Would it be easier just to put the lasers on the moon?
https://en.wikipedia.org/wiki/Lunar_soil#Moon_dust_fountains...
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.
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...
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... [1] http://breakthroughinitiatives.org/Challenges/3
Freeman Dyson co-led Project Orion.
Is there something i dont get here?
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.
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).
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.
Also: why bother decelerating at all? Take a picture as you fly by.
Lithium ion battery energy density is 265 watthours per kg. So you will need 24.9 million kg of batteries. Or 24.9 thousand tons. It's a lot, Tesla factory makes more per year.
Also known as an Astronomical Unit (AU).
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.
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%.
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.
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.
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.
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.
Hahaha, I say this partially in jest, but Hacker News is basically for people to bash on dreamers' ideas.
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.
[1] https://en.wikipedia.org/wiki/Yuri_Milner
[2] http://www.forbes.com/sites/parmyolson/2015/03/25/yuri-milne...
The other problem is that the beam would ideally go straight between earth and Alpha Centauri, and the sail just rides the beam. The Earth does rotate around the sun but the parallax this provides (over 3-6 months) is pretty low relative to the distance travelled by the probes over that time. If you try to do selective course corrections (i.e. bump one side harder than the other) then you're going to need a super fine beam that is going to miss frequently and probably hit the target when you hit.
This is pretty much inherently a shotgun approach. You don't get a mid-course correction on the birdshot that misses its target. Or rather, it's like putting out balloons into the wind and letting them drift.
The relativistic effect shifts the frequency by 22.47% for 0.2c.
Given a 445nm laser (the powerful blue ones), that's 545nm when redshifted (green). It's a bit of a jump, but that wavelength is still smack in the "solar atmospheric absorbtion spectrum window" -- you won't lose much of the energy in the atmosphere.
Reading this page diagonally [http://www.edmundoptics.com/resources/application-notes/opti...], I don't think that the idea of getting a >99.5% reflectance value for that bandwidth is that far off.
We need way better than 99.5%.
You'd probably want to do that anyway to avoid burning up your laser. Also it's easier to make a pulse at that power level vs continuous.
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 how it generalizes for longer durations.)
I conjecture that the laser profile will be "shaped", if at all possible, so the trajectory of the projectile is somewhat self-correcting: if the projectile starts to get ahead of the planned trajectory, the acceleration will decrease, while if it lags the acceleration will increase. This wouldn't require any realtime feedback by the laser emitter.
With many projectiles and a massive investment in a laser array, there will be some ability to repeatedly attempt this and iterate.
Still, this is extremely aggressive even many decades in the future.
EDIT: Their proposal confirms that the beam will need to be shaped for feedback. ("By modulating the beam modes, get greater uniformity could be attained, and feedback between the nanocrafts and the beamer array would allow real-time adjustments.") No details though.
http://breakthroughinitiatives.org/index.php?controller=Foru...
Rather than the spherical shape a supposed above, they propose a thin sail with some undefined graphene skeleton. No idea how that is supposed to survive 2-minute long explosive accelerations.
http://breakthroughinitiatives.org/index.php?controller=Foru...
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 this pressure.
Otherwise you'd end up with the light bouncing of at angles other than pi, which would direct the probe to the side.
A flat mirror coupled with crazy fast gyros (also powered by the laser) might be stable enough that the mirror holds directed toward the earth.
And then are immediately 'disposed of'. That is, quite literally, a fire-and-forget problem; those electronics don't have to keep working for months or years.
This is analogous to the way train bogies go around corners.
If the laser is tune-able, you could slowly increase the output-frequency based on how fast you expect the target to be moving at the time the light hits. This would counteract the Doppler effect and supply a consistent frequency to the target.
Yes, but maybe it could also give us the technology to quickly deflect perilous asteroids... ?
we might learn something, but we have to use resources for studying and there might just be better and/or more useful alternatives
And how does a device weighing a few grams emit a signal capable of detection back on earth? The Voyager probes are over half a light-day from earth but they have rather large antennas.
Interestingly, you also have to deal with Lorenz contraction - at .2c the ship itself will shorten by about half a percent, compounding the apparent wavelength shift. Admittedly this is a much smaller effect.
Intention leads to results (even small)
Small results lead to bigger results (bigger resources, like more knowledge)
Bigger resource leads to greater intentions
And here is why humans are still better than DeepMind, while (true) { learn, invent }
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.
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...)
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 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
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.
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.
7kT is half of the Hiroshima bomb. Still destructive, especially if you've got a bunch of them.
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.
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.
“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...
So the flash can also be used for course corrections? Cool!
It's a serious point though. Perhaps the gravity of the star will be enough to help them right?
"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...
To achieve that energy would require an array about a mile across combining thousands of lasers firing in perfect unison.
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 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).
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.
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).
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?
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...
Fly by.
Laser.
Nuclear.
I imagine/hope that they would probably do tests in the solar system before trying to send them to Allha Centauri.
In a sense, this is essentially what the universe is.
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...
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...
https://en.wikipedia.org/wiki/Interstellar_medium#Radiowave_...
Not to mention there is also additional cosmic radiation outside the heliosphere. Something the size of a chip is going to be extremely vulnerable. Slower rad-hard electronics are going to be the order of the day, but even with thousands of these things, it's going to take a toll.
I wonder if it would be easier to string ~300 of these at ~1000 AU intervals, and use them for a relay? Or perhaps ~3000 at ~100 AU intervals.
And I wonder if that would be construed as mining a common hyperspace lane by the galactic council... ;-)
How do you keep it cool with a 100GW laser pointed at it over 2 minutes?
http://link.springer.com/content/pdf/10.1007%2F978-3-642-274...
Depending on what maneuvering profile these things will have, it might be desirable to have a "semi-phased" design that can be steered to a limited degree without physically moving the spacecraft.
And, it also goes without saying that this antenna probably needs to be flexible or at least articulated in order to deploy.
NASA has previously done some really interesting antenna designs with genetic algorithms. You just need to figure out what the goodness function here should be.
https://ti.arc.nasa.gov/m/pub-archive/1244h/1244%20(Hornby)....
Similar to how the retroreflectors on the moon allow you to bounce a laser off of them and send it directly back to you.
I think this step is off by 1e5. It contains 3.9e16 hydrogen atoms at 1e6/cm^3.
So that 631 MJ becomes 63.1 TJ.
I think the volume swept out by the spacecraft is about right: (3e8m/sec)(.2)6.45e-4m^2 is about 3.9e4m^3/sec.
Then, 1e7 H atoms/m^3 * 3.9e4m^3/sec gives about 3.9e11 atoms/sec hitting the (1 inch square) spacecraft.
One of these probes, hitting the upper atmosphere, would be so light as to vapourise almost instantly, turning into a expanding plug of high-speed plasma. This would have very little ability to penetrate the atmosphere. I think all you'd see is a flash in the very high upper atmosphere as the kinetic energy was dissipated as radiation and then it'd be gone.
“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...
"For a 4m sail, for example, the diffraction limit spot size
on Earth would be on order of 1000m."
https://www.google.com/search?q=4+light+years+*+1+micron+%2F...
http://www.icarusinterstellar.org/vint-cerf-qa-on-interstell...
This is in a nutshell why me and my (MBA)boss get into arguments.
By the way, dark matter is likely passing through your body right now. It's effects can only be seen on the scale of galaxies.
I completely agree, though I prefer to look at it as "constructive criticism" of every flavor.
Dude. 9 significant digits? You don't know m to better than 1 or 2 significant digits. Just call it 5e5 and be done with it.
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?
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.
The stabilization problem sounds really hard to me.
> Otherwise you'd end up with the light bouncing of at angles other than pi, which would direct the probe to the side.
No, because these lasers won't be focused to a point smaller than the size of the satellite. I'm not sure what the waist will be, but presumably it's much larger than the mirror.
The real reason these are sails is because they maximize the momentum transfer per unit mass. But unlike sails on boats, this force can't be transmitted to a displaced payload using tethers/struts, so the payload must be dispersed and integrated within the sail itself.
Essentially, point-like sources (the laser emitter array and the target can be considered point sources when on the scale of lightyears) separated by an angle smaller than the angular resolution of the device in question cannot be resolved.
Your friend in Japan can take a picture of an apple and email it to you in the US. Your friend's camera is like the camera on the probe.
You can't see the apple from where you are, but your computer can 'see' the internet.
Your friend uses the internet to send you a digital code that your computer turns into a picture of an apple.
The optical systems used to meld the output of the lasers could be used in reverse as a vast and sensitive telescope.
But why would we be unable to see the star's planets if we are able to see a probe that small?
The laser receiver might be able to see the planets in question, but a probe with an interstellar worthy camera in the direct vicinity of the planets would obviously be able to capture a much higher resolution image.
Don't sit around complaining about people who are "doing" stuff because they're not doing the stuff you want them to be doing. Get out there and do the stuff yourself!
So then the problem is that you need a laser than can hit a 1 km^2 target from 40 trillion km away?
"Use a material with better than 99.999% reflectivity. Usually, highly reflective surfaces are dielectric mirrors, which are composed of ‘sandwiches’ of material, with each layer reflecting back a modest fraction of the total. Each layer needs to be at least a quarter of a wavelength thick. The weight can be reduced by using a monolayer with high reflectivity at the correct wavelength. Based on recent research, this could be achieved by a ‘hole-pocked’ layer, highly reflective for very specific angles where reflectivity caustics arise. (These caustics occur for wavelengths of light that are actually longer than the sheet thickness.)"
http://breakthroughinitiatives.org/index.php?controller=Foru...
Sacrificial layers may also help with heat management, I suppose.
Upon further reading, looks like you can achieve >99.9998% reflectance for a 10% wavelength band (in this case, in the infrared). [http://www.crystallinemirrors.com/wp-content/uploads/2013072...]
Still, you're right in saying that it's probably still quite a ways off.
99.999% reflective mirror - done (iphone gorilla glass ;)