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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

#151

Post by jessriedel »

gene-h wrote: post #107
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…

F=ma 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…

Yea, thanks. I think my mistake was in imagining that the sail would need to be pulling a localized payload, so that force would need to be transmitted from sail to payload using a cross-sectional area small compared to the sail. But it sounds more like the payload is integrated into the sail.

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

#152

Post by chm »

maxxxxx wrote: post #101
chm wrote: post #99

Earlier quoted context omitted.

From my poor understanding of GR, there will be no limit on the measurements. You just have to take relative motion into consideration when interpreting them.

You move the earth-sun distance in 8 minutes. I am not sure how well you can track with a camera at that speed to get any decent pictures.

Doesn't it depend on how far the object is from you and at what speed you're moving relative to it? And they plan on moving at 0.2c, not 1c.

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

#153

Post by randyrand »

antognini wrote: post #38
RIMR wrote: post #2

So, how is a tiny smartphone-sized probe supposed to transmit data back to Earth from 4.37ly away?

That is a good question. The Voyager spacecraft send out detectable signals with a ~20 Watt signal. This is larger than the ~3 W signal that a smartphone sends, but by less than an order of magnitude. Voyager is at ~130 AU. Alpha Centauri is at ~275,000 AU. With the 1/r^2 decrease in signal, that means the signal from earth will be smaller by a factor of ~3 x 10^6. Now, supposing we have ~1000 of these smartphone pro…

it seems ridiculous to use anything other than a laser IMO.

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

#154

Post by nkrisc »

fapjacks wrote: post #119
noir_lord wrote: post #47

Earlier quoted context omitted.

Travelling at 20% of the speed of light, I think it'll probably bend..

Inadvertently starting an interstellar war, provided they survive the impact on their homeworld.

According to Wolfram Alpha, an iPhone 6 Plus which weighs in at 192 grams would have about 3.558×10^14 joules of kinetic energy at .2c, or about 85 kilotons of TNT. That's roughly 4 times the energy of the Fat Man bomb dropped on Nagasaki. Not the biggest boom in the world, but certainly nothing to scoff at.

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

#155

Post by XaspR8d »

david-given wrote: post #132
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…

Apparently you can't collimate sunlight because it's not a point source. But I've been unable to find an explanation as to exactly why which I can understand, because this aspect of optics is deeply unintuitive, but the magic phrase to search for is 'conservation of éntendue'. If you find one, let me know.

Have you seen the XKCD What If on this topic? [1] It doesn't get into the details of éntendue, and even makes a joke about the topic's complexity, so it probably won't satisfy your technical demands. But I've found the arguments about reversibility (light paths through lenses are reversible, so how could one point map back to multiple points?) and area increase (lenses do not intensify image values but can change the overall image size) intuitive enough to help quell myself and my friend's arguments about the matter.

[1] - https://what-if.xkcd.com/145/

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

#156

Post by Cerium »

lunchTime42 wrote: post #87
comboy wrote: post #76

How is such thing able to communicate back with earth? Doesn't it require a lot of energy to create signal that we will be able to receive on earth (or satelite)?

Same way it got there? Laser focused on earth? Needs good aiming though.

Modulated retro-reflector. We pulse firing a lower power laser at it and it reconfigured the mirror to modulate the returned signal. Essentially an interstellar DLP projector.

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

#157

Post by ridgeguy »

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 1g/mole. A mole is 6.02e23 particles. A single H atom masses about 1.66e-27kg. And let's say the ISM has a hydrogen atom density kind of midway between the extremes: 10/cm^3 (=1e7/m^3).

Let's say the spacecraft presents 2.54cm x 2.54cm (1 square inch) = 6.45e-4 m^2 to the ISM as it moves. I think this is small compared with what the project is proposing, but we can scale as needed.

At 20% of C, the spacecraft sweeps out a volume of [(3e8.2)m/sec](6.45e-4m^2) = about 3.9e4m^3 every second, which contains about 3.9e11 hydrogen atoms, or a mass of about 6e-16kg/sec.

If all those atoms hit and stick to the spacecraft, they all get accelerated to the spacecraft's velocity. At 20%C, relativistic mass increase should be small, so let's ignore it. The energy needed to accelerate one hydrogen atom to 20%C is about 3e12J/atom.

If the spacecraft is hitting 3.9e11 atoms/sec and spending 3e-12J/atom accelerating impacting atoms to 20%C, that's slightly over 1 Watt that's decelerating the spacecraft.

Over a 20-year trip (6.31e8 seconds and assuming no deceleration), that's 6.31e8 W-sec, or 631 megaJoules of energy needed to sustain 20%C because of collisions with the interstellar medium.

A Watt isn't much, but over a 20 year trip, it integrates to a pretty big energy requirement, or a significant deceleration of an unpowered, very low mass spacecraft.

It looks to me like small, light probes won't maintain their high initial velocity very long into the cruise phase without ongoing propulsion.

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

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

#158

Post by david-given »

NoGravitas wrote: post #12
0x74696d wrote: post #9

Sending a swarm of unguided relativistic projectiles across the universe seems profoundly rude.

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 fuelless brake, and the idea was resurrected (but without the fusion drive). The maths are quite plausible and the sail itself trivially simple --- just a wire loop.

http://www.centauri-dreams.org/?p=22138

However, I don't think they'd be compatible with this idea --- I suspect you wouldn't get one big enough to be useful in a one gram package. But estimating the numbers is beyond me. Here's the paper if you want it. http://www.niac.usra.edu/files/studies/final_report/320Zubri...

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

#159

Post by tgb »

slg wrote: post #116
Carrok wrote: post #111

Earlier quoted context omitted.

Because while all we can see is the blinking light, the probe can see the things close to it far more clearly. As the above quote says the probe will "beam back pictures of the star’s planets." We can receive these pictures through the data received from that blinking light, but we can't see the planets in question. Now we can.

But why would we be unable to see the star's planets if we are able to see a probe that small?

You've gotten pretty poor answers so far. I suspect they'd try to communicate in a wavelength where there is little interference. It's one thing to notice "there is a tiny amount of wavelength X light from the vicinity of alpha centauri" and a whole lot harder to notice the difference between two light sources close together at the same wavelength. The latter requires sensitivity to magnitude AND incredible angular precision. It's like seeing a flashlight from a long way off even though you couldn't make out an object that size in daylight. You know the flashlight is on, but you can't resolve it spatially.

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

#160

Post by antognini »

randyrand wrote: post #153
antognini wrote: post #38

Earlier quoted context omitted.

That is a good question. The Voyager spacecraft send out detectable signals with a ~20 Watt signal. This is larger than the ~3 W signal that a smartphone sends, but by less than an order of magnitude. Voyager is at ~130 AU. Alpha Centauri is at ~275,000 AU. With the 1/r^2 decrease in signal, that means the signal from earth will be smaller by a factor of ~3 x 10^6. Now, supposing we have ~1000 of these smartphone pro…

it seems ridiculous to use anything other than a laser IMO.

Almost certainly. But using a laser requires much better aim than a radio transmitter.
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