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.
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.
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.
Skepticism could be constructive. Most of time people just want to improve some novel idea in question format. If someone really do not care about it they would not even bother to raise a question.
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.
Outside of Earth orbit, space objects become incredibly rare, and outside the solar system they are rarer still. There are much bigger problems to worry about, believe me.
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 also am skeptical. Why not instead try to send a probe into the interior of the sun? (because its cooler there than at the point of those lasers) Why not create a survey of the Oort cloud (because its an order of magnitude closer). This project seems cool; but maybe not well thought out.
These people are obviously very passionate about their project, and they're moving forward on something they want to do. If you're so passionate about those projects you mentioned, start making them happen!
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!
Good attitude! Still, it doesn't make their project any more likely to succeed. Those others I mentioned - easier by an order of magnitude. It makes them sound insincere.
That depends on how much energy that the space craft reflects. If it reflects all of it, the space craft would not gain any heat at all unless there is some weird quantum effect that would mean otherwise.
I'm not a physicist, but it seems like perfectly reflecting photons in a vacuum is different from being immune to conductive heat transfer in a plasma.
I'm skeptical that a guy who made his money by "transferring a variety of U.S.-pioneered internet business models to Russia" [1] and then muscling in on Silicon Valley investments like WhatsApp where he "bought a stake from the founders for $125 million within weeks of Facebook snapping up the messaging giant for $19 billion last year" [2] is capable of this kind of engineering project. He's good at making deals but he's not the kind of roll-up-your-sleeves engineer that should be involved with space exploration. I think he's just jealous of Elon Musk and Richard Branson. This seems more like the Spruce Goose than anything else, and in the same vein as Howard Hughes, I'm sure he'll be looking for a giant government grant to pay for his techno-playboy hobbies.
One thing to note is the reported mindset of the program. It doesn't really talk about their expectations around plausability of this, but rather that they seem to be targeting success through large numbers even at low probabilities. They might not care to anticipate/correct anomalous "course deviations" but rather write those off as a loss completely, hoping that just a few of many initially accelerated succeed.
It would be impossible/unlikely to correct any course deviations anyway. First off, even if the beam is collimated to within a degree or something, a degree at even a million miles is a pretty big spot, and you'd probably not be able to hit any specific unit. Even if you collimated it more finely - good luck hitting the target.
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.
"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.)"
So it sounds like we'd have a layered sequence of dialectric mirrors (themselves many layers each) each reflecting a different wavelength. The first layer lasts until the laser redshifts out of that layer's efficient reflection range, then vaporizes revealing the next layer tuned to the redshifted laser. That layer lasts until the laser redshifts even further, then vaporizes to reveal the next, and so on.
Sacrificial layers may also help with heat management, I suppose.
Some types of lasers (e.g. free electron lasers) can be tuned to arbitrary wavelengths, so you could maybe adjust the frequency of the laser as the craft accelerates.
Keep in mind that a single spaceship wouldn't take up the whole beam - for one thing, there would be a bunch of them, and for another, we simply can't focus a laser on something the size of an iPhone from even as far away as the moon. That said, it is still a massive amount of power that needs to be reflected.
I thought this also, but "[...] to keep the beam tightly focused on one probe at a time would require [...]" in the original article points in the opposite direction.
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.
(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.
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.
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.
It doesn't change the conclusion, but note that in a relativistic regime you can't use F=ma because the mass is no longer constant. You need F=dp/dt, where p is the relativistic momentum. The Lorentz factor here works out to 1.02, i.e. 510 N, not 500, so not a big effect if I got my maths right.
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.
Would a slightly conical mirror with its apex pointed towards the laser be self-stabilizing in tilt? Kind of like an aircraft with dihedral wings [1] is self-stabilizing in roll - imagine the laser illumination as an analog of the aero lift vector on the wings. If it tilts wrt the laser, the asymmetric illuminated area should counter the tilt.
> Spheres would cause a major issue - the laser would need to strike the centre of the circular visible surface.
> 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.
> The electronics in precision guided mortars in the US army survive 50,000g
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.
Yes but they do have to keep working after those 50,000G. Which is the thing: provided they survive acceleration, the remaining problem is cosmic rays. But we're talking about such small craft here that we'll be able to launch literally hundreds, probably thousands - the real challenge is getting the 100GW laser array built. Once you have it...no reason not to use it.
Similar to XorNot, I don't really see the mechanism by which high acceleration could substantially reduce long-term reliability of the solid state electronics without breaking them completely.
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 is analogous to the way train bogies go around corners.
> Edit: Just realized [...] the laser won't be green any more thanks to the Doppler effect!
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.
A laser capable of propelling an interstellar probe is also a mighty space weapon. Some people proposed constructing that on the far side of the Moon so that it hass less of a chance of being used as against Earth targets (intentionally or by accident).
Setting incredibly ambitious goals has definitely lead to impressive achievements in the past. I think it's definitely worth studying. Worst case, we'll learn something.
Side effects of these projects, even if they fail, may be massively valuable. Sometimes solving problem X that isn't a big concern for society will solve problem Y down the track. History of maths is littered with things that were useless when they where first invented, such as imaginary numbers.
I'm still not arguing that and I'm as excited as everyone else when someone comes with a bold idea as this, still, you can't gouge how well the money will be spent and downplaying risks and alternatives of undertaking a multi decade project because we are in the heat of the moment is not wise. Science justice warriors should take a deep breath before pointing fingers imo. We are inherently in a box and I doubt anyone can imagine what technology will look like in 10 years, let alone 40.
And where does the reflected laser light go? Back to earth if it's reflected 180 degree.
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.
Freeman's speech was interesting in that regard. He doesn't think Alpha Centauri is a reasonable target for this project and according to him it should not be advertised as such. But he does think that using this technology to send probes in very deep space could both tell us more about what there is over there and help us improve the technology to go beyond and thus to the actual stars.
>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.
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.
Nevermind the issue of how you either (a) transmit back over 4 light years or (b) pull off a return trajectory with just a few grams of equipment. It's easy to poke holes at ambitious projects, so I look forward to seeing what they can come up with, but man - this would be phenomenal compared to Apollo, even given how far we've come.
Even if this is possible, it'll probably be prohibitively expensive to do constant resupplies when you won't know for years if previous trips have worked. A manned mission or a resupply mission would also be orders of magnitude beyond the masses they're talking about, which strains credibility even further.
I am pretty sure you just sent out a new boat of supplies every year make it or not. Worse case the mission fails and the next mission has 20 supply ships in orbit waiting for them. I have no idea on the minimum mass required to send supplies, i.e. if you can recycle 99% of the water you need, would only need a few drops per day. If you can only recycle 50%, you are going to need a bunch.
Nah - they're already pushing some theoretical limits by talking about planetary levels of energy. You're talking about just throwing money at doing that many times in a row with no proof of success.
The plan calls for ultra-reflective coatings that are tuned to the wavelength of the laser. Apparently these exist, though I have not checked the numbers. In my class I abandoned laser propulsion quickly for just this reason (incinerated the target), but with these coatings it might work.
I'm more worried about where the reflected light is going to go once it bounces off the sails. Presumably it'll fall back on earth, where that amount of energy could cause a lot of damage.
10 light-seconds is a massive underestimate. If we assume a constant acceleration from zero to 0.2c over two years, the average speed is 0.1c. Therefore by the end of two years it will be 0.1 lightyears away. Deviations will have to be anticipated over a month in advance!
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.
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 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
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.)
I don't think you'll find 0.2c relative velocities except near very energetic phenomena, especially not of things that are too small to easily see coming.
I think it should be clear from the context that this discussion was about macroscopic objects. Cosmic rays don't carry the kinetic energy equivalent of a medium size nuke.
Their speed isn't particularly relevant, what matters more is their energy. As big as that number is to us, it's still pretty insignificant; there's a lot of objects already traveling around with us in our own solar system with 70kT (per PaulHoule in other comments) of energy. A looooooot of objects. In the grand scheme of things, this is a rounding error on a rounding error. We are small.
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?
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.
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...
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.
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.
Another problem to consider is that the interstellar medium is noisier and interferes with the signal to a greater extent. Voyager is only just reaching that region, but this signal would have to travel through that for several lightyears.
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.
Thinking out loud: You need an ingenious design that allows the laser sail to double as a directional antenna. And you pick a frequency which is not radiated by many other objects in space so it's easier to pick up the dim signal. And then you build a ginormous radio telescope bigger than any ever built before. Luckily you have a decade or two to build the telescope after the probes are launched.
And/or establish a chain of these spanning the distance between here and Alpha Centauri. They don't need to actually stop, but if enough are traveling at lower speeds or depart sufficiently late in the program then they'll be in position to act as repeaters. Possibly larger probes (which would travel slower) that have more power available for reception and retransmission.
Planar directional antennas have been done before, in EHF-type frequencies (most research has focused on 2.4 GHz and 5 GHz for obvious reasons). Lower-frequency signals would probably be harder. No idea what a nice clear frequency might be though, particularly out in the interstellar medium.
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.
Since there will be many probes sent, they will probably be able to relay the signal all along the way. It would still be difficult, but maybe manageable.
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.
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.)
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?
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.
According to a screenshot I took during the presentation (http://i.imgur.com/pncUg0Q.png), the probes are estimated to have around 200 grams. There's no way you could fit a battery, a RTG, a camera and propulsion system within 1 gram.
> 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.
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.
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.
The Chelyabinsk asteroid that blew up over Russia in 2013 was, by comparison, about 500 kT and massed about 13000 tonnes. It blew up at at 30km altitude.
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.
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?
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.
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?
“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.”
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)?
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."
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?
To answer that question, you need to know a little bit about diffraction-limited systems in optics and the Rayleigh criterion.
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.
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.
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.
We wouldn't be able to see the probe itself, rather we'd see its laser. The planet outputs (or reflects) vastly more total light than the laser, but the laser is pointed directly at the receiver and can output more light in that specific direction than the planet does. Plus, to get interesting data about the planet we need to get quite a lot of light from it, whereas we can get data from the laser over time by pulsing it so long as we can get any light from it at all.
>The planet outputs (or reflects) vastly more total light than the laser, but the laser is pointed directly at the receiver and can output more light in that specific direction than the planet does.
So then the problem is that you need a laser than can hit a 1 km^2 target from 40 trillion km away?
No, the laser would point in the general direction of the sun and as long as Earth is within the cone it has the potential to detect the signal. The laser just happens to focus all of its energy in one narrow cone as opposed to all directions.
'Seeing' the probe is like being connected to the internet.
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.
I'd be kinda hesitant about relying on a ground-based receiving station. How stupid would you feel if after 40 years flight, you missed the return signal because your receiving station happened to be on the wrong side of the Earth?
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.
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.
> 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.
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.
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).
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.
“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.”
"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.
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).
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.
For #2: What do you mean by check points? Which planets would you build the checkpoints at? When you say "atop", do you mean on the planet's surface, or in a polar orbit, or in orbit generally?
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).
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?
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.
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 ...
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?
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".
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.
Well, you could use your handy 200 GW solar array. With perfect conversion, and ~1300 W/m^2, you'd need to get your 12km x 12km panel array pointed consistently at the Sun.
You actually don't need anything crazy like that, as you don't need continuous 200GW forever, only for 2 minutes. You just need enough energy storage so you "only" need to store 6.6 gigawatthours.
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.
However, this is much clearer than firing said lasers from under the atmosphere. While charged dust is a problem for the laser equipment itself, this can be mitigated with special dust attracting equipment. Getting the lasers to the moon would be a bigger problem.
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.
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.
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%.
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?
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.
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?
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't really see the advantage of having many of them?
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.
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.
I realise this is an appeal to authority, which is bad and wrong in any argument, but I trust Stephen Hawking wouldn't back a project that falls down on the physics.
Do you seriously think these guys haven't heard of Project Orion or wouldn't use Orion's approach if they thought it was best? In case you didn't notice, Freeman Dyson is involved with Starshot.
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.
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.
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).
My guess is the sunlight reaching the earth's atmosphere is already too diffuse. This could maybe work if the collector + launchpad were closer to the sun.
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.
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.
"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!"
They're not going to put them into orbit, just a fly-by. Then they'll continue until they hit something some time in the future. In a million years some unlucky alien might get hit by an iPhone.
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.
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).
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.