back to article NASA boffin wants FRIKKIN LASERS to propel lightsails

If you're familiar with the Larry Niven / Jerry Pournelle novel The Mote in God's Eye, get ready for a dose of Déjà vu: a NASA scientist has posted a video on the agency's NASA 360 YouTube channel describing how lasers could send a spacecraft from Earth to Mars in three days. He's perfectly serious, it seems: instead of having …

  1. Joe Gurman

    California geography

    Has there been a major fault zone slip? UCLA = University of California at Los Angeles. Berkeley, aka UC Berkeley = University of California at Berkeley. The two cities are about 370 miles apart, or nearly as far Glasgow is from St. Albans.

    1. Michael Wojcik Silver badge

      Re: California geography

      I saw that and thought maybe UCLA had a Berkeley satellite campus - stranger things have happened in higher ed - but a quick search didn't turn one up. I assume it's a typo for UC Berkeley. Sloppy. Apparently the Masked Downvoter was offended, though.

  2. Ru'

    How do they propose to slow it down when it gets near Mars?

    1. Kaltern
      Trollface

      They'd turn the lasers off...

      1. timblackwell

        30% light speed? Better make sure your space is well vaccuumed.

    2. Anonymous Coward
      Anonymous Coward

      "How do they propose to slow it down when it gets near Mars?"

      Go about and drop the space anchor !

      1. albertron9000

        simple, just detach the first light sail, and bounce the laser beam off it onto the secondary light sail you've just deployed out the back. No problem!

    3. Lobrau

      Presumably, Mars wouldn't be the end goal; just a known distance. I don't think your average man on the street could imagine the distance between our solar system and Alpha Centauri. That said, don't think there'd be any problems with creating an array of lasers around Mars and applying the thrust from the opposite direction as a braking force. Any physicists (armchair or otherwise), feel free to correct me.

      Sid Meir had some good ideas about getting to Alpha Centauri. Maybe NASA should give him a buzz

      1. Rich 11 Silver badge

        That said, don't think there'd be any problems with creating an array of lasers around Mars and applying the thrust from the opposite direction as a braking force.

        How would you power them? If you use solar power then there's an efficiency problem to overcome. If you do that by storing the power ready for release in one short burst as the lightsailship approaches, then you're going to need a significant mass of batteries or capacitor arrays, with all the effort and expense that entails to get them to Mars (although of course they would be reusable). If we're talking about getting a 100kg probe to Mars in 3 days, it's difficult to imagine what that would carry that would make all that effort worthwhile. The only real value of a short trip is when the ship carries people, which would then put it in the 10,000kg class and extend the journey to maybe six weeks (which is just as well, if you care to work out the acceleration to 0.3c over a matter of days).

        1. frank ly
          Happy

          Ah, an answer from an armchair engineering accountant.

          1. Anonymous Coward
            Anonymous Coward

            Yeah, those real-world engineers - always spoiling the fun by pointing out the unicorn shortage.

        2. Dan Wilkie

          Same way you power them here? Send over a big ship first with a nuclear reactor, then use this to shuttle the people back and forth in a shorter time period later on once you've built your initial foothold.

          The runway at Bastion wasn't built by landing a C5 in the dirt.

        3. John Brown (no body) Silver badge
          Coat

          "How would you power them?"

          Supercapacitors!

          1. Michael Wojcik Silver badge

            Supercapacitors!

            Surely graphene figures in here somewhere. And nanomachines, right? Anything is possible with nanomachines.

        4. Vic
          Black Helicopters

          The only real value of a short trip is when the ship carries people

          ...Or when you're delivering a weapon...

          Vic.

    4. werdsmith Silver badge

      "How do they propose to slow it down when it gets near Mars?"

      I'm guessing if they get their navigation right then the thing will swing around Mars when it becomes affected by the gravity.

    5. Anonymous Coward
      Anonymous Coward

      Mars has a thin atmosphere

      so aerobraking is an option for stopping that doesn't rely on any fuel. Although hopefully you're not going too fast and can fold away the delicate sail before the heating starts!

      If however you are travelling at relativistic speeds I think even a thin atmosphere will probably vapourise you v. quickly.

    6. Arthur the cat Silver badge
      Meh

      How do they propose to slow it down when it gets near Mars?

      In the case of Mars and a 0.3c probe, lithobraking is the most likely technique. Pointing a spectroscope at the landing site would tell us a lot about the composition of Mars to considerable depth.

      Icon chosen as the nearest one to deadpan face.

    7. Anonymous Coward
      Anonymous Coward

      "How do they propose to slow it down when it gets near Mars?"

      Not a problem. The red tape resulting from the Environmental Impact Study of the environmental impact of an impact crater resulting from something traveling 30% of the speed of light slamming into Mars would bring it to a standstill.

  3. JimmyPage Silver badge
    Boffin

    It's true - what goes around ....

    older El Reggers may remember a very decent sci-fi/fact magazine of the 80s, called OMNI (had occasional interviews with Feynman, et al). They devoted a lot of column inches to alternative ways of getting into space - including laser sails.

    Sadly the ultimate conclusion then (as I suspect now) was aiming beam at the target - when a 1cm laser beam diverges to 400m over the 250,000 miles to the moon ....

    They even suggested bulking up the laser to shorten acceleration times !

    So, 30 year old "news".

    The upshot - and possible Holy Grail - of laser sails is you are accelerating towards the speed of light. I vaguely recall in terms of efficiency, laser sails the best bet of getting to a decent speed.

    I notice a poster has asked how you slow it down. Back in the 1980s, the solution was proposed to ..... turn it off at the halfway point, and let deceleration take over.

    1. Martin 47

      Re: It's true - what goes around ....

      and let deceleration take over.

      Well yes but unfortunately the 'deceleration' would only take effect when it made planetfall and then may be a tad too abrupt.

      1. Tom 7 Silver badge

        Re: It's true - what goes around ....

        Perhaps the idea is to blast Mars into a highly eccentric orbit so it comes closer once in a while.

        While this may sound interesting I'd rather they just experimented with solar sails first - they look quite promising on their own without sending up laser sharks to chase them around.

    2. Chemist

      Re: It's true - what goes around ....

      "Back in the 1980s, the solution was proposed to ..... turn it off at the halfway point, and let d"eceleration take over."

      Well that shows how well it was thought through !

    3. Dave 126 Silver badge

      Re: It's true - what goes around ....

      >older El Reggers may remember a very decent sci-fi/fact magazine of the 80s, called OMNI

      There's an online reboot of OMNI here:

      https://omnireboot.com/

      More recently, Buzz Aldrin's novel Encounter with Tiber explores this method of laser propulsion.

    4. VinceH Silver badge

      Re: It's true - what goes around ....

      "turn it off at the halfway point, and let deceleration take over"

      That assumes constant acceleration - by flipping it around, you decelerate at the same rate over the same amount of time. That's ideal for when accelerating at around 1g*, for example, so that any occupants can experience normal gravity.

      The suggestion in this article is a more rapid acceleration - getting the craft up to speed in around 10 minutes. Flipping it around half way and decelerating at the same rate would leave the craft drifting aimlessly at the half way point.

      * Back of envelope calculation, hopefully without dropping a decimal place anywhere, and using the average distance from Earth to Mars - which I think is 225 million KM (obviously much closer at opposition): a 1g acceleration until half way, then decelerating the rest should take about 80 hours, and the speed at the halfway point would be 1/200 x C.

      1. Anonymous Coward
        Anonymous Coward

        Re: It's true - what goes around ....

        "by flipping it around, you decelerate at the same rate over the same amount of time."

        Using what braking mechanism ?? The problem is you have the laser to get to you to whatever velocity and then no means of slowing down.

        1. VinceH Silver badge

          @AC Re: It's true - what goes around ....

          "> "by flipping it around, you decelerate at the same rate over the same amount of time."

          Using what braking mechanism ?? The problem is you have the laser to get to you to whatever velocity and then no means of slowing down."

          I wasn't making a suggestion for the method proposed in the article, I was - misreading and - answering the point made by a poster further up.

          The misread was that where the poster said "turn it off at the halfway point" I read as "turn it around at the halfway point" - so I was pointing out the use case for a steady acceleration until halfway, followed by a steady deceleration. (And that I'd misread and mentioned turning the craft around was a part of that should indicate an assumption of an board system of propulsion - not the one the article was about.)

        2. Michael Wojcik Silver badge

          Re: It's true - what goes around ....

          Using what braking mechanism ?? The problem is you have the laser to get to you to whatever velocity and then no means of slowing down.

          Geez, haven't you people ever watched Star Trek? You flip the wires around and reverse the polarity so the laser pulls rather than pushes.

          Reversing the polarity fixes everything.

          On an equally stupid note, remember when SF (including Star Trek, of course, but also e.g. Heinlein's Sixth Column and any number of other stories) was full of "tractor beams"? Ah, the magical technology of yesteryear.

    5. cray74

      Re: It's true - what goes around ....

      I notice a poster has asked how you slow it down. Back in the 1980s, the solution was proposed to ..... turn it off at the halfway point, and let deceleration take over.

      Which is fine if the 100kg probe is carrying its own rocket motor and fuel able to slow it down from 2,540,000m/s (~1% of light-speed, what you get when you head to Mars in 3 days). However, a probe being boosted to Mars from a laser on or near Earth is in a different situation because it doesn't have a laser waiting to halt it on Mars.

      The Alan Forward laser sail braking technique was to release a sail in front of the probe to reflect light back to the probe, providing the necessary reverse thrust. Still, that's a lot of delicate sail operation with 160GW of incident laser light.

  4. Anonymous Coward
    Anonymous Coward

    Would them there lasers be shark mounted ?

    The public has a right to know, and what happens when they unionise ?

    1. hplasm Silver badge
      Coat

      Re: Would them there lasers be shark mounted ?

      "...what happens when they unionise ?"

      They lose their overall charge.

      1. Destroy All Monsters Silver badge
        Coat

        Re: Would them there lasers be shark mounted ?

        The sail will move to a low-wage solar system.

  5. Blergh
    Mushroom

    Satellites

    Persumably they would need to make sure this laser doesn't accidentally target any satellites that might be in the way. Actually on second thoughts does a laser of that power have the ability to fry space debris? If so it would give it something to do while there was nobody heading out to mars.

    Ooh oh... how far is it to the nearest other solar system? About 4 light years? If we can send a satellite at 0.3 the speed of light it shouldn't take too long to go and have a proper look. I'm assuming that once it's up to speed then you stop with the laser.

    1. Paul Shirley

      Re: Satellites

      Also be a little worried about where the backscatter from the sail goes. To survive that much beam energy it has to be reflective and I'm not sure they make sunglasses dark enough ;)

      1. Martin Budden Bronze badge

        Re: Satellites

        Avoid intense backscatter by making the sail a very shallow bowl shape (convex towards us) so that the reflected light spreads out as it comes back.

  6. Paul_Murphy

    Slowing down

    The original idea, as I'm sure people are aware, is that of sending probes to other systems, using the local star as a braking system.

    Sending probes at a good fraction of the speed of light via lasers and light sails (most light sails were supposed to be a measured in tens of meters) was seen as a cheap way of sending things out and exploring the local sphere of space.

    Sending light sails around just our solar system was somewhat more difficult, thought I'm sure there was a Clarke or Niven story about a solar race that used light sails.

    I'm waiting for people to start proposing Rama-class vessels using nuclear propulsion (either bombs or electric ion) though I doubt NASA has the budget for something like that..

    1. Graham Marsden

      @Paul_Murphy - Re: Slowing down

      > I'm sure there was a Clarke or Niven story about a solar race that used light sails.

      Sunjammer aka The Wind From the Sun

      1. IvyKing

        Sunjammer

        I remember racing the story in my brother's Boy's Life magazine back in 1964 and reading "The Wind From the Sun" maybe a decade or so later. Depressing since the first read was almost 52 years ago.

        Kudos to Richard Chrigwin for mentioning "The Mote in God's Eye", though a Niven story of visiting traders had a similar plot device two to three years before "Mote" came out. This is unlike Scientific American that couldn't be bothered mentioning Niven or Pournelle with respect to laser powered lightsails back in the late 1990's - that was about the final straw leading to dropping my SciAm subscription.

  7. SeanEllis

    Starwisps

    The 30% of the speed of light in 10 minutes figure is for a spacecraft of mass 10g, assuming 100% conversion of input energy to kinetic energy (which does not happen). This is not the 100kg craft he then goes on to talk about, which would take a lot longer and have a smaller, but still usefully high, final velocity.

    Robert L. Forward popularised lightsails for interstellar exploration in the 1980s, and came up with several concepts. The "Starwisp" is an ultra-low-mass probe pushed by microwaves. See https://en.wikipedia.org/wiki/Starwisp for more details.

    1. Christoph Silver badge

      Re: Starwisps

      Forward also worked out a way to decelerate at the destination (and even fly back again). You detach the outer part of the sail and use it to reflect the beam back to the remainder of the sail.

      1. psychonaut

        Re: Starwisps

        damn - i thought id just thought of that!

        bloody sf writers, coming over here, stealing our ideas years before we've had them

      2. Captain DaFt

        Re: Starwisps

        "You detach the outer part of the sail and use it to reflect the beam back to the remainder of the sail."

        How exactly would that work?

        The outer part would still be a solar sail, thus you'd still be accelerated, and the remainder of the sail wouldn't even cancel out that acceleration, let alone over all acceleration, since the light it'd be receiving would have already lost energy to the outer part.

        In other words, wouldn't you still be accelerating, just slower with that setup?

        1. Martin Budden Bronze badge

          Re: Starwisps

          Great question. I think the answer might be something along these lines:

          The detached outer parts of the sail are angled to reflect the light *inwards* and back at the central part, which means the force on the outer parts is both forwards and outwards to the sides. The outwards forces don't move the craft because the outer parts of the sail are attached to each other and are merely tensioned by the outwards forces.

          The central parts of the sail push the craft directly backwards.

          So, add up all the forces, and we get slightly more backwards force from the central part than forwards force from the outer parts, with the difference giving tension to the outer parts. The craft decelerates.

          Someone please tell me (politely) if I've completely stuffed this up, as I'm no expert!

  8. DropBear Silver badge
    Trollface

    Well now that we know that spacetime actually ripples detectably, we just need to equip our spacecraft with suitable itty-bitty "rack-and-pinion-like" systems that engage those ripples, and off we go (I can haz the drive named after me and a tax of 0.001% on the resulting space economy boom if it works plz? I'm really modest that way)...

    1. Doctor_Wibble

      > suitable itty-bitty "rack-and-pinion-like" systems

      Better yet, since there's loads of radiation, make something to lock into the waves of that. First create an electromagnetic standing wave on the hull of the ship, then use a strobe to make the wave move backwards or forwards at whatever speed you want, and you can even add harmonics to change gear once you are going a bit faster. And no moving parts so we don't have to invent any space-proof axle grease.

      And watch how it goes, This Is True Science's Ultimate Purpose.

    2. MJI Silver badge

      I think the Culture vessels used something like this.

    3. Dan Wilkie

      To be fair, I'd buy a spaceship equipped with a DropBear Drive - that sounds badass.

      1. Adam 1 Silver badge

        One does not simply buy a space ship with drop bear drive.

        /and anyone who thinks it doesn't sound bad ass simply doesn't understand the true horror of a drop bear encounter. Bloody hell, they make our spiders look tame.

    4. Roger Varley

      Hmmm, you know, "The Drop Bear Drive" just seems to be a little lacking in the pizazz department. You just have to imagine how it would sound in a conversation between Scottie and Kirk to decide if the name would work or not.

      1. allthecoolshortnamesweretaken

        "DBD" works for me...

        1. Captain DaFt

          - "DBD" works for me... -

          Uh... nope! Doesn't work for me.

  9. thomas k

    laser mounting

    Couldn't they just mount the lasers on the rear of the craft and point them forward at the sails?

    1. James 51 Silver badge

      Re: laser mounting

      Every action has an equal an opposite reaction. Assuming everything worked at 100% efficiency the photons would push the ship back (when it leaves the laser) just as much as they pushed it forward (when it hit the sails). What you could do is use the laser as thrust pointing straight out the back but it's hard to see that being better than an ion drive. Perhaps an ion drive could provide the breaking thrust?

    2. Dave 126 Silver badge

      Re: laser mounting

      I think your sense of humour is a touch too subtle for some people!

      That (impossible) concept of 'pulling one's self up by one's bootstraps' is why we 'boot' computers.

      1. allthecoolshortnamesweretaken

        Re: laser mounting

        Münchhausen did it - he said so himself!

      2. Doctor_Wibble

        Re: laser mounting

        > That (impossible) concept of 'pulling one's self up by one's bootstraps'

        Use a pulley next time!

        And the time after that, remember to tie it to something first...

        1. John Brown (no body) Silver badge

          Re: laser mounting

          "Use a pulley next time!

          And the time after that, remember to tie it to something first..."

          ...such as a sky-hook :-)

    3. Alister Silver badge

      Re: laser mounting

      Couldn't they just mount the lasers on the rear of the craft and point them forward at the sails?

      I would be interested in seeing the results of an experiment to test this.

      Please get yourself a sailing dinghy, and set it afloat on calm water. Now, stand in the stern, and blow as hard as you can into the sail.

      Let us know how you get on...

      1. Justicesays

        Re: laser mounting

        No need to do that . Mythbusters already did it:

        http://mythresults.com/blow-your-own-sail

        Turns out it works, but you are better off blowing backwards.

  10. Tom_

    Robustness required

    My maths are probably out, but if you want to accelerate something to 0.3c in 10 minutes then you're looking at ((0.3 * c) / (600 s)) / (9.8 ((m / s) / s)) = 15 295.5336g. Highest survived g force I have heard of was Kenny Brack's 214g crash in Indy car, but of course, that wasn't for ten minutes.

    1. emmanuel goldstein

      Re: Robustness required

      With all due respect, RTFA.

      A 100Kg probe is not likely to be carrying anything alive, so chat about "Highest survived g force" is not relevant.

      1. Steven Roper

        Re: Robustness required

        The probe doesn't have to be carrying anything living for this to be a problem. An acceleration in excess of 15,000 G is going to turn pretty much any equipment on board plus the probe carting it, into an atomically thin sheet of graphene foil in very short order.

        1. cray74

          Re: Robustness required

          An acceleration in excess of 15,000 G is going to turn pretty much any equipment on board plus the probe carting it, into an atomically thin sheet of graphene foil in very short order.

          10,000 to 50,000G are commonly experienced and endured in cannon-launched guided projectiles. Of course, robust steel projectiles with solid state guidance are enduring it for a fraction of a second. I don't see a filmy laser sail carrying any payload at 15,000G.

      2. Tom_

        Re: Robustness required

        I did read the article. I just added the part about survaivable acceleration to illustrate the scale of the forces involved. I'm sorry if that was a step too far.

  11. This post has been deleted by its author

  12. M7S

    From the Aliens' point of view - Frickin' lasers....

    A little box comes hurtling towards us from the planet of the "ugly bags of mostly water".

    We send out a welcoming committee to greet it and to welcome you Earthlets to a new era of interstellar travel and co-operation (now you've cracked this we might even let you into the FTL club with a bit of tech transfer), put up a few banners, maybe some ticker-tape and all our schoolbeings having a day off to float alongside the predicted approach path, wave tentacles and study not only interstellar politics but also compare it to our own technological ancient history.

    As the box approaches, the precise aim of the previously accurate and invisible laser behind drifts infinitesimally and strikes down our peaceful (shields down) and rather delicate schoolbeings, in fact the phased array acts more like a gatling gun, "ploughing the road" as the payload hurtles towards us with the subtlety of Beagle 2.

    Wrath might be incurred. Delicate, almost ethereal beings we might be but once we don our Lemmium armour and get our emotions running high, we're pretty invincible.

    Still, it could be worse, the box could decelerate like Project Orion. Nothing says "we come in peace" like nuking the approach to the landing site.

    Icon? So many I could choose....

    1. Michael Wojcik Silver badge

      Re: From the Aliens' point of view - Frickin' lasers....

      Still, it could be worse, the box could decelerate like Project Orion. Nothing says "we come in peace" like nuking the approach to the landing site.

      Neal Stephenson wrote a little novel about that one. More or less.

      Personally, I'm not terribly worried about the scenario you describe. If the aliens are close to Earth, they'll know we have Fricken Lasers. If they're far away, the beams will be too diffuse to bother anyone. Drifting off target will be irrelevant.

      And, of course, Space Is Big. The odds of accidentally (or, for that matter, deliberately) hitting a valuable object at any significant distance from the laser are, shall we say, not large.

  13. jake Silver badge

    "photons may lack mass"

    No, they do not. Light has mass.

    Side-note: How do you put on the brakes?

  14. LisaJK

    With that amount of light energy, would't heating of the sail be a problem?

    Even the cleanest reflective surface will have imperfections and dust will stick to it, to increase energy absorption.

    One further fly in the ointment, anyone or anything getting in the way of the beams or reflected light (be careful where you point that sail), will get rather hot, to say the least!

    I guess slowing down could be achieved by swinging around the planet and using the beam to then slow it. Not sure about the G of that manoeuvre though.

    1. Destroy All Monsters Silver badge
      Headmaster

      Re: "photons may lack mass"

      Jake post coming from another universe (as usual)!

      In this universe:

      > The quantae of the electromagnetic field have zero rest mass (I hear they are actually superpositions of weak and electromagnetic bosons, so this is a bit more complex than expected)

      > The energy content of said field will manifest as gravitational mass in the usual Einstein field equations

      1. nijam

        Re: "photons may lack mass"

        > quantae

        ?

        The plural of quantum is either quanta (if you're speaking Latin) or quantums (if you're speaking English).

        If the former, then the plural of (the already plural) quanta *might* be quantae, just as the plural of data is datae... oh, wait

      2. Brewster's Angle Grinder Silver badge

        Re: "photons may lack mass"

        "(I hear they are actually superpositions of weak and electromagnetic bosons, so this is a bit more complex than expected)"

        And what universe are you in?! People, we need to agree to rendezvous in the same universe.

        1. Mark 85 Silver badge

          Re: "photons may lack mass"

          And what universe are you in?! People, we need to agree to rendezvous in the same universe.

          Why should we be in the same one? That takes all the fun out it.

          1. Brewster's Angle Grinder Silver badge

            Re: "photons may lack mass"

            Well, I'm in the quaint little one where electroweak symmetry breaking has taken place and light travels at the speed of light.

      3. jake Silver badge

        @DAM (was: Re: "photons may lack mass")

        "Jake post coming from another universe (as usual)!"

        So where do you live? I live in the observable universe ...

        "zero rest mass"

        Light, by definition, is not at rest. See: gravitational lensing.

        1. Martin Budden Bronze badge
          Joke

          Re: @DAM (was: "photons may lack mass")

          Light, by definition, is not at rest.

          It is after you turn the switch off at bed time.

          1. jake Silver badge

            Re: @DAM (was: "photons may lack mass")

            "It is after you turn the switch off at bed time."

            For very small values of "rest".

        2. Anonymous Coward
          Anonymous Coward

          Re: @DAM (was: "photons may lack mass")

          "gravitational lensing."

          'bending' of light by mass is due to the curvature of the local space-time by the mass - it doesn't require light to have mass

    2. This post has been deleted by its author

    3. Captain DaFt

      Re: "photons may lack mass"

      "No, they do not. Light has mass."

      Obligatory: "Geez, I didn't know Light was Catholic!"

    4. Michael Wojcik Silver badge

      Re: "photons may lack mass"

      Partly this is a terminological issue. In contemporary physics, "mass" is almost always used to mean "invariant mass", which is what people used to call "rest mass". In modern terminology the mass of an object is its invariant mass, which is zero for a photon.

      Partly it appears to be confusion about the effect of gravity on light, as other posters have noted. This is usually explained by noting that it's not simply mass which is coupled to gravity, but the energy-momentum vector of a particle. And a particle can have momentum without having (invariant) mass, as noted in the article.

      Sometimes people simply say something like "all particles follow geodesics in general relativity", but that's more of an observation than an explanation. (It's like saying the Pauli Exclusion Principle is primarily responsible for the volume of matter, whereas the PEP is really a description of the exclusive behavior of fermions, which is what's actually responsible. It's not like matter was a lot denser before Pauli formulated the principle.)

      Anyhoo, that's my non-physicist take on the matter. Correct away!

  15. kryptonaut
    Go

    Employment opportunity

    Maybe the pillocks who like to shine lasers at aircraft could be more gainfully employed as part of an exciting space mission.

    - "Stand here, hold this, point it... that way"

    1. noominy.noom

      Re: Employment opportunity

      Damn you Kryptonaut. You owe me a keyboard!

    2. Michael Wojcik Silver badge

      Re: Employment opportunity

      Yes, but then they could be more gainfully employed by pointing the lasers at themselves.

  16. David Roberts

    Light house?

    Thinking about this now, they are talking about a planet bound laser??

    So once you have planned the exact destination of the probe and hoisted the light sail then as the earth revolves below you once a day the beam comes on for a few seconds (to burn a path through the atmosphere and any passing seagulls) then the beam flicks across the sail and gives you a little nudge?

    Must brush up on my Motie, but I assume realistic propulsion requires a static base up in space with a large energy source to drive the laser and also enough mass that it does not fly off in the opposite direction propelled by its photon drive.

    This also makes me wonder (given that the propulsion from our sun is deemed woefully inadequate) how many times you would have to slingshot around your target system before you could capture enough photons to slow you down.

    Finally, if you can generate enough energy to accelerate your tiny probe to 1/3 light speed using a static laser, why is this better than rocket propulsion for at least the initial boost? I thought the main reason for a light sail was that it freed you from carrying a lot of reaction mass with you because it was supplied by the Universe but the downside was that you accelerated and decelerated very slowly. Laser boosting gave you an extra push out of your solar system but with issues later on when you wanted to slow down.

    1. werdsmith Silver badge

      Re: Light house?

      I would think that the laser would be deployed in space at a lagrange point like the James Webb telescope is going to be put.

      I think the right trajectory around a target planet would allow the gravity to contribute toward slowing the body down.

      I expect in practice the thing would begin its journey by rocket. It also might carry an amount of fuel to burn positional thrusters.

      1. Sweep

        Re: Light house?

        Read. The. Fucking. Article.

        1. David Roberts

          Re: Light house?

          The fucking article says honkingly large earthbound lasers.

          1. Sweep

            Re: Light house?

            "Generating all of the thrust the 100 kg spacecraft needed in ten minutes or so...."

  17. Esme

    At .3c,the only thing that'd stop it, anywhere, would be impact. Th etrajectory would barely be influenced at all if aimed for a flyby of Mars. Even Jupiter probably wouldn;t affect it much - the probe would travel 2/3 of Jupiters diamter per second, roughly. Jupiter's 'surface' gravity being about 2.5 times that of earth, then the acceleration sideways due to Jupiters gravity at the point of closest approach would be about 25m/s/s With 0.3C being about 100,000,000 m/s, even Jupiter would barely deflect the probe from its course.

    1. Francis Boyle Silver badge

      It's all done with mirrors

      Or at least it was in Charles Stoss' Acellerando. I forget the precise details but it involved dropping part of the sail and using the reflected the light for deceleration. Of course if you're just going to Mars you'd probably want to send the reflector ahead and you'd still have to solve the problem of making the system work at a distance of more than a few hundred km.

  18. Paul Slater
    Happy

    This is so simple. I mean, it's not rocket science is it?

  19. allthecoolshortnamesweretaken

    Okay, let's do this!

    BTW, I have heard it before, we actually talked about this in my physics class at school in the early 1980ies. Looking back my teacher was much more cooler than I gave him credit for at the time...

  20. Prst. V.Jeltz Silver badge

    all this based on 100kg load? its parcel delivery not space migration

    1. Anonymous Coward
      Anonymous Coward

      "its parcel delivery not space migration"

      Good thing or we would have to build a wall around the earth to stop it.

  21. Vinyl-Junkie
    Coat

    I'm very disappointed...

    Two pages into a thread full of ideas about laser-powered lightsails, of which some are definitely less sensible than others, and no-one's mentioned Crazy Eddie yet...

  22. Dabooka Silver badge

    All this talk about slowing from 30% lightspeed misses one point

    It doesn't actually HAVE to travel that quick. Surely the theory is using these figures to give an impression of what is achievable rather than feasible?

    You know, a bit like my car can do 120mph + but it sure as hell won't get anywhere near that in it's life! Otherwise all very interesting and hopefully will lead to something useful being estbalished

  23. Tikimon Silver badge
    Angel

    If you're going to reference Niven, don't forget The Kzinti Lesson!

    The Kzinti lesson is, "a reaction drive's efficiency as a weapon is in direct proportion to its efficiency as a drive." In other words, "launching lasers" would make awesome laser cannon weapons as well.

    Which would also give us some nice asteroid defense capability, would it not? We're not really likely to be attacked by aliens, but a chunk of space debris smashing down is uncomfortably possible.

    1. Vinyl-Junkie

      Re: If you're going to reference Niven, don't forget The Kzinti Lesson!

      ***SPOILERS*****

      IIRC in The Mote.... the light from the lasers went out because one of the Masters re-purposed the launching lasers as a weapon in a Motie war.

    2. Toltec

      Re: If you're going to reference Niven, don't forget The Kzinti Lesson!

      If the laser is space based and solar powered when you are not using it to launch space probes you could send the beam down to a solar collector array* on the surface. Near IR would get most of the energy through the atmosphere, though the rest might contribute to global warming and some odd weather effects...

      * Or an enemy city.

      Hey, a reason to wear tinfoil hats!

  24. Arthur the cat Silver badge
    1. Mark 85 Silver badge

      Re: Somewhat related xkcd

      And then there's Dave Barry's take from some decades ago: http://primepuzzle.com/poe/poe-1-90/ELECTCTY.TXT

  25. cray74

    Feel the Powah of the Bright Side

    The illumination required to get a 100kg probe to Mars in 3 days is "more than 100GW," give or take (I came up with 160GW based on solar sails and a 1G flight). However, that's the power of the photons landing on the probe's sail.

    Lasers are not devices noted for their efficiency in converting energy into photons. Industrial CO2 lasers run in the 1% range; some are below 0.1%; and some solid state lasers nose into the 25% range. Using the 25% and 100GW values, you need a power plant on par with most of the US's regular electrical output, or Europe's. Slight drops in laser efficiency quickly push you into "as much electricity as is generated by the entire planet."

    And that's assuming you waste no laser light with focusing issues across tens of millions of kilometers.

    Drawing from the paper:

    As an example, on the eventual upper end, a full scale DE-STAR 4 (50-70 GW) will propel a wafer scale spacecraft with a 1 m laser sail to about 26% the speed of light in about 10 minutes

    I really have to wonder what happens to a viable, 1-meter laser sail (aluminum on kapton?) when 50 to 70GW falls on it for 10 minutes. The National Ignition Facility tends to evaporate the surface of its final aluminum mirrors with its nano-second pulses.

    1. John Brown (no body) Silver badge
      Coat

      Re: Feel the Powah of the Bright Side

      Yeahbut....fusion power is only 50 years away!

      1. Anonymous Coward
        Anonymous Coward

        Re: Feel the Powah of the Bright Side

        "Yeahbut....fusion power is only 50 years away!"

        But that is only 47.7 years at 0.3c!

  26. Anonymous Coward
    Anonymous Coward

    Dangerous weapon

    What's to keep the owner of the LaGrange Laser from pointing it at the Earth and propelling the 100kg package at .3c into some.. inconvenient real-estate? This may not be built due to existing space weaponization treaties.

  27. Tempest8008

    Not sure of the math

    REALLY not sure of the math, but what would we be looking at regarding the destructive power of a 100Kg projectile moving at 0.3C?

    Is this a feasible asteroid defense?

    Or would it just be better to point the laser and shoot?

    1. cray74

      Re: Not sure of the math

      REALLY not sure of the math, but what would we be looking at regarding the destructive power of a 100Kg projectile moving at 0.3C?

      Relativistic projectiles begin approaching the yield of an equal mass of matter/anti-matter. At 0.3c, plug it into the kinetic energy equation: E = 0.5 x 100kg x (30% light speed in m/s)^2 = 4E17 Joules. With a bit of rounding, that's 100 megatons of TNT.

      Note that if you were aiming this at Earth from a LaGrange point (as suggested in another post), you wouldn't have enough distance to boost the projectile to 0.3c. 15,000G's can cross 400,000km in 73.77 seconds. (Huh, I was expecting a bit longer.) The projectile will only reach 0.036c in that time.

      It might be a viable asteroid defense, though I'd be curious to see how a c-fractional projectile interacts with a rubble pile asteroid. At 0.3c and 100 megatons per 100kg, you've kind of left the realm of merely kinetic projectiles behind and start behaving like a 100kg blob of atomic particles when they pass into the asteroid. Energy transfer would also occur quite differently than in an equivalent nuclear bomb. You'd probably end up with most of the energy transfer occurring at some depth into the hide of the asteroid like a near-surface or underground nuke, which would turn most asteroids smaller than a Dino-Killer into a killer cloud of gravel.

      The laser itself is a more versatile tool for dealing with asteroids. Direct vaporization wouldn't be completely feasible for large (1km+) asteroids without months of adiabatic heating by a ~100GW laser, and adiabatic heating won't occur in a glowing-hot asteroid. But there's a lot you could do with such a big laser.

      You can gently pressure the asteroid at lower power and low power densities, avoiding catastrophic off gassing or break ups. Or you can crank it up and burn small spots to generate a plume and thus propulsion, limited by the integrity of the asteroid. You can deal with individual fragments or even illuminate an entire cloud of gravel. And you can spend days and months working on an asteroid.

      The amount of photon propulsion from such a big laser is nice but if you have a lot of mass you don't mind vaporizing you can greatly crank up the thrust available. 100GW might deliver ~200lbs of thrust for a photon sail, but if you're vaporizing rock and water at ~1,000m/s exhaust velocity (Specific impulse of 100) you could get, like, 204 million pounds of thrust. I don't think that estimate if off by more than an order of magnitude.

      1. Chemist

        Re: Not sure of the math

        "At 0.3c, plug it into the kinetic energy equation"

        You are quite correct with the calculation but it's worth noting the relativistic energy is slightly higher even at 0.3c - at higher velocity it will be much higher.

        e.g at 0.8c it's ~2.8e18 J

  28. Allan George Dyer Silver badge
    Black Helicopters

    John Glenn had it easy...

    He was just sitting on top of two million parts -- all built by the lowest bidder. Future astronauts will be sitting on top of two million parts -- all built by the lowest bidder with a freaking big laser fired at them.

    Icon - almost the laser warning sign.

  29. Unicornpiss Silver badge
    Pint

    More useful for unmanned probes?

    I think that this would be more useful for a flyby of our closest stellar neighbors, if you could work out how to not have it destroyed by micrometeors, etc. Imagine doing a flyby of Alpha Centauri in 12-15 years instead of the hundreds it would take us with current propulsion tech. Of course the wait, including years for the data would be formidable.

  30. Sean o' bhaile na gleann

    ...and if you're not familiar with "...Larry Niven / Jerry Pournelle novel The Mote in God's Eye..." try Robert L Forward's novel "Dragon's Egg". There's a lot in there about travelling by lightsail - including a decent explanation of decelerating by detaching a large part of the sail and making it reflect light back on to the remaining part.

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