A 100GW phased laser array for interstellar lightsail propulsion (2021)

https://opg.optica.org/josab/viewmedia.cfm?uri=josab-38-5-1477&html=true

by gus_leonel • 3 years ago

106 89 3 years ago

opg.optica.org

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• 3 years ago

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• 3 years ago

They literally mention it in the first paragraph.

PaulHoule• 3 years ago

How do they deal with

https://en.wikipedia.org/wiki/Thinned-array_curse

tonmoy• 3 years ago

According to my understanding this should not apply to lasers

nippoo• 3 years ago

The thinned array curse only applies when all the sources are coherent - with the modulation system they describe, this is not the case.

osamagirl694• 3 years ago

This paper assumes a dense array (containing approximately 10^8 elements)

blincoln• 3 years ago

Is an array of 100,000,000 lasers realistic/practical, or is this more of an "assuming a spherical cow" theoretical exercise?

bagels3• 3 years ago

By not building a thinned array, presumably. They have 10^8 elements.

benfarahmand• 3 years ago

Could these ground based lasers be used for deflecting asteroids?

scrumper• 3 years ago

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• 3 years ago

I like the idea of diverting an asteroid by firing kittens at it.

rokobobo• 3 years ago

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

r00fus• 3 years ago

Maybe if said asteroid were fitted with a lightsail?

kurthr• 3 years ago

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-...

petrocrat2• 3 years ago

Interesting, I was thinking of them for another use as well: dumping surplus photovoltaic/wind collected energy into space as a thermostat of sorts to help control global average temperatures and buy more time to handle CO2 emissions. Not sure but maybe its cheaper than using batteries for time-shifting that energy to reduce fossil fuels during the transitionary period.

ricardo81• 3 years ago

Perhaps on the 'todo' list is pulsing over another 100GW laser to slow future objects back down from the other direction.

gattr• 3 years ago

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• 3 years ago

There've also been proposed flyby-only missions that don't have to worry about slowing down. (Unmanned, obviously.)

shireboy3• 3 years ago

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• 3 years ago

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

ryandvm• 3 years ago

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• 3 years ago

You would probably enjoy the Coyote trilogy. https://en.m.wikipedia.org/wiki/Coyote_(novel)#:~:text=Coyot....

ByThyGrace• 3 years ago

Now I wonder how powerful one of these arrays should be before it shoves Earth with noticeable momentum. You know, as a reaction.

adhesive_wombat2• 3 years ago

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.

ilyt• 3 years ago

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...

osamagirl694• 3 years ago

This paper is from 2021

dsr_• 3 years ago

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.

bagels3• 3 years ago

Thankfully if it's aimed at space, it's limited to shooting down planes and frying satellites.

yetihehe• 3 years ago

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.

sleepybrett• 3 years ago

because you can't turn it 90 degrees?

pajko• 3 years ago

Imagine when the beam goes off-angle, misses the sail and hits some extraterrestrial population lightyears away. Star Wars in real life.

gus_leonel• 3 years ago

https://iopscience.iop.org/article/10.3847/0004-637X/825/2/1...

postmodest• 3 years ago

If we don't use this against the MCRN, they will use it against us.

adhesive_wombat2• 3 years ago

Fusion drive torches are bad enough, but the Alcubierre drive "bow wave" is how you sterilise the entire planet face.

louwrentius7• 3 years ago

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.

andromeduck2• 3 years ago

Don't let the perfect be the enemy of the good.

louwrentius7• 3 years ago

There's nothing good about space stuff.

kraftman• 3 years ago

We can do both, they aren't mutually exclusive.

louwrentius7• 3 years ago

We shouldn't, if people are not housed or fed.

ben_w4• 3 years ago

Pay the poor people to build the vanity projects, solve two challenges at the same time.

louwrentius7• 3 years ago

Don’t build polluting vanity projects that serve no purpose and pay poor people anyway.

switchbak2• 3 years ago

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.

louwrentius7• 3 years ago

I answer with yes. People first.

philipswood• 3 years ago

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.

louwrentius7• 3 years ago

danbruc• 3 years ago

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.

civilitty• 3 years ago

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• 3 years ago

Wouldn't it likely need to charge up capacitors over time anyway?

sp332• 3 years ago

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• 3 years ago

The papers call for 10^8 individual lasers. So power connectors only need to handle a small fraction of the total power.

petrocrat2• 3 years ago

If it's meant to transfer momentum to a lightsail, then there is no requirement that the power must be delivered to a single point on the electrical grid. Several of these lasers in a smaller form factor dispersed over a rather large geographic region can all focus on a point in orbit and transfer nearly the same momentum as a single laser focused on that point would.

dredmorbius• 3 years ago

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.

jmyeet2• 3 years ago

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• 3 years ago

  > 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• 3 years ago

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 :-)

jmyeet2• 3 years ago

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.

toss1• 3 years ago

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• 3 years ago

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.

mpsprd• 3 years ago

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!

MichaelZuo2• 3 years ago

Probably a typo, otherwise the writers are very confused?

thechao• 3 years ago

Well below the G forces experienced by smart artillery. Solid state is pretty solid.

K0balt4• 3 years ago

Nope. 1 gram spacecraft, 20kg of thrust. Glass is tough.

A 1 gram flat chip of glass with about 6cm2 of area (the proposed “spaceship” design) would withstand even more than steel by nearly a factor of two.

People are freaking out about 20,000g, but you could push a flat disk of glass like that at about 60,000,000g before it failed. Good Glass has a compressive strength of about 1000 megapascals.

badrabbit• 3 years ago

Maybe 0.2%c? Either way, no gravity in outer space so isn't it like 0G?

BenjiWiebe• 3 years ago

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.

K0balt4• 3 years ago

TBF 20,000g is totally doable.

The spacecraft weigh 1 gram each, so that would be 20kg of thrust pressure.

Assuming he spacecraft are nominally 2d in construction (a silicon wafer) and the pressure is evenly distributed, I see no reason why you couldn’t expect them to withstand 20k g acceleration.

20kg isn’t a lot to stack on essentially a chunk of solid glass if evenly applied.

tekla• 3 years ago

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.

A 100GW phased laser array for interstellar lightsail propulsion (2021)

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