I've seen things you people wouldn't believe. Spacecraft with graphene sails powered by starlight and lasers Coin-sized pieces of graphene can be accelerated by firing low-powered lasers at them in micro-gravity conditions, say scientists. The technology could be a stepping stone to graphene solar sails, which could propel future spacecraft using starlight or a laser array. The material was developed at SCALE Nanotech, a startup in …
Perhaps it means that the force of gravity from the Sun will exceed the force on the sails and they will indeed slow down.
I find this kind of speculative engineering interesting but, in practical terms, extremely unconvincing. It seems to imply a level of development of civilisation at which we might have much better ways of sending long distance probes - or perhaps have even decided not to bother.
Certainly I can't think of a single government, even somewhere like Malta, that I'd trust with an 8GW orbiting laser.
I await with interest the design of your zero divergence laser.
All lasers have divergence, even if it's measured in microradians. To be exact, the minimum divergence of any laser beam is lambda/(pi * beam radius). Once the diameter of the beam exceeds the area of your sail, the inverse square law applies. The smaller you try to make the initial beam diameter, the larger the divergence angle.
Lasers are only constant diameter for distances which are short on solar system scales.
The picture is me extracting my old notes on laser physics from when I worked for an optics company.
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Because if the perforations are smaller than 1/4 wavelength of the light striking it, the radiation pressure will be the same as if it were not perforated. This would reduce the mass needed to build a sail of the same dimensions, so would allow for a larger sail. Due to the divergence of the laser light, it would be important to make the sail as large as possible.
That won't happen for anything powered by sunlight: the gravitational acceleration (ie the weight of it) will go down as the square of distance from the Sun, as will the force exerted by the sunlight. So if the force on the sails is ever more than the weight, it is always more than the weight.
On the other hand the mass of the object is constant, so the acceleration will decrease, asymptotically to zero.
Note this is for things driven by sunlight, not by giant frickin' lasers.
The problem is that the closer you get to another star the more pressure it exerts on your sail in the wrong direction. That’s why you need the laser. However you need to focus your laser on a 14m2 area at a distance of ~4 LY. Not exactly a trivial design requirement.
Then there’s the second issue - you arrive in the Alpha Centauri system at ~15% c. You need some way to slow down or you’ll just barrel straight through. If you’re spending that much time, money and effort getting to another star system you probably want to get some data back. You’ll have trouble getting through the submission phase if you’re basically proposing throwing billions of <currency> at a star using a giant **** off laser.
That’s the biggest issue with any kind of fast travel. You need to slow down without turning your payload to jam/dust. Consider the difference between going from 70mph to 0mph in a controlled fashion vs stopping fairly instantaneously through the help of a bridge support.
Re: using the destination star to slow down travel.
The problem is it tends to defeat the purpose of the idea (to get from A to B within a human lifespan). By decelerating from about the halfway point you take about 42% longer to get there (sqrt of 2, on the assumption the target star will decelerate your craft at 1ms-2).
Anyway, I’ve seen Star Wars. And whilst that *is* a moon, I’m not wholly comfortable with an 8GW laser array on it.
See "The Flight Of The Dragonfly" by Robert L Forward for a way to use lasers in the solar system to brake the interstellar craft as it arrives at the target star.
Basically the spacecraft is attached to a flat, circular lightsail thats surrounded by a much larger, slightly concave, annular sail. The combined sail is used for launch. On arrival the spacecraft and its light sail separate from the annular sail and the lasers are turned on again. The large sail focuses the light it receives on the far side of the small sail, which slows the spacecraft down while the big sail continues to accelerate past the target stellar system. This is strictly a one-way trip.
The book describes the system in some detail and gives the dimensions, masses and laser power requirements as part of the story. Worth a read if you want to understand how this type of system might work.
See also "The Mote in God's Eye" by Larry Niven and Jerry Pournelle", this time using the light sail and light from the destination's star as the brake.
And didn't George O Smith do something similar in his Venus Equilateral stories? If you haven't read them then they were written in the late 1940s and, while he had spaceships and space stations, all the electronics was based on valve technology (tubes, for left pondians).
Aha! I knew I'd read of this somewhere before...
One of Niven's ships used such a laser to maintain contact with earth. The crew didn't think of it as a weapon therefore the craft was determined to unarmed by a Kzinti telepath. However a few crew members knew better and used said laser to obliterate the Kzinti when they attacked.
Q of the day: Why is 'Kzinti' already in my phone's dictionary?
The same means that propulsed the craft toward the star, would similarly now be braking it, as Alpha Centauri would be exerting a far greater effect in opposition to whatever flickers were coming from Sol.
(Whoops! I was beaten to it. Sorry for the repetition)
On the matter of divergence, it wouldn't be too great a task to drop off the occasional lens en route, so the laser beam can be refocused. The lens too, could have its own mini sails that would give it the ability to keep it aligned.
You've basicly pointed out the answer to the deceleration problem yourself.
"The problem is that the closer you get to another star the more pressure it exerts on your sail in the wrong direction. "
So, at a certain point, stop putting energy into the spacecraft with the laser, and use the sail to start slowing down by using the pressure of the destination starsystem.
The problem is that the closer you get to another star the more pressure it exerts on your sail in the wrong direction. That’s why you need the laser. However you need to focus your laser on a 14m2 area at a distance of ~4 LY. Not exactly a trivial design requirement.
Not really, you'd use the lasers as a boost phase, not a continous acceleration until the destination star. The boost phase would be much less than a lightyears distance from our system.
Then you'd use the destination stars light-pressure to slow down. The slow down phase would be much longer, and more gradual, as you don't have the high-power lasers used for initial acceleration available to slow it down, therefore it has to rely on a lower-pressure, but longer duration, deacceleration phase.
For initial probes, they don't even need to slow down enough to enter orbit of the destination system, just slow enough to gather some data as they go through the system. Just like the Voyager and Pioneer probes, and New Horizons, haven't entered the orbit of any of their targets, they've just done flybys.
There is a huge problem, just the article doesn’t explain it well.
The sails don’t slow down, but the acceleration drops off quickly, which imposes basic limits that aren’t immediately obvious.
If you start off with a certain acceleration from the Earth (distance from the Sun 1AU), once you get to 1.4 AU from the Sun the thrust and acceleration halves (inverse square law). It halves again @2 AU, plus you spend much less time there so there’s less velocity change. Etc, etc. Almost all the velocity the velocity the sail will ever gain, is achieved within the first 0.4 AU. Light sailing is marketed as constant thrust, allowing ultra-low acceleration to get you somewhere useful, but it really doesn’t live up to that.
Some numbers: to escape the solar system, you need delta-v 12km/s from near-Earth-but-not-gravitationally-bound, on a 60 million km effective runway. This requires 0.0012 m/s2 acceleration, which for a light sail is a high hurdle to jump, but achievable. To get to the nearest star within a century needs delta-v 12,000 km/s. To achieve that on only a 60 million km runway, you need 1200 m/s2. Acceleration 120g is just impossible for solar sailing.
That’s why people want to use lasers, to increase the incident power per square meter, and get the acceleration done on a reasonable runway. These guys are achieving 0.1g acceleration, which is an amazing achievement. The problem is, it still doesn’t scale well. Remember the runway length of 0.4AU? Now you have to be able to focus your laser onto the spacecraft sail at 60 million km distance. Focusing is diffraction-limited, the tighter beam you want, the bigger the mirror. However you scale things, there’s problems.
The Breakthrough Starshot wants to make a sail of 14m2, weighing 4 grams, to reach 15% of speed of light with 8.5 GW lasers. That gives a very respectable 7000m/s2 acceleration. It also requires electronics that can survive 700g acceleration (feasible), and given that you are focusing a *8 GW laser* on it sufficient to vaporise most things, either: 99% reflective sail (feasible) with payload that can survive 2000Centigrade (not feasible), or 99.99% reflective sail (not feasible) with payload surviving 380C (marginal, payload mass <<4g, limited thermal insulation)
But the main problem is that they need to focus the lasers onto a spot-size of 4meter diameter from 1AU distance. That’s the same optics problem as a telescope resolving 4m at a 1AU distance. Atmospheric twinkle distortion prevents you doing that, so it has to be a *space* laser. And fundamental optical diffraction limit requires you to make an 18 kilometer diameter mirror to focus the laser. The largest mirror ever made is a 10 meter mirror, on the ground, even with adaptive optics, and James Webb space will be 6.5m.
An alternative mission concept has the spacecraft sail 4km diameter to have an 18 meter launch mirror. Then, the spacecraft mass x million, and even on the Starshot hyper-aggressive mass assumptions needs an 8 petawatt space laser firing for two hours continuous, which isn’t going to happen. In other words, people are tending to focus (pun intended) on the spacecraft side of laser sailing, which isn’t really the problem.
10m is a really big single mirror. With lots of mirrors and an unobtainable budget you can get to 100m. Bezos wants big manufacturing industry in space (so ambitious that I would call in SciFi). If that actually happens then the 18km mirror is "only" SciFi to the power of two.
Any time the technology for interstellar exploration is distinguishable from magic is a win. Include medical technology that seriously extends human life span the the stars feel significantly closer - at the expense of going to SciFi to the power of three.
Decide how far back in time you would have to go before people would consider our lives SciFi. I will call that 100 years so SciFi cubed is 300 years. I am glad someone is taking the baby steps now perhaps the next generation will be able build on that to make bigger steps.
If you want to go down the route of “magic just takes longer”, the problem is that there is no way to know which of the myriad magical solutions is best and closest to feasible at system-level. If you’d started Michelangelo on designing a CPU, should he work on Babbage’s Difference Engine, or help Newton play with coloured light and prisms (which ultimately turns out to be quite useful for optical resist technology)?
It’s better to set yourself nearer term goals. And frankly, having worked in the space industry, one of the biggest problems is that every project with a timeline longer than ten years turns into a boondoggle. Basically, unless the project completes in a timescale of your current job, nobody has any great interest in either finishing it, or really solving the problems. It’s just an annual budget, and people are “contributing”. They aren’t motivated to make the damn thing work.
If I want to accelerate 4 grams of payload to 15% light-speed, then we could “just” store 0.3g of antimatter in a magnetic bottle and let it annihilate as trickle, using it as thrust. That’s (today) completely infeasible, but theoretically do-able. Obviously, we need a nearly perfect gamma-ray micromirror and collimator. And ultra-vacuum technology decades beyond current, to prevent the antimatter annihilating inside the storage bottle. And ways of cooling the antimatter to keep it confinable, and miniaturised superconducting magnets to confine it without their power usage needing to increase the payload mass. But none of that sounds beyond early 22nd century technology to be honest. And definitely all closer to reality than building an 18km diameter launch mirror in space. Plus it solves the problem of being able to decelerate when we get to where we’re going.
The real point is that technologies a century away usually don’t solve problems in the way they look today, rather by discarding assumptions. Could improved sensor, CPU and comms technology mass micrograms rather than grams? Would it make more sense to scale up particle accelerator technology (which already approaches light speed, with picograms per particle bunch) than scale down rocket technology? Would a solar-system-scale coherent array optical telescope achieve better observation resolution anyway in the target star system than a 4 gram micro-spacecraft zipping through at 7000km/s a million km from any planet (Spoiler: yes it could. 25cm resolution. Tech demonstrator missions are on the horizon, and it really might be feasible within 50 years).
"Basically, unless the project completes in a timescale of your current job, nobody has any great interest in either finishing it, or really solving the problems. It’s just an annual budget, and people are “contributing”. They aren’t motivated to make the damn thing work."
That is one of the most depressing things I have heard, and it is a fault of manglement. The project should be broken down into achievable chunks (call them what you will) so that the thrill of "Yeeehaaa - we've done it!" is always in sight. The longer-term stuff needs people with a certain attitude to life - one person with a need for resolution in "only" five years can ruin a ten-year project. Fortunately, we have sufficient proof that your basic assertion is wrong - there are many examples of multi-year projects in the world - and these show that there *are* some good managers in existence.
"Include medical technology that seriously extends human life span the the stars feel significantly closer"
Human civilisation is composed of many generations usually building on each other's knowledge. Current human life-span is more than enough for successive generations to be born and serially educated en route.
Similar lines...
"Maxim 24: Any sufficiently advanced technology is indistinguishable from magic a big gun."
--from the Seventy Maxims of Maximally Effective Mercenaries, an in-universe tome of wisdom of the webcomic Schlock Mercenary.
And from said comic, we get a comedic application of that maxim with reflected sunlight in mind: part 1, part 2.
If the system you are referring to is a 1W laser-launched system, it would indeed be 54 miles per second. The problem is that the spacecraft turns out to be 2.3 million miles away at this point. And you need to keep your laser focused on that 1cm disc for the whole duration of the day.
Laser spots have divergence inversely proportional to the size of the focusing mirror, and you need to worry about atmospheric twinkle. The system you’re considering requires a focusing mirror diameter 180km in orbit around the Earth, plus means to physically swing it to keep it pointed at that 1cm object flying at 54 miles per second 2 million miles away.
The truth is, there is no way to make the numbers on a laser-launched system ever scale correctly. The Moties didn’t do their full system study correctly.
Solar sailing is actually a great near-orbit technology, in use today to save fuel on satellites. And also a great mid-future in-solar-system propulsion technology. But neither technology works for interstellar.
If after launching the craft, a second craft is then sent along the same trajectory, but purposed to refocussing the laser beam, then a third and a fourth...then the integrity of the beam could be maintained over large distances.
Similarly, the lensing apparatus could be sent ahead of the main payload and then position themselves after the payload has overtaken them, with a couple of twitches from the source laser, and an adept bit of tacking.
First solution of relaying the focusing mirrors doesn’t work: if the intermediate mirrors are 10cm, each one only extends the reach by 100,000 km, which doesn’t reach very far. Plus you’re now playing 10,000 cushion billiards onto cushions that are themselves moving at insane velocities.
The second option is slightly more viable - pre-stationing a large set of secondary focusing mirrors along the 100 million km runway track. If you could position each secondary mirror within 20,000 km of the track, these only need to be 4 meter mirrors, and the primary 100 million km away is similar’ish Requires a set of five thousand Hubble space telescopes, doing an intricate dance - things don’t stay still in space, they’re orbiting the sun, so this needs to be precalculated years in advance for them to be in the right place for only the few minutes of the shot. That’s three orders of magnitude more costly than anything we are capable of, plus unthinkable aiming dynamics, to launch one 4g spacecraft. But yes, it’s slightly better than the more obvious option which was six order of magnitude.
My next SF novella is now going to have the main protagonist, a solar-powered AI space station, buying laser boost from a handy laser "filling station" whenever it is in a hurry. What to call the station brand? It won't be BP.
Shame that any patent will have expired by the time it actually happens. You read it here first!
What if, after a while of spending all that energy propelling a (hu)manned light sail craft towards Alpha Centauri or Barnard's Star, there is a political or economic disaster here on earth, and we can no longer afford the laser? Or terrorists get at the laser and damage or destroy it? Or there is another unforeseen event, preventing the laser propelling the light sail? Of course, that could never happen could it, our fuel supplies, food supplies, economy and environment are clearly totally able to support political and economic stability for the whole period of the trip.
Actually, could we dispense with the laser and use light reflected from the Sun? Then we'd 'only' have to build and aim a really big mirror, instead of a really big mirror and a big, hungry laser?
OK time for my medication ...
Bsetr wishes, everybody and remember:
MIND THE GAP
Could we put a large solar powered array in orbit around Sol, so that it is relatively fixed with our interstellar craft, and generate the "back to the future" power necessary to spark up the laser?
It could even be initially purposed to send power back to Earth, and perhaps this be a means of funding the whole project.
Can a laser make an object accelerate in space? Absolutely. Given a sufficiently powerful laser and a sufficiently light object.
Can you use lasers to decelerate enough with current scientific knowledge and unlimited funds? Probably not. You could, however, limit the sail craft's speed to a point where it could turn around and use the sail to reflect the star's light and decelerate enough to be captured.
Is this idea practical? Not for interstellar flight but possibly for intrasystem flight.
A question for the boffins - why a polymer rather than a single atomic layer of graphene coated with a single atomic layer of silver?
Still, a light beer for the boffins.
Unless you are sending a lightsail "ark" full of settlers to Alpha Centauri, what is the point of this trip? You've just spent billions of $ to build your spaceship, sail and lasers, and you aren't getting anything back.
This effort might be worth it, if you could send a space probe to reconnoiter the Alpha Centauri system, find anything interesting, and then return to Earth. I VERY MUCH doubt that your probe would have powerful enough communications array to send any real telemetry back to Earth from 4 light years away. Plus your probe would need an ion engine or something else on top of the light sail to navigate within the Alpha Centauri system to find these interesting bits we are looking for.
So it would need to deploy its light sail, spend 30 years getting to Alpha Centauri, take down its light sail, cruise around Alpha Centauri conducting research for several years or more, deploy its lightsail again and return to Earth and then slow down enough again to be recovered, so the data can be analyzed. So on top of the engineering challenges inherent in all that, we are talking about a 70 year+ mission.
If you can't do all that, then you are not getting anything out of the mission other than a proof of concept that you can shoot something out of the Solar System using a lightsail. Humanity would be better off using that money for missions within or on the edges of our solar system.
If you are going to be practical and talk about benefits for humanity, we would be far better off spending the money on eradicating poverty and addressing many of the other ills the planet has.
However, in terms of advancing humanity in terms of the larger universe, longer term projects and thinking are things we should consider, currently, most projects are determined by politics (so results before or by the next election), or ROI which is mostly determined by share price, patience of the stake/shareholders and the state of the markets.
Space researchers are probably the most forward looking people on the planet and I am sure you would have little problem getting space researchers to embrace and work on a 70+ year project.
"... deploy its lightsail again and return to Earth ..."
Driven by what? This is necessarily a "square rigged" spacecraft. It can probably tack a bit but can't sail into the wind or even close to it.. The Earth-based laser (supposedly) got it to Alpha Centauri, but it can't bring it back.
"Unless you are sending a lightsail "ark" full of settlers to Alpha Centauri, what is the point of this trip? You've just spent billions of $ to build your spaceship, sail and lasers, and you aren't getting anything back."
We have already spent huge amounts of cash (multi-billions) to send things into space that we will never get back. Almost everything that we have thrown up there with very few exceptions was on a one-way trip.
Perhaps we could send oh, say an unmanned device to explore another star system with a means of communicating those results back to Earth? After all, 4 - 5 years to receive a transmission is acceptable given that it was nine years before the New Horizons team saw images of Pluto. Yes, it's going to take a lot longer to get to another star system than the trip to Pluto but it's not an unthinkable time if we can hit a low fraction of c. About 0.05c has been estimated as possible which gives about a century to the nearest stars.
But really, their light-sail is mostly copper with a bit of graphene on top. They did punch holes in the copper, so the graphene does fill some holes in the copper surface :).
What I found troubling was a statement in the abstract, tat: "The measured thrust is one order of magnitude larger than the theoretical calculations for radiation pressure alone. This calls for further theoretical studies and increases the interest of graphene as light-sail material."
Sounds like they are ablating some material, which would make it more of a rocket, less of a light-sail.
To my very dumb bunny non engineering mind the big problem with space flight is reaction mass. Something has to app!y a force to make your spacecraft move.
The attraction of light sails is that you have a constant (though diminishing) supply of light to push you along so you don't have to carry and burn your own reaction mass.
However the solar supply may be effectively constant but inverse square law means that the amount available gets less and less the further you are from the sun.
Lasers are an attempt to tinker with the basic fact that free but diffuse energy implies a very long time span for any interstellar mission which you want to be able to stop at the destination and then return.
Beyond a certain time span the mission starts to become pointless.
So what is the likelihood of improving the efficiency of the conversion of matter to energy?
Beyond a certain point you can presumably generate enough energy to slowly accelerate and decelerate a very large mass. Thinking Footfall style of interstellar travel, and refueling at a gas giant.
Solar energy is effectively free and constant but of limited magnitude. So in, say, 70 years time will we be able to efficiently burn rock or gas for energy and have effectively unlimited reaction mass within the solar system and other star systems?
Also, my dodgy ScFi memory suggests that you may need a force field to clear all the matter in you flight path to avoid high energy impacts. This in turn can be sucked up to provide reaction mass.
Given that we have interstellar visitors shaped like huge rocky turds might we be better off tooling up to hitch a ride on one of these? Or perhaps create our own and keep pouring energy into it as it orbits the sun until we can slingshot it towards a distant star?
I love the idea of light sails but it feels as though they are only one part of a complete solution.
Fission fragments rockets are doable with the technology available today, according to NASA studies https://www.nasa.gov/pdf/718391main_Werka_2011_PhI_FFRE.pdf (There is a lot more available on their sites.)
Able to reach speeds of several percent of light, with heavy probes, missions to the near stars at 50-100 years are doable
Looking a bit further, anti-matter assisted fission and fusion drives (not pure anti-matter) look promising.
According to the NASA studies, the main problem is producing enough anti-matter.
A couple of accelerators similar to the LHC at Cern can do it, but they don't come cheap.
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20200001904.pdf aAnd a lot more).
If I were in charge of the budget, I'd start building fission fragment rockets, instead of buying vintage chemical engines at $146 each, which is the current silliness:
https://arstechnica.com/science/2020/05/nasa-will-pay-a-staggering-146-million-for-each-sls-rocket-engine/
/Ola
"Could a muti-stage in-orbit coilgun accelerate small craft close to the speed of light?"
From Quora;
How much energy does it take to accelerate 1 gram mass to 99% of the light's speed?
2 Answers
Alexander Young
Alexander Young, Professor of Mathematics (2015-present)
Answered Sep 8, 2017 · Author has 381 answers and 1m answer views
The kinetic energy of a mass at relativistic speeds is:
KE=mc2(11−v2/c2√−1)
For m=0.001kg
and v=0.99c
,
KE=(8.988×1013J)(11−0.992√−1)=(8.988×1013J)⋅6.089=5.473×1014J
This is about 131 kilotons of TNT, which is quite a lot for something the weight of a paperclip!
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