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Space Earth Transportation Science Technology

Light Sail Propulsion Could Reach Sirius Sooner Than Alpha Centauri (arxiv.org) 171

RockDoctor writes: A recent proposition to launch probes to other star systems driven by lasers which remain in the Solar system has garnered considerable attention. But recently published work suggests that there are unexpected complexities to the system. One would think that the closest star systems would be the easiest to reach. But unless you are content with a fly-by examination of the star system, with much reduced science returns, you will need to decelerate the probe at the far end, without any infrastructure to assist with the braking. By combining both light-pressure braking and gravitational slingshots, a team of German, French and Chilean astronomers discover that the brightness of the destination star can significantly increase deceleration, and thus travel time (because higher flight velocities can be used). Slingshotting around a companion star to lengthen deceleration times can help shed flight velocity to allow capture into a stable orbit. The 4.37 light year distant binary stars Alpha Centauri A and B could be reached in 75 years from Earth. Covering the 0.24 light year distance to Proxima Centauri depends on arriving at the correct relative orientations of Alpha Centauri A and B in their mutual 80 year orbit for the sling shot to work. Without a companion star, Proxima Centauri can only absorb a final leg velocity of about 1280km/s, so that leg of the trip would take an additional 46 years. Using the same performance characteristics for the light sail, the corresponding duration for an approach to the Sirius system, almost twice as far away (8.58 lightyears), is a mere 68.9 years, making it (and it's white dwarf companion) possibly a more attractive target. Of course, none of this addresses the question of how to get any data from there to here. Or, indeed, how to manage a project that will last longer than a working lifetime. There are also issues of aiming -- the motion of the Alpha Centauri system isn't well-enough known at the moment to achieve the precise maneuvering needed without course corrections (and so, data transmission from there to here) en route.
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Light Sail Propulsion Could Reach Sirius Sooner Than Alpha Centauri

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  • This is a Thought Experiment, not a real plan to go anywhere... we aren't going to travel between the stars until we figure out something a whole lot better than chemical rockets and probably FTL drive...

    Everything else is just fantasy...

    • Thats how it begins. a "Thought Experiment" that is. 70 years is a great amount of knew tech... we will likely have something in 40 years that will sail right past our sail ships.,,, hmmm
    • by bill_mcgonigle ( 4333 ) * on Saturday April 22, 2017 @06:03AM (#54281901) Homepage Journal

      we aren't going to travel between the stars until we figure out something a whole lot better than chemical rockets

      If only somebody would put some serious effort into solar sail trajectories!

      and probably FTL drive... Everything else is just fantasy...

      Umm.....

      • Solar sail trajectories are fun to think about, but we're not going to ever actually do them... the time frame isn't useful, the data wouldn't be obtained in our lifetime, and then what? You can't go there, you can't come back...

        • Re: (Score:2, Informative)

          by wendyo ( 168574 )

          Some people think on longer time frames than you do.

          https://en.wikipedia.org/wiki/... [wikipedia.org]

        • the time frame isn't useful, the data wouldn't be obtained in our lifetime, and then what?

          Coincidentally, I was out walking today through a forest that was originally planted in the 1300s, in order to provide timber for the anticipated navy of the 1600s. Even though the people who planted the trees wouldn't see them grow to a usable size.

          Lordy! - they must have been superhumans, those Mediaeval foresters. Able to think centuries ahead, where modern people just cannot do that any more.

          • Coincidentally, I was out walking today through a forest that was originally planted in the 1300s, in order to provide timber for the anticipated navy of the 1600s.

            Was it used for that purpose? The fact that it's still there suggests not.

            • I was out walking today through a forest that was originally planted

              Was it used for that purpose? The fact that it's still there suggests not.

              I carefully used the word "originally" when I originally wrote that. Not because I expected your response, but it suffices.

              You seem to be thinking that the trees I was walking amongst were the ones that were planted in the 1300s? No - they had been harvested one at a time, according to their individual shape and size and the lumber needed for a particular ship, from

    • Getting a person there with something better than chemical rockets is just fantasy since if you got the vehicle to move fast enough even the cosmic background radiation will be shifted enough to irradiate people to death.
      Of course, a different fantasy of cryosleep plus slow travel or FTL removes that in SF at least, but not so much in reality.

      This thing on the other hand looks like a way to get a machine to another star using something that needs nothing more than some years of development (https://en.wiki
      • Getting a person there with something better than chemical rockets is just fantasy since if you got the vehicle to move fast enough even the cosmic background radiation will be shifted enough to irradiate people to death.

        You forget that anything in deep space is going to require shields, or the method of transport won't matter...

        so I do not get why you are calling it fantasy.

        You don't get it because you don't understand human nature... this simply is not going to happen...

        • by dbIII ( 701233 )

          You don't get it because you don't understand human nature

          Don't worry kid, when you are thirty years older you won't understand human nature in the same way.
          What's with the snotty put downs? Can't get laid?

          • What's with the snotty put downs? Can't get laid?

            Apparently you're not bright enough to realize that the above hypocrisy reveals how bright you aren't. And yeah; it's recursive.

      • by chihowa ( 366380 )

        Getting a person there with something better than chemical rockets is just fantasy since if you got the vehicle to move fast enough even the cosmic background radiation will be shifted enough to irradiate people to death.

        You have to be traveling at 0.999999 c before the cosmic background radiation becomes visible, but not yet even ionizing. Even with local variations in the background, you need to be well over 0.9999 c before it becomes dangerous. There's plenty of usable velocity below 0.9999 c!

    • by Applehu Akbar ( 2968043 ) on Saturday April 22, 2017 @08:35AM (#54282279)

      This is a Thought Experiment, not a real plan to go anywhere... we aren't going to travel between the stars until we figure out something a whole lot better than chemical rockets and probably FTL drive...

      Everything else is just fantasy...

      The missions being envisioned here are for small robots that can be accelerated and decelerated with reasonably foreseeable technologies, not humans with life support. Being able to decelerate into a target system would not only increase the data return, but would enable a small probe to locate accessible resources (as in not down a gravity well) to construct a transmitter large enough to return the data in the first place.

      • by religionofpeas ( 4511805 ) on Saturday April 22, 2017 @09:23AM (#54282443)

        would enable a small probe to locate accessible resources (as in not down a gravity well) to construct a transmitter large enough to return the data in the first place.

        I'm not sure how you think a small robot can find suitable resources, fly over there, and convert those resources into a large working transmitter, provide it with energy, and keep it aimed at the Earth. That's not a small robot, but a very large manufacturing base.

        • Elon Musk will build a 3D printer that can do it. It'll be powered by blockchains.

        • Use LOTS of small robots.
          • Explain how one small robot can navigate around a star. Imagine for instance that a robot is capture in orbit just outside Jupiter. How does it get to the resources it needs, somewhere in the asteroid belt, using its tiny little fuel tank ? Adding more robots doesn't help, because they all have the same problem.

            • You use a light sail. It's slow but extremely efficient.

              The "seed" that will grow into what we want could weigh less than a microgram.

              • If a light sail is slow, how can you match another body's orbital velocity ?

                The "seed" that will grow into what we want could weigh less than a microgram.

                I'm skeptical, but I'll be happy to study your design drawings.

        • The cheapest commodity to transport over interstellar distances is information, even if you have to use a physical medium to ship it in. Envision a robot with primitive foraging skills that can find a body like Enceladus and then extract metals and other base resources from its surface. Using AI directing a long series of tools-to-make-the-tools manufacturing steps, it converts a large body of ferried information into local exploration robots, a local communications network and a large transmitter.

          • First you have to have a robot that can find a suitable object in the system, and move itself into orbit, and then it has to safely land under unknown conditions. Without adding too much mass, of course.

          • Light sails may work for an interstellar cruise, but chemical rockets are the only viable theoretical way to land and take off from a planet/moon you haven't built a space elevator on. We're talking about an interstellar trip that's only theoretically feasible for a vehicle that weighs a few grams, and you're talking about adding a 200,000+ kilogram rocket to it.

            • Hence my stipulation "not down a gravity well."

              • Where do you get resources without a gravity well? Certainly not an object like Enceladus, which has a very large gravity well. Maybe a tiny asteroid, but it's highly doubtful that it's possible to land on an asteroid using a solar sail (that doesn't has a laser propulsion source anymore)... at least not in less than millions of years. Even if you did land on the tiny asteroid, extracting metal from it is not going to be possible using a few grams of spacecraft.

      • by dargaud ( 518470 )
        How about you change the shape of the lightsail to form a parabola and use that to focus the antenna signal ?
        • Shortcuts like this are great where they can be found, but there will still be a fundamental need to scavenge local energy and local materials at the target system, leveraging the knowledge and processing power you bring with you.

      • The missions being envisioned here are for small robots that can be accelerated and decelerated with reasonably foreseeable technologies

        No. The only forces being modelled for deceleration in the target systems are those of light pressure and gravity - which we can calculate from the light flux (observed at Earth), the range (parallax), and orbital mechanics.

        Once someone has a design for a probe (mass, sail area, reflectivity) then the analysis can be re-done to calculate the travel times (and important thi

    • by Solandri ( 704621 ) on Saturday April 22, 2017 @11:45AM (#54283001)
      Thought experiments are how you come up with an idea that nobody has thought of before [snopes.com].

      Back in the late 1980sI was on an email discussion group for Traveller (a sci-fi RPG). Someone asked why hydrogen fuel (for fusion) was stored as water aboard ships. Someone answered that water stores hydrogen atoms more densely than hydrogen gas, and the energy needed to chemically break off the hydrogen atoms off of water was trivial compared to the energy you could get from fusing them into helium. That spawned a discussion about whether there were other molecules which stored hydrogen even more compactly. Methane (CH4) was an obvious choice - 4 hydrogen atoms per non-hydrogen base, compared to just 2 for water (H2O). But eventually we settled on ammonia (NH4) because it's liquid at room temperature and wouldn't require pressurization or cryogenic storage in a vehicle sharing space with a life support environment for humans.

      It's totally useless info right now (and probably the next few decades). But it's something that will be important in the future.
      • by Cyberax ( 705495 )
        Ammonia is NH3 (ammonium ion is NH4+). The densest hydrogen storage is BH3 (boron hydride) with 23% of hydrogen by weight compared to 17% in ammonia.
    • we aren't going to travel between the stars until we figure out something a whole lot better than chemical rockets and probably FTL drive...

      And just where did anyone involved with this make any suggestion that it involved any human - or even any mammal - ever reaching another star system? I as sure as hell didn't see that, and I did read the fucking paper.

      It is a moot point (cue grammar Nazis who think that it's "mute") whether a VonNeumann robot with the pinnacle of 22nd century software counts as a human

  • a team of German, French and Chilean astronomers discover that the brightness of the destination star can significantly increase deceleration, and thus travel time

    Sounds like they've discovered a way to significantly increase travel time.

    • by dwsobw ( 2723483 )
      Presumably, if you can decelerate faster you can also accelerate faster. Given that you have to take all the fuel for deceleration with you but not (or at least less) the one for acceleration, I can see how having the destination help decelerate is useful ...
    • by JDevers ( 83155 ) on Saturday April 22, 2017 @06:44AM (#54281987)

      No, this is a way to significant increase BRAKING power. The speed up is tied to Sol no matter where we point a light sail, by changing the destination we can slow down a LOT faster.

      • significantly increase deceleration, and thus travel time

        Significantly increase deceleration, and thus travel time = Significantly increase deceleration therefore significantly increase travel time, no?

        "Significantly increase deceleration, and thus decrease travel time" would imo imply a journey taking less time, but that's not what's written in the summary.

  • by TheOuterLinux ( 4778741 ) on Saturday April 22, 2017 @08:05AM (#54282171)
    With current technology, that's all you'll be doing. Focus on Mars or Moon bases for now.
  • Why is light sail considered entirely possible, while EM-propulsion remains in the domain of evolution-denying (and even Trump-voting)? Aren't they both using light (of some frequency or another) as, uhm, tangible? Something, against which it is possible to push, however slightly?

    • by Anonymous Coward

      all of the peices of light sail have been demonstrated at some scale and the physics is extremely well understood (with the exception of some high power laser wierdness). EM drives seem to be magic, and the mechanism that creates the thrust isn't understood at all.

      Oh, add in that the force from an EM drive is incredibly tiny.

    • by Anonymous Coward

      I assume you're joking. Or you haven't done high school physics yet. It's the difference between using your arms to push yourself against a wall, and holding both your hands together and claiming that pushing harder with the left arm your entire body will move right.

      • I assume you're joking. Or you haven't done high school physics yet. It's the difference between using your arms to push yourself against a wall, and holding both your hands together and claiming that pushing harder with the left arm your entire body will move right.

        Technically speaking, if you hold your hands together and yet push harder with the left arm then that would imply a net acceleration of your left arm to the right. Except in the case of massless arms, this would imply a net force acting to push on the body, moving it left in the absense of any other forces. Simply severing the arms near the end of their travel should suffice to keep the body in motion until it is altered by additional forces.

    • Why is light sail considered entirely possible, while EM-propulsion remains in the domain of evolution-denying (and even Trump-voting)? Aren't they both using light (of some frequency or another) as, uhm, tangible? Something, against which it is possible to push, however slightly?

      The difference is not that the momentum of light is different, it is in fact the same for both cases. The problem lies in the weight of the craft itself. With an onboard em source you would need to carry your own fuel and have a massive em emitter. You may perhaps use solar cells to supplement some of the fuel needs at the expense of lower efficiency per photon at the craft and even more mass is needed. With a light sail you can construct a massive array of base laser stations with huge power supplies,

      • by mi ( 197448 )

        With an onboard em source you would need to carry your own fuel and have a massive em emitter.

        That's not necessarily a problem — even a kilo of material contains enormous amounts of power (m*c*c), we just don't yet know, how to extract it...

        It would seem, the first such craft — if any are built — will have both. The sail for long distance travel, and EM for shorter-distance maneuvering, when nearby stars may not be sufficiently "bright". Not at all unlike the first coal-powered ships, whi

        • If we're going to use an EMDrive, why not just have a wizard make it go poof and reappear at the desired destination?

        • The EmDrive is not EM propulsion, it's pseudoscientific nonsense, confirmation bias in action. As claimed to operate, the EmDrive is a closed system that does not emit anything, and so does not conserve momentum or energy. EM propulsion using on-board power would be a photon rocket, and while it might have some exotic applications in things like precision formation flying, it's not going to be good for any more than that without the ability to convert matter into EM radiation with high efficiency.

          Also, even

    • Are you referring to the EM drive using an asymmetric chamber to bounce microwaves around in that allegedly violates conservation laws without exotic technology? It is exceedingly unlikely that that works as described. It may wind up having interesting physics, but there is so much physics that works so well that's based on conservation of energy and momentum to make it extremely improbable that they're wrong. Moreover, by Noether's Theorem, this would imply that physical laws change over relatively sho

  • We're doing it all backwards. In order to avoid navigational errors we have to get the star to draw us towards it or vicey versy. In other words, use attractive forces instead of repulsive ones.

  • IIRC the one test we tried to do of a light sail didn't work very well. We seem to be pretty good about getting stuff into space at the moment, maybe we should try again. Kickstarter, anyone?
  • So when are we expecting Alpha Centauri to get there?

  • I find it hard to believe that something going that far is not going to collide with some other object. The idea is cool though, maybe we should use robots to see how well it goes?
    • Was thinking the same thing. Even apart from the threats of crashing into tiny specs of dust and particularly large molecules at relativistic speeds, the probe would be subject to extreme radiation throughout the journey just as a byproduct of is speed, which can't be good for the electronics.

      • On top of that, what if it springs a tiny air leak? They'll never find it! They need to send robots first and place repair and rescue packages and maybe even extra fully functional spaceships. Outer Space is mostly like a desert. Not much to rely on.
  • by Anonymous Coward

    The solution is a two sail solution. The sail will break into two sails on approaching the destination star. The big primary sail will act as a mirror and beam light backward unto the smaller secondary sail slowing down the probe.

  • By not needing to bring the bulk of the fuel needed for interstellar transportation, and by efficiently utilizing em from nearby stars, it will be very hard to beat its speed for the foreseeable future. Even things like the em drive, if it's real (tldr extremely unlikely is being nice) require onboard fuel and would take far longer to make the same trip.

    Ultimately the best setup possible to move through space in a normal fashion would be to create or mine a miniature black hole of roughly a billion metr
    • It's nice that you can shoot an ultra light sail craft through the galaxy but without communication, it will be rather pointless. And it will be tricky to add communication equipment that can send a signal across a few light years without messing up the "ultra light" property.

  • The cover article in the March 2017 issue of Scientific American was about using multiple light sails and miniature sensors to visit Alpha Centauri, with a large array of lasers -- either earth-based or space-based -- as the primary accelerant. The use of light sails, however, can be problematical.

    First of all, consider solar wind, the stream of gases and particles emitted by the sun. If solar wind is faster than the sails, it will accelerate them beyond the force of the proposed laser array. If the sola

    • In either case, the solar wind and the sun's gravity can alter the trajectory of the sails.

      The influence of other star's gravity is calculable. (Unless there's something gravitating and dark out there.) The influence of interstellar winds ... is a fair question. So you send your first few probes off to see how they behave. It's not as if they'll contain anything you're more emotionaly attached to than some bits of wiring and (maybe) an AI.

      The Oort cloud also requires consideration. If the sails are not punc

    • Sunlight carries about a thousand times as much momentum as the solar wind. The drag produced by solar winds and interstellar gases isn't insignificant, but it'd be a very small source of error. Compensating for wrinkles in the sails will probably require bigger adjustments.

  • While I didn't work on this one, the Galileo mission was started at JPL in 1977 (there were lots of study work done before that) and was ended in 2003 with the planned plunge into the Jovian atmosphere. That's 26 years, which was at least for many of the engineers, scientists, technicians and managers on the project a good portion of their careers. 74 years or more (added 5 years for planning and implementation) isn't bad for an interstellar robot mission. The challenge is always the stability of the fundin

  • how to manage a project that will last longer than a working lifetime

    This is not a problem we cannot solve: Most engineers that witnessed Voyager 1 launch in 1977 are likely to be retired now. We still collect information from the probe, thank to younger engineers that joined after launch.

  • Assuming you could include on the spacecraft the necessary detection equipment to figure out where the relevant stars are, you could have it compute its own course corrections, right?

  • Is the point to just get to another solar system?
    Or, get to a close one with the highest probability of life?

    In any case, it seems that even unmanned probes are a waste of time until faster communication (Quantum Communication?) is developed.
    A manned mission seems the best option if we must be going soonest.
    Then, while that mission is in route, we will have (likely) developed faster technology and can pick them up on a subsequent mission.
    (Yeah yeah yeah. I know. Greater challenges with generations in

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