A 100GW phased laser array for interstellar lightsail propulsion (2021)
opg.optica.org
opg.optica.org
That probe must be made in unobtainium to handle that acceleration. For 0.2c in 5 minutes it's 20394G!
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.
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.
How much worse would it be if sustained over a long period?
People have a bias against high forces because they’re made of meat.
Or their friends.
Or threaten to do so.
That's probably unfair. I assume they'll be spread across deserts and other places with low population density.
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.
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.
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.
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...?
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.
https://en.wikipedia.org/wiki/Thinned-array_curse
?
This paper is from 2021 which is right about when the breakthrough starshot program ended.
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
> The Breakthrough Starshot Initiative [1] aims to send a probe to image a planet orbiting Proxima Centauri and return data
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.
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)?
* 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.
- 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
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.
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...
> 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.
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.
- [1] https://www.quora.com/What-is-the-total-radiation-pressure-o...
Enough to deflect? A kiss from a kitten is enough to deflect if you do it far enough in advance…
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
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-...
I’m done with all those rich people vanity projects, we have enough issues here on the ground.
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.
> 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.
?
But big sports make people happy and they're only expensive compared to individuals, not compared to economies, so I don't try to campaign against what (sometimes little) joy other people can get in their lives.
Films are kinda weird now Starship — a billionaire's vanity project that doesn't have any current reasonable economic purpose (given SpaceX is already 75% of total planet-wide tonnage to orbit not just US) — is getting close to the point that Moon (2009) could reasonably be filmed on the actual Moon for the film's original ($5m) budget.