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.

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

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 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.

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.

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!

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

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

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.

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.

We need way better than 99.5%.

"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...

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

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

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.

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

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.

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...

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.

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.

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.

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

The stabilization problem sounds really hard to me.

> 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.

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.

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

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.

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

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

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.

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.

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 }