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

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?<p>And whatever laser sources we&#x27;re using must track this smartphone-sized object during these two minutes across several million kilometers. Perfectly. (It also means that whatever course deviation the object experiences must be anticipated <i>seconds before it happens</i>, because at the end of the two minutes the object will be ~10 light-seconds away.)<p>If they could pull this off it will be the next Apollo, but I&#x27;m skeptical.<p>Edit: Just realized we have even more problems, about the reflective coatings. If the laser is green, by the time it&#x27;s flying away at 0.2c, the laser won&#x27;t be green any more thanks to the Doppler effect! I&#x27;m too lazy to calculate, but it will definitely shift toward the red. So whatever reflective coating we use must be able to work near-perfectly over a wide range of spectrum.

This is a variation of the old Starwisp idea from 1985:<p><a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Starwisp" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Starwisp</a><p>tl;dr: a lightweight (about 1kg) vehicle made out of carbon wire mesh. It acts as a microwave mirror. You both use this for propulsion, by blasting it with microwaves from a Sol-system maser cannon, and for data recovery; its sensors cause the reflected microwave signal to be perturbed based on what its sensors see.<p>So, you accelerate them (in bulk) at about 2G up to .1c. You ignore it until it&#x27;s about 80% of the way there, and then for fire the maser at it again; the beam reaches the starwisp as it passes through the target system and powers the sensors. A few years later you read back the return signal.<p>There were a whole bunch of technical problems with it, not least how to build a microwave lens 560km across, but it&#x27;s still vastly more plausible than trying to push steel cans across the interstellar gulf. I&#x27;d be really interested to see if this version works.

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 <i>very</i> challenging though.<p>For a quite detailed recent treatment of optical&#x2F;IR propulsion see this paper by Philip Lubim:(<a href="http:&#x2F;&#x2F;www.deepspace.ucsb.edu&#x2F;wp-content&#x2F;uploads&#x2F;2015&#x2F;04&#x2F;A-Roadmap-to-Interstellar-Flight-15-h.pdf" rel="nofollow">http:&#x2F;&#x2F;www.deepspace.ucsb.edu&#x2F;wp-content&#x2F;uploads&#x2F;2015&#x2F;04&#x2F;A-R...</a>)<p>For a thorough, if somewhat outdated, treatment of the “starwisp” idea using microwaves rather than optical&#x2F;IR lasers, see this paper: <a href="http:&#x2F;&#x2F;path-2.narod.ru&#x2F;design&#x2F;base_e&#x2F;starwisp.pdf" rel="nofollow">http:&#x2F;&#x2F;path-2.narod.ru&#x2F;design&#x2F;base_e&#x2F;starwisp.pdf</a> by Robert Forward.<p>To poll the success of this overall endeavor, as well as start to make predictions about which components will&#x2F;won’t work, Metaculus is launching a series of questions —check it out if you have expertise or opinion: <a href="http:&#x2F;&#x2F;www.metaculus.com&#x2F;questions&#x2F;#&#x2F;?order_by=-publish_time" rel="nofollow">http:&#x2F;&#x2F;www.metaculus.com&#x2F;questions&#x2F;#&#x2F;?order_by=-publish_time</a>

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

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

What about the energy dissipated by collision with the interstellar medium? Won&#x27;t that slow the probe considerably?<p>Apologies for significant errors and appreciation for corrections in the following hasty and unchecked calcs.<p>Wikipedia [1] says the ISM density ranges from as little as 1e-4&#x2F;cm^3 for hot, ionized regions to 1e6&#x2F;cm^3 for cool, dense regions.<p>Let&#x27;s say it&#x27;s all neutral hydrogen, which conveniently masses 1g&#x2F;mole. A mole is 6.02e23 particles. A single H atom masses about 1.66e-27kg. And let&#x27;s say the ISM has a hydrogen atom density kind of midway between the extremes: 10&#x2F;cm^3 (=1e7&#x2F;m^3).<p>Let&#x27;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.<p>At 20% of C, the spacecraft sweeps out a volume of [(3e8<i>.2)m&#x2F;sec]</i>(6.45e-4m^2) = about 3.9e4m^3 every second, which contains about 3.9e11 hydrogen atoms, or a mass of about 6e-16kg&#x2F;sec.<p>If all those atoms hit and stick to the spacecraft, they all get accelerated to the spacecraft&#x27;s velocity. At 20%C, relativistic mass increase should be small, so let&#x27;s ignore it. The energy needed to accelerate one hydrogen atom to 20%C is about 3e12J&#x2F;atom.<p>If the spacecraft is hitting 3.9e11 atoms&#x2F;sec and spending 3e-12J&#x2F;atom accelerating impacting atoms to 20%C, that&#x27;s slightly over 1 Watt that&#x27;s decelerating the spacecraft.<p>Over a 20-year trip (6.31e8 seconds and assuming no deceleration), that&#x27;s 6.31e8 W-sec, or 631 megaJoules of energy needed to sustain 20%C because of collisions with the interstellar medium.<p>A Watt isn&#x27;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.<p>It looks to me like small, light probes won&#x27;t maintain their high initial velocity very long into the cruise phase without ongoing propulsion.<p>[1] <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Interstellar_medium" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Interstellar_medium</a>

Oddly the most practical use for this is as an interstellar weapon. You don&#x27;t need to solve the problem of communicating back to Earth, you just need a little terminal guidance.<p>If an iPhone weighs 100 g and we use a non-relativistic formula for energy at 1&#x2F;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 the Columb barrier as well, so you might get a nuclear boost to the yield as well.

How close does a tiny cell-phone camera objective lens have to get to an interstellar object before it can resolve detail better than the High-Def Space Telescope will?

It seems to me that the accuracy required to aim an unguided projectile anywhere near the target stars will be impossible. They&#x27;ll fly by pretty far. At what distance to Alpha Centauri does an iPhone quality camera outmatch the Hubble&#x27;s photos?

How is such thing able to communicate back with earth? Doesn&#x27;t it require a lot of energy to create signal that we will be able to receive on earth (or satelite)?

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&#x27;m sure they&#x27;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.

This is quite an interesting proposal, although there are some major technological challenges to address:<p><i>To achieve that energy would require an array about a mile across combining thousands of lasers firing in perfect unison.</i>

&gt; Estimating that the project could cost $5 billion to $10 billion, Mr. Milner is initially investing $100 million for research and development. He said he was hoping to lure other investors, especially from international sources.<p>It would be cool if there was one or more endowments for this sort of thing, to perpetuate the mission(s), similar to how schools have endowments.

I know we don&#x27;t currently have the technology for this, but it makes complete sense. Get small things moving really fast instead of big things moving slow.

Zuck to fund interstellar travel w&#x2F;in decades? 2040 Breaking News: Alpha Centauri Rejects Facebook&#x27;s &#x27;Free&#x27; Internet

A step towards such a project is to send such probes to every object in the solar system.

Say you could accelerate a ~1g camera to 0.2c.<p>I don&#x27;t know how to do general (hard) AI, but if I did, I can&#x27;t think of a fundamental reason I couldn&#x27;t shrink the AIs down to ~1g and make them able to survive 60,000G. So you could send a bunch of AIs on a 20 year trip to Alpha Centauri.<p>I would imagine Stephen Hawking has already put two and two together in this way; he often warns that humans should leave Earth to avoid extinction.<p>Even if we can&#x27;t colonize other stars with people within this century, AIs could be thriving there within that timeframe. At least our &quot;descendants&quot; (the AIs) would be protected from extinction (by redundancy across stars).

A quite similar proposal was discussed on Hacker News two months ago: <a href="https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=11151497" rel="nofollow">https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=11151497</a>

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&#x27;s still sending back data it gathered in 2015 from an encounter mere light-hours away.<p>Even if the laser propulsion aspect of this works out, I think communicating with something that far away is fantastically beyond state-of-the-art in wireless communications.

I am a self-proclaimed &#x27;newbie&#x27; when it comes to understanding energy, astronomy, and physics in general, but I have a few questions and any answers or opinions would be appreciated:<p>1.) Why in the world does the &#x27;Light &#x2F; Laser Beamer&#x27; need to be physically located on earth? Why not in space?<p>2.) Is building &#x27;check-points&#x27; for both data and power atop the planets not a possibility? (Solar, chemical, etc).

I wonder how they intend for the probe to actually <i>communicate</i> 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.<p>Perhaps it could be done with laser? Having said that, I&#x27;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&#x27;t work with lasers and may be off on this, so take this paragraph on principle rather than factual).<p>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: <a href="http:&#x2F;&#x2F;www.nature.com&#x2F;news&#x2F;quantum-communications-leap-out-of-the-lab-1.15093" rel="nofollow">http:&#x2F;&#x2F;www.nature.com&#x2F;news&#x2F;quantum-communications-leap-out-o...</a><p>And how to keep this iphone sized device powered?

Is it going to just do a fly by or will it somehow enter orbit once it reaches Alpha Centauri? How is it going to transmit data back to earth in a manner that would overcome the signal to noise ratio? Will power be solar or nuclear? How would it know when it is there to begin operating, especially if it uses solar power?

Related article at <a href="https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=11481351" rel="nofollow">https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=11481351</a> (but comments moved here).

If this works, it would probably be more useful for probing objects in our solar system. You could send a probe to Jupiter and hear back in ~4 hours. Waiting ~25 years for results from Alpha Centauri would be rather painful in comparison. Getting anything interesting back would be a long shot as the instruments would be rather limited and solar systems tend to be rather large.<p>I imagine&#x2F;hope that they would probably do tests in the solar system before trying to send them to Allha Centauri.

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&#x27;t see how it can be sufficiently corrected for the beam and sail asymmetries. It is like kicking a soccer ball - you&#x27;d like to hit a perfect &quot;9&quot; (&quot;upper 90&quot; in US), yet ...

<i>Moreover, to keep the beam tightly focused on one probe at a time would require an adaptive optics system that compensated for atmospheric turbulence — something astronomers know how to do over a span of 10 meters, the size of a big telescope mirror now, but not over a mile.</i><p>Would it be easier just to put the lasers on the moon?

Did this remind anyone of the Polis&#x27; spreading and searching the stars in Diaspora (by Greg Egan)

How significant are the relativistic effects at 0.2c (the probe speed they are aiming for)?

Here&#x27;s my big question: How do you slow down? 4.37 light years does not take into account all the time required to slow the hell down. Am I missing something? Isn&#x27;t this the biggest fundamental issue with long range space travel?

I&#x27;ll join the skeptics line with this observation: <i>even if</i> we are able to put this little device near Alpha Centauri, how the heck will we get a signal back from it? Will it have the juice to beam a picture back to Earth?

We should send even smaller nano-probes that would be even easier to accelerate and would have the capability to collect dust and build it into larger probes that would talk back to us when they come online!

Why do we need thousands of theses things, not just a couple of big ones? There seems to be some concern over the size constraints, but don&#x27;t really see the advantage of having many of them?

Why not go to Proxima Centauri? It&#x27;s closer!<p><a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Proxima_Centauri" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Proxima_Centauri</a>

The project estimates are completely off. Building a 1gigawatt power plant costs 10billion. Let alone building 100 of them. Also making the laser array. Also you can&#x27;t have reflection without absorption. Also we don&#x27;t have solar sails that fit the bill.<p>This project is barely physically and economically plausible. Its not a question of scaling, or engineering. The physics just don&#x27;t work.<p>We have a way to get a space craft to 20% of C. Its called Project Orion. <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Project_Orion_%28nuclear_propulsion%29" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Project_Orion_%28nuclear_propu...</a>

Ahem.. why does the object have to be behind the solar sail? Cant it be, one object hanging between two solar sails? Or Three?<p>Is there something i dont get here?

Why don&#x27;t we focus and reflect the sun&#x27;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&#x27;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. Better yet, we can use SpaceX&#x27;s reusable rockets and save even more on launch costs.

Would they slow down when they reach another star? What kind of observations can you do if you move at that speed?

So basically project something that we are never going to know or measure whether or not it has succeeded？

Why not accelerate slower with smaller lasers? What harm in taking 2 days? 2 months? to get to speed.

Bah... I think that it&#x27;s better to make a magnetic luncher on the Moon.

Let&#x27;s build a mass driver around the circumference of the Moon. Like a cannon in a particle accelerator shape.

Does that mean that the US will be the winning civilization?

Mr. Ha

I thought very strange things start to happen at even 10% the speed of light (mass increase, etc.) so 20% sounds impossibly ambitious?<p>Also the fastest physical object we&#x27;ve ever observed is only 1000 miles per second. A far cry from even 10% of c<p>Then there is that pesky problem of deceleration.