[HN Gopher] A 100GW phased laser array for interstellar lightsai...
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A 100GW phased laser array for interstellar lightsail propulsion
(2021)
Author : gus_leonel
Score : 99 points
Date : 2023-08-18 11:16 UTC (11 hours ago)
(HTM) web link (opg.optica.org)
(TXT) w3m dump (opg.optica.org)
| osamagirl69 wrote:
| This paper is from 2021
| [deleted]
| osamagirl69 wrote:
| Kind of odd that they did not reference any of the existing work
| done by the breakthrough foundation/UCSB
| https://www.deepspace.ucsb.edu/projects/starshot
|
| In particular they have a few paper about lab demos of phased
| arrays specifically for breakthrough starshot
|
| https://opg.optica.org/abstract.cfm?uri=ASSL-2021-JM3A.43
|
| https://arxiv.org/pdf/2107.00568.pdf
|
| etc
| xibbie wrote:
| Isn't it literally the first thing they reference in the
| introduction?
|
| > The Breakthrough Starshot Initiative [1] aims to send a probe
| to image a planet orbiting Proxima Centauri and return data
| whimsicalism wrote:
| They literally mention it in the first paragraph.
| tekla wrote:
| Bullshit. We all know that the only reason for this is, is to
| justify Angel killing weapons. I'm all on board for that
| actually.
| danbruc wrote:
| For reference, 100 GW are 3.3 % of the average world electricity
| production in 2020 of 3062 GW, but this is supposedly only
| required for a few minutes.
| dredmorbius wrote:
| The Saturn V S-IC first stage generated 166 GW of power output,
| roughly equivalent to the electrical generation capacity of
| France at the time. Though that was sustained for 168 seconds
| (2m48s).
|
| Power _output_ vs. _time_ comparisons can be interesting. The
| Hiroshima "Little Boy" bomb output about 18 GWh worth of
| energy, in a few milliseconds. A large electrical generation
| plant of 1 GW capacity generates the same amount of electrical
| power in about 18 hours, and is releasing the _thermal_ energy
| in about 2--3 hours at typical Carnot efficiencies.
| civilitty wrote:
| So it only requires 40% of all power produced by California,
| Oregon, Washington, and British Columbia combined [1]? For a
| few minutes? No problem then!
|
| Jokes aside, I don't think there are any interconnectors on the
| planet that can handle that amount of power for even a split
| second, let alone the infrastructure to supply it for minutes
| at a time to a single location. The biggest grid
| interconnectors in the world cap out at a few gigawatts.
|
| [1] https://en.wikipedia.org/wiki/Western_Interconnection
| cma wrote:
| Wouldn't it likely need to charge up capacitors over time
| anyway?
| sp332 wrote:
| You would probably charge some on-site capacitors using a
| slower interconnect. That way you would only need a small
| distance to handle the discharge from the capacitors to the
| lasers.
| teruakohatu wrote:
| The papers call for 10^8 individual lasers. So power
| connectors only need to handle a small fraction of the total
| power.
| PaulHoule wrote:
| How do they deal with
|
| https://en.wikipedia.org/wiki/Thinned-array_curse
|
| ?
| tonmoy wrote:
| According to my understanding this should not apply to lasers
| nippoo wrote:
| The thinned array curse only applies when all the sources are
| coherent - with the modulation system they describe, this is
| not the case.
| bagels wrote:
| By not building a thinned array, presumably. They have 10^8
| elements.
| osamagirl69 wrote:
| This paper assumes a dense array (containing approximately 10^8
| elements)
| blincoln wrote:
| Is an array of 100,000,000 lasers realistic/practical, or is
| this more of an "assuming a spherical cow" theoretical
| exercise?
| osamagirl69 wrote:
| Not really, with current technology such a system would be
| astronomically expensive (~trillions of usd to build the
| laser).
|
| This paper is from 2021 which is right about when the
| breakthrough starshot program ended.
| jacquesm wrote:
| No, this is all theory. There is no way this kind of effort
| will be funded, but the theoretical exercise is
| interesting.
| dsr_ wrote:
| Remember to keep control of the death-ray in the hands of people
| who will not point it at their enemies.
|
| Or their friends.
|
| Or threaten to do so.
| postmodest wrote:
| If we don't use this against the MCRN, they will use it against
| us.
| bagels wrote:
| Thankfully if it's aimed at space, it's limited to shooting
| down planes and frying satellites.
| sleepybrett wrote:
| because you can't turn it 90 degrees?
| yetihehe wrote:
| 100GW? More like vaporising planes and satellites. 100GW will
| heat about 50T of aluminum to boiling temperature in about
| 1s. Of course you won't have perfect heat transfer, but if
| you can send that power for more than 5s, most satellites
| will start actually boiling and vaporising.
| ericbarrett wrote:
| Could you even operate this laser terrestrially? The
| article says optical frequencies (visible light). I think
| the backscatter from atmospheric dust alone would blind
| anybody who glanced at it, and it would be visible for
| miles around.
| [deleted]
| bagels wrote:
| I didn't do the math, but on an array this big, the 'near
| field' might be outside of the atmosphere anyways.
| jandrese wrote:
| It's not one laser, it's 100,000,000 single kW lasers
| spread across the surface of the Earth. A single kW laser
| is already very dangerous and stuff like reflecting off
| of a passing airplane will be a problem, but they'll be
| everywhere so you can't hide.
|
| That's probably unfair. I assume they'll be spread across
| deserts and other places with low population density.
| ericbarrett wrote:
| Missed that, thanks.
|
| I thought maybe attenuation in the atmosphere would be a
| problematic amount of heat, but it turns out the Earth
| gets roughly 200,000 TW of solar irradiance so 100 GW
| would have a negligible (immediate) effect even if fully
| absorbed.
| pajko wrote:
| Imagine when the beam goes off-angle, misses the sail and hits
| some extraterrestrial population lightyears away. Star Wars in
| real life.
| gus_leonel wrote:
| https://iopscience.iop.org/article/10.3847/0004-637X/825/2/1.
| ..
| adhesive_wombat wrote:
| Fusion drive torches are bad enough, but the Alcubierre drive
| "bow wave" is how you sterilise the entire planet face.
| benfarahmand wrote:
| Could these ground based lasers be used for deflecting asteroids?
| r00fus wrote:
| Maybe if said asteroid were fitted with a lightsail?
| kurthr wrote:
| You only lose ~50% using ablation rather than reflection so
| it would work fine without a lightsail.
|
| Long ago there was discussion of using X-Ray lasers pumped
| with nukes as well.
|
| https://www.nytimes.com/1992/07/21/us/star-wars-x-ray-
| laser-...
| scrumper wrote:
| I think far less efficiently than propelling a light sail -
| asteroids being not very reflective. There would be some
| momentum imparted by the photons impacting and more by ablation
| of the surface though (if the laser is powerful enough).
|
| Enough to deflect? A kiss from a kitten is enough to deflect if
| you do it far enough in advance...
| SoftTalker wrote:
| I like the idea of diverting an asteroid by firing kittens at
| it.
| jacquesm wrote:
| Your idea of kissing is a bit violent.
| rokobobo wrote:
| To divert an asteroid, you'd attempt to ablate a part of it,
| rather than just rely on its reflectivity:
|
| https://en.m.wikipedia.org/wiki/Asteroid_laser_ablation
|
| I imagine that tracking an asteroid and continuously pointing
| the laser array at a specific portion of it, will be harder
| than tracking a shiny light sail that's continuously
| illuminated by the laser array--but as a whole, I think
| shooting a laser at an asteroid is one of our best bets, so
| having that array would be a good tool of planetary defense
| jmyeet wrote:
| People often don't realize just how much energy is required to
| reach even the nearest star in reasonable time. Of course it
| depends on how long you're willing to take to get there.
|
| I forget the exact numbers but if you start making assumptions
| like 1G of acceleration up to somewhere between 0.01c and 0.1c
| you have a travel time between 40 and 400 years to reach Alpha
| Centauri and an energy cost per kg of payload in the exajoule
| range. That's as much as 1% of the mass as energy assuming you
| have a perfect conversion of mass to energy. This is getting into
| the realm of all the energy we produce and use for the entire
| planet in a year.
|
| Of course we have nothing like that. We don't even have a
| theoretical framework for that other than possible matter-
| antimatter annihilation but even then you have containment issues
| and have to perfectly convert that to motion in the desired
| direction. That is nontrivial.
|
| Our best hope for doing this and carrying our own fuel is nuclear
| fusion (or possibly by detonating nuclear bombs behind you; yes,
| seriously). Even here the tyranny of the rocket equation [1] just
| kills you from the weight of the fuel you have to carry. Also,
| exhaust velocity limits how fast you can get no matter how
| efficient your energy production.
|
| The only viable way may in fact be to capture energy from the sun
| and focus it at your ship to accelerate it. Photos have no rest
| mass but they do have (and can impart) momentum, which is what
| this article is talking about.
|
| I don't know why they'd be using ground-based arrays. You'll end
| up cooking the atmosphere this way and losing energy if you
| don't. A laser array like this really has to be space-based,
| practically speaking.
|
| Of course you have the issue of how you decelerate at the other
| end. This is a not-insignificant problem that probably rquires
| sending automated probes ahead to construct the necessary
| infrastructure to decelerate you.
|
| So this article uses gigawatt level power to accelerate a
| "spaceship" of a few grams. I'm not sure what you do with that
| exactly but this goes to just how high the energy budget
| requirements are.
|
| Even all the hopium over various FTL methods (eg Alcubierre
| drive, wormholes, space-folding) have no theoretical basis (eg
| requiring negative mass and/or energy) but even if they did the
| energy requirements are completely ignored. If you look into it
| and arrive at any numbers at all it's things like "convert 1
| Solar mass into energy".
|
| [1]: https://www.kallmorris.com/columns/tyranny-of-the-rocket-
| equ...
| semiquaver wrote:
| > Of course you have the issue of how you decelerate at the
| other end. This is a not-insignificant problem that probably
| rquires sending automated probes ahead to construct the
| necessary infrastructure to decelerate you.
|
| But how do you get the probes where you're going in a
| reasonable time frame? You can't use the light sail method
| because of the same deceleration problem. So you're stuck with
| the whole operation taking tens of thousands of years, which
| means you might as well have sent the original payload with
| whatever sublightspeed mechanism you would use to send the
| probes.
| pmontra wrote:
| I can't remember the book (maybe one by Egan?), they
| transported the endpoint of a wormhole to its final
| destination with a normal slow spacecraft and then started
| using the tunnel for FTL. I guess the crew used the wormhole
| all the time to refuel the ship and to go sleeping at home
| :-)
| jmyeet wrote:
| People require life support and don't want to spend 80,000
| years travelling 4 light years. Automated probes don't have
| that problem, although they still need to last that long in
| the hostile conditions of space, which is another not-
| insignificant issue.
|
| At the other end, an automated probe could both accelerate
| and decelerate at significantly higher than 1G if desired.
|
| So automated probes have a vastly wider set of possible
| mission parameters available to them.
|
| You can deploy a solar sail to decelerate as you approach
| another star. This limits how fast you can go as you need
| sufficient time to decelerate. Additionally, you have drag in
| interstellar space anyway just from dust and gas particles
| slowing you down.
|
| Fun fact: to accelerate beyond about 0.86-0.9c your ship
| would need to be aerodynamic because of the decceleration
| effect of the interstellar medium.
|
| What you're really building here is what's often called an
| "interstellar highway". If fusion ever becomes viable you can
| extend this to having waypoints to accelerate or decelerate
| (as required) a spaceship along the path but that's not
| strictly required.
|
| So building such a system will likely take hundreds, possibly
| thousands, of years but ultimately result in something where
| you can travel between the endpoints in a few decades.
| louwrentius wrote:
| I can't be enthusiastic about anything related to space knowing
| that many people - especially in the USA - are homeless or are
| living poverty and have nothing to look forward to, and no death
| doesn't count.
|
| I'm done with all those rich people vanity projects, we have
| enough issues here on the ground.
| switchbak wrote:
| You can use such a justification to argue against any far flung
| efforts of humanity - and many have for a very long time.
|
| This sounds like: we must solve the disparity between rich and
| poor before we invest any efforts elsewhere. Since that's
| clearly not happening, doesn't that imply that we can't make
| progress in science and space?
|
| I find it interesting that often those making such a claim
| don't devote their lives to fixing the problem in the way that
| they're asking for. Sounds like a recipe for cynicism and
| despair.
| louwrentius wrote:
| I answer with yes. People first.
| andromeduck wrote:
| Don't let the perfect be the enemy of the good.
| louwrentius wrote:
| There's nothing good about space stuff.
| philipswood wrote:
| Do you feel the same way about sport:
|
| > Sports-related costs for the Summer Games since 1960 is on
| average $5.2 billion (USD) and for the Winter Games $393.1
| million dollars. The highest recorded total cost was the 2014
| Sochi Winter Olympics, costing approximately US$55 billion.
|
| Or entertainment:
|
| > Whereas the average Hollywood film costs around $100 million
| to produce, the expenditures of the top 11 most expensive
| movies exceeded a whopping $300 million, adjusted for
| inflation.
|
| ?
| louwrentius wrote:
| No
| kraftman wrote:
| We can do both, they aren't mutually exclusive.
| louwrentius wrote:
| We shouldn't, if people are not housed or fed.
| ben_w wrote:
| Pay the poor people to build the vanity projects, solve two
| challenges at the same time.
| ricardo81 wrote:
| Perhaps on the 'todo' list is pulsing over another 100GW laser to
| slow future objects back down from the other direction.
| shireboy wrote:
| This is something my kids and I have wondered about too and
| were hoping some more physics-minded soul here could rule
| in/out:
|
| * Base laser "pushers" on moon, Lagrange points, etc.
|
| * Use moon/earth pushers to push 1st wave lightsail craft(s)
| towards destination. 1st wave craft are also themselves
| equipped with laser pushers aimed back towards earth.
|
| * Use moon/earth pushers to push 2nd wave lightsail craft
| towards destination
|
| * 1/2 way to arrival, 1st wave starts pushing _back_ 2nd wave
| and moon/earth pushers stop pushing.
|
| * 2nd wave "stops" at destination or slows enough to be
| captured in destination orbit.
| cannedbeets wrote:
| For that to work, the 1st wave ships would need to carry a
| power source capable of driving a pushing laser. This
| presents a number of significant challenges:
|
| - the current plans for Breakthrough Starshot[0] are to send
| a centimeter-sized, ~1 gram mass spacecraft.
|
| - even such a tiny spacecraft would require a 4x4-meter solar
| sail and 100 GW of laser power, because photons carry such
| small amounts of momentum
|
| - a ground based nuclear power plant produces a couple GW of
| power. 100 GW of continuous laser power would require the
| entire output of 50+ nuclear power plants
|
| - that amount of power is difficult enough on the ground,
| trying to put 50+ nuclear power plants worth of energy
| generation on a space craft would alone be a tremendous
| challenge
|
| - assuming we could do that, it would weigh unfathomably more
| than a gram. Just the fuel alone in a ground based station is
| several tons; the infrastructure to turn that fuel into power
| is many thousands of tons more... times 50+. It would not be
| impossible for that amount of power generation to weigh a
| megaton.
|
| - assuming it could be launched from earth or built in space,
| and pushed by solar sail, the solar sail needed for a ship
| weighing a megaton would be another level of challenge. If 16
| square meters of sail are needed for every gram, then a
| megaton would require 16x10^12 square meters. That would be a
| manufactured object 4,000km on a side, or roughly the size of
| Russia.
|
| - We haven't accounted for the increased power needed to
| power this craft by sail from the ground. Momentum (let's
| ignore relativity even though we are talking about reaching
| speeds where that starts to be important) is mass*velocity,
| so in order for our megaton-class spacecraft to be pushed
| with the same performance as a 1 gram spacecraft, we'd need
| to scale the momentum delivered by photons with the mass of
| the craft. That would require 10^12 (one trillion) times more
| laser photons, which would require one trillion times more
| energy to produce and 50+ trillion ground-based nuclear power
| plants.
|
| - I think it's safe to say that pushing a megaton-class
| starship with lasers and a solar sail is unfeasible. But
| wait, if we've managed to assemble and launch the craft, we
| don't need the ground based lasers or the sail, we can just
| power the ship using all those power plants on board. [1]
|
| - with a ship that large, we could take a city worth of
| people along for the ride, so is there still a use for the
| giant laser we've brought with us? Maybe interstellar package
| delivery?
|
| [0] https://en.wikipedia.org/wiki/Breakthrough_Starshot [1]
| https://en.wikipedia.org/wiki/Project_Daedalus
| shireboy wrote:
| Nice. Exactly the kind of thought exercise I was hoping
| for.
| danenania wrote:
| How about thinking in terms of solar rather than nuclear? I
| think space-based solar can have similar output to earth-
| based nuclear plants. Very expensive to build and launch,
| of course, and for terrestrial power there's the issue of
| getting the energy back to earth, but for a space-based
| laser it may be the best option?
|
| And for the deceleration with a backward laser idea, could
| the initial sail store some percentage of the laser energy
| that's used to propel it, along with solar energy from the
| target system's star, and use that for the deceleration
| laser? I'm sure that's still prohibitive in terms of mass,
| but probably much better than nuclear or any other fuel-
| based option.
| shireboy wrote:
| Yeah, I was thinking space or moon based solar or solar +
| nuclear on this side. Why beam that energy down through
| atmosphere to earth, convert it to laser, then beam it
| back out? On destination side, I was thinking solar-
| powered (or whatever you call it when the star isn't
| Sol), but I like the idea of the sail storing laser
| energy (or would that break down - can a photon both
| impart momentum and have it's energy stored?) Your waves
| wouldn't have to be equal or limited to 2 waves : 1000
| 100MW wave 1 reverse-pushers could aim at slowing down 1
| wave 2 probe, or a series of waves until wave N is slow
| enough to be caught by the star. Sounds like we're not
| slowing down at another star any time soon, but
| interesting to think about.
| ryandvm wrote:
| They can just rely on technological progress.
|
| By the time the original craft arrives, we will have solved
| high speed interstellar travel and will have been there for a
| couple generations with plenty of time to build the optical
| catcher's mitt.
|
| Hopefully anti-depressants will have made similar gains,
| because there is going to be nothing so depressing as waking up
| from your 300 year interstellar nap to find out there's a
| housing crunch in your new star system.
| sgtnoodle wrote:
| You would probably enjoy the Coyote trilogy. https://en.m.wik
| ipedia.org/wiki/Coyote_(novel)#:~:text=Coyot....
| gattr wrote:
| That's (hopefully) not necessary. The main idea for braking is
| detaching the sail from the payload, then the sail focuses the
| light on a smaller sail/surface on payload's forward end to
| brake it.
|
| Besides, let's not get ahead of ourselves with interstellar
| missions. Scaling this all down for intrasystem propulsion
| would be great as well. Anyone liked what New Horizons has
| shown? How about sending one small probe to Pluto per month
| (with travel time measured in months/weeks instead of years)?
| ceejayoz wrote:
| There've also been proposed flyby-only missions that don't
| have to worry about slowing down. (Unmanned, obviously.)
| toss1 wrote:
| The biggest question is what is the point? A gram-scale probe is
| nice, but how is it going to return _ANY_ useful data at all from
| even the nearest star system 4ly distant? Skimming the paper
| seems to make no mention of it
|
| Also, they're saying that it is 100GW of total power and the
| "ground-based laser array will be need to be kilometers in
| scale". Say we're talking 10 square kilometers, that's 10GW/Sqkm.
|
| With 1,000,000 m^2 per km^2, that's pumping 10 kilowatts per
| square meter up through the atmosphere.
|
| That means a crow-sized bird, with a wingspan of ~18"/50cm x
| 7"/20cm is 0.1 m^2 and will absorb a kilowatt of radiant energy.
| Like spreading it out on top of ten 100-watt incandescent bulbs,
| or directing a 1000-watt hairdryer at it. It won't fry instantly,
| but will rapidly overheat in the few minutes they claim it will
| take to accelerate the craft to 0.2c.
|
| Since they're detecting atmospheric disturbances, perhaps they
| could route around birds by momentarily turning off beams that
| would be wasted anyway?
|
| Even assuming this is a cover for an array to fry satellites or
| incoming ICBMs, it seems kind of frivolous... I'd love to be
| wrong because it'd be cool to generate those kinds of speeds,
| but...?
| aftbit wrote:
| >Accompanied by military escort vehicles and helicopters, an
| ambulance departed with the Fourth Wallfacer. Against the
| lights of New York City, Wade's figure appeared as a black
| ghost, his eyes glinting with a cold light. >"We'll send only a
| brain," he said.
|
| Still too heavy by a factor of 1000 or so. Perhaps we'd be
| better off building this laser array in space, using self-
| replicating robots to assemble cables around a moon of Jupiter
| to siphon power from its orbit in the magnetic field.
|
| >Electricity surges through the cable loops as they slice
| through Jupiter's magnetosphere, slowly converting the rock's
| momentum into power. Small robots grovel in the orange dirt,
| scooping up raw material to feed to the fractionating oven.
| Amber's garden of machinery flourishes slowly, unpacking itself
| according to a schema designed by preteens at an industrial
| school in Poland, with barely any need for human guidance.
|
| From one of the Dark Forest books and Accelerando respectively.
| ByThyGrace wrote:
| Now I wonder how powerful one of these arrays should be before it
| shoves Earth with noticeable momentum. You know, as a reaction.
| ilyt wrote:
| Assuming this guy on quora didn't fuck up math [1], "multiple
| suns worth of photons"
|
| - [1] https://www.quora.com/What-is-the-total-radiation-
| pressure-o...
| adhesive_wombat wrote:
| Incident solar radiation of 1kW/m2 is something like 100
| million gigawatts and the force from that is about half a
| billion Newtons, or the weight of an Iowa-class battleship.
| mpsprd wrote:
| >accelerating it via radiation pressure to 0.2c within a few
| minutes [7].
|
| That probe must be made in unobtainium to handle that
| acceleration. For 0.2c in 5 minutes it's 20394G!
| badrabbit wrote:
| Maybe 0.2%c? Either way, no gravity in outer space so isn't it
| like 0G?
| BenjiWiebe wrote:
| Nope, G forces don't need gravity. If you are in a "0G"
| environment but accelerating at 9.8 m/s2, that's
| indistinguishable from being "at rest" in a 1G environment.
| MichaelZuo wrote:
| Probably a typo, otherwise the writers are very confused?
| thechao wrote:
| Well below the G forces experienced by smart artillery. Solid
| state is pretty solid.
| MichaelZuo wrote:
| Not for sustained minutes, a few miliseconds.
| zardo wrote:
| That's probably worse as the jerk must also be very high
| FredPret wrote:
| I'm not going to do the calculation right now, but I
| wonder what G-force would be needed to deform a steel
| shell, for example.
|
| How much worse would it be if sustained over a long
| period?
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