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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

#161

Post by maverick_iceman »

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.

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

#162

Post by trhway »

to me the most intimidating problem is to make sure that the the beam is symmetric and sail is perfectly symmetrical reflecting the beam, otherwise it will go in very different direction than intended. Given the high acceleration during the short time period, i don't see how it can be sufficiently corrected for the beam and sail asymmetries. It is like kicking a soccer ball - you'd like to hit a perfect "9" ("upper 90" in US), yet ...

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

#163

Post by KKKKkkkk1 »

Messrs Milner and Zuckerberg are surely aware that their business is perceived as an occasionally creepy timesink that made them enormously rich while adding little value to society. I'm sure they're not happy about this and would want to change this perception. One way to accomplish this is to pay a team of celebrity scientists for the privilege of associating your names with theirs.

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

#164

Post by throwaway_yy2Di »

nilstycho wrote: post #55
avian wrote: post #42

I wonder what plans they have for communicating with a probe light-years away. Consider New Horizons. It has a power budget and antenna gain way beyond something that can be crammed into a few grams. Yet, the bit rate is so low it's still sending back data it gathered in 2015 from an encounter mere light-hours away. Even if the laser propulsion aspect of this works out, I think communicating with something that far a…

“Images of the target planet could be transmitted by a 1Watt laser onboard the nanocraft, in a ‘burst mode’ which uses the energy storage unit to rapidly draw power for the power-intensive laser communications mode. Upon approach to the target, the sail would be used to focus the laser communication signal. “For a 4m sail, for example, the diffraction limit spot size on Earth would be on order of 1000m. A kilometer-s…

    "For a 4m sail, for example, the diffraction limit spot size
    on Earth would be on order of 1000m."
That must be a misprint; the diffraction-limited spot size would be on the order of 10 million kilometers. It's correct when it says a 1 km^2 collector would intercept ~10^{-14} of the transmitted signal.

https://www.google.com/search?q=4+light+years+*+1+micron+%2F...

https://en.wikipedia.org/wiki/Airy_disk#Size

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

#165

Post by david-given »

Zikes wrote: post #120
drabiega wrote: post #74

Earlier quoted context omitted.

As for #1 it's just a matter of logistics. It would actually be a little easier to do from space because you don't have to worry about the effects of the atmosphere, but it would be a pretty massive piece of infrastructure and it's likely that within this time frame it would be vastly simpler and cheaper to build it on earth.

For a laser of the proposed magnitude surely a little bit of air isn't going to make a significant difference. From what I can gather the thing would be classified as a WMD and capable of carving a smiley face on Pluto.

Heat air up enough and it turns into plasma. Plasma's opaque. That's going to do terrible things to your beam focus.

I remember hearing that this is a problem with high-energy laser weaponry, which is why it tends to be pulsed, to give the hot air time to dissipate. But I can't find a reference now.

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

#166

Post by maverick_iceman »

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?

Why decelerate? At 0.2c it takes around 40 minutes to cross 1AU. (Ignoring time dilation which is not super significant at 0.2c.) That gives ample time to take photos from a moderate distance.

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

#167

Post by igravious »

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?

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

#168

Post by yk »

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…

Well a 1 g spacecraft at .2 c has roughly 10^12 J kinetic energy, so it just loses something in the order of 10^-3 of its energy due to collisions with the ISM. (Which should have an order of magnitude less density in the solar neighborhood.)

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

#169

Post by eduren »

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

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