Questions about how Solar Sails work:

While at Nextfest, I saw the solar sail exhibit, but unfortunately didn’t get a chance to have my questions answered.

I understand the basic principle: that the sail is effectively pushed by photons bouncing off of its surface and that while the object starts off at a slow speed, the more photons that bounce off of it, the faster it goes.

Here are my questions:

  1. Since there are stars all over the place, wouldn’t photons bounce off of the sail from all directions, causing it to go in unintended directions? Or would this not happen since there are more photons coming from our sun and these are more 'powerful" than photons coming from other stars? I also presume that, like a wind sail, the solar sail will be positioned so as to reflect photons from the sun and the odd angle would minimize the effect of photons coming from other stars. Is this correct?

  2. If the sail is in space and there is no gravity in space, wouldn’t it get up to top speed fairly quickly (the guy at the event described acceleration as a very gradual process)? If f = ma, would acceleration be slow because the photons have very little mass in and of themselves? I presume that the gravity of the planets would also have to be overcome.

2a) I assume that the size of the object being pulled by the sail is more-or-less irrelevant. That is, a Mir space station-sized thing would accelerate at the same speed as a satellite-sized thing.

  1. Is the theoretical top speed of this thing the speed of light? Or slightly under?

  2. Can one steer these things? Would they just deploy the sail in another direction and then have to wait for the momentum to swing them back to zero velocity and then start all over again in the opposite direction? I presume that NASA would just shoot the thing into space and build the appropriate transmitters for what data they were looking for and once it gets out of range, figure it had served its purpose.

  3. Indulge me on this one: I presume that whatever the sail is fitted on is to send back data of varying sorts. Would it be possible for this thing to start accelerating to such a degree, stop and turn around and accelerate back that it would pass its own transmissions and make it back to Earth before the transmissions? What speeds would we be talking about (this would, of course, require us to know what sort of transmitters it has, something I have no idea of). This sounds like the instigating plot element of a sci fi movie, but I just thought it would be interesting.

The sun is much closer than other stars so for a given area of sail more photons from the sun will hit than from other stars. The intensity of any radiating source varies inversely with the square of distance. That is to say if the intensity of light is measured at a given distance and you double that distance the intensity will be 1/4 as much. This is called the law of inverse squares and applies to any radiating energy that diverges such as light or sound.

Dunno the answers to all of your questions, but here’s some pages on solar sails.

If the bounced photons can be angled then some directional control seems possiable.

Reversing direction may be possiable, but would require another star if you want to just use the solar sail (Launched from the sum (Sol?) to Alpha Proxima. After the 4.x lightyears of travel Alpha Promima should be able toturn this thing back to us.

Also you could take in the sail and let gravity take you back to the sun.

About 2/3 of the way down this page is the diagram I recall for return-voyage solar sails. (This model requires a stationary laser power source, and would be used interplanetarily.) Basically the outer part of the sail breaks off and is used as a reflector to reverse the thrust.

If there is directional control, is it not possible to “tack,” cut back and forth at oblique angles with a solar sail?

Photons transfer momentum to the sail (and suffer some slight reduction in frequency in the process). The amount and vector of the momentum is dependant upon their source and the orientation of the sail toward it; in near-Sol space, the vast majority of photons will be coming straight out from the Sun, where as light from stars will be coming in from all directions and will be much less intense. Also, some of the thrust may come from “solar wind”; charged particles ejected from the sun, as well as from photons.

First of all, there is gravity in space; in the area near the sun and planets the gravitational field is signficiant; even in interstellar space, and for that matter intergalactic space, gravitational fields (or gravitons, if you like) exist, although they are much less significant owing to the rather weak interaction of gravity (compared to electromagnetism). In any case, I believe the concept you are searching for is actually inertia, or the resistance of a body to change velocity (speed or vector). Photons do indeed have a very small momentum or equivilent mass compared to the “normal matter” of baryons (protons and neutrons) and electrons. A single photon with a 520nm wavelength (visible “green” light), for instance, will have a mass of about 2.4eV. An electron has a mass of 511keV, or a difference of greater than five orders of magnitude; protons and neutrons have a mass of around 940MeV, or more than eight orders of magnitude difference. One eV is equal to 1.6x10[sup]-19[/sup] joules, and one joule is enough to raise a 0.1kg object–say, a golf ball–1 meter on the surface of the Earth.

So, even if you could fully transfer the energy of photon to your sail, you’d need, in the words of the late Carl Sagan, billions and billions of photons per single atom of your vessel in order to get it going at all. Fortunately, the sun wastes photons like they’re going out of style, and you’re free to grab all you want, but even with a large, lightweight sail, it will take time to accelerate your cargo. The advantage of a solar sail, of course, is that you don’t have to drag your fuel source with you, nor are you limited by the amount of fuel you can ccarry. Instead, you use the Sun’s exhaust, or (potentially) even a very powerful laser to propel your craft, albeit slowly.

In absense of other forces, an object would accelerate in porportion to the surface area of the sail and in inverse proportion to its mass. The effect of gravity by local objects will, of course, affect its trajectory.

Actually, the theoretical top speed would be considerably less than light; you need to consider not only the effect of diminished light as you move away from your energy source (as a factor of distance squared), but as you accelerate to relativistic speeds (say, in excess of .1c) you are going to have to deal with reduced efficiencies from the redshift (difference between your velocity and the incoming speed of photons, which makes the photons appear less energetic) and the drag resulting from the sparce but everpresent gases in space. Just as an off-the-cuff guestimate, I’d speculate that your effective maximum velocity would be somewhere around <1% of c for even the smallest payload, using the Sun as your power source. Using a directed, high powered laser (as Dr. Robert Forward has suggested) might give you a much higher velocity, owing to the vast reduction of energy dropoff with distance; however, assuming that you are going to want to stop someplace (say, Beta Hydrae, to have some jynntnnix with the hideous-looking but genial Ablogmothian Space Slug), you are going to want to limit your speed such that you can slow to interplanetary velocities using the repulsion and gravitationally “reverse slingshotting” of your target system.

If you’re willing to keep going indefinitely (and presuming some kind of variable frequency, highly directed laser beam that your friends back on Planet Earth are going to keep running for you for centuries) then you could presumably get up into the energy ranges of x-rays or even gamma rays (which would be downshifted to the visible spectrum owing to your relativistic speeds); again, the limitation would be drag forces from the interstellar medium and the ability of your craft to protect you from damage due to gas clouds and the incoming, highly blueshifted electromagnetic radiation coming dead ahead. You’re not going to be able to make a roundabout and get back home for Happy Hour, though.

In-system, you could “steer” your craft in a way somewhat analogous to how you steer a sailing vessel; by altering the orientation of the sail to the direction of the incoming light “wind”. You wouldn’t be able to “head up” into a close hauled velocity; you could, however, use the principles of orbital ballistics to “slow yourself” down into a tighter, faster orbit, or speed up into a slower, outbound spiral. Nothing “in orbit” has a zero velocity; if it did, it would accelerate directly toward the Sun (or other significant local mass). As with boat handling, maintaining steerage (in the case or orbital ballistics, orbital velocity) is paramount to maintaining control and maneuverability. It certainly wouldn’t behave like a Star Trek spaceship, but then, nothing really does, except in the visions of (some) science fiction writers.

No. Assuming that its transmissions are of the electromagnetic variety and not written on, say, rocks or old copies of Mad magazine, your theoretical craft could not accelerate faster than the signal (which would be moving at c), even if it had very powerful engines instead of weak solar propulsion.

I can’t believe I’m going to pimp for Carl Sagan, but his book Cosmos(and the PBS series on which is was based) are an excellent nontechnical primer for learning about space physics and transportation; as I recall, he has an entire chapter on spacecraft concepts, including solar sail craft, magnetic (Bussard-type) ramscoops, Orion nuclear propulsion, et cetera. Robert Forward’s Indistinguishable From Magic, while awful as literature, presents numerous speculative concepts for space travel.



Great job “Stranger On A Train” you beat me to it and I couldn’t have explained it so well

I recommend the book “The Starflight Handbook” by Eugene Mallove & Gregory Matloff.

It has all the type of craft you describe complete with math formulas. I don’t do
the math, but it explains everything!

I personally think the “Bussard Ramscoop” is how we will get to the stars :slight_smile:

I also loved the Orion nuclear propulsion idea, where they detonate atomic charges
behind a craft (short explanation) to propel it. It was actually tested with a craft they
named “Put-Put.” Freeman Dyson, of Dyson Sphere fame, was on the team. Not
really very practical, but hey … nice try!

All the above is why Larry Niven is my favorite Sci-fi author. All his ships in his books are
based on actual, plausible theories. Plausible as in it should work but we might not
have the technology for a while … quite a while!

Stranger, that was simply wonderful! That should be made into a Staff Report.

No. A tacking sailing vessel relies on the same principle as an airplane (or it’s wings, to be more exact): the velocity of a fluid is inversely proportional to its pressure. The sail on a tacking vessel is set up in such a way that the wind pushes air faster around one side of the sail than the other. Since there is no equivalent property of space, where the only movement is the protons, the principle would not apply.

Well, **stranger ** answered my question about as thoroughly as one could hope. Thanks dude.

Except he’s raised more. What’s “jynntnnix?” :slight_smile:

Oooh, ooh, I think I know this one. Is jynntnnix the one that’s two percent of nitric acid, mixed into water at about seventy-five degrees C, with a few drops of fanallan rum and a cube of benzene added to give it an extra kick?

Or am I thinking of gziinoontahniggz?

[sub]confused? See ‘restaurant at the end of the universe’, ch 24[/sub]

Well, actually it’s Chapter 25:


Hmmm… that’s weird, my ebook edition and that website seem to be differing on the chapter numbers. Will check into this further when I get home from work. :slight_smile:

F = ma, so if you put the same force on a larger (more massive) object, you get less acceleration. The force on a solar sail depends on the area of the sail, so for a sail of a given area, yes, the mass of the payload does matter, and the lighter the better.

One often-missed point about solar sails, by the way: It’s theoretically possible, but in practice impossible, to make a sail so large and so lightweight that it would be able to counteract the gravitational force on it. The net force on a practical solar-sail craft in the Solar System would be pointed towards the Sun at all times. In fact, its path would be a Keplerian orbit, just as if it were orbiting without a sail around a star slightly less massive than the Sun.

In order to get anywhere with a solar sail, you need to regularly change the configuration of the sail. For instance, suppose you start with the craft in a circular orbit at 1 AU from the Sun (more or less the Earth’s orbit). Now suppose that, from January 1 to July 1, you deploy the sail, but from July 1 to January 1, you pull it in. Keep up this pattern for long enough, and you’ll find yourself in an elliptical orbit, with the sail furled while you’re approaching the Sun, and deployed when you’re moving away from the Sun. Keep up the pattern even longer, and the apohelion will get further and further away from the Sun, until eventually you reach escape energy, and can travel as far as you like from the Sun.

I think you’re mistaken in that (not simply your typo of “protons” for “photons”). Arthur C. Clarke, who knows a thing or two (and certainly a great deal more than I) wrote a short story called Sunjammer IIRC, in which contestants in a solar sail race do tack by moving their solar sails. Given that Clarke tends to get his science right, I’m gonna guess that you can tack.

Yeah, I don’t think those chapter numbers are accurate. They list the introductory notes about the universe disappearing as ‘chapter 1’, and then has chapter 2 as ‘the story so far’.

Both my ebook and audiobook versions have the introductory statements as being without a chapter, and chapter 1 starting with ‘the story so far’. I don’t have a print copy to check at the moment.

shrugs Funny bit, anyway. Don’t get me started on remmanqox :wink:

No it doesn’t. Or, at least, it doesn’t matter very much. Tacking relies on the fact that the shape of the boat in the water allows for much easier travel forward/backward along the heading of the boat. Try taking sometime without the centerboard up and see how little the sailshape will help; the wind just blows you sideways.

And I think you could tack in space if you had some extra reflective surfaces to work with and a rigid structure to mount them on, but I haven’t figured out how, yet.

Oh, good lord. I started thinking about solar sail tacking and forgot how to write. This should be:

Tacking relies on the fact that the shape of the boat in the water allows for much easier travel forward/backward along the heading of the boat than it does sideways. Try tacking sometime with the centerboard up and see how little the sailshape will help; the wind just blows you sideways.

This isn’t entirely correct. An aircraft wing works simply by Newton’s law of action and reaction: it redirects air downwards (action), and this pushes the wing upwards (reaction).

And a solar sail can control thrust for the same reason. If the wing is tilted so that the light is reflected to the left, the sail is pushed towards the right. Of couse you can’t actually have thrust towards the sun, but that doesn’t matter for interplanetary flight where everything is in orbit. The spacecraft tilts its sail so that the light is reflected “forward” (direction of its orbital motion). This acts to slow down its orbital motion. The result is the spacecraft goes into a lower orbit, i.e. closer to the sun. You could say the sun’s gravity performs the same function as the centerboard of a sailing boat.

By the way, the short story by Clarke is also published as The Wind from the Sun. (And it’s one of my favorite sci-fi short stories ever.)