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Space Science

The Flight of the Solar Sail 143

Posted by CowboyNeal from the kon-tiki-in-space dept.
N3wsByt3 writes "After months of uncertainty, the final verdict has fallen: The Planetary Society has reveiled that it will launch its Cosmos 1 on June 21. Cosmos 1 will be the first non-governmental spaceship that makes use of solar sails as main propulsion mechanism - it is pushed along by light particles from the Sun, instead of bringing its fuel along for the ride - which makes this a unique experiment in more then one way." This was supposed to have happened already, so here's hoping things get off the ground this time.
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The Flight of the Solar Sail More

The Flight of the Solar Sail

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  • Uhm (Score:5, Funny)

    by Mr_Icon ( 124425 ) on Saturday June 04, 2005 @11:48AM (#12724000) Homepage
    "reveiled:" is that like the opposite of "unveiled?" So, are you saying that they have tried to cover up the news? And if so, then how are we finding out? Or is it from the French "reveil:" the awakening. Did they wake up to the news? Is that why they tried to cover it up?

    So confused.

    • Re:Uhm (Score:1)

      by nxtr ( 813179 )
      It's a perfectly cromulent word. Jeez!
    • "reveiled:" is that like the opposite of "unveiled?"


      Sort of. It started in the "veiled" state, then someone unveiled it. But after seeing it for the first time they decided it looked better with the veil on, so they "reveiled" it.

  • As opposed to... (Score:2, Insightful)

    by Vox Humana ( 580533 )
    all the governmental solar sail spacecraft? WTF?
    • The principles of solar sails have been perfectly well understood for decades, and used in varying degrees for the entire time. Note the sails at the ends of the solar array tips on Mariner 4 (little blue squares) :

      http://www.jpl.nasa.gov/missions/past/mariner3-4. h tml [nasa.gov]

      That was in 1964.

      The only unique thing about this flight is that it's the only purpose of the mission.

      Brett

      • Re:As opposed to... (Score:4, Informative)

        by BlackStar ( 106064 ) on Saturday June 04, 2005 @12:20PM (#12724193) Homepage
        Perhaps if you actually looked into it rather than knee-jerk looking to prove it wrong, you'd find that Cosmos-1 is ONLY powered by the solar sails after orbit insertion. No hydrazine corrective thrusters, no other propellant capabilities. The tips on the Mariner series aided in studying the solar wind environment, but they did not assist in propulsion or stabilization to any measurable extent.

        This is the first spacecraft propelled completely by solar wind.

        • Errm... I read the grandparent as sarcastic.
          As in "What Gov't Solar Sail craft???".

          Perhaps if you actually looked into it rather than knee-jerk looking to prove it wrong, you'd have seen that.

  • This article made me wonder when was the last time the .gov used solar sails... I couldn't remember any (I am not that much of a space guy). So I decided to look up "solar sail" on Google. The first link that came up... planetary.org I have never heard of the site before (almost makes me wonder if it is fake) so I got to thinking... could slashdot have upped the ranks of that website on google? Could this be a newly found slashdot effect?

    Please direct all flames to /dev/null

  • My prediction (Score:1, Funny)

    by Anonymous Coward
    This solar sail is really going to blow.

  • This seems silly (Score:3, Insightful)

    by Albinofrenchy ( 844079 ) on Saturday June 04, 2005 @12:03PM (#12724075)
    Honestly the idea that because it doesn't carry fuel makes it easy to 'travel to the stars' is kinda stupid. For one thing, almost by definition you can't travel towards a star on this thing because as you approach it, it will start to slow down, and eventually stop.

    The idea of using the suns energy is good, but maybe they should find a way to harness that energy so one could move the direction one pleases.

    • Re:This seems silly (Score:5, Informative)

      by spune ( 715782 ) on Saturday June 04, 2005 @12:11PM (#12724128)
      The sail ideally will reach 0.1c exiting the solar system. After that point, the sail does ditttysquat and can be removed from the craft the sail is bearing. Combining solar sails with conventional space manuvering systems isn't all that difficult; this would allow all the steering one would like.

      As the sail would approach another star, the sail would slow if both sides of the sail were mirrored. This would merely mean that were we traveling to another star, we could actually stop rather than keep on going. Doesn't that kinda sound handy at all?

    • Re:This seems silly (Score:3, Insightful)

      by imsabbel ( 611519 )
      Well, isnt slowing down and stopping exactly the thing you WANT when approaching the destination star? :)

      But yes, its not that easy. And yeah, at least in earth orbit, solar wind dominates over photon pressure. Also, the interstellar medium would be a much too big drag factor.

    • Re:This seems silly (Score:4, Insightful)

      by rsynnott ( 886713 ) <synnottr@tcd.ie> on Saturday June 04, 2005 @12:13PM (#12724139) Homepage
      Actually, that's one of the nice things about it. Assuming the other star has the same solar wind as the original (which it probably won't) your spacecraft may actually come to a complete halt at the other star, which is what you want. A classic problem with interstellar travel is that not only do you need to carry fuel to accellerate you towards the objective, but to slow you once you get there as well.
      • I'd say the most classic problem is interstellar travel itself; since our most far-out space craft are just now breaching what we know to be the end of our solar system, we have no idea about any of the dangers of interstellar travel. And my guess is that we won't for another two hundred or so years, once everything in our solar system has been "decently" explored and the technology exists to cheaply put things into space.

        Besides, slowing down isn't as difficult as speeding up; use the slingshot effect i

        • Re:This seems silly (Score:2, Insightful)

          by rsynnott ( 886713 )
          If you're willing to go there REALLY slowly, then yes, this would work. The slingshot effect won't help much for very high velocities. Basically, for sensible, remotely useful interstellar travel, we'll have to wait for either some magic source of energy, or a Bussard ramjet, or some other magic thing. I'll be a while :)
      • Actually, I doubt you'd want to come to a complete stop near your destination star. Then you'd go into freefall as the star's gravity pulls you in. What you'd really want to do is establish an orbit around either the star or one of it's planets.

    • Tacking (Score:5, Informative)

      by Beryllium Sphere(tm) ( 193358 ) on Saturday June 04, 2005 @12:22PM (#12724201) Journal
      You could use a solar sail to go from Earth to Mercury if you wanted.

      Angle it so that thrust is opposite the orbital motion of the earth. You slow down relative to the sun. You fall to a lower orbit. Nifty, huh?

      If you're really in a hurry to slow down you detach a reflector, let it fly in front of you, and have a planet-based launching laser fire at the reflector, bounce back to you and slow you down.
      • Angle it so that thrust is opposite the orbital motion of the earth. You slow down relative to the sun. You fall to a lower orbit. Nifty, huh?

        How fast exactly would that be?

        The sun is 1.50x10^11m away from the earth. The mass of the sun is 1.99x10^30kg. Using the equation a = Gm/r^2, we get acceleration = 6.67x10^-11 x 1.99x10^30 / (1.5x10^11)^2, which means acceleration starts at 5.9x10^-3 ms^-2 towards the sun.

        I know that as you get closer to the sun the gravity and acceleration would increase, but I
    • Another consideration to take into account is that the destination star also exerts a gravitional pull on the space craft, so you may not be able to stop at all (most likely to occur if your navigation system uses both S.I. and Imperial measurements)
    • You could easily move towards a start with solar sailing but it would require constant course corrections to be able to accurately reach your destination. This is a fairly basic sailing technique used since the 16th century by sailors to sail into the wind, you can read about it on wikipedia.

      <url:http://en.wikipedia.org/wiki/Sailing#Runnin g_ Downwind>

      it really interesting how simple it is once you look at it.

    • Re:This seems silly (Score:3, Informative)

      by tylernt ( 581794 )
      "as you approach it, it will start to slow down, and eventually stop"

      It may seem counterintuitive, but sailboats here on Earth can actually sail against the wind. Not directly -- most sailboats can point about 45 degrees into the wind (i.e., if the wind is blowing from north to south, a sailboat can travel northeast or northwest as well as anything more southerly than that). When sailing against the wind, the sails (which are allowed to curve) become airfoils and "pull" the boat upwind in exactly the same
      • I don't think this approach would work with space sails for one single reason: you are neglecting the effect of a sailboat's keel or daggerboard in the mechanics of upwind sailing. Without a keel, you will slide sideways because there is nothing in the stationary medium of water to resist the pressure differential produced by the sails acting as airfoils.
      • While sailing upwind by pointing into the solar wind is not feasible, what you can do is move up-orbit or down-orbit--changing your distance from the sun by either accelerating or decelerating. Remember, you start out with some pretty good speed due to the earth's orbital velocity.

        If you go faster, you move away from the sun. If you angle the solar sails to slow down, gravity pulls you back in. Thus, you can acutally make round trips within the solar system.

        For all the idjits posting in this thread: th

    • Re:This seems silly (Score:4, Informative)

      by RichardX ( 457979 ) on Saturday June 04, 2005 @03:39PM (#12725196) Homepage
      For one thing, almost by definition you can't travel towards a star on this thing because as you approach it, it will start to slow down, and eventually stop.

      From the Planetary.org solar sail FAQ [planetary.org]

      9 . Can a solar sail only provide thrust away from the sun?

      No, thrust can be generated inward or outward with respect to the sun. By turning the sail at different angles, we can add or subtract velocity to the spacecraft. When we add velocity, the sail flies away from the Sun. When we subtract velocity, its orbit spirals inward.

      • Re:This seems silly (Score:3, Informative)

        by RichardX ( 457979 )
        Yeah, replying to my own post, but there's another misconception a lot of people here seem to have that needs addressed.

        Also from the Planetary.org Solar Sail FAQ [planetary.org]

        5. Does a solar sail fly on the solar wind?

        No! The solar wind is made up of ionized particles ejected by the Sun. These particles move much slower than light. A solar sail does not stop or reflect them, although they also may impart some of their momentum to the solar sail. However, the force from the solar wind is less than one percent of th
    • And you can't sail into the wind with a sailing ship.

      Oh. Wait......
    • The idea of using the suns energy is good, but maybe they should find a way to harness that energy so one could move the direction one pleases.

      They did, and it's called solar sailing. It may be slow, but you can quite easily maneuver a solar sail either toward or away from a star. Orbital mechanics keeps many secrets from the mathematically uninitiated, but it's vaguely analogous to tacking a sailboat into the wind.

      rj

  • So.. uhh.. speed? I haven't RTFA but I guess the question would be, why. What is the voyager probe propelled by, that's just rockets to correct trajectory and drift, no? Why solar sail, when a probe can reach the edge of our solar system the running out of fuel problem when reaching long distances is solved so.. is the solar sail fast? (over a very long time allowing acceleration like an ion drive that is).

    • Re:So umm.. (Score:2, Informative)

      by spune ( 715782 )
      As the solar sail travels the solar system, it would slowly accellerate, reaching approximately 0.1c exiting the system, according to Carl Sagan's wife, who is funding the project.

    • Re:So umm.. (Score:1, Informative)

      by Anonymous Coward
      The Voyagers took advantadge of a very special and unfrequent planetary relative position that allowed to accelerate the probes by gravitational assistance "jumping" from planet to planet in a spiral path.
    • As long as it's traveling in one direction for long enough speed isn't going to be a problem (vacuum = no resistance). What it's going to lack is acceleration.
    • Solar sails are (in theory) the maximum way to effeciently propell yourself. They simply sit out in space, speeding you up, verses having to provide your own thrust, thus keeping more fuel onboard, thus building momentum faster.

      Voyager and the gang took a good twenty-thirty years getting to where they're going. A sailed ship could spend that same twenty years, get to the same place, and still have a full load of fuel left.

  • Solar Sails (Score:2, Insightful)

    by DeadlyBanana ( 888719 )
    so apparantly we have the technology to propel objects via sunlight... but we still cant make our cars to utilize a resource besides gasoline ( with a little hydrogen somestimes)?
    • You could have a solar car, if you didn't mind it looking like a black pop-tart [americanso...llenge.org] and other problems.
    • we still cant make our cars to utilize a resource besides gasoline


      We have the technology, the only problem is that sunlight is too weak. To power a small car we would need a solar panel bigger than a tennis court. Our roads are too small for any practical solar powered car.


      A spacecraft can be big because space, well they don't call it "space" for nothing, right?

    • Sure we can - it's just that we have to find a well-lit (from one end), airless, frictionless, weightless highway to use them on.

      Seriously, what would make you think these problems are even distantly related? Last time I checked there wasn't a massive international rocket fuel infrastructure that has to be considered when developing new spacecraft propulsion methods. Inventing a clean-fuel car would be trivial if all politics were completely removed from the situation and you were given a fresh start.

  • We know the Voyager probes took 30 years to reach the edge of the solar, give or take a few stops to snap pictures at some of the more famous landmarks. But how long would it take the solar sails to reach the edge (assuming that it was making a beeline out of the system)?

    In laymen terms, please. For us dumb Americans. :P
    • This particular sail is not expected to escape from earth. From the article: " It is possible that by this time the spacecraft will have risen to a high enough orbit that it will remain there, forever orbiting the Earth. It is more likely, however, that the orbit will slowly decay, and Cosmos 1 will end its days as a fireball in the Earth's atmosphere."

      That's because the material of the sails is not very resistant and will break after about a month. I suppose that a stronger sail could eventually reach the

    • Short answer: a few days (Score:5, Informative)

      by mangu ( 126918 ) on Saturday June 04, 2005 @01:08PM (#12724466)
      how long would it take the solar sails to reach the edge


      A solar sail could, theoretically, pass the Voyagers in less than a week. That's the advantage of having continuous thrust along the way, rather than one impulse at the start and then coasting as the Voyagers did.


      The Voyagers are now at a distance of about 13 light-hours from the sun. A thin and lightweight solar sail would quickly accelerate to almost the speed of light, so it's possible to cover that distance in a few days at most.

      • "quickly" is a highly subjective term :) Assuming no relativistic effects, a one earth-gravity acceleration would take almost a year to reach 1.0c. The solar sail is nowhere near 1 earth-gravity acceleration

        Relativistic effects means that once you start to approach 1.0c, time dialation reduces the 'real' effectiveness of such acceleration.

        As your speeds become higher and higher, interstellar gas, sparse as it is, gives you a SIGNIFICANT friction force, again reducing the effectiveness of acceleration, and

        • Re:Short answer: a few days (Score:4, Informative)

          by david.given ( 6740 ) <dg@cowlark.com> on Saturday June 04, 2005 @01:35PM (#12724621) Homepage Journal
          what would be REALLY effective is Ramjets

          Alas, ramjets don't work in Sol's part of the galaxy --- the interstellar medium's not right. (I don't know the details, but I believe that you can't get enough thrust from a certain collector area to overcome the drag needed by your collector.)

          And you're quite right, it takes about a year's worth of 1g acceleration to reach relativistic velocities. Which is why Starwisp [wikipedia.org], the closest thing we've got to a decent design for an interstellar probe, will accelerate at 115g.

          The entire probe only weighs 16 grams. It is, in effect, a microwave-frequency light sail, a kilometre across, powered by a 10 gigawatt maser based in Earth orbit; the maser provides both propulsion and power for the probe to return data once it reaches its target.

          Starwisps should be mass producable, and only require a few days of thrust to launch, so you could use one maser to power practically any number of them. Since they cruise at 0.2c, we might also end up getting data back within our lifetimes...

          • Which is why Starwisp, the closest thing we've got to a decent design for an interstellar probe, will accelerate at 115g.

            Another design is now over 25 year old --- Daedalus. A Google search will tell you much more about it, but it's basically a very big rocket.

            The reference design accelerates a 500 tonne payload to around 0.12c using a two stage rocket. The acceleration is around 0.1g and the rocket runs continuously for something over a year. When the probe reaches the target star (the reference missi

        • Ramjets are no better than sails really.

          Ramjets, as they are currently designed, require input medium, a way to accellerate it (ala, adding energy), and a way to output it.

          In our solar system, there aren't too many fields of gas we can use to accellerate our device, so we end up carrying a good portion of it onboard, or saturating a field before hand in preperation. Then, we have to be able to add energy to the material as we pass through it, which requires us carrying the ability to produce massive q
        • The other problem is that as you near 1.0c, that interstellar gas becomes cosmic rays. You'd quickly become fried.

          Though I suspect your sails would become eroded from friction long before you reached significant values of c anyway.

          Interesting thought though. Use sails to accelerate to where ramjets become effective.
          • A very good moderator for radiation is ordinary H2O, water. There are multiple reasons why you find quite a bit of water in nuclear power plants, and the moderation effects of water are just one of them.

            Another good reason to have water is for life support on a spacecraft, as it is a very dense material to supply emergency oxygen if necessary. Or to drink with your Tang that you brought with you. :)

            Still, the radiation hazards for prolonged travel at even a minor fraction of the speed of light is going
      • If this is true, why aren't we sending a probe to Pluto, or Alpha Centauri, for that matter?

        What's the secret drawback that you're not mentioning thats preventing this from being used?
        • It's obvious; surface area.

          A sail encompasses a LOT of surface area, which means that as the sail scales to larger sizes, it becomes a much more attractive target to whatever strikes it.

          Now, granted a sail in space with a hole doesn't "lose pressure" like it would here on earth, but after a while, the strikes would render the sail uneffective, and the sail would need to be replaced.

          Couple this with traditional space craft design and it means you have to do a lot of discarding along the way, which i
        • What's the secret drawback that you're not mentioning thats preventing this from being used?

          The fact that such a solar sail wouldn't be able to carry much of a payload.

          OK, here's the long answer: a solar sail made of a reflective membrane a few atoms thick would be able to accelerate very quickly to high speeds. But if you attach any sort of payload to that membrane, then you have to accelerate the mass of the payload as well as the solar sail itself.

          There's a technology limit here, the bigger and thi

          • I have a book of science fiction short stories containing the story "Sail 25" by Jack Vance.

            It's about the training of six space cadets aboard a solar-sail powered vessel. However, the instructor - ol' Henry Belt - sees to it that everything that can go wrong DOES go wrong! To make matters worse, he's drunk and tells 'em he thoroughly expects to die in space - so the cadets are basically on their own if they ever want to see home again. What they thought was just an interplanetary jant to Mars and back

      • Long answer: a few years. (Score:4, Interesting)

        by ciroknight ( 601098 ) on Saturday June 04, 2005 @01:29PM (#12724587)
        The problem with the parent's post is the sail itself. Our solar system is a rocky and dangerous space, and so far, we have no idea what the area outside of it is like either.

        The sail would quickly break apart as it gets struck by all kinds of space debris, some left by us, others by more natural occurances. Thus, for an effective craft, multiple sails would be kept on board, being deployed stratigically when the previous sails are no longer providing maximal thrust, and when the coast is clear.

        Next, between those times when the sails are not up, the ship will probably want to keep thrust, so it will have to carry onboard some propellant to keep its thrust up during the times it is without sails. Thus, the ship will lose a significant portion of weight during its travel.

        Lastly, unless we align everything like we did with the Voyager launches, gravity will not be so forth coming for this space craft. This will probably mean multiple near-sun passes to build up the speed nessicary to exit the solar system and continue on to the next star. This means some clever routing by computer simulations, along with a computer figuring when to discard and open new sails along the way.

        Not only will all of this cost a lot, it will likely make it take much longer to get out of the solar system. Lucky for us, as we can pile on the goodies like cameras, radio antenna, and do some exploration as it passes through the planets building up gravitational accelleration.
        • Isn't all that stuff located in the same (sort of horizontal) plane. Couldn't we send the ship 'up'?
          • A sail probe to Pluto would probably be the easiest one to do since it's off the plane. Everything else would be a more interesting problem to solve. This is nothing new since they had this concern with the Pioneer [nasa.gov] probes and the asteroid belt.

        • Next, between those times when the sails are not up, the ship will probably want to keep thrust, so it will have to carry onboard some propellant to keep its thrust up during the times it is without sails. Thus, the ship will lose a significant portion of weight during its travel.


          What do you mean by "keep thrust?" It's not like it's going to start slowing down in the middle of space or anything...
          • If we want this thing to reach relevistic speeds, then every second it wastes not accellerating is a second you can't get back. Take that with the fact this thing is shedding weight (and therefore momentus potential) and you end up with a seriously inefficient vehicle.
        • The problem with the parent's post is the sail itself. Our solar system is a rocky and dangerous space, and so far, we have no idea what the area outside of it is like either.

          news flash: space is fairly empty. the statistical chance of being hit by debri AWAY FROM EARTH is small.

          Next, between those times when the sails are not up, the ship will probably want to keep thrust, so it will have to carry onboard some propellant to keep its thrust up during the times it is without sails. Thus, the ship will
          • The point of the solar sail is that it will be able to attain a much higher velocity then voyager due to the constant acceleration of the sail.

            MicroNewtons of force does not equate to much acceleration.

            You still have to do flybys if you want to get anywhere within a human lifetime. Plus, by going near the sun, you achieve much much higher acceleration on the outward legs. So long as you don't melt your sail ;~)

            This does require a storable/deployable sail for highest efficiency.
            • small acceleration over a long time period still produces a large velocity. and from what i understand a earth based/orbiting laser could give an initial push to the craft.
              • Assume 10Kg craft.

                F = (S*A*sin(theta) / c

                Theta = incident angle
                A = area
                S = solar flux

                Let theta = 0, A = 100m^2, S = 1.36KW/m^2

                F = 0.000454 N

                a = F/m = .000454N/(10kg) = 0.000045365 _m/_s^2

                22043.6 Seconds to reach 1 meter per second (6.12 hours)

                So about 6 hr/(m/s)*4500(m/s)(about that needed to get to mars on a nice, almost hohmann, orbit from earth orbit). = 1125 days.

                Presently we can get to mars in about 180 days.

                So, yes. Over a *LOOOOONNNG* time.

                I believe that one must achieve about 44km/sec de
                • Your arguement is elegant and well thought out but let me pose 2 questions. First, is there any reason a MUCH larger sail couldn't be used? the sail material is light and would be easy to get into orbit. Second, i wouldn't think that a solar sail would suit going to mars and i don't believe i ever asserted that. But it could be more effective then Voyager for long term missions?
                  • Oh yes! You are correct. I am very sorry if I sound like I am against solar sails!

                    My arguement was actually rather inelegant in the regard that I certainly sounded like I was arguing against sails.

                    For reference, I prefer the concept of mag-loops; but regarding sails:

                    1.) Larger sails: Yes, we would use larger sails. But the force supplied only rises with direct proportion to the sail area. And it adds weight in tie-lines etc.

                    2.) I only used mars as a reference. The use of a sail is indeed ideal for deep
        • The solar system is a pretty empty place. There was some "concern" that the asteroid belt may have large amounts of "dust", but most of the outer solar system missions (Pioneer, Voyager, Cassini, Magellan) have more than demonstrated that this is not the case.

          Yes, the asteroid belt has a lot of pieces, but you would be foolish to run into an asteroid, especially anything that would cause real problems to a solar sail.

          As to debris in LEO, yeah, there is a bunch of that right now, primarily due to space ac
      • AARGH!

        No. It can't: for one thing, the solar wind is *NOT* moving at the speed of light.

        For another, it would take YEARS or to accelerate to any significant velocity... like even a tenth of a percent of C (which is still hauling ass).

        Off the top of my head I recall seeing a decade brandied about. That would be via a number of trips into the inner solar system. Why inner? Because the solar flux is far more dense, and your acceleration higher (flux drops off with the square of distance; thus thrust decreas
        • It is not the solar wind that is captured by solar sails... it is sunlight.

          And last time I checked, sunlight was traveling at, you guessed it, the speed of light.

          While I would agree that it would take years to accelerate to a significant velocity, you still can't dismiss the constant acceleration that would be able to assist the propulsion of a spacecraft. And this is something "free" as well, as you don't need to bring any extra payload for fuel. Once you get to your destination, you can return to your
          • It is not the solar wind that is captured by solar sails... it is sunlight.

            Moments after I posted that I wanted to kick myself. I do recall that some portion of your acceleration comes from the solar wind as well; less than a few percent though.

            As to most of the rest, I was just really don't like seeing massively incorrect posts marked as '+5, Informative'. I am a big fan of solar sails, and think they have a strong future. I like mag-sails MORE, as they are easier to control (turn on and off, no stowing
      • Second reply to this post. I did some math, to show how naive the parent post is:

        Assume 10Kg craft.

        F = (S*A*sin(theta) / c

        Theta = incident angle
        A = area
        S = solar flux

        Let theta = 0, A = 100m^2, S = 1.36KW/m^2

        F = 0.000454 N

        a = F/m = .000454N/(10kg) = 0.000045365 _m/_s^2

        22043.6 Seconds to reach 1 meter per second (6.12 hours)

        So about 6 hr/(m/s)*4500(m/s)(about that needed to get to mars on a nice, almost hohmann, orbit from earth orbit). = 1125 days.

        Presently we can get to mars in about 180 days.

        V_
  • Most discussions about solar sails tend to discuss things like interplanetary or interstellar travel. While this is cool, it is a rather long term goal. The more interesting uses would be orbital stationkeeping. Currently, if you want a geostationary orbit, you either have to set the satellite at 41,000km orbit. If you want it at a different altitude (this is a 200ms round trip lag for light signals) you have to constantly burn fuel to remain geostationary. Once we have viable solar sails, they would be use
    • You are close to what I would think would be interesting, the fact that satellites could position or reposition themselves with sails instead of rockets, and thus save costly refuel missions. This would apply for even geo-stationary orbits in case they get out of correct positions...

    • You could also use solar sails for other stationkeeping like the L1,L2, & L3 Lagrange points which is needed for certain telescopes.

      For those who don't know, L1 -L3 are the quasi-stable lagrange points.

      (L4 and L5 are far more stable)

      I believe the quote is 'It's a great idea.... it just won't work'. If you have ever seen an orbit of an L(1-3) point you would know why. *shudder*. The dynamics involved are pretty short term; which is to say, the tiny tiny tiny amounts of thrust you would get from a sol
  • So what happens when it gets close to another star? If the Sun will push it now, the next star will push it too I would assume - but which way? The sail could go backwards, wrap around what its pulling, and then due to momentum you have a giant missle.
    • I was wondering about that myself; someone caught it up-thread (here [slashdot.org]), though.

      -PS

    • Because the accelleration effect of the solar wind falls off over distance, and the effect becomes effectively zero looooooong before a space craft comes under the influence of another star (assuming you aren't in an area of space where the stars are really close together (galactic core, stellar nursery)). Of course, in such areas, you have bigger problems that getting blown around by stellar wind.
  • Okay, I understand that something can be pushed by light. This makes sense. Yet I came up with what I consider to be a paradox. Can someone find the hole in my logic and help me understand things better?

    Okay... I have two solar sails with perfect reflecting ability. I place them so that they are facing away from each other. I turn on my handy-dandy perfectly unidirectional light source so it hits one of the sails square on.

    Bounce! The light pushes the sail a bit. Light reflects to other sail. Bounce. The
    • Because you can never have a materialthat will reflect absolutely 100% of the light that's going in. Pretty soon, the light source will have diminished, transferring it's energy to the sails. Incidentally, the absorption of light is what gives solar sails their 'push' (due to conservation of momentum, the light comes in with a certain amount of momentum, and that momentum is transferred to the sail when it gets absorbed)

      Hope that helps.
      • No. There is no "absorption". The reflection of the light is what give it the push. And, no there isn't a 100% perfect reflector. However, theorietical thought experiments do allow for such things (think about Einstein's theories). If, in theory, I have broken the laws of thermodyncamics, there must be some other fllacy in my logic.

        • Yes, there is absorption and reflection. Think of absorption and reflection analagous to inelastic and elastic colisions, respectively.

          Supposing that you did have a perfect reflector, I fail to see how you'd be violating laws of thermodynamics. You'd have light oscillating back and forth between two mirrors, which isn't a bad thing, per se. Since they are both connected, they would want to stretch apart, but would remain still.
          • It would be violating the rules of thermodynamics as energy would be being created if my original thought model were correct.

            As other have pointed out, with each reflection, the photons would be red shifted... that is the light itself loses a bit of energy with each reflection. The loss of energy equals the amount gained minus any "friction" in the system. If any "absorption" occurs, this would be part of the friction, not what drives the solar sail.

            If absorption is what drove the system, the engineers wo
    • I think the light will go redder and redder.

      So now I have a question, does redder light have a lower impact strength (less oomph) to power the sail?

      Bert
      • You hit it Bert. The a photon's momentum is equal to Planck's constant times the frequency. As the particle gives up momentum to the sails, the frequency becomes lower, the light becomes redder, and eventually the high energy UV light you started with becomes really low energy radio waves as the energy keeps dropping.

    • Re:Need explanation (Score:3, Informative)

      by borg ( 95568 )
      first of all, there's no such thing as a perfect reflector, so there's always going to be be some loss. but there's a grand tradition of frictionless planes and other such things in these though experiments, so we'll give you that.

      second, unless you have perfectly coherent light (super laser), you'll have dissipation of the beam. we'll spot that, too (see above).

      once those have been taken care of, the dealbreaker is going to be that the reflected photons are red-shifted when they bounce off each sail.

      t
      • Thanks for the explanation. I am aware thre is no "perfect" reflector or "perfectly" coherent light. They only exist in thought experiments. However, thought experiments are useful in finding the limitations of understanding. This is how I figured out I didn't really understand the concept of the solar sail.

        Your explanation of the red-shift filled in the gap. What the problem boiled down to is "what energy decreases as the solar sail energy increases?"

        Thanks!
    • Err. This probably won't help much, but I've seen this exact question asked, and answered on Physlink [physlink.com] which is an "ask the experts" type physics board
      .
      Unfortunately I can't seem to find it again, or remember what the answer was, so you'll just have to take my word for it - though if I can track it down I'll paste a link in here later.

      But I remember the answer basically stated that no, that scenario doesn't work. I think possibly it was because if the light is reflected perfectly, no energy is imparted into
    • The light will redshift on each bounce. Each red-shift is a little bit of energy lost. Red light is less energetic than blue light, and thus will provide less impulse over time due to the continuous red-shift.
  • Is this thing pushed by solar wind or photons?
  • While I think that solar sails are a neat technology, I would like to hear what some other people here think of their potential as manned spacecraft.

    My contention of what I've seen from all of the numbers that get used to describe velocities and transit times for solar sails, is that this will be used primarily for transfer of bulk goods between different places in space.

    A comparison would be how bulk goods are shipped today in industrial countries. Right now the most common methods are by barge (or equi

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