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Comment by bufferoverflow | Hacker News Reader

Every high power laser propulsion proposal I have seen requires magical materials that don't exist, and aren't likely to exist any time soon.

Even if you have a mirror capable of reflecting 99.999% of light (best dielectric mirror), hitting it with 100GW means it will still absorb 1 million watts. That will melt anything tiny near instantly.

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But you don't need to hit the sails with 100GW to get a few grams of weight up to relativistic speeds right?

1 gram at 0.2c has 1030MWh of energy. So at 1Mw of received power it would take 1030 hours or about 60 days to accelerate 2g to 0.2c.

I believe most plans call for much more than 60 days of acceleration. So less than 1Mw of power needs to be delivered to the solar sail. Realistically the mass will be more than 2g. Lets say they roughly cancel out.

At 99.99% efficiency that would be 100w to dissapate. Seems like a lot, but could be doable.

> 1 gram at 0.2c has 1030MWh of energy. So at 1Mw of received power it would take 1030 hours or about 60 days to accelerate 2g to 0.2c.

If only we could perfectly convert laser light into kinetic energy, this kind of thing would be much easier.

Light has momentum: 1 GW/c is ~3.336 N, but that's when absorbed, by reflecting it (and because of conservation of momentum) you can double that.

6.672 N / 2 grams = 3336 m/s^2 => 5 hours

1 MW/c makes that 60 weeks:

https://www.wolframalpha.com/input?i=0.2c+%2F+%28%281+MW%2Fc...

(I assume the researchers have done all the relevant details or it wouldn't have gotten this far).

You obviously forgot to calculate how far your accelerators would have to be spaced out if you're accelerating that slow.

At 2g and 60 days, your system would have to span 263,632,527,360 km.

That's more than 10 times further than Voyager 1, which is tiny space probe that has been traveling for 47 years.

And that also assumes that your light to speed conversion is 100% efficient and that you can hit the 1-gram target perfectly over 263 billion km.

Sorry, it doesn't work no matter how you slice it.

I've thought the better idea would be to tune the laser beam to resonantly scatter off certain ions that are kept trapped in a magnetic field.

Singly ionized alkaline earth elements (magnesium, calcium) should have very strong resonant absorption, just like neutral sodium, due to the single outer shell electron. If the laser is tuned properly it could even cool the ions, preferentially scattering off ions moving toward the laser beam, reducing their kinetic energy in the rest frame of the vehicle.

The idea of laser cooling might also apply to a solid laser sail.

Is a single probe subject to 100 GW? Isn't that just the output power of the laser array.

If that 100GW is over 1km2, the incident light is 10W/cm2 and mW levels of heating.

1km2 is typical for these ideal to minimise the laser dispersion

And that megawatt will be absorbed in a tiny surface area, no? Given that it's a laser? So even though the sail has plenty of surface area to reject heat, it won't be able to conduct it faster than it vaporizes.

But maybe you could use 500x 1GW lasers distributed around the sail, or use the plume of vaporized material as your propulsion, or have a sacrificial layer of material. I don't have relevant expertise, to be clear, I'm spit balling.

1 megawatt of continuously absorbed power would require a lot of mass to dissipate without melting. But since we're talking about gram-sized objects, there's no chance.

Even kilowatt would be a problem for object that small.

> 1 megawatt of continuously absorbed power would require a lot of mass to dissipate without melting

Hum... I would require a lot of surface area, that's certain. There's no constraint at all at the mass.

Yes, there's a constraint on mass. Surface area is for dissipation. Mass will have to store the absorbed energy until it's dissipated. Mass determines the temperature. And every material has a limit on that temperature.

Starshot which is the above proposal is likely based uses 10 meter square solar sails that are 100 atoms thick.

> In order to reach relativistic speeds, the Starshot lightsail should have an area of ~10 m2 and be kept to a mass of under ~1 gram, which translates into an equivalent thickness of approximately 100 atomic layers ... With radiative cooling being the sole mechanism for passive thermal management in space, we quantify stringent requirements on material absorptivity that enable the lightsail to withstand high laser intensity and prevent excessive heating and mechanical failure.

They seem to think that heat dissipating is within the realm of plausibility

Materials challenges for the Starshot lightsail, Nature Materials, 2018, https://daedalus.caltech.edu/files/2018/05/Materials-challne...

It's a nice idea but surely any variation in mass of the lightsail will result in significant forces which will literally pull the sail apart. And with a thickness of 100 atoms that variation might be just a few atoms. I can't see how this can be manufactured to take such high forces and be so light and thin.

That's why it is a research project, it is hard to do.

No, lasers are rather inaccurate over large distances. So it would be very uniformly spread.

"Laser" actually refers only to the generation technique and the resulting phase coherence of the resulting photons. Lasers don't have to be particularly tightly focused. In fact if you've got a laser pointer at home, there's may be a lens on it you can take off, and there will be quite a spread on it. It is focused down by a lens and if you look carefully at the resulting spot you can see interference speckles from the focusing lens. Without the lens the laser will lack those speckles and you'll get a uniform, much larger spot from the raw laser.

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