
A Visionary Project Aims for Alpha Centauri, a Star 4.37 Light-Years Away - sravfeyn
http://www.nytimes.com/2016/04/13/science/alpha-centauri-breakthrough-starshot-yuri-milner-stephen-hawking.html?mabReward=A6&moduleDetail=recommendations-2&action=click&contentCollection=Americas&region=Footer&module=WhatsNext&version=WhatsNext&contentID=WhatsNext&src=recg&pgtype=article
======
yongjik
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. (It
also means that whatever course deviation the object experiences must be
anticipated _seconds before it happens_ , because at the end of the two
minutes the object will be ~10 light-seconds away.)

If they could pull this off it will be the next Apollo, but I'm skeptical.

Edit: Just realized we have even more problems, about the reflective coatings.
If the laser is green, by the time it's flying away at 0.2c, the laser won't
be green any more thanks to the Doppler effect! I'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.

~~~
grecy
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 about how it couldn't be done.

Don't be one of those people.

Even if they "fail" to make all of this work and get something to Alpha
Centauri, they're still going to vastly expand mankind's knowledge of a lot of
difficult space-related technologies, which is a huge win.

This is hacker news, where people are trying to do difficult things to change
the world. That's the whole point.

~~~
rayiner
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.

~~~
IndianAstronaut
>Skepticism is the natural worldview of an engineer

This is in a nutshell why me and my (MBA)boss get into arguments.

------
david-given
This is a variation of the old Starwisp idea from 1985:

[https://en.wikipedia.org/wiki/Starwisp](https://en.wikipedia.org/wiki/Starwisp)

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.

So, you accelerate them (in bulk) at about 2G up to .1c. You ignore it until
it'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.

There were a whole bunch of technical problems with it, not least how to build
a microwave lens 560km across, but it's still vastly more plausible than
trying to push steel cans across the interstellar gulf. I'd be really
interested to see if this version works.

------
AnthonyAguirre
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/uploads/2015/04/A-R...](http://www.deepspace.ucsb.edu/wp-
content/uploads/2015/04/A-Roadmap-to-Interstellar-Flight-15-h.pdf))

For a thorough, if somewhat outdated, treatment of the “starwisp” idea using
microwaves rather than optical/IR lasers, see this paper:
[http://path-2.narod.ru/design/base_e/starwisp.pdf](http://path-2.narod.ru/design/base_e/starwisp.pdf)
by Robert Forward.

To poll the success of this overall endeavor, as well as start to make
predictions about which components will/won’t work, Metaculus is launching a
series of questions —check it out if you have expertise or opinion:
[http://www.metaculus.com/questions/#/?order_by=-publish_time](http://www.metaculus.com/questions/#/?order_by=-publish_time)

~~~
stcredzero
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.)

~~~
AnthonyAguirre
agreed! The starwisp paper is really very nice.

------
0x74696d
Sending a swarm of unguided relativistic projectiles across the universe seems
profoundly rude.

~~~
NoGravitas
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?

~~~
david-given
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](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...](http://www.niac.usra.edu/files/studies/final_report/320Zubrin.pdf)

~~~
mrfusion
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?

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

~~~
antognini
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 probes,
this means that their combined signal will be weaker than the Voyager signal
by a factor of ~3000. We might imagine that they are somewhat more optimized
to send a stronger signal than an ordinary smartphone. Let's say they have a
30 W radio instead of 3 W. This means that they will have a weaker signal by a
factor of ~300. This is a lot, but if they manage to build a detector that has
a larger effective diameter than the Deep Space Network by a factor of ~15.
Given the budget of the project, and the fact that they have 20 years of time
to prepare after the launch, this could be feasible.

Looking at the project's website, though, it looks like they are taking the
approach of using a laser onboard the probe instead of using a traditional
radio transmitter [1]. This would increase the efficiency of the transmission
by several orders of magnitude. It might even be possible to detect the signal
from a single probe with this approach.

[1]:
[http://breakthroughinitiatives.org/index.php?controller=Foru...](http://breakthroughinitiatives.org/index.php?controller=Forum&action=viewforum&id=14&page=1)

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

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

------
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](https://en.wikipedia.org/wiki/Interstellar_medium)

~~~
eduren
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](http://breakthroughinitiatives.org/Challenges/3)

~~~
ridgeguy
Perhaps they might. Looking forward to reading the entire project description
when I get home this evening.

------
PaulHoule
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 the Columb barrier as well, so you might get a nuclear boost to the
yield as well.

~~~
zeveb
> Oddly the most practical use for this is as an interstellar weapon.

I completely don't advocate this, but the 9-year-old-boy part of my brain
thinks it'd be pretty awesome to announce our presence to the universe by
attacking a star system.

~~~
PaulHoule
One of the explanations of why we are not seeing evidence of ETs is that
everybody is hiding and afraid they might get bombed by their neighbors;
that's because bombing your neighbors is orders of magnitude easier to do than
visiting them -- you don't need to solve the problem of decelerating on the
other end for one thing.

------
pklausler
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?

~~~
antognini
A cell phone camera probably has a resolution of ~1 arcminute (about the
resolution of the human eye). The Hubble has a resolution of ~0.05 arcseconds.
The HDST would have a resolution of ~0.002 arcseconds. So to get comparable
resolution to Hubble the probe would have to be at a distance of (.05 / 60) *
270,000 AU ~ 225 AU and a distance of (.002 / 60) * 270,000 AU ~ 9 AU to beat
HDST. A camera on such a telescope would probably be of somewhat higher
resolution, however. In fact, it would probably be at the diffraction limit,
which for a ~1 cm aperture in visible light would be closer to ~10". This
would increase the distances above by a factor of 6 or so.

Resolution isn't the only thing, though. The contrast is extremely important.
A little probe in the Alpha Centauri system would be able to take clean images
of any planets in the system. A telescope in our solar system trying to
directly image these planets has to contend with contamination of light from
the stars.

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

~~~
nilstycho
“The light beamer must focus a spot smaller than the sail onto the sail, as it
orbits 60,000km above the Earth’s surface. This alignment must be produced
when the target star system (Alpha Centauri) has the correct configuration
with respect to all planetary and stellar bodies in the intervening space,
such that the flyby occurs within 2 AU of the target planet. Using on-board
photon or others thrusters, the nanocraft will have the ability to make some
modest mid-course corrections, on the order of 1-2 AU.

“The task of pointing the array is dominated by the problem of keeping the
sail on the beam. This problem is defined by the width of the sail and the
distance to it. As an example, for meter-scale sail size the launch distance
is on the order of a few million km. The pointing accuracy required for beam
stability at this distance is on the order of a milliarcsecond. There are
several mitigation approaches that could be used to counter these effects. A
model of the atmosphere, calibrated with radar, laser beam, and optical
measurements in real time, would enable the required beam precision to be
achieved. Targets such as Alpha Centauri are bright star systems that will
inform pointing requirements.

“Monitoring the laser beam output provides the information needed to form the
beam. The Starshot system would be very different than a conventional
telescope, and specialized to its purpose. For example, most ground-based
telescopes, such as the Keck telescope, point to within a few arcseconds and
can track in a closed loop mode to better than 100 milliarcseconds. For the
purposes of Starshot, a significant improvement on this precision is required.
However, the beam synthesis inherent in the phased array system provides
considerable fine-pointing capability, supplemented by closed loop tracking of
the beacon on the spacecraft.”

[http://breakthroughinitiatives.org/index.php?controller=Foru...](http://breakthroughinitiatives.org/index.php?controller=Forum&action=viewforum&id=2&page=1)

~~~
lucaspiller
> on-board photon or others thrusters

So the flash can also be used for course corrections? Cool!

------
comboy
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)?

~~~
rm_-rf_slash
Not a scientist but I first read this on The Economist and they summed it up
pretty well:

"At its destination it would beam back pictures of the star’s planets with its
on-board laser. No current observatory could possibly pick up such a
signal—but the kilometre-wide launch array should be able to. The optical
systems used to meld the output of the lasers could be used in reverse as a
vast and sensitive telescope."

[http://www.economist.com/news/science-and-
technology/2169687...](http://www.economist.com/news/science-and-
technology/21696876-interstellar-travel-means-thinking-both-very-big-and-very-
small-new-plan)

~~~
slg
This might be a stupid question, but if we have the technology to see a
blinking light the size of a postage stamp trillions of miles away, why would
we need to send a probe with a camera all the way out there in the first
place?

~~~
Carrok
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.

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

~~~
tgb
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.

~~~
warrenpj
Would it be an accurate analogy to say this is why stars in the night sky
appear the same size (point-like), but vary in brightness?

------
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.

------
ColinCochrane
This is quite an interesting proposal, although there are some major
technological challenges to address:

 _To achieve that energy would require an array about a mile across combining
thousands of lasers firing in perfect unison._

------
a3n
> 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.

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.

------
sna1l
I know we don't currently have the technology for this, but it makes complete
sense. Get small things moving really fast instead of big things moving slow.

------
jgianakopoulos
Zuck to fund interstellar travel w/in decades? 2040 Breaking News: Alpha
Centauri Rejects Facebook's 'Free' Internet

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

------
mikepalmer
Say you could accelerate a ~1g camera to 0.2c.

I don't know how to do general (hard) AI, but if I did, I can't think of a
fundamental reason I couldn'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.

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.

Even if we can't colonize other stars with people within this century, AIs
could be thriving there within that timeframe. At least our "descendants" (the
AIs) would be protected from extinction (by redundancy across stars).

------
vilhelm_s
A quite similar proposal was discussed on Hacker News two months ago:
[https://news.ycombinator.com/item?id=11151497](https://news.ycombinator.com/item?id=11151497)

------
avian
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 away is fantastically beyond state-of-the-art in
wireless communications.

~~~
nilstycho
“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-scale receiving array would intercept 10-14 of
the transmitted signal. The main challenge is to use the sail as diffraction
limited optics for the laser communication system. This would be achieved by
shaping the sail into a ‘Fresnel lens’ upon approach to the target. The sail
structure could be different at the launch and communication phases. In order
to maintain a high transmission through the Earth’s atmosphere, the
communication would need to operate at a wavelength shorter than that used by
the launch laser system, due to the Doppler shift of the nanocraft relative to
the Earth.”

[http://breakthroughinitiatives.org/index.php?controller=Foru...](http://breakthroughinitiatives.org/index.php?controller=Forum&action=viewforum&id=13&page=1)

~~~
throwaway_yy2Di

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

[https://en.wikipedia.org/wiki/Airy_disk#Size](https://en.wikipedia.org/wiki/Airy_disk#Size)

------
LockandByte
I am a self-proclaimed 'newbie' 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:

1.) Why in the world does the 'Light / Laser Beamer' need to be physically
located on earth? Why not in space?

2.) Is building 'check-points' for both data and power atop the planets not a
possibility? (Solar, chemical, etc).

~~~
drabiega
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.

~~~
Zikes
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.

~~~
david-given
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.

------
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...](http://www.nature.com/news/quantum-communications-leap-out-of-the-
lab-1.15093)

And how to keep this iphone sized device powered?

~~~
firethief
> Maybe with some form of quantum entanglement?

[https://en.m.wikipedia.org/wiki/No-
communication_theorem](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...](http://www.theatlantic.com/science/archive/2016/04/yuri-milner-
zuckerberg-starshot-interstellar-centauri/477669/?single_page=true)

------
ryao
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?

~~~
ars
Answers to your questions are mostly in the article. Did you read it?

Fly by.

Laser.

Nuclear.

------
dang
Related article at
[https://news.ycombinator.com/item?id=11481351](https://news.ycombinator.com/item?id=11481351)
(but comments moved here).

------
ryao
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.

I imagine/hope that they would probably do tests in the solar system before
trying to send them to Allha Centauri.

------
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 ...

------
NoGravitas
_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._

Would it be easier just to put the lasers on the moon?

~~~
oabm
It might be easier to fire the lasers from the moon, but getting the lasers to
the moon would be a huge pain.

~~~
stdgy
Not to mention supplying these hypothetical moon lasers with enough power to
blast these mini-probes into another star system.

~~~
Zikes
[https://en.wikipedia.org/wiki/Helium-3](https://en.wikipedia.org/wiki/Helium-3)
ought to do the trick.

~~~
Zikes
I'm a bit surprised at the downvotes. I'm no scientist, but I meant this
suggestion in earnest. As far as my layman's knowledge goes, I've always
understood the Moon's He-3 reserves to be a "vast, as-yet-untapped power
resource".

~~~
marcosdumay
People have to learn how to harvest power from fusion before.

~~~
ars
You know how they have chemically pumped lasers?

So it would be awesome to directly pump the laser from H3 fusion.

H3 fusion is actually very easy - I think we could do it today. We just don't
have a readily available source of H3 for fuel.

~~~
Ankaios
3He fusion is easy—you can do it in a garage with some hacked-together
equipment.

However, 3He fusion with _net power output_ is _exceptionally_ hard, even with
complete disregard to cost. Much harder than deuterium-tritium fusion. We
don't know how to do it yet.

~~~
ars
Thanks for the correction. Seems the rosy predictions I remembered are not
accurate.

------
georgehm
Did this remind anyone of the Polis' spreading and searching the stars in
Diaspora (by Greg Egan)

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

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

~~~
mikeash
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.

~~~
gus_massa
(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.

~~~
mikeash
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.

~~~
gus_massa
Yes, this is the first two terms of the Taylor series near 0, so it works only
at "low" speed.

For speeds that are close to c, you have to use another "Taylor" series to get
a good approximation. If v is close to c, then the approximation of the
correction is sqrt(2(c-v)/c). So at 99%c you get 4.5%.

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jondiggsit
Here'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't this the biggest fundamental issue with long range space
travel?

~~~
hughes
You don't. You get a few hours (tops) to look around at anything interesting
in Alpha Centauri before zooming out of the target system and off into really
deep space.

Kind of like the New Horizons mission, which never stopped at Pluto. It just
flew right on by, but still got a ton of useful scientific data.

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1024core
I'll join the skeptics line with this observation: _even if_ 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?

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digsmahler
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!

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a_imho
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't really see
the advantage of having many of them?

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retrogradeorbit
Why not go to Proxima Centauri? It's closer!

[https://en.wikipedia.org/wiki/Proxima_Centauri](https://en.wikipedia.org/wiki/Proxima_Centauri)

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valarauca1
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't have reflection without absorption. Also we don't have solar
sails that fit the bill.

This project is barely physically and economically plausible. Its not a
question of scaling, or engineering. The physics just don't work.

We have a way to get a space craft to 20% of C. Its called Project Orion.
[https://en.wikipedia.org/wiki/Project_Orion_%28nuclear_propu...](https://en.wikipedia.org/wiki/Project_Orion_%28nuclear_propulsion%29)

~~~
gene-h
You are putting out 100 gigawatts for 2 minutes. One does not need 100 one
gigawatt powerplants, just some way to store and release a lot of energy
really fast.

Second, we have mirrors that can reflect up to 99.999% of a single wavelength
of light(such as that emitted by a laser)[0]. There is a detailed analysis of
the concept in [0] and the Breakthrough Foundation has a detailed list of
challenges associated with this project at [1].

Getting the laser out of the atmosphere is probably one of the biggest
challenges with this.

[0] [http://www.deepspace.ucsb.edu/wp-
content/uploads/2015/04/A-R...](http://www.deepspace.ucsb.edu/wp-
content/uploads/2015/04/A-Roadmap-to-Interstellar-Flight-15-u.pdf) [1]
[http://breakthroughinitiatives.org/Challenges/3](http://breakthroughinitiatives.org/Challenges/3)

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lunchTime42
Ahem.. why does the object have to be behind the solar sail? Cant it be, one
object hanging between two solar sails? Or Three?

Is there something i dont get here?

~~~
amdavidson
The spot size of the laser will presumably be considerable after 10 light-
seconds and we wouldn't want to irradiate the nano satellite.

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rm_-rf_slash
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. Better yet, we can use SpaceX's reusable
rockets and save even more on launch costs.

~~~
david-given
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.

~~~
XaspR8d
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/](https://what-if.xkcd.com/145/)

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

~~~
chm
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.

~~~
maxxxxx
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.

~~~
chm
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.

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eva1984
So basically project something that we are never going to know or measure
whether or not it has succeeded？

~~~
simonh
They describe a plan for sending a signal back.

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

~~~
tim333
The longer you take the further away it will be and the range of the lasers is
only so far. The average speed during acceleration is 0.1c so if you take 2
days it would be 51 billion km away which is out of range.

~~~
JoeAltmaier
Ok. 10 minutes then.

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andreapaiola
Bah... I think that it's better to make a magnetic luncher on the Moon.

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ant6n
Let's build a mass driver around the circumference of the Moon. Like a cannon
in a particle accelerator shape.

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

~~~
mikro2nd
"civilization"? You mean like when Ghandi was (allegedly) asked what he
thought of Western Civilization, and replied, "I think it would be a good
idea!"

~~~
yaacov
I think they meant the video game. Space race victory

~~~
labster
Unlikely. I think these spacecraft won't fare so well against the mind-worms.

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llomlup
Mr. Ha

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ck2
I thought very strange things start to happen at even 10% the speed of light
(mass increase, etc.) so 20% sounds impossibly ambitious?

Also the fastest physical object we've ever observed is only 1000 miles per
second. A far cry from even 10% of c

Then there is that pesky problem of deceleration.

~~~
mabbo
At 20% the speed of light, time would dilate by around 2%, but that's about
the extent of the effects, afaik.

Also: why bother decelerating at all? Take a picture as you fly by.

~~~
onesun
Have you ever tried to take a picture from a moving car? They never turn out.

~~~
fsloth
The sun is 8 light minutes away from earth. These babies would span that
distance in 40 minutes. If the plan is to take planetary images I would call
that basically crawling speed in terms of angular velocity (unless they are on
actual collision course).

