC)
See photon or, for the classical model, Poynting vector. Using conservation of energy and momentum you can easily calculate the energy of the reflected photon. For experimental verification see P. Lebedev, 1901, "Untersuchungen über die Druckräfte des Lichtes", Annalen der Physik, 1901. Icek 17:07, 8 October 2006 (UTC)
The correct title of Lebedev's paper should be "Untersuchungen über die Druckkräfte des Lichtes" (note the double k). Icek 00:29, 2 November 2006 (UTC)

It's basic physics that photons have momentum. Photons are both particles and waves. You can apply de Broglie's famous theorem to photons to calculate the momentum of photons.

[edit] solar wind or photons?

"Another false claim is that solar sails capture energy from the solar wind. The solar wind, composed of charged particles, would indeed apply a small amount of pressure to a solar sail, but this is small compared to the pressure exerted by light that would be reflected from the sail."

Everyone seems to think it's the solar wind. Can we have some references?
What is the mechanism by which photons impart momentum if they have no mass? (In detail, with references) - Omegatron 20:37, Jun 23, 2005 (UTC)

Albeit the photons have zero mass at zero velocity, they posses a momentum, equal to h.nu (Planck's constant multiplied by frequency). Therefore, in the case of total absorption (i.e. albedo = 0), this momentum is transferred totally to the sail. In the case of total reflection (albedo = 1), the momentum of photon is reversed, and therefore t w i c e the value of its momentum is transferred to the sail.

Of course, the solar wind (particles from Sun, mainly protons) are imparting also some pressure on sail, but this is in order of 0.1 nanopascals (compare it with 4.6 micropascals of solar radiation pressure), cf. McDowells site.

Also, if my memory is right, the Pioneer 10 space probe is not using the "solar sail" or sola pressure at all. On other side, Mariner 10 (probe to Mercury), was using a solar vanes for controlling the attitude (ie. orientation, not for changing its path). Probably confusion. Antonin Vitek, (avitek) Czech wiki

Some physics can be found e.g. at:

http://www.inspacepropulsion.com/tech/sails_physics.html

In my last quote, the value 4.6 micropascals is for totally absorbing sail (black-body-like), for an totally reflective one is twice as big, i.e. 9.2 micropascals.

The quotes about levelling the weight of 0.8 grams per meter squared (black-body) or 1.6 grams per m.sq. ("white-body") by solar pressure are valid for interplanetary space only(without perturbing forces of planets). It is interesting, that this value (mass per area) is independent on distance from Sun, as the pressure diminishes with the square of distance from Sun as well as the radiation pressure, i.e. both forces remains in an equilibrium for such sail.

Antonin Vitek, Czech wiki


[edit] physics

"The craft would have been gradually accelerating during each orbit as a result of the radiation pressure of photons colliding with the sails."

I thought it was from the solar wind.
Aha. "Another false claim is that solar sails capture energy from the solar wind. The solar wind, composed of charged particles, would indeed apply a small amount of pressure to a solar sail, but this is small compared to the pressure exerted by light that would be reflected from the sail."

"Photons are the sub-atomic particles that make up light, and travel at the speed of light. As photons reflect off the surface of the sails, they transfer momentum to the object."

How does this work, anyway? The photons would have to lose velocity if they transferred momentum, no? Are they absorbed and re-reflected with lower energy (frequency)? - Omegatron 18:07, Jun 22, 2005 (UTC)
They are indeed reflected with lower frequency. The easiest way to think about it is to assume it's something similar to an elastic collision, but with the photon moving at C before and after impact. These assumptions are realistic under most conditions. --Christopher Thomas 19:28, 22 Jun 2005 (UTC)
Careful! Even if the reflected photon had the same energy, a net momentum is imparted on the sail. As long as the photons are reflected from their original direction, the law of conservation of momentum will "enforce" this. Awolf002 19:57, 22 Jun 2005 (UTC)
The change in momentum of the sail, however, results in a change in kinetic enery of the sail. This is balanced by a change in energy of the photon. It isn't always redshifted; for a decellerating sail, for instance, it would be blueshifted.--Christopher Thomas 20:05, 22 Jun 2005 (UTC)
Well, read the Laser cooling article, where all photons have the same energy. You still change the momentum of the object interacting with those photons. Awolf002 20:13, 22 Jun 2005 (UTC)
I am very familiar with laser cooling. The photons are absorbed and emitted at the transition frequency in the atom's rest frame. The frequency of the lasers is actually tuned to slightly below the transition frequency, with the extra energy for the low-to-high transition supplied by the atom's kinetic energy. From the lab's rest frame, it looks like the atom emitted a photon of higher energy than it absorbed. This is why laser cooling is effective.--Christopher Thomas 20:22, 22 Jun 2005 (UTC)

(moved from Talk:Spacecraft propulsion)

I heard that solar sails are propelled by photons hitting the 'sails'. But photons have no mass and therefore no momentum, so in a collision they provide no impulse? -- sodium

Actually, photons do have a momentum. The momentum of a photon equals &#8462;/&lambda;, or Planck's constant divided by the photon's wavelength.
Thanks, yes. Since I asked the question we have covered it in my A level course.


Photons have no rest mass. Photons have energy, however, so their "mass" is E/c**2, so their momentum is E/c. This momentum is imparted to the sail upon impact. - RjLesch.

It may be interesting to note that if you make the sail reflect the photons, you actually get 2E/c per photon. --BlackGriffen


yes the photons do provide the impulse to the sails. According to the Planetary Society who have built the first solar sail powered craft 99% of its propulsion should come from reflecting photons, only 1% from the solar wind. See also http://www.physlink.com/ae270.cfm and http://www.u3p.net/tipe/phot_jp.htm --rmhermen


[edit] Sailing upwind

Would it be possible to sail upwind with a solar sail? --EnSamulili 28 June 2005 21:13 (UTC)

Not really, at least not in the sense that sailboats do. Sailing into the wind at all is only possible because a boat is in water. In space, the sail is only pushed directly away from the source of the incoming photons, and can't be redirected or caught in any other way without some other force acting on it. siafu 28 June 2005 21:43 (UTC)
Yes, you *can* "tack" with a solar sail, pretty much exactly the same way sailboats do.
  • To accelerate *away* from the sun, you point the sail so it bounces photons directly back towards the sun. (Much like when sailing downwind, you point the sail so air particles bounce directly back from where they came from, like a parachute).
  • Assuming you're in orbit around the sun, to accelerate *forward* in your orbit, you tilt the sail about 45% so that it bounces photons *behind* you. (Much like when the wind is blowing from the west, and I accelerate *forward* to the North by tilting my sail about 45% so that air particles bounce off my sail heading *behind* me, heading South). (This indirectly leads to a *larger* orbit, *further* from the sun).
  • Assuming you're in orbit around the sun, to accelerate *backward* in your orbit, you tilt the sail about 45% the *other* way, so it bounces photons *ahead* of you. (Much like when I turn my sail so that air particles bounce off heading North, pushing me to the South). (This indirectly leads to a *smaller* orbit, *closer* to the sun).
  • To *move* directly towards the sun, use one of the above techniques to accelerate *backward* in your orbit for some time, until you are no longer moving relative to the sun. Then fold up the sail (or turn it edge-wise) and drop straight down towards the sun.
  • Much like the counter-intuitive fact that sailboats can go much faster *across* the wind than in the *same* direction the wind blows, the "forward in orbit" and "backward in orbit" maneuvers work even with craft that are much too heavy (more than 1.6 g/m²) for the "accelerate away from the sun" maneuver to do anything useful.
--DavidCary 03:16, 29 July 2005 (UTC)
Folding up the sail and dropping straight towards the sun is not using the sail to move upwind. It's using gravity; one might as well say, "Sure, just come to a stop and fire the rockets." siafu 21:59, 1 August 2005 (UTC)

Ah, you are exactly right. I said a lot of words, but I never actually answered the original question.

  • Say you turn the sail nearly edge-wise to the sun. Tilt it just a little, so that photons from the sun bounce at a glancing angle, deflected slightly to the right. Then the net force of the photons is almost entirely to the left. The net acceleration on the sail (including gravity) is towards a point just to the left of the sun. (Much like when the wind is blowing from the west, and I turn my sail almost edge-wise to the wind, but deflecting the wind slightly to the north. Then the force of the wind is almost entirely to the south. The *combination* of the force of the wind in the sail and the pressure of the water on the keel / centreboard moves the entire sailboat in a direction just to the south of directly west, just to the south of directly upwind).
  • If I turn everything around, I get ... photons deflected slightly to the left ... force ... almost entirely to the right ... net acceleration ... towards a point just to the right of the sun ... moves the entire sailboat ... just to the north of directly west, just to the north of directly upwind.
  • Alternating back and forth between left and right allows a sailboat to "tack", reaching a point directly upwind of its initial position.

The combination of the force of the wind on the sail, and the force on the keel/centreboard, allows a sailboat to accelerate/move in (nearly) any direction.

The combination of the force of photons on a solar sail, and the force of gravity, allows a solar sail to accelerate/move in (nearly) any direction.

If there's any way to not use gravity, I would sure like to know :-).

There's several ways to think about the forces on a solar sail. All give the same results.

One way:

  • Whenever a photon bounces off a mirror, the net force on the mirror is from the silver side towards the dark side. Imagine a spike pointing directly out from the dark side of the sail. The force of photons bouncing off the sail is always in the direction of that spike, no matter which direction the photon came from. We have one force directly towards the sun (gravity), and one force we can point half of all directions (reflection). (By turning the sail, we can point that spike almost any direction we choose, as long as we keep the spike in the dark).

A different way:

  • There are 2 parts to the force of a photon bouncing off a solar sail. The first part is the force of the photon hitting the solar sail. Since the photon always comes directly from the sun to hit the sail, this part of the force is always directly away from the sun. (This would be the *only* force if the solar sail were black, but no one wants that -- everyone's solar sail design is reflective / mirrored). The second part of the force comes from the photon shooting out of the sail. We control exactly which direction the photon goes when it leaves. We can tilt the sail to send a beam of photons practically any direction (except *directly* away from the sun). Shooting out those photons in *that* direction pushes the sail in the exact opposite direction. We end up with one force directly away from the sun (absorption (optics)), one force directly towards the sun (gravity), and one force that we can point in nearly any direction we choose (emission).

If we put one or the other of these ways of explaining it into the main article, which one do you think would be less confusing? Even better, could you write an even less confusing way of explaining it?

--DavidCary 03:11, 15 August 2005 (UTC)

"We can tilt the sail to send a beam of photons practically any direction (except *directly* away from the sun)"

What if you bounce it off another solar sail? :-) - Omegatron 03:20, August 15, 2005 (UTC)
The reason I objected to your well-reasoned explanation is because a sailboat using only the force of the wind to move upwind, taking advantage of the resistance of water to pressurization, or more generally, the resistance to lateral motion provided by the keel which forces the boat forward (mostly) rather than directly away from the wind, and not because your explanation was wrong in any direct way. Outside of a gravity well with only one source of photons (obviously a purely hypothetical situation) it wouldn't be possible to move "upwind" with a solar sail. However, just about any situation wherein a solar sail would be useful requires a powerful light source, which in our universe is also almost always a powerful source of gravity at the same time. The only situation that I can think of, practically, where such would be eliminated is with a craft in interstellar space being struck with a carefully aimed maser/laser/whatever that is focused on a very narrow angle, allowing it to stay intense despite great distance.
I'd say your second explanation is more accessible to the lay audience, personally, but it might be helpful to refer to and/or explain the nature of vectors, but that's just a suggestion.
In response to Omegatron, if you manage to reflect the photon directly away from the sun again, the net force transferred to the object holding the sails (assuming it's one rigid object) would be zero, given that the photon receives the same force in being reflected out on the same vector it was originally on, so you COULD do that, but it would defeat the purpose of using a solar sail to begin with. siafu 04:03, 15 August 2005 (UTC)

[edit] Removed yacht text

I removed the following:

"Aspiring engineer Kyle Caskey has proposed to use designs of manned spacecraft with solar sails as the main means of propulsion for a small pleasure crafts, somewhat like yachts. These solar sail yachts could move small groups of family and friends between the Moon and Earth and other destinations at varying speeds."

Text from an aspiring engineer shouldn't really be included in this case. kmccoy (talk) 21:08, 13 July 2005 (UTC)

[edit] Question

What kind of acceleration are we talking about, here? It would be great if we could translate some of the mechanisms, etc, above into measures that the average sci-fi fan can grok. Do solar-sailed craft whiz about the solar system, or just kinda putter? If HAL had been sail-powered, how long would it have taken Dave to get to Jupiter? Thanks!

I agree that it would be nice to put acceleration, earth-to-mars time, earth-to-jupiter time, etc. into the article. --70.189.75.148 00:54, 29 July 2005 (UTC)
I enthusiastically also agree. Therealhrw 00:23, 29 December 2005 (UTC)
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