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Thursday, July 24, 2008

Solar Sailing in Space

NASA prepares to test a satellite that can be propelled by light particles from the sun.

By Brittany Sauser
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Photon flight: NASA’s NanoSail-D is a small satellite that can move through space using a solar sail propelled by pressure from photons. The sail, fully expanded to 10 square meters in this image, is a very thin sheet of plastic coated with aluminum.
Credit: NASA
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video  Watch a demonstration of the solar sail being deployed.

For the first time, NASA is preparing to send into orbit a small satellite that can be propelled by solar sails. When light particles from the sun strike the surface of the sail, the energy is transferred to it, providing a propulsive force that moves the satellite through space.

NASA's goal is to test the complex deployment mechanism of the 10-square-meter sails, says Dean Alhorn, an engineer at NASA's Marshall Space Flight Center, in Huntsville, AL, and the lead engineer on the project. "A successful flight will not only make for a unique historical event, but will show that we have a reliable mechanism to deploy a solar sail in space for future missions," says Alhorn.

The satellite, called NanoSail-D, is scheduled to launch from Omelek Island, in the Pacific Ocean, on July 29 onboard the Falcon 1 rocket developed by Space Exploration Technologies (SpaceX), of Hawthorn, CA. The NanoSail-D satellite's main frame is only 30 centimeters long and weighs nine pounds. Its solar sail is made of a custom polymer that is thinner than a piece of paper and coated with aluminum to reflect the photons. "It looks like Saran Wrap with a metalized surface but is stronger and suited for the space environment," says Alhorn.

In theory, a solar sail could be used as propulsion for round-trip missions in the solar system. In friction-free space, the tiny propulsive force of photons could conceivably get a craft up to about 100 miles per hour in a day and nearly 100,000 miles per hour in three years. Changing the sail's angle can change the craft's trajectory. "There is a lot of potential for solar-sail propulsion once we show that this can be deployed in space," says Alhorn. "Already we are working on ways to maneuver the sails, and we can theorize better designs that are based on proven technology."

The concept of solar sailing was invented in the 1920s by two Russian scientists, and it has been the subject of a few projects over the years, says Louis Friedman, the executive director of the Planetary Society, a public space organization based in Pasadena, CA. The Russians deployed a large reflective sheet of material outside their Mir space station in 1992, and the Japanese did something similar in 2004, but neither was used for solar-sail propulsion.
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energy NASA photons satellite space travel sunlight

Comments

  • Space, the final frontier.
    Handshake on 07/24/2008 at 3:08 AM
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    Very interesting.But...
    This is a very complicated and non-efficient way of doing things.
    In space in no gravity, no winds, why don't use that information?

    Why don't they use a round material and fill edges (like a tor) of that round piece of material with a gas(small pressure) or with a liquid that will freeze and give that material the desire shape?

    They will be able to squeeze in a small box a very big piece of material and let it inflate in space like a balloon and form a big "thing"(mirror, antenna, space craft) and fill it after that with another stuff "glue", or water to give a permanent shape to that thing.

    When I see mechanical devices used in space, I wander... !

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    • Re: Space, the final frontier.
      kmunng on 07/24/2008 at 6:23 AM
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      Handsake, You're wrong about no gravity in space. Gravitational force exists between planets and galaxies. Utilizing phonons from Sun (or stars) in space to propelling long distance (n light years) space travel is efficient enough as there are many stars and suns in space. 
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      • Re: Space, the final frontier.
        mkogrady on 07/24/2008 at 1:41 PM
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        What happens if the solar sail reaches an area of space where photons from one sun are equal to the one propelling the device - will it cause the solar sail to stall?

        Kind of like Interstellar Doldrums?
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        • Re: Space, the final frontier.
          Brittany Sauser on 07/24/2008 at 4:28 PM          		 Technology Review TR Staff
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          Hi mkogrady,

          I relayed your question to NanoSail-D's lead engineer Dean Alhorn and his team. Here is their response:

          "There are various methods to accommodate this situation. First, by the time the spacecraft experiences the effect of another sun, the spacecraft will be traveling at a great velocity and will slowly decelerate from the new source.  The energy from any source depends upon the distance to the source, so the farther the spacecraft is away from a source, the less the effect of the photons on the sail.  Second, the sail could be turned such that the reflective surface is not in the source path so the incoming photons do not have a large area to affect the spacecraft’s momentum.  Likewise, turning the sail slightly would cause the spacecraft to turn with energy from the new source and travel in a different direction. Another option would be to jettison the sail after the spacecraft had sufficient velocity to exit the solar system. Any one of these ideas or a combination would prevent a “stall.”  Even in a double star system, these methods would prevent a “stall” from occurring."
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          • Re: Space, the final frontier.
            jrarchibald on 07/25/2008 at 7:44 AM
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            Hi,

            There seems to be the impression that by altering the angle of the sails to the Sun there will be a change in the path of the vehicle.  In objects that have no independent  guidance reference i.e. such as a keel, the unbalanced force of the wind cannot be applied as a force offset to the direction of the wind.  As a result the object will continue to be blown down wind irrespective of the angle of the sails to the wind.
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            • Re: Space, the final frontier.
              cjameshuff2 on 07/25/2008 at 9:34 AM
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              There *will* be a change in the path of the vehicle. The force applied to a solar sail is very nearly perpendicular to the sail regardless of its orientation, due to the fact that it reflects light rather than simply absorbing it. There is a small always-outward component due to the sail being an imperfect reflector, but reflectivities exceeding 90% are not difficult to achieve. So no keel necessary...if you have not exceeded solar escape velocity, tilting the sail to one side can cancel your orbital velocity and allow you to spiral closer in to the sun.

              If you *have* exceeded escape velocity, you're going to have to drop something. Either throw enough reaction mass outward to drop yourself below escape velocity, or jettison the larger part of your sail and use it as a mirror to provide thrust in a direction opposite to the sun.
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  • free
    asdar on 07/24/2008 at 7:55 AM
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    I don't see how you can get more efficient than free. I do take your point about forming the desired shape, but complications in space make for failure.

    They're keeping it pretty simple, which I think is smart.
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  • Efficient, yes. Effective...?
    dtutelman on 07/24/2008 at 9:10 AM
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    I don't think the question is efficiency. But intuition keeps saying there must be more you can get out of photons than simple momentum transfer (which is what solar sailing is).

    On earth, we use solar power perhaps less efficiently, but more effectively -- more joules per photon. But I don't know how to turn electricity or heat into propulsion in a vacuum, without expending mass somehow. And, if you expend mass, you eventually run out of fuel.
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    • Re: Efficient, yes. Effective...?
      cjameshuff2 on 07/24/2008 at 10:16 AM
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      There is a tradeoff between mass efficiency and energy efficiency. Ion drives are quite power hungry devices and use enormous amounts of power to produce rather little thrust...but they can produce that thrust for years on end with a tiny tank of fuel. Compressed air driving a stream of water out the back could achieve enormous amounts of thrust, but the exhaust velocity will be so low that utterly impractical amounts of water and compressed air would be required to keep it up for any amount of time.

      Chemical rockets are somewhere in the middle, trying for a high "specific impulse" so they have to lift as little fuel off Earth as possible, but also requiring high thrust so they can exceed one full gravity of acceleration. Once in orbit, you don't need to exceed any particular amount of acceleration to get anywhere, because even the smallest gains add up, so highly fuel-efficient engines become desirable (except when trip time is a particular problem...a manned craft has requirements that supersede fuel efficiency).

      Higher fuel efficiency is achieved by using a higher exhaust velocity, but since (non-relativistic) momentum is m*v and kinetic energy is 0.5*m*v^2, energy requirements increase faster than the thrust produced for a given mass of fuel. You can not have an exhaust velocity equal to the speed of light with any fuel possessing mass, so a photon drive is the limit of fuel efficiency, requiring only energy and no fuel.

      You could collect light with solar panels and use the electricity to power an ion drive, but you lose most of the light energy and require fuel, even if it is small amounts. (This *is* actually done, however, due to the relative simplicity and the lack of development of solar sails, and the occasional need for the higher thrust of an ion drive.) You could do the same, using the energy to power a photon drive to eliminate the fuel requirement, but photon drives and solar panels are far heavier than a thin film mirror of the same area, on top of the inefficiencies of conversion along the way. Solar sails are really the best way to harness sunlight for propulsion without requiring fuel.
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      • Re: Efficient, yes. Effective...?
        tsaidak on 07/24/2008 at 11:17 AM
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        Solar sails have an additional efficiency available.  You can use "ground based" lasers to add photonic energy, making the space craft accelerate even more.  This in effect allows you to "add fuel" without adding mass the the craft.  This can be done even at interstellar distances as the space craft will always be moving at very low speeds.
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        • Re: Efficient, yes. Effective...?
          cjameshuff2 on 07/24/2008 at 3:54 PM
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          You could do the same with solar panels, really...but solar sails can be much larger and lighter, allowing them to gather more of the beamed power. (diffraction limits are likely going to make the beam much wider than any likely solar panel array, but a large sail might still get all the beam at extreme distances)

          You're off on the speed, though. Solar sail craft would not have high acceleration, but could accelerate continuously and indefinitely while being lower in mass than solar panels + ion drives. They can reach very high velocities over the course of a few years, even without the assistance of a laser beam. However, this should not cause any particular problems with laser propulsion, given the distances involved and the predictability of the sailcraft's trajectory...it won't be moving particularly quickly across the sky from the laser's point of view, and its path will be easy to anticipate.
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  • Solar Sailing?
    phlyerphil on 07/24/2008 at 12:29 PM
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    Am I missing something here?  I have many years of sailing experiance (not in space fortunately), beginning with my first attempt as a youth when I commandeered an old flat-bottomed row boat, rigged it with a half a pup tent for a sail and set off for what I hoped would be a pleasant sail.

    With the wind (Photons) at my back, I charged downwind.  It was exciting until I decided that I'd had enough fun for the day and should start back.

    Opps!  Having no keel to to provide a means of resistance to provide a resultant change of force to effect a change of direction, I remained headed downwind until I dropped the whole rig and broke out the oars.  A hour or so later, totally exhausted, I returned to the point of my departure.

    So, please tell me how your space sailboat is to be navigated with no keel or other means to effect a change of direction (or even to return to your origin). 

    I suggest that you be prepared for a lo-o-ong trip downsun with your insurance policies paid-up.
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    • Re: Solar Sailing?
      levit8r on 07/24/2008 at 2:13 PM
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      phlyerphil, you are indeed missing something very large -- the sun, and its gravitational attraction. Solar sails can be used to increase orbital velocity, and spiral out from the sun. They can also be used to decrease orbital velocity, simply by tilting them in the opposite direction, causing the spacecraft to spiral in toward the sun. No need to row.
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  • sun sail
    momark2 on 07/24/2008 at 1:33 PM
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    "In theory, a solar sail could be used as propulsion for round-trip missions in the solar system. In friction-free space, the tiny propulsive force of photons could conceivably get a craft up to about 100 miles per hour."

    Typo? Else it's a LONG ride anywhere in space...
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    • Re: sun sail
      Brittany Sauser on 07/24/2008 at 2:07 PM      		 Technology Review TR Staff
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      Maximum speeds for solar sailing spacecrafts are not exactly known; 100 miles per hour is just the speed the spacecraft could reach in a day. The Planetary Society estimates that in 100 days a sail could reach 10,000 miles per hour, in one year 36,000 miles per hour, and in 3 years could reach a speed over 100,000 miles per hour. This means it could reach Pluto in less than five years.

      I have added this information to the article.
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      • Re: sun sail
        momark2 on 07/25/2008 at 1:26 PM
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        Thanks for clarification.
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      • Re: sun sail
        Manuvidya on 07/28/2008 at 7:05 AM
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        I'm wondering what counter figures have been used to account for the loss of propulsion once the craft gets farther away from our sun. One would need another sun in roughly the same direction to pick up the slack and give the travelingspeed another boost. Slingshotting your craft from gravitational pull to gravitational pull is.. creative maneuvring at best :P
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        • Re: sun sail
          cjameshuff2 on 07/28/2008 at 10:05 AM
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          First, this is a spacecraft you're talking about...it's not going to slow down and get stuck in interstellar space, it'll keep going until it is stopped by something.

          Second, why the aversion to slingshotting? It's a well-understood and well-proven procedure, and solar sailcraft would be particularly well equipped to do it against stars.

          Finally, you only seem to consider the uses for interstellar propulsion. A sailcraft within the solar system could carry non-perishable cargo between planets and asteroids for very low cost. Sailcraft probes could operate indefinitely, surveying asteroids, comets, and moons close up until their electronics finally fail. And satellites equipped with sails can use their thrust to maintain drastically non-standard orbits, holding positions out of the orbital plane, or even stationary hovering, balanced between photon pressure and gravity.
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          • Re: sun sail
            Manuvidya on 08/01/2008 at 6:38 AM
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            Thanks for the reply :)

            I don't have an aversion to slingshotting at all, it's just that it would cost considerable amounts of time to position the craft, approach a star, speed up, escape gravity at the right time, etc.
            Not a bad way to move if it's your only chance but I merely figured that traveling as straight as possible would give us an advantage, be it in delivering goods or exploration via unmanned crafts or in manned flights. I guess I can't wait for us (humanity) to start terraforming something ;)

            I see your point about no slowing down due to the extremely-near-vacuum nature of space, but gravity and sunlight (or any other radiation that might be picked up by the sail) will surely have it's effects on course, speed, etc.

            But hey, I barely know how to get dressed in the morning so I'll leave all that stuff to NASA :)
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            • Re: sun sail
              cjameshuff2 on 08/01/2008 at 2:18 PM
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              "it's just that it would cost considerable amounts of time to position the craft, approach a star, speed up, escape gravity at the right time, etc."

              Such a course would obviously be planned ahead of time. It would take no additional time to set up for a flyby, as you would have plenty of time to make any trimming maneuvers during the decades or centuries it takes to reach the flyby system. A straight line path at the same average velocity would of course be shorter, but achieving that velocity without the sail and flybys would require enormous amounts of fuel.


              "I see your point about no slowing down due to the extremely-near-vacuum nature of space, but gravity and sunlight (or any other radiation that might be picked up by the sail) will surely have it's effects on course, speed, etc."

              Those are the main things that make flybys work. The craft falls into the gravity well, accelerating in the process, and then burns/uses the solar sail to shorten the time in the gravity well on the outbound side, allowing it to keep some of the velocity it gained. They won't normally have anything resembling a drag effect, and can be exploited to accelerate the craft further on its journey or slow it down for arrival. Also, if its an interstellar probe, the latter stars it visits might not be any further from Earth, it might even turn back toward Earth to make transmitting data easier. Flybys allow this to be done without losing the speed gained during the trip to the destination.

              These maneuvers are complex, but well understood and easy to model, and there'll be no shortage of time in which to plan them out in an interstellar journey. Realistically, we're unlikely to ever bother with such things for the foreseeable future...they will likely have been in use for a very long time for transport between points within the solar system before we send off an interstellar probe.
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              • Re: sun sail
                Manuvidya on 08/05/2008 at 4:46 AM
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                Yet again I owe you my thanks for the additional explanation. I guess my reasoning just shows not only how little I know about space and traveling through it, and how limited my 'vision' is :)
                It is however, one of the reason that I hope to live to be at least 300 ;) To find out what mankind can and will do in the future :)
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  • Reactions so far
    Manuvidya on 07/28/2008 at 6:59 AM
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    Hi all, first off, I'm new here, so please bare with me as I learn this sites and it's peoples way of communicating :)

    What I think shows from the reaction to solar propulsion by means of 'sailing' is the limited view most humans have. This is not a judgmental statement at all, but merely an observation.

    Most comments are put in contexts of traveling distances on earth, specifically the efficiency/effectiveness responses. Today is the 28th and whoever proves himself right or wrong, I'll try to see what this technology does tomorrow. If this works, we'll have a method of 'cheap' transport without the need for tons of costly fuel etc. Think sails of 500x500 meters, moving ships that are built to sustain life for decades, carrying terraforming equipment.. Ok, I'm getting ahead of things :) I wish this mission all the best and hope to be able to spend some more time here, talking about technology from (as far as I go anyways) laymens point of view.
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  • A couple of thoughts
    shomas on 08/02/2008 at 9:58 AM
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    One thing that hasn't been discussed hear in TR is how much of an effect fluctuating solar winds might have. I am guessing very little when about 1 astronomical unit from our sun but does the effect change much when in orbits close to Venus or Mercury.

    When it was stated that Pluto could be reached in 4 years, did that take into account the intensity of light drops by the inverse square of its distance from the source. How long would it take to get to the nearest star other then our own of course orbit, and take photos, and then return.

    Last yet not least its important to note that a sail with only one side reflective in interstellar space will have photons approach it from both sides of the sail.
    1. Photons that hit the reflective side will hit the sail giving momentum to the sail and then bounce off giving it yet more momentum.
    2. Photons that hit the side that absorbs photons will give some energy canceling out some of the forward momentum and then radiate the extra heat back into space as photons on probably both sides equally.

    If the last statement is only partially true, then hypothetically if all stars were evenly spaced, and gave off the same amount of light, and were equally distant from our spacecraft that starts its journey motionless in relation to all the stars, our sail would still have a net propulsion in the direction of the side that absorbs the photons.
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    • Re: A couple of thoughts
      cjameshuff2 on 08/03/2008 at 4:27 PM
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      Fluctuating solar winds won't do much, the solar wind doesn't carry much momentum in comparison to the light that solar sails use. The luminance of the sun does vary over time, but by a fraction of a percent. It would certainly be taken into account, though.

      Yes, the numbers do account for the loss of sunlight with distance. Trajectories have even been developed which involve starting out in another direction and then backtracking to do a flyby of the sun, allowing more time close to the sun during which to accelerate. An interesting and related fact: the intensity of light from the sun drops with the inverse square of distance, but so does the gravity of the sun. A sail that exactly counters the force of gravity of the sun on a payload at Mercury will also do so at Pluto. A sailcraft can thus carry just enough sail to counter the sun's gravity, drop toward the sun with the sail on edge, and turn it face on for the trip out, keeping all the velocity it gained during the flyby. It needn't even have enough sail to fully support it against the sun's gravity, just enough to provide that asymmetry between the incoming and outgoing trips.

      The sail with one reflective side and one absorbing side would accelerate until it reached equilibrium temperature, at which point the emission from the black side would match reflection from the mirrored side. Unfortunately for spacecraft, good reflectors also tend to be poor radiators. If the sail were a heat pump and actively heated the mirrored side and cooled the black side to compensate for the difference in emissivity, or if the entire sail were actively cooled, it would continue to produce net thrust but would require energy input.

      edit: And something more immediately relevant...the launch failed. It appears the second stage did not separate when it was supposed to. Experimental solar sail craft seem to have hard luck with launch vehicles...
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  • Solar Sails
    PeterC2 on 08/10/2008 at 10:53 AM
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    A more efficient and navigable solution might be to have an umbrella-like structure blackened on the concave (back) face and silvered on the front.
    • Have the sail sectors built as inside-out capacitors, with the static charge held on the outside back surface with the charge control circuitry on the inside between the front and back surfaces, doubling as a support frame.
    • Have the sectors divided into four quadrants that are insulated from one another so as to provide both collective and differential charge.
    • When deployed and fully charged, release a small amount of inert gas from the centre of the back face.
    • It's only my theory, but the gas molecules could be thus trapped by the static charge and held against the back surface. This could dramatically boost forward propulsion, by adding the pressure of gas excitement to photon pressure. The heat from the dark surface generated by photon impact could be utilized by the energized trapped gas molecules, to create forward pressure. The pressure could be differentially controlled across the surface by concentrating the static charge and hence the gas in any combination of quadrants.

    This sail would store energy more like a hot-air balloon and would require minute quantities of trapped gas to create a significant enough pressure differential between its leading and trailing faces.
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