Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

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

nytimes.com

Post reply on HN
Print view View original

201–210 of 274 posts

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

#201

Post by maxaf »

xbmcuser wrote: post #199
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…

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?

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

#202

Post by nardi »

JoeAltmaier wrote: post #197
ryao wrote: post #188

Earlier quoted context omitted.

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.

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

#203

Post by a_imho »

tachyonbeam wrote: post #30
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…

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

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

#204

Post by AnthonyAguirre »

stcredzero wrote: post #115
AnthonyAguirre wrote: post #22

This is a pretty cool initiative — I looked into beamed propulsion a bit while teaching a course this past fall, and it seems to me that if we (or human technologies) are going to reach a star in our lifetimes, this is by far the most likely way. Still very challenging though. For a quite detailed recent treatment of optical/IR propulsion see this paper by Philip Lubim:( http://www.deepspace.ucsb.edu/wp-content/uploa…

I'm disappointed that more people don't mention Robert Forward when this idea is being covered in the media. (Probably because his Usenet posts, papers, and books predate the World Wide Web.)

agreed! The starwisp paper is really very nice.

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

#206

Post by gunn »

nardi wrote: post #202
JoeAltmaier wrote: post #197

Earlier quoted context omitted.

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.

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

#207

Post by gus_massa »

mikeash wrote: post #20
cwkoss wrote: post #16

Earlier quoted context omitted.

To the probes, wouldn't the 20-year flight 'feel' like 16?

Not that much. Time dilation due to speed is sqrt(1 - v^2/c^2). At 0.2c, that's 98%, so 20 years would "feel like" 19.6 years.

(I Agree.)

As a rule of thumb, you can approximate sqrt(1 - v^2/c^2) = 1 - 1/2 (v/c)^2. So if v/c = 2, then the correction is approximately 1 - 1/2 .2^2 = 98% as you said. This formula is easier for back of the envelope calculations and the important point is that it's quadratic. (I always forget the 1/2 :(. )

The GP comment used the wrong correction of 1 - v/c, that is not quadratic so the correction is much bigger.

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

#208

Post by rotexo »

LyndsySimon wrote: post #62
1812Overture wrote: post #26

Earlier quoted context omitted.

Just flip the battery in the laser around and it will pull instead of pushing.

You mean we should... reverse the polarity?

excuse me, but I think you forgot to take the opportunity provided by that ellipsis to put on your visor.

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

#209

Post by paulmd »

0bfus wrote: post #33
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…

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.

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

#210

Post by vacri »

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…

> 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.

Post reply on HN