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posted by janrinok on Saturday February 10, @09:48PM   Printer-friendly

https://phys.org/news/2024-02-laser-driven-lightsails-stable.html

It's a long way to the nearest star, which means conventional rockets won't get us there. The fuel requirements would make our ship prohibitively heavy. So an alternative is to travel light. Literally. Rather than carrying your fuel with you, simply attach your tiny starship to a large reflective sail, and shine a powerful laser at it.

The impulse of photons would push the starship to a fraction of light speed. Riding a beam of light, a lightsail mission could reach Proxima Centauri in a couple of decades. But while the idea is simple, the engineering challenges are significant, because, across decades and light-years, even the smallest problem can be difficult to solve.

One example of this can be seen in a recent arXiv preprint paper. It looks at the problem of how to balance a lightsail on a laser beam. Although the laser could be aimed directly toward a star, or where it will be in a couple of decades, the lightsail would only follow the beam if it is perfectly balanced.

If a sail is slightly tilted relative to the beam, the reflected laser light would give the lightsail a slight transverse push. No matter how small this deviation is, it would grow over time, causing its path to drift ever away from its target. We will never align a lightsail perfectly, so we need some way to correct small deviations.

For traditional rockets, this can be done with internal gyroscopes to stabilize the rocket, and engines that can dynamically adjust their thrust to restore balance. But a gyro system would be too heavy for an interstellar lightsail, and adjustments of the beam would take months or years to reach the lightsail, making quick changes impossible. So the authors suggest using a radiative trick known as the Poynting–Robertson effect.

The effect was first studied in the early 1900s and is caused by the relative motion between an object and a light source. For example, a dust grain orbiting the sun sees light coming at a slight forward angle due to its motion through sunlight. That little forward component of light can slow down the asteroid ever so slightly. This effect causes dust to drift toward the inner solar system over time.

In this paper, the authors consider a two-dimensional model to see how the Poynting–Robertson effect might be used to keep our lightsail probe on course. To keep things simple, they assumed the light beam to be a simple monochromatic plane wave. Real lasers are more complex, but the assumption is reasonable for a proof of concept. They then showed how a simple two-sail system can use the effects of relative motion to keep the craft in balance. As the sails tilt off course slightly, a restorative force from the beam counters it. Thus proving the concept could work.

More information: Rhys Mackintosh et al, Poynting-Robertson damping of laser beam driven lightsails, arXiv (2024). DOI: 10.48550/arxiv.2401.16924

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  • (Score: 4, Insightful) by JoeMerchant on Saturday February 10, @10:24PM (4 children)

    by JoeMerchant (3937) on Saturday February 10, @10:24PM (#1343912)

    The laser won't be adjustable worth a damn, but the ship holding the lightsail should be able to angle the sail to adjust the net thrust vector applied to the ship-sail system by the beam, keeping it on course and centered in the beam.

    Now, if there's anybody at Proxima Centauri or any system "down beam" from the ship's course, we will be giving them a couple of decades' of beacon in the sky notice that we're coming...

    --
    🌻🌻 [google.com]
    • (Score: 3, Interesting) by ElizabethGreene on Monday February 12, @06:03PM (3 children)

      by ElizabethGreene (6748) Subscriber Badge on Monday February 12, @06:03PM (#1344126) Journal

      It turns out you don't have to physically tilt the light sail, just reduce the reflectivity of one side or the other. Absorbed photons impart less net momentum than reflected photons.*

      * This is not a violation of conservation of momentum. The momentum of the absorbed photon is the same, but when it's re-emitted as IR/heat it can be emitted in the opposite direction, cancelling the absorbed momentum, in the same direction doubling it, or moved orthogonal to the direction of travel via an active cooling mechanism. If those are perpendicular to the direction of travel and distributed on both sides of the craft, the net effect on momentum should be zero in aggregate.

      • (Score: 2) by JoeMerchant on Monday February 12, @08:21PM (1 child)

        by JoeMerchant (3937) on Monday February 12, @08:21PM (#1344136)

        Cool trick, if the sail supports LCD display like twisting mirrors.

        --
        🌻🌻 [google.com]
        • (Score: 2) by ElizabethGreene on Monday February 12, @10:24PM

          by ElizabethGreene (6748) Subscriber Badge on Monday February 12, @10:24PM (#1344147) Journal

          Using Liquid Crystal Panels for this was one of the technologies demonstrated by IKAROS (Wikipedia) [wikipedia.org]. The funny business with using cooling to move around the radiation pressure from radiative cooling hasn't been demonstrated yet. I can't imagine it's a practical mass trade-off unless you need active cooling for other reasons, e.g. with a laser pusher or opening the sail close to the sun.

      • (Score: 1) by rumata on Tuesday February 13, @12:47AM

        by rumata (2034) on Tuesday February 13, @12:47AM (#1344163)

        > It turns out you don't have to physically tilt the light sail, just reduce the reflectivity of one side or the other.

        This is probably aimed at project starshoot or similar, where the base premise is to point several GW of laser at low single digit of square meters of sail to get to an appreciable fraction of c in a hurry.

        Reducing reflectivity is the one thing you absolutely don't want to do under those conditions lest your ship turns to plasma in pico seconds :-).

  • (Score: 2) by Gaaark on Saturday February 10, @10:46PM

    by Gaaark (41) on Saturday February 10, @10:46PM (#1343916) Journal

    how about 2 sails: one you direct the laser at, which bounces the laser to the other sail, thus balancing the effect?

    Probable problem: the bounce would have less 'push' than the 'push' against the first sail, but the in-balance would be slower and should thus be easier to correct over time.

    Might even make the in-balance more apparent earlier?

    Probably have to make the first sail a mirror of some kind, which might add to weight....

    --
    --- Please remind me if I haven't been civil to you: I'm channeling MDC. ---Gaaark 2.0 ---
  • (Score: 3, Interesting) by Anonymous Coward on Sunday February 11, @01:04AM (6 children)

    by Anonymous Coward on Sunday February 11, @01:04AM (#1343922)

    They're going at a substantial fraction of the speed of light. How do they stop?

    They turn their sales around and let the remote sun's light slow them down. However this is much less intense than the laser directed at them at the beginning. They get closer to the star, where the light becomes more intense, however now they're subject to the star's gravity. They could enter a tight loop around the sun for some duration as they use the sun's light vs the sun's gravity to slow down - however they start out with incredible speed. They would enter an orbit something like Mercury, or closer.

    • (Score: 4, Interesting) by JoeMerchant on Sunday February 11, @01:19AM (1 child)

      by JoeMerchant (3937) on Sunday February 11, @01:19AM (#1343925)

      Aero-braking in the heliosphere, of course.

      I did have a thought about "shaping" the pusher laser - not a great one, but, if the center of the pusher beam were slightly weaker than the ring around it and the ship could stay in that weaker central region, that could have a self-centering effect - as you drift to the outside, the outside gets pushed a bit harder - directing you back towards the center. Certainly you would need some active adjustment in the sail itself to damp oscillations and such, but better to start with a quasi-stable situation than an inherently unstable one and try to ride it for decades.

      --
      🌻🌻 [google.com]
      • (Score: 2) by coolgopher on Sunday February 11, @02:22AM

        by coolgopher (1157) on Sunday February 11, @02:22AM (#1343936)

        +1 Funny for "Aero-breaking in the heliosphere"

    • (Score: 3, Informative) by mhajicek on Sunday February 11, @08:14AM

      by mhajicek (51) on Sunday February 11, @08:14AM (#1343949)

      They don't stop; they take some pics and scans, and beam the data back before continuing off into the void, like the Voyagers did with the outer planets.

      --
      The spacelike surfaces of time foliations can have a cusp at the surface of discontinuity. - P. Hajicek
    • (Score: 0) by Anonymous Coward on Sunday February 11, @12:27PM

      by Anonymous Coward on Sunday February 11, @12:27PM (#1343962)

      Most of the mass is actually the light sail. There is one proposal where the sail is in the form of a disk with a very wide ring around it. The ring is reflective and has much more area than the the disk. As they get near the destination they separate, the ring continues to accelerate while reflecting light back on the smaller disk which slows the payload.

    • (Score: 1) by Chrontius on Monday February 12, @01:54AM

      by Chrontius (5246) on Monday February 12, @01:54AM (#1344025)

      https://newatlas.com/magnetic-sails-spacecraft-brakes/52275/

      Magsails would be powered by either laser beam power or by solar power from the star being approached. Once they hit the heliopause, it would be like a parachute opening; the forces would be considerable until the ship has slowed down to sub-relativistic speeds. If you’re clever (and this isn’t a starwisp) you used the stardust you’re using to slow down to refill your fuel tanks. Then, once the magsail is producing relatively minimal braking force, you can fire your fusion engine for final deceleration, orbital insertion, and maneuvering.

      In the near term, your best performing fusion rocket is going to be Positron Dynamics’ design. You don’t need big magnets or crazy lasers; just a radioactive rock which spontaneously produces infinitesimal amounts of antimatter for millennia at a go. When you turn on the electrostatic “moderator” device, suddenly these spontaneously-emitted positrons get slow enough to interact with the fuel you’re injecting into the engine, and the antimatter is potent enough to initiate fusion with only minimal confinement. Even better, most of the system is potentially very lightweight.

    • (Score: 2) by ElizabethGreene on Monday February 12, @10:43PM

      by ElizabethGreene (6748) Subscriber Badge on Monday February 12, @10:43PM (#1344148) Journal

      It's unlikely they'll be able to stop; These would be fly-by missions unless there is a substantial improvement in the state of the art.

      The last paper I read on this was ages ago, but IIRC there was a whiteboard design of a >1 km^2 solar sail flying down to about halfway between Mercury and the Sun. Then the sail would unfurl to yeet a small payload at > 1G for hours to place it on an interstellar trajectory. The presumption was the trip out of the solar system would destroy the sail via heating and dust collisions.

      It's difficult to imagine we could put the spacecraft on a trajectory precise enough to fly near enough the far star for significant deceleration. The closest star we could reach for is 4 light years away. That's a long leg on the angle so even microarcsecond errors could cause a significant miss.

  • (Score: 2) by Beryllium Sphere (r) on Sunday February 11, @07:20AM (3 children)

    by Beryllium Sphere (r) (5062) on Sunday February 11, @07:20AM (#1343947)

    There are people trying to design a system with lasers ramming light sails to relativistic speeds. https://breakthroughinitiatives.org/initiative/3. [breakthroughinitiatives.org]

    • (Score: 2) by JoeMerchant on Sunday February 11, @02:32PM (2 children)

      by JoeMerchant (3937) on Sunday February 11, @02:32PM (#1343971)

      I'm curious what they use to collimate these ultra-powerful lasers.

      The lasers I have had actual experience with are far from perfectly straight non-diverging beams, and anything I know about that focuses the energy into a less divergent beam also diminishes the total power output significantly - the tighter beam is more powerful in its center, but all that extra energy goes somewhere...

      --
      🌻🌻 [google.com]
      • (Score: 3, Informative) by hendrikboom on Sunday February 11, @10:56PM

        by hendrikboom (1125) Subscriber Badge on Sunday February 11, @10:56PM (#1344008) Homepage Journal

        Ant the uncertainty principle limits the amount we can keep the laser beam from spreading.

      • (Score: 1) by Chrontius on Monday February 12, @01:57AM

        by Chrontius (5246) on Monday February 12, @01:57AM (#1344026)

        Kilometers-wide Fresnel lenses balancing on photon thrust and gravity, I imagine. They’d be gossamer structures of carbon fiber, mylar, and kapton, spun like rifle bullets to keep them from falling out of station and minimize the amount of station-keeping which would require thrusters to handle.

  • (Score: 2) by VLM on Sunday February 11, @06:06PM (3 children)

    by VLM (445) on Sunday February 11, @06:06PM (#1343991)

    The artists have always portrayed solar sails from an 1800 sailboat perspective. So a single large ship and arty looking sails.

    Perhaps another strategy would work better, a cloud of many small vehicles and the sail looks like a crumpled ball of paper. Certainly lower efficiency on a small scale but the overall system, or space probe program, may scale better.

    • (Score: 1) by Chrontius on Monday February 12, @02:04AM (1 child)

      by Chrontius (5246) on Monday February 12, @02:04AM (#1344028)

      You’re sort of recapitulating the Starwisp concept. Each ship would mass only a kilogram, between the massive sail and an instrument package the size of a Coke can. They would be light enough and cheap enough that you could launch entire swarms of the fuckers, which will be nice to have when inevitable attrition causes the failure of a portion of your swarm. Because a single speck of dust won’t kill the entire mission, you can use considerably less shielding per probe which makes them, again, lighter faster (both to make, and their cruising speed) and cheaper. This gets you into factory-style mass production, which again cuts down the cost of your space probe program.

      Let’s conservatively say that a Starship can launch 100 metric tons to a parking orbit. That’s 100,000 Starwisps! For the price of one bigass interstellar probe, you could blanket every system within ~20 light years with precursor probes. Plus, because they’re so small and can accelerate so fast, you can use them as improvised relativistic missiles in case you need to shoot down the alien mothership or something! ;)

      • (Score: 2) by VLM on Monday February 12, @12:32PM

        by VLM (445) on Monday February 12, @12:32PM (#1344068)

        Plus, because they’re so small and can accelerate so fast

        Also you could vary the mass very slightly resulting in slightly different acceleration and, eventually, arrival times, so as to snap multiple pics of possible planets, etc.

    • (Score: 2, Insightful) by khallow on Monday February 12, @05:15AM

      by khallow (3766) Subscriber Badge on Monday February 12, @05:15AM (#1344042) Journal

      and the sail looks like a crumpled ball of paper.

      The artsy sail will catch more light per unit mass. It's cross section area that you want. Crumpling reduces that area. Also you can angle an artsy sail and thus maneuver the vehicle towards the center of the beam.

  • (Score: 2, Insightful) by Moof123 on Sunday February 11, @10:32PM (1 child)

    by Moof123 (5927) on Sunday February 11, @10:32PM (#1344005)

    Seems like a large corner reflector out of mylar out in front of the center of mass would get the job done? Too simple?

    • (Score: 1) by Chrontius on Monday February 12, @02:06AM

      by Chrontius (5246) on Monday February 12, @02:06AM (#1344029)

      That's bloody brilliant. And I bet you can tweak the geometry a little such that it would compensate for drifting out of the beam, too…

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