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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

#171

Post by TallGuyShort »

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…

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.

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

#172

Post by david-given »

maverick_iceman wrote: post #161
PaulHoule wrote: post #24

Oddly the most practical use for this is as an interstellar weapon. You don't need to solve the problem of communicating back to Earth, you just need a little terminal guidance. If an iPhone weighs 100 g and we use a non-relativistic formula for energy at 1/4c, that is 2.8 x 10^14 joules, a ton of TNT equivalent is about 4.1 x 10^9 joules, so that is a cool 70kT -- The impact velocity would be high enough to break th…

It's actually 0.7kT. A 0.7kT asteroid will burn in the atmosphere of an earth like planet or for an atmosphere less body like moon it will cause a crater with diameter ~10m. Asteroids this powerful are pretty common.

The Chelyabinsk asteroid that blew up over Russia in 2013 was, by comparison, about 500 kT and massed about 13000 tonnes. It blew up at at 30km altitude.

One of these probes, hitting the upper atmosphere, would be so light as to vapourise almost instantly, turning into a expanding plug of high-speed plasma. This would have very little ability to penetrate the atmosphere. I think all you'd see is a flash in the very high upper atmosphere as the kinetic energy was dissipated as radiation and then it'd be gone.

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

#173

Post by ridgeguy »

igravious wrote: post #167
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…

So you're saying they'll slow down significantly due to the friction of the interstellar medium so they'll need some form of propulsion of their own?

Yes, but yk's note that the friction load is a small fraction of the craft's kinetic energy makes me rethink whether the effect will matter much.

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

#175

Post by ridgeguy »

eduren wrote: post #169
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…

Good point. On their website, they list the specific challenges that they need to address, including a page for "4 Photon Thrusters". Would those be able to counteract the deceleration? http://breakthroughinitiatives.org/Challenges/3

Perhaps they might. Looking forward to reading the entire project description when I get home this evening.

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

#176

Post by brianwawok »

TallGuyShort wrote: post #171
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…

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.

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

#177

Post by dogmatix »

jessriedel wrote: post #48
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…

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.

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

#178

Post by TallGuyShort »

brianwawok wrote: post #176
TallGuyShort wrote: post #171

Earlier quoted context omitted.

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.

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

#179

Post by grecy »

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 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.

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

#180

Post by Zikes »

Zikes wrote: post #113
stdgy wrote: post #29

Earlier quoted context omitted.

Not to mention supplying these hypothetical moon lasers with enough power to blast these mini-probes into another star system.

https://en.wikipedia.org/wiki/Helium-3 ought to do the trick.

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".
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