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Comment by yongjik | Hacker News Reader

So the whole thing should weigh in the order of several grams, yet it should be able to deflect 100GW of incoming radiation near perfectly (otherwise it will instantly vaporize) while maintaining structural integrity under the acceleration of 60,000g for two minutes?

And whatever laser sources we're using must track this smartphone-sized object during these two minutes across several million kilometers. Perfectly. (It also means that whatever course deviation the object experiences must be anticipated seconds before it happens, because at the end of the two minutes the object will be ~10 light-seconds away.)

If they could pull this off it will be the next Apollo, but I'm skeptical.

Edit: Just realized we have even more problems, about the reflective coatings. If the laser is green, by the time it's flying away at 0.2c, the laser won't be green any more thanks to the Doppler effect! I'm too lazy to calculate, but it will definitely shift toward the red. So whatever reflective coating we use must be able to work near-perfectly over a wide range of spectrum.

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You're right to be skeptical, because this is going to be very difficult and has many challenges that need to be overcome.

But that doesn't mean we should sit here and say "nope, can't be done".

People were skeptical when planes first flew, when it was attempted to put people on the moon, and so on.

Jeez, people were skeptical when SpaceX said they could land a first stage on a barge at sea, and posted endlessly about how it couldn't be done.

Don't be one of those people.

Even if they "fail" to make all of this work and get something to Alpha Centauri, they're still going to vastly expand mankind's knowledge of a lot of difficult space-related technologies, which is a huge win.

This is hacker news, where people are trying to do difficult things to change the world. That's the whole point.

I also am skeptical. Why not instead try to send a probe into the interior of the sun? (because its cooler there than at the point of those lasers) Why not create a survey of the Oort cloud (because its an order of magnitude closer). This project seems cool; but maybe not well thought out.

That depends on how much energy that the space craft reflects. If it reflects all of it, the space craft would not gain any heat at all unless there is some weird quantum effect that would mean otherwise.

Likewise, such a spacecraft could survive the inside of the sun.

I'm not a physicist, but it seems like perfectly reflecting photons in a vacuum is different from being immune to conductive heat transfer in a plasma.

That and reflecting one wavelength vs a wide spectrum.

These people are obviously very passionate about their project, and they're moving forward on something they want to do. If you're so passionate about those projects you mentioned, start making them happen!

Don't sit around complaining about people who are "doing" stuff because they're not doing the stuff you want them to be doing. Get out there and do the stuff yourself!

Good attitude! Still, it doesn't make their project any more likely to succeed. Those others I mentioned - easier by an order of magnitude. It makes them sound insincere.

If nothing else they might work out how to send larger probes to far out parts of our solar system that reach in days instead of months and years.

This is exactly the point of long shot R&D: lesser milestones are useful in their own right. Who knows what else might come of it?

Skepticism could be constructive. Most of time people just want to improve some novel idea in question format. If someone really do not care about it they would not even bother to raise a question.

I do want to raise my hand about how the sails is going to avoid space objects (meteoroid, space dusts... visible, non-visible, or even dark matters) given its large surface.

Outside of Earth orbit, space objects become incredibly rare, and outside the solar system they are rarer still. There are much bigger problems to worry about, believe me.

By the way, dark matter is likely passing through your body right now. It's effects can only be seen on the scale of galaxies.

People were also skeptical of all the people before the Wright brothers who jumped off cliffs and killed themselves. Skepticism is the natural worldview of an engineer, because the universe is incredibly hostile to our ideas.

>Skepticism is the natural worldview of an engineer

This is in a nutshell why me and my (MBA)boss get into arguments.

If those people weren't optimists, they wouldn't have jumped off the cliffs.

> "This is hacker news, where people are trying to do difficult things to change the world."

Hahaha, I say this partially in jest, but Hacker News is basically for people to bash on dreamers' ideas.

> Hahaha, I say this partially in jest, but Hacker News is basically for people to bash on dreamers' ideas.

I completely agree, though I prefer to look at it as "constructive criticism" of every flavor.

I'm skeptical that a guy who made his money by "transferring a variety of U.S.-pioneered internet business models to Russia" [1] and then muscling in on Silicon Valley investments like WhatsApp where he "bought a stake from the founders for $125 million within weeks of Facebook snapping up the messaging giant for $19 billion last year" [2] is capable of this kind of engineering project. He's good at making deals but he's not the kind of roll-up-your-sleeves engineer that should be involved with space exploration. I think he's just jealous of Elon Musk and Richard Branson. This seems more like the Spruce Goose than anything else, and in the same vein as Howard Hughes, I'm sure he'll be looking for a giant government grant to pay for his techno-playboy hobbies.

[1] https://en.wikipedia.org/wiki/Yuri_Milner

[2] http://www.forbes.com/sites/parmyolson/2015/03/25/yuri-milne...

One thing to note is the reported mindset of the program. It doesn't really talk about their expectations around plausability of this, but rather that they seem to be targeting success through large numbers even at low probabilities. They might not care to anticipate/correct anomalous "course deviations" but rather write those off as a loss completely, hoping that just a few of many initially accelerated succeed.

It would be impossible/unlikely to correct any course deviations anyway. First off, even if the beam is collimated to within a degree or something, a degree at even a million miles is a pretty big spot, and you'd probably not be able to hit any specific unit. Even if you collimated it more finely - good luck hitting the target.

The other problem is that the beam would ideally go straight between earth and Alpha Centauri, and the sail just rides the beam. The Earth does rotate around the sun but the parallax this provides (over 3-6 months) is pretty low relative to the distance travelled by the probes over that time. If you try to do selective course corrections (i.e. bump one side harder than the other) then you're going to need a super fine beam that is going to miss frequently and probably hit the target when you hit.

This is pretty much inherently a shotgun approach. You don't get a mid-course correction on the birdshot that misses its target. Or rather, it's like putting out balloons into the wind and letting them drift.

I love these napkin calculations.

The relativistic effect shifts the frequency by 22.47% for 0.2c.

Given a 445nm laser (the powerful blue ones), that's 545nm when redshifted (green). It's a bit of a jump, but that wavelength is still smack in the "solar atmospheric absorbtion spectrum window" -- you won't lose much of the energy in the atmosphere.

Reading this page diagonally [http://www.edmundoptics.com/resources/application-notes/opti...], I don't think that the idea of getting a >99.5% reflectance value for that bandwidth is that far off.

Umm... 0.5% of 100GW is 500MW, or about the power of a nuclear reactor. The spaceship won't survive a millisecond.

We need way better than 99.5%.

Article says it needs to be better than 99.999%

iphone innards - done

99.999% reflective mirror - done (iphone gorilla glass ;)

Yep.

"Use a material with better than 99.999% reflectivity. Usually, highly reflective surfaces are dielectric mirrors, which are composed of ‘sandwiches’ of material, with each layer reflecting back a modest fraction of the total. Each layer needs to be at least a quarter of a wavelength thick. The weight can be reduced by using a monolayer with high reflectivity at the correct wavelength. Based on recent research, this could be achieved by a ‘hole-pocked’ layer, highly reflective for very specific angles where reflectivity caustics arise. (These caustics occur for wavelengths of light that are actually longer than the sheet thickness.)"

http://breakthroughinitiatives.org/index.php?controller=Foru...

IIRC, Robert Forward was writing about sails using dielectric mirrors with holes in them back in the 80's.

Yea, david-given links to one of Forward's old paper elsewhere in this thread.

http://path-2.narod.ru/design/base_e/starwisp.pdf

So it sounds like we'd have a layered sequence of dialectric mirrors (themselves many layers each) each reflecting a different wavelength. The first layer lasts until the laser redshifts out of that layer's efficient reflection range, then vaporizes revealing the next layer tuned to the redshifted laser. That layer lasts until the laser redshifts even further, then vaporizes to reveal the next, and so on.

Sacrificial layers may also help with heat management, I suppose.

Some types of lasers (e.g. free electron lasers) can be tuned to arbitrary wavelengths, so you could maybe adjust the frequency of the laser as the craft accelerates.

Well, that sure is a good point.

Upon further reading, looks like you can achieve >99.9998% reflectance for a 10% wavelength band (in this case, in the infrared). [http://www.crystallinemirrors.com/wp-content/uploads/2013072...]

Still, you're right in saying that it's probably still quite a ways off.

Keep in mind that a single spaceship wouldn't take up the whole beam - for one thing, there would be a bunch of them, and for another, we simply can't focus a laser on something the size of an iPhone from even as far away as the moon. That said, it is still a massive amount of power that needs to be reflected.

I thought this also, but "[...] to keep the beam tightly focused on one probe at a time would require [...]" in the original article points in the opposite direction.

You could have a sequence of lasers each tuned to a slightly different color calculated to reverse the relativistic shift.

You'd probably want to do that anyway to avoid burning up your laser. Also it's easier to make a pulse at that power level vs continuous.

Agreed on the extreme skepticism warranted.

I am imagining these "solar sails" to be essentially spherical, both for structural reasons and so that their response to applied laser power does not depend on orientation. The electronics in precision guided mortars in the US army survive 50,000g.

http://proceedings.ndia.org/C347/Davis.pdf

(This is only for the fraction of a second when they are fired, and I don't know how it generalizes for longer durations.)

I conjecture that the laser profile will be "shaped", if at all possible, so the trajectory of the projectile is somewhat self-correcting: if the projectile starts to get ahead of the planned trajectory, the acceleration will decrease, while if it lags the acceleration will increase. This wouldn't require any realtime feedback by the laser emitter.

With many projectiles and a massive investment in a laser array, there will be some ability to repeatedly attempt this and iterate.

Still, this is extremely aggressive even many decades in the future.

EDIT: Their proposal confirms that the beam will need to be shaped for feedback. ("By modulating the beam modes, get greater uniformity could be attained, and feedback between the nanocrafts and the beamer array would allow real-time adjustments.") No details though.

http://breakthroughinitiatives.org/index.php?controller=Foru...

Rather than the spherical shape a supposed above, they propose a thin sail with some undefined graphene skeleton. No idea how that is supposed to survive 2-minute long explosive accelerations.

http://breakthroughinitiatives.org/index.php?controller=Foru...

F=ma

In this case m ~ 1 gram and a = 0.2c / 120 seconds =499654.097 m/s^2

F = 499654.097 m/s^2 * 1 gram = 500 newtons

However this force is distributed over the whole sail which has an area is 16 m^2, so pressure applied to the sail is 500 N/16 m^2 = 31.25 pascals. This is a fairly low pressure. The pressure change going up or down a couple meters is about equal to this. Your typical cheap garbage bags can withstand this pressure.

Yea, thanks. I think my mistake was in imagining that the sail would need to be pulling a localized payload, so that force would need to be transmitted from sail to payload using a cross-sectional area small compared to the sail. But it sounds more like the payload is integrated into the sail.

> 499654.097

Dude. 9 significant digits? You don't know m to better than 1 or 2 significant digits. Just call it 5e5 and be done with it.

It doesn't change the conclusion, but note that in a relativistic regime you can't use F=ma because the mass is no longer constant. You need F=dp/dt, where p is the relativistic momentum. The Lorentz factor here works out to 1.02, i.e. 510 N, not 500, so not a big effect if I got my maths right.

Spheres would cause a major issue - the laser would need to strike the centre of the circular visible surface.

Otherwise you'd end up with the light bouncing of at angles other than pi, which would direct the probe to the side.

A flat mirror coupled with crazy fast gyros (also powered by the laser) might be stable enough that the mirror holds directed toward the earth.

Would a slightly conical mirror with its apex pointed towards the laser be self-stabilizing in tilt? Kind of like an aircraft with dihedral wings [1] is self-stabilizing in roll - imagine the laser illumination as an analog of the aero lift vector on the wings. If it tilts wrt the laser, the asymmetric illuminated area should counter the tilt.

[1] https://en.wikipedia.org/wiki/Dihedral_(aeronautics)

An excellent idea. But then you have to worry about oscillation and damping.

The stabilization problem sounds really hard to me.

> Spheres would cause a major issue - the laser would need to strike the centre of the circular visible surface.

> Otherwise you'd end up with the light bouncing of at angles other than pi, which would direct the probe to the side.

No, because these lasers won't be focused to a point smaller than the size of the satellite. I'm not sure what the waist will be, but presumably it's much larger than the mirror.

The real reason these are sails is because they maximize the momentum transfer per unit mass. But unlike sails on boats, this force can't be transmitted to a displaced payload using tethers/struts, so the payload must be dispersed and integrated within the sail itself.

> The electronics in precision guided mortars in the US army survive 50,000g

And then are immediately 'disposed of'. That is, quite literally, a fire-and-forget problem; those electronics don't have to keep working for months or years.

Yes but they do have to keep working after those 50,000G. Which is the thing: provided they survive acceleration, the remaining problem is cosmic rays. But we're talking about such small craft here that we'll be able to launch literally hundreds, probably thousands - the real challenge is getting the 100GW laser array built. Once you have it...no reason not to use it.

Similar to XorNot, I don't really see the mechanism by which high acceleration could substantially reduce long-term reliability of the solid state electronics without breaking them completely.

This is analogous to the way train bogies go around corners.

> Edit: Just realized [...] the laser won't be green any more thanks to the Doppler effect!

If the laser is tune-able, you could slowly increase the output-frequency based on how fast you expect the target to be moving at the time the light hits. This would counteract the Doppler effect and supply a consistent frequency to the target.

A laser capable of propelling an interstellar probe is also a mighty space weapon. Some people proposed constructing that on the far side of the Moon so that it hass less of a chance of being used as against Earth targets (intentionally or by accident).

> A laser capable of propelling an interstellar probe is also a mighty space weapon.

Yes, but maybe it could also give us the technology to quickly deflect perilous asteroids... ?

or at least those that weigh about the same as a piece of paper...

That sounds like a good idea... as long as no one develops mirrors :)

The laser is definitely a weapon at the powers they're talking about

Setting incredibly ambitious goals has definitely lead to impressive achievements in the past. I think it's definitely worth studying. Worst case, we'll learn something.

>Worst case, we'll learn something

we might learn something, but we have to use resources for studying and there might just be better and/or more useful alternatives

Side effects of these projects, even if they fail, may be massively valuable. Sometimes solving problem X that isn't a big concern for society will solve problem Y down the track. History of maths is littered with things that were useless when they where first invented, such as imaginary numbers.

I'm still not arguing that and I'm as excited as everyone else when someone comes with a bold idea as this, still, you can't gouge how well the money will be spent and downplaying risks and alternatives of undertaking a multi decade project because we are in the heat of the moment is not wise. Science justice warriors should take a deep breath before pointing fingers imo. We are inherently in a box and I doubt anyone can imagine what technology will look like in 10 years, let alone 40.

And where does the reflected laser light go? Back to earth if it's reflected 180 degree.

And how does a device weighing a few grams emit a signal capable of detection back on earth? The Voyager probes are over half a light-day from earth but they have rather large antennas.

Relevant comment below: https://news.ycombinator.com/item?id=11481689

It goes back in the general direction of Earth. The reflector needs to be highly efficient, but it doesn't need to be perfectly flat.

Freeman's speech was interesting in that regard. He doesn't think Alpha Centauri is a reasonable target for this project and according to him it should not be advertised as such. But he does think that using this technology to send probes in very deep space could both tell us more about what there is over there and help us improve the technology to go beyond and thus to the actual stars.

>If the laser is green, by the time it's flying away at 0.2c, the laser won't be green any more thanks to the Doppler effect! I'm too lazy to calculate, but it will definitely shift toward the red.

Interestingly, you also have to deal with Lorenz contraction - at .2c the ship itself will shorten by about half a percent, compounding the apparent wavelength shift. Admittedly this is a much smaller effect.

Nevermind the issue of how you either (a) transmit back over 4 light years or (b) pull off a return trajectory with just a few grams of equipment. It's easy to poke holes at ambitious projects, so I look forward to seeing what they can come up with, but man - this would be phenomenal compared to Apollo, even given how far we've come.

Why return? I bet you can find a lot of volunteers for a one way mission, especially if you offer to follow them with supply trains every year or so.

Even if this is possible, it'll probably be prohibitively expensive to do constant resupplies when you won't know for years if previous trips have worked. A manned mission or a resupply mission would also be orders of magnitude beyond the masses they're talking about, which strains credibility even further.

I am pretty sure you just sent out a new boat of supplies every year make it or not. Worse case the mission fails and the next mission has 20 supply ships in orbit waiting for them. I have no idea on the minimum mass required to send supplies, i.e. if you can recycle 99% of the water you need, would only need a few drops per day. If you can only recycle 50%, you are going to need a bunch.

Nah - they're already pushing some theoretical limits by talking about planetary levels of energy. You're talking about just throwing money at doing that many times in a row with no proof of success.

The plan calls for ultra-reflective coatings that are tuned to the wavelength of the laser. Apparently these exist, though I have not checked the numbers. In my class I abandoned laser propulsion quickly for just this reason (incinerated the target), but with these coatings it might work.

I'm more worried about where the reflected light is going to go once it bounces off the sails. Presumably it'll fall back on earth, where that amount of energy could cause a lot of damage.

Worst case, if it doesn't work, maybe we can at least use built infrastructure to send new iPhones to Mars expedition.

10 light-seconds is a massive underestimate. If we assume a constant acceleration from zero to 0.2c over two years, the average speed is 0.1c. Therefore by the end of two years it will be 0.1 lightyears away. Deviations will have to be anticipated over a month in advance!

Not 2 years, they want to fully accelerate to 0.2c in 2 minutes!

o.O I misread that, that's incredible!

And what exactly happens with reflected laser?

Solar sails seems more feasible to me.

Its not about result, its about intention.

Intention leads to results (even small)

Small results lead to bigger results (bigger resources, like more knowledge)

Bigger resource leads to greater intentions

And here is why humans are still better than DeepMind, while (true) { learn, invent }

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