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A Visionary Project Aims for Alpha Centauri, a Star 4.37 Light-Years Away

nytimes.com

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Re: A Visionary Project Aims for Alpha Centauri, a Star 4.37 Light-Years Away

#231

Post by mliker »

grecy wrote: post #179
yongjik wrote: post #23

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. (I…

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 abou…

> "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.

Re: A Visionary Project Aims for Alpha Centauri, a Star 4.37 Light-Years Away

#232

Post by williamcotton »

grecy wrote: post #179
yongjik wrote: post #23

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. (I…

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 abou…

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...

Re: A Visionary Project Aims for Alpha Centauri, a Star 4.37 Light-Years Away

#233

Post by ars »

leblancfg wrote: post #59
yongjik wrote: post #23

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. (I…

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/a…

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.

Re: A Visionary Project Aims for Alpha Centauri, a Star 4.37 Light-Years Away

#234

Post by ars »

ryao wrote: post #184

Is it going to just do a fly by or will it somehow enter orbit once it reaches Alpha Centauri? How is it going to transmit data back to earth in a manner that would overcome the signal to noise ratio? Will power be solar or nuclear? How would it know when it is there to begin operating, especially if it uses solar power?

Answers to your questions are mostly in the article. Did you read it?

Fly by.

Laser.

Nuclear.

Re: A Visionary Project Aims for Alpha Centauri, a Star 4.37 Light-Years Away

#235

Post by ars »

marcosdumay wrote: post #218
Zikes wrote: post #180

Earlier quoted context omitted.

I'm a bit surprised at the downvotes. I'm no scientist, but I meant this suggestion in earnest. As far as my layman's knowledge goes, I've always understood the Moon's He-3 reserves to be a "vast, as-yet-untapped power resource".

People have to learn how to harvest power from fusion before.

You know how they have chemically pumped lasers?

So it would be awesome to directly pump the laser from H3 fusion.

H3 fusion is actually very easy - I think we could do it today. We just don't have a readily available source of H3 for fuel.

Re: A Visionary Project Aims for Alpha Centauri, a Star 4.37 Light-Years Away

#237

Post by duaneb »

mliker wrote: post #231
grecy wrote: post #179

Earlier quoted context omitted.

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 abou…

> "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.

Re: A Visionary Project Aims for Alpha Centauri, a Star 4.37 Light-Years Away

#238

Post by ridgeguy »

DavidSJ wrote: post #189
ridgeguy wrote: post #157

What about the energy dissipated by collision with the interstellar medium? Won't that slow the probe considerably? Apologies for significant errors and appreciation for corrections in the following hasty and unchecked calcs. Wikipedia [1] says the ISM density ranges from as little as 1e-4/cm^3 for hot, ionized regions to 1e6/cm^3 for cool, dense regions. Let's say it's all neutral hydrogen, which conveniently masses…

> about 3.9e4m^3 every second, which contains about 3.9e11 hydrogen atoms I think this step is off by 1e5. It contains 3.9e16 hydrogen atoms at 1e6/cm^3. So that 631 MJ becomes 63.1 TJ.

I assumed an interstellar medium density of 10 H atoms/cm^3, which is 1e7 atoms/m^3, given 1e6 cm^3 per m^3.

I think the volume swept out by the spacecraft is about right: (3e8m/sec)(.2)6.45e-4m^2 is about 3.9e4m^3/sec.

Then, 1e7 H atoms/m^3 * 3.9e4m^3/sec gives about 3.9e11 atoms/sec hitting the (1 inch square) spacecraft.

Re: A Visionary Project Aims for Alpha Centauri, a Star 4.37 Light-Years Away

#239

Post by ridgeguy »

dogmatix wrote: post #177
jessriedel wrote: post #48

Earlier quoted context omitted.

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 ho…

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)

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