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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

#221

Post by lholden »

I wonder how they intend for the probe to actually communicate back to earth. Flying by Jupiter... The data downlink feed from New Horizons was about 38 kilobits per second. After the Pluto flyby... it was down to 2 kilobits per second and has been decreasing the further away it gets.

Perhaps it could be done with laser? Having said that, I'd think the beams focus would be quite wide by the time it reaches earth. It would have to be both perfectly formed and aimed with absolute accuracy. (I don't work with lasers and may be off on this, so take this paragraph on principle rather than factual).

Maybe with some form of quantum entanglement? Current forms of usable quantum communication still require mediums like fiber optics as far as I understand. Ex: http://www.nature.com/news/quantum-communications-leap-out-o...

And how to keep this iphone sized device powered?

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

#222

Post by firethief »

lholden wrote: post #221

I wonder how they intend for the probe to actually communicate back to earth. Flying by Jupiter... The data downlink feed from New Horizons was about 38 kilobits per second. After the Pluto flyby... it was down to 2 kilobits per second and has been decreasing the further away it gets. Perhaps it could be done with laser? Having said that, I'd think the beams focus would be quite wide by the time it reaches earth. It…

> Maybe with some form of quantum entanglement?

https://en.m.wikipedia.org/wiki/No-communication_theorem

Your other questions are all addressed in an article in The Atlantic: http://www.theatlantic.com/science/archive/2016/04/yuri-miln...

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

#223

Post by jerryhuang100 »

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…

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.

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

#224

Post by imron »

rm_-rf_slash wrote: post #83

Why don't we focus and reflect the sun's rays instead? Or even combine this and the traditional rocket method by putting the emitters in space to catch the sun and push these tiny craft after they've been shot from the Earth by the zillions in a single satellite. That way we deal with two big problems at once: generating laser energy sufficient for escape velocity, and focusing lasers through an atmosphere layer. Bet…

> by putting the emitters in space

They cover this somewhat in the article by saying that emitters powerful enough to power the craft, and that are essentially able to position themselves to point in any direction pose a potential military threat and they're unlikely to get approval for launching such things.

Presumably other space-faring nations would also take issue and look to destroy them (China has already demonstrated it's capable of destroying satellites in orbit using ground based lasers).

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

#225

Post by mrfusion »

david-given wrote: post #158
NoGravitas wrote: post #12

Earlier quoted context omitted.

It would be nice if they could use solar wind to decelerate as they approached. I know there's not enough energy there to insert them into an orbit, but they might get time for a few more pictures?

You can --- you use a thing called a magsail. The short version is: remember the old Bussard ramscoop idea? You use a magnetic field to collect interstellar hydrogen which you then fuse for thrust? Turns out that in our part of the galaxy, you get more drag from the sail than you do from the fusion thrust, so the idea was scrapped. An embarrassingly long time later people finally realised that they'd invented a fuell…

Very fascinating! I had no idea we had such a viable option for braking.

Two questions for you if you're up for it.

Could we use this concept for braking during earth re entry to depend less on complex heat shields?

Why is the idea of a bussard ramscoop not viable?

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

#226

Post by mikeash »

gus_massa wrote: post #207
mikeash wrote: post #20

Earlier quoted context omitted.

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.

Cool trick. It starts to fall apart at higher speeds (at c, the approximation produces 1/2, when it should be 0), but it looks like up to 0.5 or 0.5 it's decent.

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

#227

Post by skykooler »

ChrisClark wrote: post #102
skykooler wrote: post #90

Earlier quoted context omitted.

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!

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

#228

Post by rayiner »

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…

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.

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

#229

Post by bbcbasic »

a_imho wrote: post #203
tachyonbeam wrote: post #30

Earlier quoted context omitted.

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.

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

#230

Post by amiralul »

jim-greer wrote: post #64
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…

The probes would be 1g, not 100g. They're sending the "guts" of an iPhone. 7kT is half of the Hiroshima bomb. Still destructive, especially if you've got a bunch of them.

According to a screenshot I took during the presentation (http://i.imgur.com/pncUg0Q.png), the probes are estimated to have around 200 grams. There's no way you could fit a battery, a RTG, a camera and propulsion system within 1 gram.
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