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Space

How Solar Sails Work 83

Posted by Hemos from the learning-more-about-the-world dept.
Gary writes "You can also learn more about How Solar Sails work. In mid-April 2001, The Planetary Society, in collaboration with several Russian space organizations, will test launch the deployment mechanism for the first solar-sail mission, Cosmos-1, in a sub-orbital flight. It is a rounded solar sail that is divided into eight triangular blades with inflatable booms for support. Maneuvering a solar-sail spacecraft requires balancing two factors: the direction of the solar sail relative to the sun and the orbital speed of the spacecraft."
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How Solar Sails Work More

How Solar Sails Work

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  • One point you forgot to refute...

    The solar wind has almost nothing to do with solar sails. They derive power from the impact of photon, not from the impact of the charged particle known as solar wind.

    The power from photons is orders of magitude greater. If they relied on the solar wind, they wouldn't go anywhere.


    Don Negro

  • planetary society has more on their mission (Score:3)

    by Don Negro ( 1069 ) on Friday April 06, 2001 @06:53AM (#311249)
    Here [planetary.org].

    They also have a good high-level overview of solar sails, and theirs in particular.

    Good to see Carl's people still working to promote his dreams. I got all nostalgic reading the site.

    Don Negro

  • You are now leaving the Sol system. Please obey all interstellar traffic laws. Thank you for visiting and come again soon.

    Mile Marker 18,000,000,111
  • It's called inertia. If the same engine powers two cars, and everything between the two cars is equal, but one of the cars weighs twice as much, the lighter car will accelerate faster.

    Yup, good ol' f=ma. Or, in this case, a=f/m.

    And why the hell was this post moderated as Off Topic?? Are moderaters that stupid?

    Who knows? Maybe meta-moderation will get 'em, maybe not. [shrug] The system's not perfect, but it does ok, overall.

    (That's off-topic, though f=ma isn't.)

  • Oh well, I am sure some viable use will be found for it. Either that or we will dump millions of dollars into it and then decide that it isn't viable.

    One of the interesting things about this particular project is that it's privately funded. (By Cosmos Studios, which is a joint venture of Carl Sagan Productions, Inc. and Intend Change, Inc.) "We" aren't "dumping" any money into it, in this case.

  • 1. The further you are from the sun, the less force is exerted on the sails, until the forces from other stars/suns outweighs the force of our sun. Remembering that our sun is relatively small, this will not necessarily be at the midpoint between solar systems.

    Nod. Using these for interstellar travel would be trickier than just sailing aroung the solar system. Some possible work-arounds have been proposed, though.

    2. Travelling towards the sun is difficult and requires some sort of "tacking" process.

    Travelling toward the sun is cake. "Tack" to lower any orbital velocity "around" the sun, and the sun's own gravity will do the rest.

    3. Travelling in any direction other than away from the sun requires a centerboard and rudder for maintaining a direction.

    Oops. No. These are work great in the context of sailing on top of a fluid medium, but there are other ways of maintaining orientation -- gyros, small thrusters, auxilary "guide sails", etc.

    4. Sun-spots change the intensity of the solar winds, creating 'gusts'. Gusts will be difficult to control.

    "Gusts" of solar wind aren't really closely comparable to gusts of air. With reasonable sensors and feedback systems, a solar sail would manage just fine.

    From what I can see, there is only one direction the vessel can travel, and that will be determined by the release time from the earth, and the velocity at releas. After that, the only form of control is to change the "sail area" by either reducing sail size, or by changing orientation. Neither of these processes change direction, only speed.

    Changing the orientation of the sail can change the direction and/or speed of the craft, especially when used in combination with gravity fields.

  • Given the state of the Russian economy, modifying one of their ICBMs -- especially since it's certainly one already designed to be sub-launched -- and renting a sub to launch it probably is cheap, in comparison to other methods.

    (Thanks, NASA et al, for screwing up the DC-X and generally continuing to smackdown any other chance for cheaper launch systems.)

  • Re:no yet very feasible (Score:3)

    by Bearpaw ( 13080 ) on Friday April 06, 2001 @06:16AM (#311255)
    Solar Sails dont hold much promise yet though ...

    I think it depends a lot on the mission characteristics. Some propulsions systems are good for some kinds of missions, some for others.

    Besides, how are solar sails ever going to become feasible unless someone starts actively working on them?

  • Oopsie (Score:4)

    by Bearpaw ( 13080 ) on Friday April 06, 2001 @06:09AM (#311256)
    The sail must be thin and have a minimal mass, because the more mass, the less force that sunlight imparts to the sail.

    Um. I think the force imparted by the sunlight is the same. The force imparted just results in a lower acceleration if the mass of the sailcraft is higher.

  • I don't know if they actually did this, but a sea launch allows you to launch much closer to the equator than most land-based launches, since most launch sites are fairly far away from the equator. A boat you can move anywhere.

  • Real sailboats aren't really "pushed" by the air, really the sail is used as a vertical wing-shape, so that the passage of air over the sail creates "lift" in a forward direction. If sailboats were just pushed by the wind, they wouldn't be able to tack upwind at all.

    As far as the solar sail, I wonder if they could alter the direction of the reflection in order to steer. Does the direction in which the light bounces off of the mirror affect the resultant acceleration?

    At the very least you could angle the aspect ration of the sail to the sun in order to modify your acceleration; if your sail catches less photons then you won't accelerate as fast.

  • Think about it this way. You've got a target free to move in two dimensions sitting on a pool table. Roll pool balls at said target. The direction of travel of the target will vary depending on the angle of the target to the incoming track of the pool ball. The pool ball will rebound off the target (angle of incidence equals angle of reflection) and the target will move away from the impact in the direction of the bisector of the angle of the ball's incoming and outgoing tracks. Cornfused yet? : )

    Most sails (unless you're running directly away from the wind with your main on one side and your jib on the other, or using a spinnaker) work by the Bernoulli effect...they're airfoils. Solar sails are pure Newtonian kinematics...f=ma.
  • When you've got a solar sail many kilometers on a side, you're not at all worried about holes from even medium-large (think basketball) sized meteors.

    Interacting with something that far away is just not gonna happen. Keep in mind that once the signal comes from the spacecraft and is received by the ground station, the spacecraft is already FAR past the article of interest, and moving at Ludicrous Speed (tm). Turning around for another look is not an option. Imagine trying to make a cannonball go back for another look. : )
  • A few comments on this:

    1) Consider the possibility of driving similar sails not with solar light but with high-powered Earth-based or orbital lasers.

    2) This sail is quite small; just large enough to test the theory. Interplanetary solar sails would require MUCH larger surfaces, and, as you say, much better/lighter materials.

    3) All ratios of mass vs. light pressure aside, a solar sail is impractical for interstellar travel because the pressure imparted by the solar wind ceases at the heliopause (by definition, the boundary between our solar system and interstellar space), where solar particles form a shock wave as our solar system moves through interstellar dust. Pioneer 10 and both Voyager 1 and 2 are believed to be approaching the heliopause [exosci.com].

  • This page [caltech.edu] has a decent visual explanation of how one might tack in a spacecraft with a solar sail. The fundamental concept is that the photons are reflected off of the sail, allowing the spacecraft to impart positive or negative delta-v to itself, thus spiralling into higher or lower orbits.
  • The point many people seemed to miss, and, the correct aspect of the animation (which someone said was wrong) is that the Solarsail is only used to effect orbit.

    Period.

    That is all a Solarsail is designed to do. It does it extreemly efficiently. To the point where if the craft is in a stable orbit it can easily be accelterated (increasing it's orbit) or decelterated (decreasing it's orbit). No further push required as it will maintain that orbit like any other object in the solar system.

    (Hence the reason the animation shows the sail at the 'wrong' angle to reflect any energy while it continues to orbit. It is only when the orbit is increased, or decreased the the sail is used to reflect the energy received. (either at an angle to accelerate, or decelerate the craft; again increasing or decreasing it's orbit.).

  • Actually, you are forgetting about the fact that a spacecraft would be in orbit around the sun. It is true that there will be no keel, and you won't be able to tack to control your direction.

    However, you will still be able to go towards the sun just as easily as going away from it, because the sun's gravity will still be the dominant force in play. To travel towards the sun, you angle your sail to slow you down, while to travel away from the sun you angle it to increase your orbital velocity.

    If you reflect the sunlight directly back at the sun, you won't travel away from it at all, even though the light will be exerting a maximum force on your spacecraft.

    This is because regardless of your distance from the sun, the gravitational force will always be greater than the light force. Actually, the ratio between the two is determined by the area/mass ratio of your spacecraft, but for those that are proposed, gravity will dominate. This also means they won't be practical for leaving the solar system, since they would have to spiral out rather slowly.
  • Not necessarily. There are designs for sails which will have a "lightness" as high as 3 (lightness = photon pressure / gravitational force). Nobody's built one yet, of course.

    I would be curious to see information on these designs. Certainly with such a design one could leave the solar system (although it would take an awfully long time to get to another star...).

    You should look up the heliogyro concept for the Halley rendezvous probe, with careful attention paid to the planned trajectory. It makes a jog out, falls back in, and catches the comet shortly after perihelion... and stays with it. That's not "spiralling out slowly" by any definition, and that probe could have been built 20 years ago. (Should have been, too.)

    I'm looking but haven't found any helpful info on the Halley rendezvous probe, but what you're describing sounds like the probe would slingshot around either a planet or the comet itself, in which case the solar sail isn't providing the primary power to the probe which is allowing it to do anything other than "slowly spiral". The primary power would be coming either from the kinetic energy of the planet which is being slighshotted around, which is the conventional way of doing intra-solar travel.

    However, if you wanted to leave the solar system, it seems unlikely that you could get enough energy for it by slingshotting around planets, and even if you could, there would be very little reason to use a solar sail to do it, since all of your maneuvering would be within the solar system, and could be done using conventional propulsion.

  • Will not work for interstellar travel. :( (Score:3)

    by David Roundy ( 34889 ) on Friday April 06, 2001 @07:24AM (#311266) Homepage
    In case anyone's interested, I worked out some numbers on the force of the light on this thing relative to that of the sun's gravity. Both forces drop as inverse squares of the distance from the sun, so the ratio between them remains constant.

    The force due to the light is determined by the area of the sail and the luminosity of the sun (which I looked up), while the force due to gravity is just determined by the mass of the spacecraft and the mass of the sun.

    When you work out the numbers using the area and mass quoted in the article, the force of gravity from the sun turns out to be about 200,000 times greater than that of the light.

    This means (as the article states) that the best such a spacecraft can do is remain in orbit around the sun, and angle the light in such a way as to gradually spiral out to a higher orbit.

    Alas, no interstellar travel with one of these guys, since we can never break out of solar orbit. For a spacecraft with this size solar sail to achieve interstellar travel, it would need to have a mass of less than 0.3 grams! That would require quite an impressive materials breakthrough, I'm afraid.
  • Right on the money.

  • You can find more information on solar sails here [ex.ac.uk].

    Al.
    --

  • Re:Sail Direction (Score:3)

    by jonnythan ( 79727 ) on Friday April 06, 2001 @07:24AM (#311269)
    It's like this, if i can explain it.

    The sail gets its acceleration from the inertia imparted on it by bouncing photons, essentially. So imagine a tennis ball bouncing off a wall. Whatever direction you throw the ball, the net force on the wall is directly perpendicular to it - it changed the ball's velocity only in the perpendicular direction. The ball maintained all of its momentum parallel to the wall, but its momentum perpendicular to it changed. Hence, the wall wants to accelerate perpendicularly to its own face.

    The same is the case with the solar sail. It will accelerate perpendicularly to its own sails. The magnitude of the acceleration will be greatest when the sails are perpendicular to the "light beams" or however you want to refer to them - that way, the sail absorbs all of the momentum times two of each photon. when at an angle, the sail will only absorb (total momentum) sin (whatever the angle is) times two.
  • I'm not sure if it makes sense to me. The surface has to be reflective. I understand this. If it absorbed the photons, then it would not receive the inertia that the photons impart.

    Now, that said, does it receive it's energy from the impact of the photons or the deflection of the photons (similar to say, a rocket engine's exhaust)? It seems to me the actual impact of the photon, combined with the fact that the photon is not absorbed, would provide the inertia. If that's the case, then angle to the sun has no effect.

    I'd like to hear what a physicist has to say on the topic. I don't know. I'm just going from my basic understanding of physics.

  • Okay, I just read some physics on this. It makes intuitive sense, but I'm not one to always buy into intuition. Basically, the force is the combination of the force of the particles plus the force of the deflection (somewhat reduced, depending on the effectiveness of the reflecting surface). Anyway, I guess I can buy that. The web page: Tacking Solar Sails [caltech.edu] has a good description of it.

    The cueball analogy works, I suppose, but here's why I didn't buy it. Take the cueball analogy down to the atomic level. The photon is the cueball, and each molecule of the sail is an object ball.

    Take any of those object balls and place it mid table, in the center. Now hit it with a cueball, any side, doesn't matter exactly where. Chances are, it's deflected, right? Now repeat this a million times, and take the average of where the object ball ends up. It ends up in the middle of the table. That was my take on it.

    Since the deflected photons act as a force in themselves though, that does change things. I just don't really understand how a deflected photon can provide any additional force, since the inertia appears to be transferred at impact. That's my confusion.

  • Sail Direction (Score:3)

    by Pedrito ( 94783 ) on Friday April 06, 2001 @07:08AM (#311272)
    I don't see how the angle of the sail relative to the sun would have any effect on the direction of the vehicle. It's not a sail boat. The difference between this and a sailboat is that a sail has a rudder which has drag against the water to control direction.

    With no atmosphere and no water, a solar sail has nothing to drag against to control direction. It can simply be pushed one way: Away from the sun, and then affected by gravitation of other objects. The angle of the sail to the sun just can't have any effect that I can fathom. Maybe I'm wrong.

  • #2 is easy to solve, just collapse the sail once you start getting negative accelleration, it gives the added effect of acting like a break when you want to stop also.
    =\=\=\=\=\=\=\=\=\=\=\=\=\=\=\=\=\=\=\=\=\= \=\=\=\
  • Sorry about the spelling, but I don't think you understand. A Solar sail traveling from one star to another could you the sail to stop it's motion. Once the object gets close to the other star deploy the sail and the star's radiation pushes the other way, slowing you down. Think about it.
    =\=\=\=\=\=\=\=\=\=\=\=\=\=\=\=\=\=\=\=\=\=\= \=\=\
  • Would it be possible to mount lasers on the spacecraft itself? This would require a powersource, and I don't know if a small enought amount of solar cells could provide this, but you should be able to bounce the light off of the sail. It's analgous to you blowing into your own sails, except that here you might actually be able to affect the motiion of the craft. Would this work? I'm really just brainstorming.

    Nate

  • Negative Refractive Index stuff and Solar Sails (Score:3)

    by DeepDarkSky ( 111382 ) on Friday April 06, 2001 @06:36AM (#311276)
    Instead of aluminum coated mylar, maybe they'd like to investigate the use of 3M's (no, I don't work for 3M or have stock in 3M) Radiant Mirror Film [3m.com], which was first covered last year in this article [usnews.com] which explains 3M's research into the birefringent effect. This film would be able to reflect more light than conventional light reflective material such as metal film coated mirrors.

    On a separate though, I wonder if they could use the negative refractive index composite material in making the solar sails, since they are probably still investigating the material to use for the sail?

    From yesterday's linked article:
    "``negative refraction'' would make possible the construction of a lens capable of focusing light to limits not currently achievable. "

    and a related article [economist.com] on the Economist about how negative refractive index material can possibly help make a "perfect lens"

    Perhaps they could use the phenomenon of the negative refractive index to make more efficient or more maneuverable sails?

  • Speaking of things you forgot in high school, how about algebra. 'F = ma' is the same as 'a = f/m'. Thank you, thank you. please hold your applause.

  • The first time I ran across the idea of solar sails was in some of Cordwainer Smith's sci-fi short stories written, I think, in the 50's & 60's. It seemed to show up in the Instrumentality of Mankind stores before the sail ships were replaced with planoforming. Anyone else remember those books? When did Cordwainer Smith first write about solar sails? More to the point, anyone know of a reference to solar sails which predates Cordwainer Smith. As far as I can tell, he invented the idea, though no one seems to read him much today.
  • Smith did it for sure in 1963 with Think Blue, Count Two. Some of his earlier stories I suspect dealt with solar sails. Sunjammer was published in 1964, according to www.sfsite.com, so it looks like Smith beat him to the idea.

    Anyone more familiar with Smith's short stories remember if any of these earlier works dealt with solar sails? I included Think Blue on the list as the first one I could think of.
    War No. 81-Q (1928)[as Karloman Jungahr] - This story was first published
    in 1928 in a school publication when 'Smith' was 15.
    Scanners Live in Vain (1950)
    The Game of Rat and Dragon (1955)
    Mark Elf
    [vt Mark XI] (1957)
    The Burning of the Brain (1958)
    Western Science Is So Wonderful (1958)
    Angerhelm (1959)
    The Fife of Bodidharma (1959)
    Golden the Ship Was - Oh! Oh! Oh! (1959)
    Nancy
    [vt The Nancy Routine] (1959)
    No, No, Not Rogov! (1959)
    When the People Fell (1959)
    The Lady Who Sailed the Soul (1960)
    Alpha Ralpha Boulevard (1961)
    Mother Hitton's Littul Kittons (1961)
    A Planet Named Shayol (1961)
    The Ballad of Lost C'Mell (1962)
    From Gustible's Planet (1962)
    Drunkboat (1963)
    The Good Friends (1963)
    On the Gem Planet (1963)
    Think Blue, Count Two (1963)
  • Thanks for that link. sort of obviates the need for my later post. The name was popularised by Arthur C Clarke, in his short story Sunjammer of 1964 (reprinted as the title story of The Wind from the Sun in 1972), though the concept in science fiction goes back at least as far as Cordwainer Smith's The Lady who Sailed the Soul of 1960. In factual speculation it is even older: the Russian aeronautics pioneer Konstantin Tsiolkovsky and his colleague Fridrickh Tsander wrote in 1924 of "using tremendous mirrors of very thin sheets" and "using the pressure of sunlight to attain cosmic velocities". The term itself seems to have been coined in the late 1950s by the American engineer Richard Garwin.
  • I find it interesting that the earth was once explored by people in ships sailing around it, and that it could be possible that the universe will be explored by people in ships sailing through it.
    Watch out when you get to the end of the universe, i hear there's a great restaurant there ;)

    -fohat
  • I attended a lecture by doctor Frisbee, who is from JPL, and he said that they are developing AI systems to navigate the ships. He also mentioned something that i had never really thought about before. The impact from dust is quite strong when travelling really fast.
  • With these facts:
    1. Solar sails are low acceleration, high top speed system.
    2. Rockets are high acceleration, low top speed.

    The obvious solution is to start a voyage with a rocket to get speed up, boost the rockets (for more momentum and less mass to push), unfold the sail, and keep accelerating.
  • Looks like we are moving more and more towards a weird variation of the world of Max Headroom [maxheadroom.com]. Reading this story, I was reminded of the episode where there was a brief mention of a Solar Yacht race.

    What Is Interesting is to take look at the series, which was so prescient in so many ways, which also missed the Internet as being a part of world culture. It was really too hot for TV, and took more than a nibble out of the hand that fed it. For those who do not remember it, it was a 1987 TV show based on a world run entirely by the multimedia moguls, and where the Internet did not exist, and TV was mandatory.

    Talk about hitting a little close to home.

    Check out the Vinny the Vampire [eplugz.com] comic strip

  • I thought that a Russian physicist came up with the idea sometime around the 1920s. I don't have the book with the reference with me however.
  • It's called inertia. If the same engine powers two cars, and everything between the two cars is equal, but one of the cars weighs twice as much, the lighter car will accelerate faster.

    And why the hell was this post moderated as Off Topic?? Are moderaters that stupid?

    ---

  • Solar Sails dont hold much promise yet though, debris can tear the sails to shreds, the payloads are insanely small and maneuvering is (as this article shows) very hard. Im more interested in the plasma engines bound for mars in 5 years.
  • Man those scientologists were advanced. What am I doing not bowing before them? Oh yeah know I remember - not buying into crap.
  • Just wait 'til Dennis Conner gets word of this! And we thought the airfoiled cat was contraversial!
  • 2010: From English to Japanes and back again, in the spirit of Zerowing:

    All fifth planet and moon are belong to us, all other planet are belong to you.

    --

  • but once it got too far from the sun it would not be getting enough photons of light to accelerate any more.

    The farther it gets from the sun the less impacted it is from solar gravity, even though the light energy gets weaker, thus acelleration is expected to remain constant.
  • First of all it's F=ma.
    Second, F=ma is WRONG. It is a non-relativistic equation and only applies to low velocities. The more accurate version is F=dp/dt where p is momentum, i.e. p=m*v/sqrt(1-v*v/c*c). Physics actually considers momentum to be a quantity that is more fundamental than velocity.
    Third, you implicitly seem to be confusing fundamental with primitive. Why would high-tech progress imply that we wouldn't still be using F=ma anymore? I guess I don't see what is so "amazing" about our continuing to use it. Even though it's wrong, it's still accurate enough for most human endeavors which take place at small fractions of c.
  • very unfeasible - likely to be the recreational equivalent for a sailboat of the future, where the rest of us will burn energy like mad on the going to and from like we do today. Very Romantic though. Reminds me of the journey scene from TRON.
  • What do you do at night?

  • Um, no.

    Conversation of momentum. Any push your sails get from your laser photons is cancelled by the push your laser photons gave the laser upon leaving the laser.

    Chris
  • In a couple of Larry Niven stories, they used lasers to launch an interstellar lightsailer, but to brake at the destination, it had to almost dive into the sun...
    Why not bring along chemical (or nuclear, or whatever) rockets just for the breaking?

    Chris
  • solar sails are pretty much out of date
    instead the new approach now is to use a magnetic field which can also experience solar wind.
    • since it's a field there obviously is no wear or tear.
    • since it's generated it can be easily enlarged or reduced in size
    to find out more google: solar sails magnetic fields [google.com]


    here's a nice abstract from http://www.ugcs.caltech.edu/~diedrich/cgi/search.c gi?andrews%2C+d [caltech.edu]
    • Abstract:

    • The magnetic sail, or magsail, is a field effect device which interacts with the ambient solar wind or interstellar medium over a considerable volume of space to generate drag and lift forces. Two theories describing the method of thrust generation are analyzed and data results are presented. The techniques for maintaining superconductor temperatures in interplanetary space are analyzed and low risk options presented. Comparisons are presented showing mission performance differences between currently proposed spacecraft using chemical and electric propulsion systems, and a Magsail propelled spacecraft capable of generating an average thrust of 250 Newtons at a radius of one A.U. The magsail also provides unique capabilities for interstellar missions, in that at relativistic speeds the magnetic field would ionize and deflect the interstellar medium producing a large drag force. This would make it an ideal brake for decelerating a spacecraft from relativistic speeds and then maneuvering within the target star system.
    _________________________________________
  • of course using real sails leads to wear and tear, as also insanely cumbersome maintenance.
    a recent novel alternative is experiencing the force of solar wind with a generated magnetic field.
    see post #79 [slashdot.org]
  • they mention quite a few drawbacks of real material sails (wear & tear, launch, weight, construction),
    drawbacks that can be circumvented with the help of a magnetic field that acts as a sail,
    see post #79 [slashdot.org]
  • A good explanation... except that the site does not mention anything about gravity and it's effects.

    At just one place it mentions that the solar pressure is about 9m/s^2, less than 9.8m/s^2, which is lesser than what is needed. Then the article mentions that solar pressure could be used for powering crafts in the initial stages as it nears the sun.

    Yeah, but getting it up there? And if the craft is that near to the sun, wouldn't it burn up or something?

    So you would need something to put it up there, accurately, and then use an initial momentum from the sun by being close to it, but not very close, and evade both the gravities, and then get a velocity from all this. Wow!

    I think we are missing something here...Any idea ppl?

    "...Fear the people who fear your computer"
  • Heinlein's Orphans of the Sky was about a huge ship
    containing a community that had forgotten they were in a
    ship, untill the control room was discovered by a curious teen.
    A short, great read. Great socialogical thoughts as well.
  • Wouldn't taking momentum from photons through reflection/refraction slow the photons, therefore slowing the relative time? If there were a solar sail big enough, it wouldn't appear to move at all! Nevermind, it would probably just equalize with the gravity well of earth (if that's where we're viewing it from...) and appear only minutely slower than normal sattelites. Hee hee. A tiny bit of mis-understanding can go a long way. :)
  • :: http://www.terroristsupply.com ::

    And we're supposed to trust a site which proudly displays this on their front page? Maybe the author should've wrote a better article submitted it to SecurityFocus [securityfocus.com], and the Associated Press.

    Just because this may or may not affect Ameritech it has nothing to do with news being posted here. Imagine if /. accepted every single security based qualm in the world, we'd have no room for linux or MS stuff homeslice ;)

  • 2 cents (Score:3)

    by deran9ed ( 300694 ) on Friday April 06, 2001 @06:14AM (#311304) Homepage
    Would be really useful if they could find a material that the sails could withstand damage done by getting hit by meteors, maybe people at NASA could actually put them to extreme uses and send them billions of years in space.

    It would've also been nice if included in the article, there was a link to show how astronomers at places like NASA control these things when they're so damn far. I know they would pre-program directions, but what if something was spotted close by, how the heck do they manage to change the course of something millions of miles away to investigate it?

    millions of bandwidth bits away [antioffline.com]
  • I found a reference to that here [stp-gateway.de]. The negative refractive lens can (in theory) be created with a 40 nanometre thick silver film. But to work, it must be at a "distance of 20 nanometres from the light source, since the system uses its near field. [stp-gateway.de]"
  • Because you'd need something like 99.9% of the weight of the vehicle to be fuel. That's with chemical rockets. With fusion, you could do considerably better, maybe only 90% fuel. And that fuel should be something like hydrogen or deuterium -- pretty hard to keep it from leaking away in a ten year flight...
  • It _is_ like blowing into your own sails. But you would get a little thrust by shining the laser backwards. The thrust in newtons is power in watts divided by the speed of light (3E8 m/s), you need a heck of a good source of power, like a big fusion generator or tons of antimatter. And it doesn't have to be visible light -- microwaves, gamma rays, or infrared would do just as well as long as it's directional. Note that all practical energy sources emit more power as waste heat than as usable energy -- so maybe the best light-rocket would simply radiate heat out one end.

    However, if we do get fusion powered craft, I expect they would put the usable energy into accelerating the fusion products. You get more thrust from a given power by accelerating helium than by shining a light. For interstellar flight, most of your craft would have to be fusion fuel, so you might as well use the byproducts... Light-propulsion makes sense only as a way to use some power supply you don't have to take along. (Except maybe in a warcraft where the big laser could do double-duty?)
  • It receives MOMENTUM from the photons bouncing off it. Momentum change = mass * change in velocity (and it's a vector). If the mirror is at right angles, you get twice the momentum of each reflected photon, because you get both the incoming momentum and the reaction to the outgoing momentum. The photon gets mass from e=mc^2, or m = e/c^2. c^2 being very big, the mass of even a whole lot of photons isn't much, hence thrust is rather low. But it's free (aside from the cost of the sail, lifting it to space, and repairing meteor holes....)

    The spacecraft receives ENERGY (mv^2) only to the extent that it changes velocity in response to that momentum transfer. It's going to be a small fraction of the energy in the light.

    As for how you angle the thrust: someone used a tennis ball analogy, that's probably the easiest way for most non-physicists to see it. Or if you want a mathematical treatment: let the mirror be angled at 45 degree to the incoming light. Incoming photons are reflected at 90 degrees to incident. The momentum transfer to the mirror is the vector sum of the momentum of the incoming light (x at 0 degrees) and the outgoing light (x at 90 degrees). The resultant is sqrt(2)* x at 45 degrees. But notice that you've reduced both the amount of light you intercept by angling the mirror and the impulse you get from each photon intercepted (2x at 90) -- so a 45 degree angle cuts the thrust in half. (This assumes 100% reflectivity -- with a real mirror, your results are somewhat worse, since some photons are absorbed and so only contribute outwards thrust. And outwards thrust won't help you spiral outwards -- you aren't going to get enough thrust from sunlight to just fly straight out. The animation in How Things Work is wrong.)

    At arbitrary angles (still assuming 100% reflection): The thrust will be right down the axis of the mirror. With the mirror and thrust axis at angle T to the sun (that is, 0 degrees is dead-on and 90 is turned edge to the light), the thrust is (cos(T))^2 times the maximum thrust. 60 degrees cuts the thrust to 1/4 of maximum, but it directs most of it along the orbit where it will actually be useful. And at high angles, you get better reflection. Possibly it would be best to plan on never setting the sail closer than 60 degrees, so you can make the spars weaker and lighter.
  • Correct. One thing is that sailboats use two "wings", the sail above and the keel below, in fluids with different velocities, and so the vector sum of the "lifts" can be partly into the wind. Lightsails cannot create thrust towards the source of the light, and in practice they'll always have significant thrust away. But orbital dynamics comes to the rescue: angle the sail to reduce your orbital velocity and you will drop, even though part of the thrust is still outwards.

  • Re:Sailing Issues (Score:4)

    by markmoss ( 301064 ) on Friday April 06, 2001 @07:40AM (#311310)
    The sails are reflective (mirror-like), so by angling them you can get sideways thrust -- a combination of the incoming momentum of the sunlight and the reaction from the momentum of the reflected light. So angling the sails at 45 degrees gives you thrust at 45 degrees to the sun, but it cuts the thrust by half. (sin(45) squared -- once for the reduction in light intercepted by angling the sail, once for the off-axis thrust.)

    You can't tack like a ship tacking into the wind (where the vector sum of forces on the sails, keel, and rudder gives a net force partially into the wind) because you don't have any equivalent to the keel. But you can use an angled sail to reduce the orbital velocity, so the ship drops sunward. And the best way to go outwards is to use sideways thrust to increase the orbital velocity.

    The real problem with solar sails is the very tiny force per square meter that is possible. It should be enough for minor orbital adjustments. If you plan way ahead, it might be possible to use light sails to slowly spiral in or out (like weeks or months to the moon, years or decades to Mars). Or you find a way to augment the thrust -- like building a giant battery of lasers on the moon to provide much more intense light. In a couple of Larry Niven stories, they used lasers to launch an interstellar lightsailer, but to brake at the destination, it had to almost dive into the sun...
  • I can see this as being practical for maybe keeping something at one place in orbit around the sun or for moving very slowly around our solar system, but once it got too far from the sun it would not be getting enough photons of light to accelerate any more. I guess the trick would be to gain enough speed before it gets too far away from the sun. The problem with that would be that, at least it seems logically, you have to increase the size of the sail a large amount to gain even a little acceleration. Oh well, I am sure some viable use will be found for it. Either that or we will dump millions of dollars into it and then decide that it isn't viable.
  • As a sailor, I can see some major issues with solar sail travel...

    1. The further you are from the sun, the less force is exerted on the sails, until the forces from other stars/suns outweighs the force of our sun. Remembering that our sun is relatively small, this will not necessarily be at the midpoint between solar systems.
    2. Travelling towards the sun is difficult and requires some sort of "tacking" process.
    3. Travelling in any direction other than away from the sun requires a centerboard and rudder for maintaining a direction. Both these systems rely on resistance. In space there is no resistance. There sill need to be something to exert force away from the sun to keep the correct heading.
    4. Sun-spots change the intensity of the solar winds, creating 'gusts'. Gusts will be difficult to control.

    My understanding is that solar-wind is best considered as only part of a spacial travel system. Combining solar-winds with other power mecnahinsms. From what I can see, there is only one direction the vessel can travel, and that will be determined by the release time from the earth, and the velocity at releas. After that, the only form of control is to change the "sail area" by either reducing sail size, or by changing orientation. Neither of these processes change direction, only speed.

    Thus, for useful travel, an intersection trajectory must be set at the beginning of the flight, and then the speed of the craft must be carefully managed so that the destination point and the craft intersect at the correct time.

  • Acceleration (a) = F/M

    Is Newton's 2nd Law for those of you who forgot what you learned in high school. It's amazing that even after all the high tech progress and all the major advances in theoretical physics, we are STILL using the good old formula F=MA.

    It's Beautiful!!

  • To get Cosmos-1 into Earth orbit, the spacecraft will be loaded into a modified intercontinental ballistic missile (ICBM) of Russian design, called the Volna.The ICBM will be launched from a Russian submarine in the Barents Sea.

    Why would they resort to doing this? It seems more costly since water launched rockets need additional equipment to exit the water properly and renting out a russian sub can't be that cheap. And i'm no rocket scientist but i can't think of any technical plus for a sub based launch. Any ideas?

  • ... and COlumbus set sail seeking a faster trade route to Mars...
  • Maneuvering a solar-sail spacecraft requires balancing two factors: the direction of the solar sail relative to the sun and the orbital speed of the spacecraft."
    That should be the direction of motion of the spacecraft; its orbital speed is a function of the altitude and eccentricity of the orbit. The attitude of the sail vs. the line to the sun determines the direction of thrust, the orbit is raised, lowered or changes plane depending on whether the thrust adds to, subtracts from, or is crosswise to the spacecraft velocity vector.
    --
    Give a man a fish and he eats for a day.
  • If you reflect the sunlight directly back at the sun, you won't travel away from it at all, even though the light will be exerting a maximum force on your spacecraft.
    Not true. If you reflect directly back at the Sun the thrust will offset gravity and will cause the orbit to expand, as if gravity had decreased. As gravity and photon pressure both scale as the inverse square of R, that can be taken as a rule.
    This is because regardless of your distance from the sun, the gravitational force will always be greater than the light force.
    Not necessarily. There are designs for sails which will have a "lightness" as high as 3 (lightness = photon pressure / gravitational force). Nobody's built one yet, of course.
    This also means they won't be practical for leaving the solar system, since they would have to spiral out rather slowly.
    You should look up the heliogyro concept for the Halley rendezvous probe, with careful attention paid to the planned trajectory. It makes a jog out, falls back in, and catches the comet shortly after perihelion... and stays with it. That's not "spiralling out slowly" by any definition, and that probe could have been built 20 years ago. (Should have been, too.)
    --
    Give a man a fish and he eats for a day.
  • I would be curious to see information on these designs. Certainly with such a design one could leave the solar system (although it would take an awfully long time to get to another star...).
    You can find some short info on heliogyros here [caltech.edu], and the history here [google.com]. (The sites aren't loading well at the moment, look in Google's cache.)

    Google says that Drexler's "lightsail" paper is at http://www.aeiveos.com/~bradbury/Authors/Engineeri ng/Drexler-KE/MMfSSAO.html [aeiveos.com], but I'm having difficulty getting the page to load right now (try Google's cache [google.com]). Anyway, Drexler writes "A 20 nm thickness of aluminium has a reflectivity approaching that of the bulk material (~ 0.9). Lightsails constructed on the multikilometre scale can have structural masses that are small compared to the reflector mass, if a suitable pure-tension structure is employed to transmit forces from the sail to the payload. At Earth's distance from the Sun, the outward acceleration of an unloaded sail using 20 nm aluminium reflectors is ~0.16 m/s^2, or ~ 14 km/s per day." The acceleration of the Sun's gravity at Earth is only 0.0059 m/sec^2, unless I slipped a decimal point.

    I'm looking but haven't found any helpful info on the Halley rendezvous probe, but what you're describing sounds like the probe would slingshot around either a planet or the comet itself...
    No, that wouldn't have worked anyway because Halley's orbit is inclined too far to the ecliptic. The maneuver went something like this:
    1. Thrust out and change plane to get into the plane of Halley's orbit.
    2. From the top of this ellipse, thrust back to get into a very elliptical retrograde orbit.
    3. Fall in toward the Sun, edge-on to minimize thrust.
    4. Once past perihelion, thrust to match velocities with the comet which is just about to come by.
    You can do this with arbitrarily little thrust so long as you start long enough in advance and have no limit on total impulse. That describes a solar sail pretty well.
    However, if you wanted to leave the solar system, it seems unlikely that you could get enough energy for it by slingshotting around planets...
    Excuse me? That's exactly how Pioneers 10 and 11 and Voyagers 1 and 2 have already left the solar system: slingshotting Jupiter. Are you having a really bad day at the keyboard, or have I just been trolled?
    --
    Give a man a fish and he eats for a day.
  • The problem with the standard magsail (and artificial magnetosphere devices) is that they are pretty much pure drag devices. They can push you along the solar-wind vector, and that's just about it. You have to do something fancier to add or subtract velocity to circularize your new orbit, or to change your orbital plane. This is something a solar sail can do easily.

    Artificial magnetosphere gadgets have the further disadvantage that they require a source of plasma to inflate the magnetic field, so they consume their mass over time. Again, not a problem for a lot of missions, but hardly the be-all and end-all of propulsion technology.
    --
    Give a man a fish and he eats for a day.

  • Since the deflected photons act as a force in themselves though, that does change things. I just don't really understand how a deflected photon can provide any additional force, since the inertia appears to be transferred at impact.
    Think of playing pool with a cueball made of modelling clay. (Use a slingshot instead of a cue, of course.) You throw it at the target ball, and when it hits, it sticks. The target ball takes on the momentum of the clay. If you use a real, elastic cueball instead, when it hits it bounces. From the perspective of momentum, you could consider this as a lump of clay hitting and sticking to the target, then expelling the clay in another direction (giving another boost to the target). Or you can just measure the difference in momentum (mass * velocity) of the cueball before and after and by conservation of momentum you know what was transferred to the target ball.

    Photons are no different. Photons carry momentum P=E/c. If you shine photons onto a blackbody they transfer momentum on impact (and make the blackbody hotter); if they bounce off instead it's equivalent to re-emitting the photon in another direction, with the momentum changed. The delta-P is transferred to the mirror. Does that help?
    --
    Give a man a fish and he eats for a day.

  • This means (as the article states) that the best such a spacecraft can do is remain in orbit around the sun, and angle the light in such a way as to gradually spiral out to a higher orbit.
    Maybe for that one, but not in general.

    Back of the envelope: Let's take a sheet of aluminum foil 1 kilometer square and 5 micrometers thick. The total volume is 5 cubic meters, total mass about 13,500 kg. It receives about 1.35 gigawatts of sunlight at 1 AU. If it has 100% reflectivity, by P=E/c it can develop a best-case thrust of 2.7*10^9/3*10^8 = 9 N (4.5 N at a 45 degree angle, of which 3.2 N would be outward and 3.2 N would be tangential).

    4.5 N doesn't sound like much, but it's an acceleration of about 0.33 mm/sec^2. By a = w^2r, the acceleration of Sol's gravity at 1 AU is only about (2*pi/(365.25*86400s))^2*150,000,000,000m = 5.9 mm/s^2. More to the point, Earth's orbital velocity around the Sun is about 30 km/sec. To get to solar escape velocity, you need about 42 km/sec. 12,000m/s/.00033m/s^2 = 36 million seconds = 421 days, ignoring the decrease of sunlight with distance. If you dropped in to the distance of Venus you could cut that in half, if you built your orbit into an ellipse that took you in to about the orbit of Mercury and then boosted like hell on the way out you could shave even more. You wouldn't get to another star in any reasonable amount of time, but you could sent a probe to literally anywhere in the solar system in a few years without any fancy gravity assists (flyby, not rendezvous).
    --
    Give a man a fish and he eats for a day.

  • Non-issues (Score:4)

    by Spamalamadingdong ( 323207 ) on Friday April 06, 2001 @07:04AM (#311322) Homepage Journal
    Addressing the points in order:
    1. The further you are from the sun, the less force is exerted on the sails, until the forces from other stars/suns outweighs the force of our sun.
    True. However, this occurs at such an absurdly vast distance from Sol that it is not a factor for any near-term mission.
    2. Travelling towards the sun is difficult and requires some sort of "tacking" process.
    False. All you have to do is reflect light in the direction of orbital motion, which causes a braking thrust. This causes the orbit to move closer to the Sun.
    3. Travelling in any direction other than away from the sun [a] requires a centerboard and rudder for maintaining a direction [b]. Both these systems rely on resistance [c]. In space there is no resistance [d]. There sill need to be something to exert force away from the sun to keep the correct heading [e].
    [a]False, [b] true, [c] irrelevant, [d] no shit, Sherlock, [e] false.

    You can generate thrust in any direction in the half-sphere centered on the Sun-spacecraft line, with the thrust falling off according to cos^2 theta. So long as your trajectory changes can be performed without a thrust vector aiming away from the Sun, you can (theoretically) perform them with a solar sail. In space, gravity (especially from a third body, like Earth) and inertia allow tack-like maneuvers.

    4. Sun-spots change the intensity of the solar winds, creating 'gusts'. Gusts will be difficult to control.
    Solar sails use the pressure of the photons, not the plasma wind. There may be some small influence from plasma impingement, but it is just that: small.
    From what I can see, there is only one direction the vessel can travel, and that will be determined by the release time from the earth, and the velocity at releas.
    You should look up the "heliogyro" concept for a Comet Halley rendezvous probe (it was never built, of course). The details will show you where your thinking is faulty.
    --
    Give a man a fish and he eats for a day.
  • A solar sail may have two useful applications (IMO):

    1. It can be used to correct trajectories of satellites and such without using too much energy and/or without requiring these sats to carry a lot of fuel prior to their departure;
    2. It can be used to put some big structure out of the solar system, maybe carrying thousands of people in a self-recycling environment, to what would be a true interstellar travel, only without a `coming back' prevision.

      I believe it was Isaac Asimov who first thought about their use to the latter, in one of his Early Asimov collection. (Must check, haven't read them in a long time...)

  • It can simply be pushed one way: Away from the sun, and then affected by gravitation of other objects. The angle of the sail to the sun just can't have any effect that I can fathom. Maybe I'm wrong.

    You are wrong. If you weren't, there would be no way to hit a billiard ball in a direction different from the cue ball.

    Sun's photons => cue ball.
    Sail => billiard ball.

  • This is nothing new (Score:5)

    by Magumbo ( 414471 ) on Friday April 06, 2001 @06:28AM (#311325)
    The scientologists used a similar technology when they came to earth 65,000,000,000 billion years ago. Everybody knows that.

    --

  • The scientific community has a responsibility to the public to limit the information which it reveals, or to release the information in a more controlled medium. When I first came online , I was excited. I was going to be able to teach my children interesting things via multimedia, we could explore together!

    Then my youngest came to me and asked to explore on her own. I saw no problems with that. Sadly, that child has been lost to me. She came across the site of a man who believed that "Information was to be free" and began to browse. My daughter was naieve. She was a victim, one of the first victims of this 'information revoloution.'

    On this site she learned about the republican party. I don't know if I can put across the loss our whole family experienced when she joined that same party.

    And now my eldest boy is downstairs, with our family cat in tow. He's been using my tools, and making a racket. I was worried and so I checked in his room, and what did I find, this slashdot article on how to make a solar sail.

    Take some responsibility for your actions.
    I won't lose another child because of this 'revoloution'

  • This is actually a pretty regular sort of launch these days. The missile itself is watertight, so there really isn't any need to waterproof the spacecraft. Most of the launchers we use (the largest exception being Ariadne) are modified ICBMs of one sort or another. The Russians are selling off old hardware, that they can no longer maintain, very cheaply; the technical capabilities for, say, launching a comsat into a very specific orbit are limited, but for this mission it'll do.
  • So is the reason this works is because the sun BLOWS?

  • The discussion on solar sails is interesting, but it has diverged along two different routes which makes it confusing. Solar sails can generate thrust by two different mechanisms. First, they can absorb the solar wind and generate thrust by "braking" the motion of these particles. Second, it can interact with solar light, i.e. use photons to produce thrust. Material sails can interact with both. Solar sails using a magnetic field can only interact with the solar wind. Even so, one analysis showed generating a thrust of 250Newtons at a distance from the sun equaling the Earth's orbit. If the space craft weighed, say, 1000Kg then it would accelerate at 0.25meters/second squared, and could reach a speed of 100,000 meters per second in 400,000 seconds (5 days roughly). Need more speed? Accelerate for a longer time. I don't see any limitation to interstellar travel via this mechanism. Using photons to propel a solar sail is more complicated than simply providing thrust perpendicular to the plane of the sail. The reason is that the sail not only reflects photons, it also absorbs them. The thrust generated now depends on the ulimate disposition of the absorbed photons. Look for information on the Poynting-Robertson effect to learn something about the mechanics of material propelled by solar photons.
  • I co-edited the definitive book on the subject, with the listed editors ARTHUR C. CLARKE and DAVID BRIN. The title was "Project Solar Sail", and there are still about 75,000 used copies of the paperback out there in North America... The book, published by the ROC division of New American Library, had a selection of fiction about Solar Sails -- from some famous authors -- plus new nonfiction, illustrations, and short version of a longer poem on the subject which I coauthored with Ray Bradbury. It is listed on Amazon, with rave reviews... The time for Solar Sailing has come... As Clarke says, for most of the long history (including the future) of humanity, "ship" will more often mean vessels in space than vessels in water...

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