Breakthrough Starshot - Mission to Alpha Centauri

Moving that 100-gigawatt laser out of Earth's atmosphere would be a huge plus. That much power and you can be sure it will induce considerable atmospheric turbulence above and beyond what they'll already have to deal with. Adaptive optics will help but won't eliminate the problem entirely.

If the laser were solar powered they could orbit it the L1 Lagrangian point so that it is continuous sunlight. The solar array would be huge any way you look at it. Not including other losses and assuming the fill factor is 100%, at the best PV cell efficiencies (NREL research cells at 46%) a square array would be nearly 15 kilometres on a side. They could also power it with a nuclear reactor assembled in orbit, but probably at huge cost far exceeding that of a solar array.

Interesting idea to use chips and solar sails, though the technical challenges they're facing on nearly all fronts are enormous. Not insurmountable, just very very difficult!

As Bert - my old mentor and an absolutely stellar engineer - used to say: "One's reach should always exceed one's grasp."

The linked article referred to by Bayes clearly indicates the laser will have to be in space. I do not recall what that laser is to be powered by. However, this is not a beam required for twenty years, but only a matter of minutes to accelerate the sail and chip to 20% c (0.2c). They do mention the sail itself must be (1) 99.9999% reflective (no such material yet exists), and also must withstand 60,000 g acceleration (no material has been found with this strength, nor has it been tested).

It would be fantastic for a chip less than the size of a dime to be able to transmit data back from 4-5 light-years away and have a signal/noise ratio at Earth that could be detected. Totally amazing technology is afoot here if we have it or can find a way to make it.

I don't even believe graphene as a single atom thick sail can meet the challenge ahead for the light sail, but I do wish the team good look with this, as the spin-offs would probably translate into a boon for mankind back here on Earth.

According the article, the 100 GW laser at the propulsion end of things is ground-based.

ahem, were we even looking at the same article?

I don't know what article you're looking at, but the diagram is from SciAm's article that Bayes linked to his post (first link).

I went back, apparently, I had the completely wrong impression of the laser array.

I like the solar reflector array idea. There is also a thing called the HELIOS laser that has a pretty high efficiency for any laser. It is a solar pumped Nd:YAG crystal laser. Modifications of the crystal to include Cr, result in even higher pumping efficiency (broader band absorption, still in the range to result in lasing of the base transition).

The Japanese were wanting to install HELIOS on American soil for their proposed magnesium energy economy. It is the only thing hot enough to convert MgO directly back to Mg metal.

Large numbers of small lasers in a large array makes it possible to finely-tune the phasing. It is hard to phase large (probably Q-switched) Nd:YAG lasers and you still need the large aperture to minimise the beam divergence. Otherwise you're back to the same situation as when using independent mirrors and monochromatic light. However they pump the laser is secondary; it's the phasing and the aperture size (and of course the the power) that's key to making it work. As long as they meet the other requirements but pump it directly with sunlight? That'd be cool.

OK, but the other hurdle(s) remain, not the least of which is the ultra-high reflectivity of the sail, ultra-light weight of the sail, and configuration that absolutely prevents the laser beam from hitting the chips and overheating them. Then there are immense hurdles to communicating back from the flyby.

No kidding.

They could deploy the sails 'chip-forward' to protect the chips from the laser light and then turn the sails around after the boost phase so the the chips can communicate with Earth.

Also, what they're showing as a 'representative' chip can't even be close to its actual size - however it's done - considering, for example, that the 'chip' includes a 150 mg radioisotope source. If that's on-chip, that's one damn big chip.

Speaking of radioisotope sources, 150 mg of plutonium (for example) produces a net heat output of about 75 mW. Converting that heat into electricity will incur losses of course, and so we're talking maybe 20-30 mW on a good day. But, on the other hand, they're saying the photonic thrusters and the lasers used for communications will be on the order of 1 watt. Looks like they're going to need to store up that energy in some sort of capacitor and so we've got an even a bigger 'chip.' I don't know about you, but this is beginning to look like a nanosat to me - one needing a bigger sail.

Yep. I think this is a non-starter as such.

I'm wondering what keeps these sails oriented. Any minor variation in reflectance, I would think, would spin them around. The force from all those photons bouncing off the sails would be directed perpendicular to the surface of the sails, which might not be exactly in the direction of Alpha Centauri.

With 60000 g acceleration, things could go awry very quickly.

This discusses some techniques. Also the 'chips' will be equipped with photonic thrusters.

These photonic thrusters could something as simple as an LCD with a big change from specular to diffuse reflectivity, correct? I see that sail orientation and deployment arises from spin during deployment of each chip/sail.

New material: Tantalum carbide - Hafnium carbide has been proven to remain solid up to 4200-4300 K. That should make it easier to tolerate slightly less than 99.9999% reflectivity of the sail itself. Although any temperature in that range would defeat the sail by re-radiation, correct?

The sail is only needed during the boost phase during which anything even slightly light-absorbent in the beam path would heat up, very likely enough so to vaporise - such as the 'chip'? LCDs certainly would.

After deployment from the 'mother ship' it is unlikely the craft will be oriented correctly and will probably have a slight spin as well. These will have to be corrected by the photonic thrusters before they turn the boost laser on it (except where are they getting the power for their photonic thrusters? From that 150 mg radioisotope source? Uh huh). Thing is, even if the sail is oriented correctly, the thrust vector at each point on the sail is aligned with the surface normal at that point (picture a light ray striking a mirror (whether curved or flat), the incoming and reflected light and their momentum vectors symmetric about the surface normal at that point and so the net momentum vector is co-linear with it but pointing in the opposite direction).

As these sails are a thin film it is guaranteed they will have variable curvature - especially under thrust - and those surface normals will look like a concave pin cushion. If the net curvature is asymmetric with respect to the beam axis there will be a torque that will spin the craft around, exposing parts like the 'chip' that aren't supposed to be in the beam. Not only spin, but the craft will buffed about like a dry leaf in the wind. The only way they're going to stop this is by shaping the sail itself in real time. Good bloody luck.

I don't think reradiation would defeat the sail. The sail would be radiating from both sides.

To the extent that the sail is made of a real material of nonzero, the sail should be slightly hotter on the side being heated, suggesting a miniscule net thrust addition might be possible, almost certainly negligible.

The thrust from radiation would be dwarfed by the thrust from the reflected photons not only because of the relatively small amount absorbed but also because twice the momentum is obtained from a reflected photon than from a similar photon either absorbed or emitted.

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As far as materials for sails....perhaps an opaque (reflective? one can hope) plasma generated by the satellite and supported on magnetic fields.

If I understand correctly, photonic thrusters generate equal and opposite forces on two reflecting mirrors. There's no big mystery there. But I don't see how that can be used to propel these light sails unless the light reflects from the sails back to the 100 GW laser and back again multiple times. (This is even more farfetched than the rest of this scheme! )

No, as I understand it they're using them to maintain flight configuration amongst themselves long after the boost phase. During the boost phase only the sail is involved as anything else in the beam would be fried on-the-spot

The ability to fly in an extremely well-controlled configuration is critical to their ability to communicate back to Earth, as the configuration will act as a (3D) phased array transmitter with dispersed elements, one per craft. As each craft cannot itself transmit more than a watt (plus having a tiny aperture), they'll all have to work in concert as a phased array in order to make the resultant beam sufficiently powerful and directional enough to communicate. The communications lasers are separate lasers from the photonic thrusters, btw. Lots of lasers involved at both ends.

Almost sounds like it could be less difficult to quantum entangle transmit the entire array to orbit around Alpha Centauri. (I am surely jesting here, no idea how to make that work.)

"With 60000 g acceleration, things could go awry very quickly."

Yup.

They mention a 150 mg radioisotope source powering the 'chip' (which looks less and less like a 'chip' the more I read).

Now imagine just this source tethered to or attached directly to the sail, a sail perhaps a dozen or so atoms thick spanning 16 square metres and accelerating at 60,000 gees. At that acceleration those 150 mg will exert a force of 9 kilograms on that sail and we're speaking only of accelerating the mass of the source; nothing else. What's going to stop that sail from tearing or buckling as it tries to drag that mass along?

If they put the 'chip' ahead of the sail to protect it from the laser, they will have to mount it to the sail using a rigid framework to keep it from plunging backward through the sail and tearing it. No matter how they do it, that framework is going to deform and unless that deformation (not to mention orientation) is perfectly symmetric with respect to the beam there will be lateral acceleration and torque to deal with as well. Photonic thrusters won't do it any good during the boost phase; a few watts competing with megawatts. They would have to control the deformation and that means actuators, though possibly the sail material itself could do that using some sort of piezoelectric effect.

46% efficient solar cells? How efficient are the lasers? Why not cut out the "middleman" and set up a few square miles of adjustable mirrors on the moon and zap them Archimedes style?

I think that is more plausible than the Earth-based laser system... still need materials that could withstand the heat from non-reflectivity. At those power levels it does require 99.9999% reflectivity, anything less and it goes {poof}.

One little speck of dust on the reflector would generate a lot of local heat. Whether this would destroy the mysterious substance the solar sail is made of, or locally degrade the 99.9999% reflectivity is uncertain, but I would think it probable, and the resulting cascade of laser heating would most likely destroy the sail.

Agreed. A speck of dust it's Game Over - and there will be dust.

Personally I feel they need to re-think this boost phase a bit; possibly a larger sail together with a gentler laser and a longer boost phase. This would also give them more time to detect and make corrections meanwhile. As it stands it sounds like hitting a bathroom tissue with a firehose and hoping it goes where they want - intact.

OK, but set it up like the HELIOS laser, so that sunlight is all that is needed to power the laser.

Ha, I have to give you a GA for shooting down my idea!

"100 million small lasers" is enough to make me skeptical.

Few, if any, of us will ever live to know if it worked, even if it ever "gets off the ground".

If only 7% of China's population built them, one worker per laser, they'd be done in a week. Of course you'd have to order 800 million units to get 100 million working ones but hey!

Been there with a (complex) 4 up shielded-twisted pair cable.

Guaranteed 100% inspected. I guess they didn't understand that we didn't want the 25-35% that failed.

Did they say what constitutes an 'inspection'? I sometimes wonder if 'inspection' to them means "Yes, we counted them as they went into the shipping container," or "Yep, we looked at them and, yep, they're all blue and have those little plastic thingies on the ends."

In other cases they may do a cursory test but then do nothing with the results. "Oh, you mean we were supposed to separate the wheat from the chaff? Why didn't you say so?" '100% inspected' doesn't mean anything on its own.

No, they were supposed to be 100% working.

I had visions of hundreds of peasants in hovels bent over smoking solder pots, making dozens of bad solder joints.

That was long long ago in another life far far away.

Yep. Be careful what you wish for.

Back the phased array 100 GW laser: Why can they not have a laser system in space, thus avoiding atmospheric distortion all together? Why not make each one 2-3 orders of magnitude more powerful? Of course the numbers are still way too much.

The money to put all that expensive hardware in space just isn't worth the "bang" of the project, sorry, but a good telescope beats going there in this case.

We need to have sustained impulse drives running on some sort of nuclear power that can accelerate the chips (as a cluster or not) up to near the speed required by this plan, but be more patient, so that 20 years becomes 200 years? Unfortunately, by the end of 200 years, this whole thing could blow out of proportion, become some sort of weird religion where the followers sit around their space radio (just one I hope), waiting for a ping from the god probes.

Light coming to us from Alpha Centauri, and Proxima Centauri is already coming here at 100% the speed of light. Use it.

I have not worked it all out, but I suspect that even 1 mN force on a 1 gram sized chip could reach amazing speed after about a year. Let me see:

F=ma, thus a= F/m, F= 1g-m/s², m= 1 g, thus a= 1 m/s²

one year = 31,557,600 seconds. v(t)= 1/2 at² +v₀t, for the sake of argument v₀ is negligible, since the velocity of departure can be arbitrarily set to just above that of stable orbit. v(1 year) = 0.5*(31557600)², or 4.9794106 x 10¹⁴ m/sec (not correcting for relativity). c= 2.99792458 x 10⁸ m/sec, so obviously we do not need to accelerate for a full year to reach 0.2c.

t = (0.2c/a)^1/2 = 12,243.2 seconds (give or take), that is 204.054 minutes, 3.4 hours only.

The problem is mass, since the only propulsion system available would require much more mass, or have to develop many, many N of force to arrive at something more than 1 m/sec² , perhaps 1.5 g with a plasma engine up to a certain speed, then utilize the new microwave drive (pulls thrust out of the cosmos?), to achieve final speed. OR maybe once the bulk of the acceleration has taken place, release the chips and sails out of a pod, and then blast them with the high power laser. Right now, I think the sails would disintegrate, and if the laser hit the chip, then that would disintegrate.

doggone it! I forgot to cancel the off-topic marking for my post which, no way is off-topic.

I just want to say one of my favorite things about CR4 is that I'll read an interesting article, put it up in a post on CR4 to share with you guys, and come back a day later and learn all this cool stuff related to the article in the comments. I may not say it every time, but thanks!

Thank you for continuing to post thought provoking topics.

This is an interesting idea on many levels.

I do think there should be some more discussion and eventual buy-in on a global basis if this kind of thing is to proceed.

I'm not opposed to this kind of thing, it just deserves discussion and acceptance by more people that inhabit this planet...more than just a silicon Valley billionaire and a theoretical physicist (to begin with, how about an 'actual' physicist? ).

I realize Voyager 1 and 2, are already speeding away from our solar system, but these are moving much more slowly and not beaming info back pointing to Earth at anywhere near the intensity of this plan.

There is a lot to be said for passive listening. Giving away your existence and location without any idea of what you are giving it away to deserves serious discussion and should not be left to just a couple of guys who think it'll be a hoot.

Yes, but I think if they are there, and hostile, they have had more than enough time to listen in on our communications, radio, and television signals for more than 50 years.

Your point is nevertheless a good one, considering this would put the source of signals nearer their backyard. Perhaps they are even reading this thread. I do not believe we have intercepted the signals, so they have not attempted to reply.

Moving anything with photons is a very inefficient process. Force = Power/c, where c is the speed of light. 1 Gigawatt will produce about 3.3 newtons of force, or about 1/3 pounds. I suppose you can double this if the photons reflect and change direction in keeping with conservation of momentum.

https://en.wikipedia.org/wiki/Solar_sail

Force per unit area = Power per unit area / velocity of light. 100 GW of power will produce a total of about 333 newtons, about 35 pounds of force. To accelerate a mass to .2c within minutes with 35 pounds of thrust means that the mass has to be exceedingly small. I leave the calculation as an exercise to the student as they say in the textbooks.

My bad... 1 GW produces about 3.3 newton or 3/4 pounds, 333 newtons = about 75 lb

We have all been there before. Calculate, check, repeat, then answer. Sort of like measure twice or thrice, then cut.

I measure once, cut twice, then go back to the hardware store for another board.

They're saying an acceleration of 60,000 gees. This together with an applied force of 333 Newtons (just stick with metric. it's easier ) and we get a mass of 5.55 grams for the whole shootin' match. 999.3 seconds to accelerate it to 0.2c (neglecting relativistic mass gain which only accounts for 2% at 0.2c).

Of course it's all going to accelerate in the right direction because everything will work pǝrteɔfly.

Now that's keeping that tongue in cheek, and a civil tongue in that head!

Nice math skills.

Any fine pitch IC technology sent out of the protective envelope of our planet's atmosphere would not survive. The shielding necessary to allow it to survive would be excessive.