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NASA Demonstrates Space Sails (In The Lab)
caelumcrux writes: "Once again, technology repeats itself. NASA's jet propulsion laboratory released information on their latest testing: the use of lightweight sails and beamed-energy propulsion for space travel. It's only a matter of time before kids are flying photon kites." The laser sails have been covered here before but the microwave experiments seem like a greater success. Thankfully, this has a few more details than many NASA releases, and even features a link to a (not very good, artists-conception type) picture of the thing.
The source is pushed back. But the source may have a lot more energy available than the sail, and/or being setup on something of a high mass (like the moon, for instance).
The momentum of a photon is independant of its velocity (which is c no matter what) or its mass (0 no matter what either). It is known as h/l, where h is the Planck's constant and l the wavelength. So if there was a loss of momentum what would happen is a frequency shift.
But, in practice, there is no noticeable loss in the absolute value of the momentum, only a change in the direction of the vector, which becomes the mirror image of what it was when coming in. The difference between the two vectors is the amount of reaction given to the sail.
OG.
That's if you're trying to blast something into space using rockets. How about using a "space elevator" as detailed in a million-and-one sci-fi books, such as the Red/Green/Blue Mars trilogy?
The concept is that you have a gigantic cable with one end fixed to a point on Earth, the other having a massive weight on it, somewhere outside of our atmosphere. Anything you need to get into space is simply pulled up the cable. It'd be a pretty big engineering project for sure, and don't ask me what effect it would have on tides or what would happen if the cable broke, but it'd be safe and cheap (on fuel) once it's there.
That'd still be a few years off though...
AFAIK, "solar wind" is actually a stream of highly charged particles streaming away form the Sun. Particles have more mass than photons, so they could potentially push a sail.
...richie - It is a good day to code.
If you are really able to accelerate these sails by a couple of g, then would it be possible to accelerate a payload through the atmosphere into orbit?
So I happen to think about it like Feynman did. That is hardly the worst possible sin.
What I said is perfectly correct, and the formulation that I used is mathematically cleaner, albeit less useful for particle physicists.
Regards,
Ben
My usual seat in the cluetrain is at A HREF="http://pub4.ezboard.com/biwethey.ht
And decided that the moderators would likely misunderstand the response I had in mind.
:-/
I was thinking something like, "You are no Einstein" and pointing him at E=MC^2. Then following up with the particle-wave duality and mentioning the experimental issues with diffracting a herd of elephants.
All true, but given the level of science around here, some moderator would be sure to misunderstand.
Cheers,
Ben
My usual seat in the cluetrain is at A HREF="http://pub4.ezboard.com/biwethey.ht
Why do I use it? hbar is more convenient than carrying around that 2*pi in many circumstances. E=hbar*omega, where omega is the angular frequency. (note there's a 2*pi in hbar, and one in omega too that cancel out) hbar is also unambiguous. The alphabet gets used and re-used (and greek alphabet) in any science, and the meaning of variables becomes obscured. But there's only one hbar!
--Bob
1^2=1; (-1)^2=1; 1^2=(-1)^2; 1=-1; 1=0.
I'm not an expert on this. It's called Mini-Magnetospheric Plasma Propulsion (M2P2) and they claim solar panels and about 3kW are enough to sustain it.
Well...all of it. There isn't anything that "absorbs" the solar wind (except obvious things like planets). The solar wind ends at the heliopause where is where the interstellar medium takes over. The heliopause doesn't happen until well past Pluto, the Voyager and Pioneer probes will pass through it soon, and scientists are eager to see if they send back any interesting data about it.
I haven't run this calculation, but yes, atmospheric distortion would be a problem, as would diffraction (at millions of miles, diffraction effects from the opening end of the laser can be large!) Compensating for atmospheric distortion may be possible. Extremely precise aiming devices would be required. It would also be a given that a large portion of your laser would miss the craft entirely. But even if 1% of the laser's power hits the craft, I'd guess that it's still cheaper than launching an equivalent power source.
--Bob
1^2=1; (-1)^2=1; 1^2=(-1)^2; 1=-1; 1=0.
Some 25 years ago, at the tender age of 10 I invented, at least for myself :-) the idea of space sales. I remember it quite clearly as I'd been round a friends house when I spotted this gadget on their window sill. It had a thin metal pole in the middle, with four paddles mounted horizontally to a rotating joint at the top of the pole. Each paddle was painted black on one side and white on the other. The whole thing was enclosed in a glass dome (I'm guessing there was a small vaccuum inside). When exposed to sunlight, the paddles would spin. It took me a little while to find out how it worked, and then a little longer to transfer that idea to a space ship, and resulted in me drawing a picture of The Solar Sail Powered Space Ship for a school project.
Not long after that I started to discover girls in a big way and all thoughts of sailing through space evapourated into the mists of childish fantasy.
Macka
ROFL...
I've been watching McGyver lately (they show reruns on WGN, and I don't have much to do because school's out), and it's a really neat show - but he ALWAYS seems to build a bomb in each episode. Either that or disarm one. Kinda cool tho...
True. However, my typical methods on Slashdot for correcting people are usually to just try to calmly correct people without trying to be insulting.
I decided a while ago that people are really truly fascinated with physics and astrophysics for some reason (I wrote a paper on this in college, actually, it has to do with something I call the "carpenter effect": people are somehow mystified and at the same time scared of whatever or whomever created the world. It shows up in almost every major religions, except a few) and therefore, considering that they're both my field, and that bad science in both of them is rampant (see also xxx.lanl.gov's gr-qc list, which has a number of total-bull papers) that I should do my best to inform people of the truth.
A professor of mine told me once that he thought that quantum mechanics should be taught before classical mechanics, because then we'd never even think about these dumb ideas like absolute locality, etc. I'm inclined to believe him. People who don't complete physics the entire way through get more misinformation that information, quite often.
Man, this is offtopic. But, eh.
Actually, as New Mexicans are quite fond of saying, they're Americans, rather than Mexicans. It's one of only two states/provinces in US/Canada that actually has the name of its country on the license plate.
You are correct about the atmosphere diffracting and spreading the beam, however, this is an engineering problem rather than a theoretical limitation. The beam would scatter uniformly, and diffuse so that the main effect of the laser would be to heat the atmosphere slightly rather than doing any serious danger.
New Mexico has one of the lowest population per square meter in the country (Alaska and Wyoming are ahead of it, I'm almost sure). I didn't say that "the" people there wouldn't be an issue. I said that "people" wouldn't be an issue - because there wouldn't be any around. It's the same reason that nuclear testing grounds were in those areas.
Considering the pathetically minimal impact on environment, I'm surprised you're even concerned. Myself, personally, I'd be more upset about the rapid urbanization in many of the cities than what's being done out in the unpopulated desert.
And the air being dusty means little - it depends whether or not those dust particles absorb/reflect the wavelength of light being used. Most likely not.
Sigh. I didn't actually mean to propose New Mexico - I was just there recently, and there is a lot of scientific activity there simply because the population density is so low, which is why I used it as an example. (See also Los Alamos, and nuclear research)
What I was trying to get across is that this would be situated in an area with virtually no one around, so the whole "public danger" argument is crap.
Okay. Find out for yourself then. Do the research, and post here what you find.
A stupid man will believe everything someone tells him.
Another stupid man will not believe anything anyone tells him.
An intelligent man will find out the truth.
One thing that science absolutely loves is alternatives. There are multiple methods to solve any problem. Look at semiconductors - for most purposes, silicon is far superior to germanium as a basis for semiconductor devices. However, germanium devices are still made. Why? Because they are useful in certain tasks, and they are improving.
Ion drives have limitations. Slow acceleration, for one. Second, they still require weight to be carried, so that liftoff is still heavy. Third, they do contain slightly expensive materials (if memory serves, the one on DS1 used xenon ions - xenon is pretty expensive. Not that expensive, but still expensive). Fourth is again, the lack of acceleration - which means you can't change course. At all. Or slow down. You'd need to slow down at the same rate you sped up.
Sails have many advantages, but by far the weight requirement is the greatest. Low weight means little energy needed for fast acceleration and high speed. Therefore it is more efficient to use sails rather than ion drives for many applications.
Ion drives are not a panacea for space travel. Far from it! Sails actually offer a very good possibility for intra-system transport, and ion drives do not.
On a personal note, I do not think it is a good idea to second-guess the research decisions of people who are in a field you are not in. If they are working in your field, then, yes, you can have a valid criticism. But if you are not working in that field, you do not know all the information, and therefore, cannot make an informed comment.
I highly doubt that NASA is financing anything with the word 'space' in it - and research like this is ridiculously cheap compared to actual space projects - a pathetically small amount (thousands compared to millions). Spending a fraction of your money on research that could potentially save you a large fraction of your money isn't bad business - it's good business.
Actually, momentum is not the product of mass times velocity solely. Fields can carry momentum and energy, and a photon is a quantum of oscillating electric and magnetic field. For a photon, the momentum carried is directly proportional to the energy of the photon, with a constant of proportionality of the inverse of the speed of light (energy = momentum * speed of light).
The sails reflect light. Which is beneficial, since it means that twice the momentum is transferred into the sail as would occur if the sail absorbed it.
Except for the fact that none of the radiation seeps. It's a laser - it would be collimated. People would have no idea that it is anything other than a large satellite dish.
Radio antennas give off radiation too - radio radiation. No one worries about radio radiation - there are plenty of people right beside a radio tower - hell, some are built in the middle of cities. Somehow I've got a feeling no one will really care, at least, after a few years of no one dying because of it.
You are in more danger from your microwave oven than you would be from these things.
Besides, NASA isn't stupid. They'd probably build it in New Mexico, where it's flatter anyway and easier to build a directional array. People wouldn't be an issue.
Yup. However, I also took the time to realise that other people might not. This way, when they read that message, they'll also see mine.
In my opinion, far too many people don't realise dry sarcasm and take it at face value, so it's a bit of a safety margin.
I also realised that I forgot to add the "I'm pretty sure this was a joke, but just in case..." line that I normally stick at the bottom.
I really don't like bad science. I do my best to prevent its propagation by any means possible.
He's right. Either leads to a consistent formulation of special relativity. That being said....
I can also point out that from the point of view of a purist it is cleaner to think that way.
Others may argue just as convincingly that it is cleaner to interpret mass as a Lorentz invariant and incorporate the extra gamma factor into the momentum. Ultimately it seems to depend upon what one intends to do with the SR formulation. Since much of the more modern physics literature (e.g. Jackson's Classical Electrodynamics, Landau and Lifshitz's The Classical Theory of Fields) interprets mass as the Lorentz invariant quantity m showing up in E^2 = m^2c^4 + p^2 c^2, using the more conventional definition has some pragmatic value.
I must confess that I've never done any calculations in general relativity using your convention for mass, and my interest is piqued. Do you find GR is easier to interpret using your mass convention?
s/mass/rest mass/g
*Wipes the egg off his own face.*
...and are supposed to show that photons have mass, well they don't actually work.
:)
No surprise. No measurements have demonstrated that photons have mass--in fact, they have been shown to have, if any mass, only an extremely small amount of mass. If the science-fair prop were to show that, then a call from Stockholm would follow not long after.
They rotate the wrong way as the heating effect caused by the solar radiation is vastly greater than the push from the photons
This doesn't make any sense. What is the "heating effect" to which you refer?
I understand that photons have momentum (or, classically, that a nonzero Poynting flux impinges on the vane surfaces), and that for optical frequencies roughly twice the momentum is imparted to the reflecting surface than the black surface by conservation of momentum. So one might guess that the vanes would rotate in the direction with the black faces moving forward. What is the mysterious "heating effect" that invalidates this argument?
But it looks cool anyway.
Well, we agree on this anyway.
Wrong! As we all know, light is a wave and as such has no mass, therefore by definition it always has zero momentum...
This is not true. While I'm sure you will be inundated with all the quantum theory of light "Hey bonehead, E = h nu and E = sqrt[(mc^2)^2 + (pc)^2], so for light (which has m = 0) h nu = p c, or p = h nu / c" posts, I would like to comment that you can also get momentum flux from classical electromagnetic theory as well. (It is clear to me from your post that you have no background in relativity, and--I'm guessing--quantum theory, so this may be more relevant to you):
Basically, from Poynting's theorem one can show that the volume integral of (E x B) is proportional to the electromagnetic momentum.
Absorption of the light is the only way to make these sails work - the light heats up the sail, thermionic emission of electrons occurs and it is this emission of electrons which is responsible for the change in momentum which propels the spacecraft.
Not really; if desired you could just have the sails be reflecting surfaces. If you reflect the incident wave, then the momentum exchange will be twice (roughly, allowing for dissipative effects) that of the incident wave.
A little knowledge is a dangerous thing.
Whoooeeee! Stick the pig, Pa, we're havin' dinner!
Someone obviously MISSED Physics 101. Too much time watching WWF maybe? Or out tippin' cows maybe?
As per BASIC HIGH SCHOOL PHYSICS,light has properties of both particles and waves, and is almost, but not completely, massless.
Please wipe that egg off your face, BadERA, and go tell your high school physics instructor that he failed miserably in teaching you basic high school physics.
Light has momentum but has never been shown to have any measurable amount of mass; it has been verified experimentally to an enormously high degree of accuracy that light does not have mass.
This is really a cool development. I wonder what level of acceleration these methods of propulsion have, and at what distance beaming the required energy becomes infeasible.
They had a also better watch out for those Grid Bugs, too.
Washington, DC: It's like Hollywood for ugly people.
The concept is sound, and remember in space you have perpetual motion, you only need the sail and some sunlight to start you off, then your flying and you can litterally "surf" on the orbits created by celestial bodies so as to remain in motion.
um. space does have mass and gravity. therefore, it is by no means perpetual motion.
Because `kinetic mass' does not exsist. Saying `the mass of an object increases when it goes faster' is wrong. Mass has absolutely nothing to do with speed. The mass of an object is the same in any system, inertial or not. That's the way it's defined. You can't change that.
From now on I will refer to mass as the physical defenition of mass.
So photons do not have mass. They do not have kinetic energy either. Their energy is actually in an electro-magnetic field. So their energy is dependant on their frequency.
There is no problem in stating energy and momentum conservation laws in Special Relativity, or General Relativity. If there were, the theory would have been abandoned by Einstein himself. In General Relativity the conservation law of energy and momentum is... P^{\mu\nu}P_{\mu\nu}=0 (tensor math). Done. In Quantum Mechanics, yes, there is a possibility of `borrowing' energy from nowhere really for such a time that it cannot be detected in any way. Every (sane) physisist you will talk to will tell you that energy is globally conserved.
The sail temperature issue is non-trivial.
Let's look at this sail temperature problem, shall we? It's not all that difficult really. The total power emitted by an object of temperature T and surface area A is A \sigma T^4 where \sigma equals the Stefan-Boltzman constant. In the article, they say approximately 10 kilowatts was beamed to the sail. In equilibrium these figures must be the same, which yields T=1152 Kelvin for a sail of 1 square meter total, that's front and back. Yeah, that's probably hotter than you'd want it to be.
So no, the energy is not 0.5mv^2
No, it's not. With your defenition of mass, it would be (m-m0)c^2, m0 the rest mass. Your own strange defenition of mass is killing you here. Use E=Gmc^2, G=1/sqrt(1-v^2/c^2) instead. If you don't believe this results in Ekin=0.5mv^2 for v small compared to the speed of light (Newtonian), make a Taylor series to the linear term, and substract mc^2.
The relation of energy to momentum is E=c*sqrt(p^2+m^2 c^2). Assuming p to be small in comparison to mc and v again small compared to the speed of light (Newtonian), yes this results in Ekin=0.5pv.
Incidentally the wavelength is irrelevant to how efficient light sails are.
True. The efficiency is only dependant on the percentage of photons missing the target, the percentage of photons being absorbed in the interstellar matter, and the reflection coefficient of the sail.
Newton's statement of his three laws is actually completely correct in SR.
I can't come up with a reason why Einstein would come up with his Theory of Relativity if this statement is correct. Can you?
I've just read your follow up on a reply to this post, and here's what I have to say about it.
First off, I read the "random decent reference", and found it to be in accordance with my remarks, and not yours. Read it again, and remember that when it talks about mass, it is talking about what you would call rest mass. Then note that for v=c the gamma factor goes to infinity. Since the rest mass is zero, gamma*m is undefined. Your background in math should tell you that much. By the way, a mathematician should be able to apperciate an invariant quantity. I know, because I am not only a physics student, I am also a mathematics student. And a astrophysics student.
And from a purist's point of view it is not better to look at mass as the relativistic mass. The energy in your body as heat is most certainly not measured when you step on a scale. What you need to do is forget mass, and go for energy! Mass is not conserved. If it were, nuclear fusion or fission wouldn't work. Stars wouldn't exsist. Energy is conserved, and a far better quantity to work with when relativity is needed.
Cheers, Alfred.
You can find an introductory article to solar and laser driven lightsails on my website. It was written more than a few years ago now, back when I was an undergraduate, but its aged fairly well (unfortunately). If you want a really in-depth look at this topic I'd recommend Colin McInnes' book Solar Sailing: Technology, Dynamics and Mission Applications .
Al.--
The Daily ACK - Eclectic posts by yet another hacker
One of the reasons for going to the carbon composite sail it that it's highly refractory (resists heat very well). If it radiates as a blackbody and it can tolerate a temperature of 1500 Kelvin (around 2200 F), it can dissipate about 290 KW per square meter - in each direction. If it can radiate both forward and aft, that's over half a megawatt per square meter. With that kind of radiating capability, it doesn't much matter if you reflect the incident power or absorb it (though it does affect your energy efficiency).
--
Ancient Goth: Someone who overthrew the Roman Empire.
Time is Nature's way of keeping everything from happening at once... the bitch.
Just out of curiosity, why do you use hbar? I always thought hbar = h/(2*pi)
The momentum of a photon is given by p = h/lambda, or hc/f (where lambda is the wavelength). This can be derived using E=mc^2 (from Einstein), E=hf (the energy of a photon, where h is Planck's constant) and p=mv (or mc) and doing some algebra. This will also give you a variable mass depending on the wavelength, but I've heard that that's only valid relativistically.
There are some solar sails among the images i have at Elfwood.
;)
:)
Like Captain Poppins kite and the U.N. solarsail 'Chinook'. Please check them out and leave me a comment or two. Maybe my images are good enough for NASA, if that is the case then feel free to give me a job...
There are other pictures in the gallery too...
Try this one on for size.
Lasers produce COHERENT beams of light. Yes, they emit visible radiation. However, as they emit this radiation in a directional beam, and shed very few photons away from that axis, Mr. Erikson has very little to fear, unless he chooses to stick his pinhead in front of the wrong end of the laser.
I am, therefore you think.
Jon, let me thank YOU for keeping ME amused at work. So when's the next binge of fearmongering?
I am, therefore you think.
Whoooeeee! Stick the pig, Pa, we're havin' dinner!
Someone obviously MISSED Physics 101. Too much time watching WWF maybe? Or out tippin' cows maybe?
As per BASIC HIGH SCHOOL PHYSICS, light has properties of both particles and waves, and is almost, but not completely, massless.
I am, therefore you think.
Assuming, of course, it becomes even remotely feasible to do either one within the next few decades...heh. What's next?
Maybe this...
Remember, kids, it's only premarital if you plan on getting married.
It would be too long to wait for a proper betting pool, but any guesses as to how long it will be before they try this in space?
I've heard that the Russians tried it in the 50s, but back then there weren't materials that are light enough, so it didn't work as well.
I do not believe that photons (i.e. light) can travel at any speed other than C...
You see light travel at speeds slower than C every day, light slows down in water, even in our athmosfere light doesnt travel at C but a tiny bit slower
42
I know it makes some problems simpler, but how about this one: if I apply force F in x coordinates to particle at reset with mass m and x(0)=0 then x=ln(cosh(Ft/m)), where F/m is in terms of c/s, and x is in light-seconds. I took a senior-level course in relativity, and I don't think it was covered. Usually I learn mathematical concepts pretty fast, but it took me a while to get used to raising/lowering indices on tensors.
he interesting thing about /. is how little basic science a lot of the people who think they are qualified to answer your question have. Anyways, here are the answers: 1. I have no idea how long until they try this in space. 2. Yes, by Newton's third law the source is pushed back. As long as that is a planet, you won't care much. 3. a) Think ping-pong balls. When you hit them you can change the direction of motion without changing their speed. That is a transfer of momentum. b) Momentum is mass*velocity. The velocity of light in a vacuum is constant. The mass is dependent on the frequency. So light coming from a gravity well, or from something receeding from us loses energy, and the photon is "red-shifted". c) They would want to avoid absorbing photons since that would burn up the sail very fast. Energy scales as velocity squared, and light moves pretty fast, so it delivers a *lot* of heat for the momentum. About your 3 point, that's not what I learned in my relativity course. momentum is mv/sqrt(1-(v/c)^2), where m is rest mass. Last I knew, photons did not have rest mass. If I remember correctly, energy is sqrt(mc^2 + (0.5mv^2)^2/(1-(v/c)^2)), which is not quadratic.
Momemtum is not mass times veloity for velocities approcaching the spead of light. And photons don't have mass. Why are all these posts with incorrect information rated so high?
Your energy formula isn't quite right (it's somewhat more complicated than that), but at least you got the correct momentum formula.
Was it Bruce Perens who wrote the sci-fi story about a space..eh..sail..eh spacysailships holding a race around the earth ?
Of course their sails were solar powered, but it's kina scary how sci-fi can sometimes hit home ?
I just hope apocalypse now wasn't that acurate...unless I get to be the hero and save the world of course.
PS. Slashdot has now surpassed scandinavia in it's troll population !!!!
"Semper in excretum set alta variant"
sigh
If you read the New Scientist at all, you might have seen an article in it about the e-bomb, which basically is a cheap (and easily) constructed weapon that uses microwave radiation to generate a set of pulses to frizzle electronic hardware. My immediate thoughts are that a misdirected beam of microwave radiation could be fatal, frizzling the internal circuitry of the craft, should it ever get into space. I just guess that they'll have to provide some pretty effective shielding. Impressive nonetheless, though. M.
It would be too long to wait for a proper betting pool, but any guesses as to how long it will be before they try this in space?
Also, by Newtons Third Law, shouldn't the source be pushed back as well? And what about conservation of momentum? If the photons have given up some of their velocity (momentum) does that mean they drop below the speed of light? How does this work?
Russell gave a superior smile before asking what the turtle was standing on. "You're very clever young man, very clever," replied the old woman, "but it's turtles all the way down."
-- told by Stephen Hawking
If you MUST spam EVERY discusion with talk of MDMA, could you please, at least fix the ascii art?
--Josh
There are exactly 42,935,718 letter sized sheets in a square mile.
The guys who did the microwave sail have a web page at Microwave Sciences, Inc. There are some abstracts on there, but not the full papers, unfortunately.
--
WordSocket Voice BBS Software
62.5 megajoules/kg would give you a velocity of about 11 km/s, which is approximately escape velocity for the earth. This sounds like a lot of energy, but it really isn't all that much. 62.5 megajoules is the same as 62,500 kilowatt-seconds, or about 17 kWH.
Including taxes, Illinois Power is currently charging me around 13.7 cents per kWH at residential rates, so this much electricity would cost only around $2.00. A 100 kg person-sized mass could be launched for a couple of hundred bucks in electric charges.
Of course, you'd have to add on the cost to lift a life-support system, and take into account that this thing is not going to be anywhere near 100% efficient. On the other side of the coin, big industrial users can get electricity for a *lot* less than the residential rate.
That's equivalent, per pound, to taking something that weighs a million pounds and lifting it twenty feet!
I think your calculations are off a little bit here. I'm getting more like 9.4 feet.
--
WordSocket Voice BBS Software
So this is what they needed the spider silk milking "goat system" for. I just hope the Shadows won't get mad when encountered out there...after all, they DO own our spiders.
I was reading through the NASA press release and read that they characterized their sail material as 'stiff'. Ignoring jokes about dead people, this sure makes it difficult to fold it together nicely in a spacecraft launched from Earth and then unfolding it in near space which, as I understand it, is the way this is perceived to work.
Also, I'd like to know what they mean to do at arrival? I mean, you've got to apply the brakes at some point in time - just turning off the beam will keep you floating indefinately. Aero-braking perhaps? Or maybe a retro-rocket? Maybe one of those brand new ion-drives? Let's just skip the sail and use the ion-drive to get there :-)
It's fascinating techology, but I'd REALLY like to know what practical purpose this could have, that wouldn't be easier to solve with other technologies.
Black holes are where God divided by zero
Actually, depending on the range needed, a sail configuration may end up being the fastest method. That's because a big sail can not only produce more push than an ion drive, it will never run out of fuel - so you can accelerate all the way to the halfway point, and then turn around to decelerate (or, on an interstellar trip, perhaps use a magnetic sail to decelerate faster, as someone else pointed out). An ion drive will invariably run out of fuel at some point and then you'll be coasting forever.
The advantage of an ion drive is that it's much smaller - thus easier to deploy (there are several theoretical deployment designs for unfurling sails by rotation, mostly, but such designs add weight, and you want it to be as lightweight as possible to use the push for the cargo, not for the sail), and also independent of remote sources of power (for a solar sail, you lose push and risk destabilizing the sail configuration if your orbit takes you in a shadow of a planet - for microwave mesh sails, you need the microwave transmitter aimed and powering the sail all the time).
Ask a silly person, get a silly answer.
Well regarding the unlimited range of laser I was more interested in what is cheaper . At least I thought refuelling stations would be better. After all I can build a car with a 5000 gallon gas tank but I prefer Shell;)
**Life is too short to be serious**
The press reease was a bit sketchy on a few pts:
1>has Nasa perfected the method of how to keep the beam always on the sail and keep it focussed over long distances.
2>is the sail material capable of taking the heat over long periods of time?
3> wouldnt it be better to spread the whole energy over the whole sail by varying the pt at which the beam hits the sail
4> Does NASA propose a series of refuelling stations along common space routes which will boost spacecraft on their journey. As soon as a spacecraft goes out of range of a boosting station the next one will start beaming to it
5>What will the beam do to any spacecraft coming in its way close to the source (Whoooosh!!!)
**Life is too short to be serious**
I stand corrected ;o
Bagpuss
Your friendly cloth cat
Grid bugs follow floating eyes around, just in the hopes of dealing the final, humiliating blow to some hapless adventurer.
You are frozen by the floating eye's gaze!
The grid bug bites! You get zapped! --more--
The grid bug bites! You get zapped! --more--
The grid bug bites! You get zapped! --more--
The grid bug bites! You get zapped! --more--
You die.
Would you like to identify your posessions? [ynq]
Then you get the final shot of your tombstone, and you are recorded for eternity in the logfile as:
Emrys-W level 6 - killed by a Grid Bug.
In post-9/11 America, the CIA interrogates YOU!
Let me see: you're pushing drugs, so you're either a psychiatrist or a DEA agent. Right?
--
--
What? WHAT?!! Oh.
By use of a remote laser or microwave source, beamed energy can be directed toward a space sail. In space, the laser or microwave source may be provided by a satellite or other type of spacecraft.
..and the craft providing the laser or microwave source will us space sails powered by another satellite or other type of spacecraft, which in turn will be powered by another satellite or.. Etc etc..
I can just picture a long chain of space sail crafts providing energy to one another and stretching from earth to destination.. Watch out for them meteors!
air and light and time and space
The laser would be earth bound, and nothing more than a a large peace of equipment. A laser pointing towards the sky shouldn't scare anyone.
= =
I openly admit that I am not a physics major and I haven't spent very much time with anything related to aerospace, but if you mount the laser on earth, wouldn't you then be required to have multiple lasers to account for the rotation of the earth? This would also cause problems as the laser responsible for primary propulsion was switched, as the velocity would change, and therefore result in a constantly changing velocity/trajectory, dramatically increasing the difficulty of a simple mission. It seems that the earlier comment of a space based laser would be more appropriate, but even that must orbit something... so wouldn't it still face similar (although possibly substantially less) problems?
Please correct me if I'm wrong.
===============================================
If ignorance is bliss, wipe the smile off my face
Mooniacs for iOS and Android
wouldnt it being a laser or microwave not really move it at all since it is just a electromagnetic emmission of some sort?
Yes. The whole space sail idea must be a lie, and was apparently posted for that reason. There are a number of other examples of apparent NASA fake science, like Wormholes. I am not certain Wormholes are completely bogus, but there are much easier and simpler ways to achieve the same results making Wormholes superflous even if they were real. Generally, NASA seems to advertise the junk science as supposed work in progress and the real (classified) science as impossible (i.e. time machines).
To explain the falacy of space sails, you need only observe that photons have no mass (right?) and this must be completely obvious to any real physicist. This is easy to see because photons, which move at the speed of light, would need to obtain infinite mass (and momentum), according to special relativity, in order to reach the speed of light (if they had mass). Hence, the speed of light is the speed limit for matter according to special relativity.
Therefore, space sails would not accellerate at all when reflecting or absorbing photons, providing the space sails did not expel matter. The only possible propulsion would be caused if the photons were absorbed, the sails became hotter, and this caused the sails to expel matter (propellant) thereby causing (very very little) propulsion, but this would be much much less efficient than otherwise harnessing the energy from the photons to expel propellant.
Note that since space ships will be unable to travel at the speed of light (or probably even close to it), it would take a LONG time for any human to travel outside the solar system (at least, the first time). Interstellar travel will probably be primarily via teleportation between colonies using teleportation devices since the only alternative would be spending many years in space ships. Today's "phone calls" would be replaced by telepresence using brain interfaces to provide virtual sensory input comparable to "being there." However, it will initially take a LONG time to colonize outside of our solar system in comparison to the comparably speedy development of technology to do so because of the distance and speed limitations. Therefore, knowledge of the future of space travel (a relatively slow process) can only remind us to preserve Earth's environment rather than worry about leaving Earth.
It appears the only way to speed up space travel is to contact extraterrestrials (ET's) and use teleportation technology to jump onto worlds that we have not colonized (otherwise we would have to wait for our ship to arrive before we could explore or teleport there). I believe this was the scenerio in the movie "Contact." Time would probably not be a problem when dealing with ET's because there would be time machines incorporated into the teleportation devices to compensate for time in transit back and forth.
Finally, teleportation technology would, of course, make time travel possible. The AI (aka "God") would be needed as a time cop (and regular "supercop" of course) to prevent us from harming the timeline, however we legally define that concept. Finally, since we were not the first intelligent life in the universe, we will have to abide by any "universal" laws of the older extraterrestrials because they could easily destroy or subdue humanity...our "God" must be subordinate to theirs.
Artificial Intelligence = "Eye in the Sky" = One Dollar Bill = "The Force" = The "Martin Luther" King "God"
Cool... I can't wait to go surfin' on the magnetic waves in the rings of Saturn
let me guess - they're going to attach a sail to the front of the craft and then attach some sort of fan on the back so it blows whatever it's meant to blow into the sail and hey-presto, the craft will move forwards.
------------------
Launching payload from the surface of the Earth is one of the biggest problems in space travel, since it takes so much energy, which is compounded by the need for the larger engines and fuel storage. The other problem is time, since space is so gosh darn big.
What are the possible launch methods?
Traditionally; solid or liquid accelerant (rocket fuel.) Huge rocket boosters push against the ground in a massive show of force, wasting huge amounts of energy in lifting a heavy craft, which mostly consists of fuel and the machinery for using fuel.
Focused laser on the bottom of a craft can cause it to rise. I saw a special on TDC or TLC last year about someone using a laser burst to launch what looked like pie tins dozens of feet into the air. The problem was that because of air currents and eddies, the laser quickly lost focus and the pie tin went off track and lost its fuel, falling to the Earth again. In space, this wouldn't be such a big deal. Or maybe with a larger craft and larger laser, focusing would be easier.
Magnetic rings. I remember reading about this as a child. Some kind of magnetic ring accelerator... not sure how exactly this works. If someone with some brains could post on the practicality of this, I'd like to see some debunking (or even bunking).
I have an idea; instead of using all kinds of liquid fuel to lift (for example) the space shuttles, why not use some kind of mechanical launcher? Use whatever energy source to fuel a big device like a Ferris Wheel. Load your payload into a slot on it, get it up to speed, and then throw your craft up in the sky. What's wrong with that idea, besides the problems with accuracy in launching? It seems to me like it would be a bit more efficient than the current method, along with having other benefits; smaller craft since most of the launch power is coming externally, slower acceleration times to make it easier on astronauts, no tons of rocket fuel sitting around ready to explode, etc.
:) It had a scene with some kind of laser beam transport. Amusing science fiction.
-- CompTechNews Message Board: http://comptechnews.com/ --
Radiation from lasers? Which type would that be? You know full well thats a load of rubbish.
Microwave radiation idiot - read the article. You know, like mobile phones give off but a hell of a lot stronger? If there's so much of a scare over using phones, think about how much of a scare there'll be over a fuck off big array of microwave lasers.
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Jon E. Erikson
Jon Erikson, IT guru
Why on Earth would I want to reply to someone who is trying to "out" me as a troll for some reason known only to themselves. OTOH, your posts do serve to keep me amused at work, so I hope you continue.
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Jon E. Erikson
Jon Erikson, IT guru
What a load of nonsense :)
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Jon E. Erikson
Jon Erikson, IT guru
Eh? What is the practical use of this? I'm not disagreeing with the concept, which obviously works, but surely this is just another one of NASA's research projects that never go anywhere. They've got ion drives now, so why are they bothering with this?
The idea that they're going to be able to push anything of any use, such as cargo or satellites, using lasers is rediculous - the energy required is more than enough to destroy the object in question, which is hardly a useful end result is it? And as for power consumption, I doubt this will be any more efficient than the Saturn V once all the costs are factored in.
And even if they solve this "little" problem, who is going to want to have a huge laser sited in their territory, beaming out dangerous radiation 24 hours a day? People are worried enough about nuclear power, the idea of huge lasers and microwaves beaming across the sky is sure to provoke hordes of protestors.
NASA have a promising drive technology in the form of their ion drive, why bother with something that isn't practical? They need to stop financing anything with the word "space" in it on the off chance it'll pan out and spend their money on real space projects.
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Jon E. Erikson
Jon Erikson, IT guru
I better not forget to take the oars before leaving the shore in such a craft.
In particular, it has a link to this site, which tells all about solar sailing. It has an introduction section with "Solar Sailing 101". It should explain your questions.
"Let your heart soar as high as it will. Refuse to be average." - A. W. Tozer
Eh, you know, there is this little thing called the atmosphere through which the beam has to travel. There's all sorts of minuscule dust and crap on it that reflects and dissipates light.
No one worries about radio radiation - there are plenty of people right beside a radio tower - hell, some are built in the middle of cities.
You must work for the radio tower business. This is the standard line they give when communities complain about having antenna towers in them.
Besides, NASA isn't stupid.
Aha.
They'd probably build it in New Mexico, where it's flatter anyway and easier to build a directional array. People wouldn't be an issue.
Yeah, those people wouldn't be an issue, since they're just Mexicans, eh?
The air in New Mexico is particularly dusty, BTW, which means that the beam would reflect a lot in the low atmosphere, in unpredictable angles...
Anyway, I must warn you all against trying to use the Aztlán homeland for these schemes. We are organized, and we will resist you.
Are you adequate?
And we are to believe you?
The technology mongers have always lied to have things their way. Have you never heard of the story of the Haitian Creole Pigs? This is an important story everyone should know, and meditate on its consequences:
Thus wrote Jean-Bertrand Aristide. Thus we have the lesson: when the "experts" come telling you that you need new pigs, or that the huge laser they will put in your ancestral homeland won't hurt, they are lying.
Are you adequate?
Okay. Find out for yourself then. Do the research, and post here what you find.
Research? Is this the euphemism for what the US has done in places like Central America and the Caribbean, where they give women untested pills that result in their giving birth to deformed babies? Or when the expose political prisoners to strong doses of radiation?
A stupid man will believe everything someone tells him. Another stupid man will not believe anything anyone tells him. An intelligent man will find out the truth.
Precisely. Which involves knowing history, and knowing the nature of technology, and, more importantly, the nature of the technologies constructed by capitalism. This is the essential truth you have failed to grasp.
Are you adequate?
YEAH! An' put in some dancing women!! NAKED Dancing women! Well, ok, put 'em in bikinis at least, in case there are kids logged on (We know they never do, right? :-} )
It's a good idea in theory, but Kelly Johnson (Skunk Works ex-head) did some number-crunching, and figured out that it would take a balloon with a one-mile diameter to carry that thing around.
As he put it, "That's one hell of a lot of hot air." To put it in the context of this whole solar sail business, "That's one hell of a huge sail."
(taken from "Skunk Works" by Ben Rich and Leo Janos)
Firstly, the Sun is emitting photons in the wrong direction - you'd get a force exerted towards the Earth, not away from it.
Secondly, the force exerted by the photons striking the sail would probably be too weak to overcome the weight (i.e. m*g) of the whole system (the sails and the payload).
Thirdly, even if the force was enough to overcome the weight, you would have a huge opposing force from air resistance.
Not to mention the fact that the sails were designed for photons striking them; I don't think they'd hold up very well against trillions of nitrogen/oxygen/etc. molecules and dust particles =)
If you mean the little wheels that are silver on one side and black on the other and are supposed to show that photons have mass, well they don't actually work.
:)
They rotate the wrong way as the heating effect caused by the solar radiation is vastly greater than the push from the photons
But it looks cool anyway
troc
Troc's dubious podcast and blog: http://www.trocnet.net
I'm guessing that to launch a craft of any reasonale size you would probably need a huge solar sail.
.0001% weaker, and you won't even notice.
No, to launch a craft of any size a solar sail wouldn't work. We're talking accelerations of a thousandth of a gee here; that won't get you off the ground.
Even in low earth orbit a solar sail wouldn't work; atmospheric drag would outweight light pressure.
Where a solar sail would work great would be, say, ferrying nonperishable cargo over interplanetary distances. It may take you a couple years to get the cargo there, but the fact that you didn't need any fuel for the trip and can send the sail back for more makes up for that.
Now i've got no evidence whatsoever to back this up but personally i'd imagine huge football pitch sized things.
Imagine larger. A square kilometer of sail would be required to give a 10 ton payload good acceleration.
With all the crap floating about in space surely the chance that these would rupture is unacceptably high
There actually isn't all that much crap floating about in space, except in low earth orbit where we put it there. And with a sail, floating crap isn't a problem. So a micrometeorite hits your sail? It'll go right through; the sail will become
It's still a cool idea though.
Definitely.
1. The focus issue depends on the distance. To focus an optical beam (500nm wavelength) on a 1km sail at distance d meters, you need an aperture over which the laser is coherent of:
5*d*10^-10 m
so, at d = 40 000km (Geostatioery orbit distances) you need 20 mm aperture (perfectly standard laser aperture, off-the-shelf)
at d = 400 000km (lunar distances) you need 20cm aperture, so some reasonable optics, but nothing silly
at d = 60 000 000 km (Mars orbit insertion, using Earth based lasers) you need 30m aperture. This could probably be done, but is exceeding proven technology
at d = 10^9km (outer solar system) you need a 500m diameter coherent laser source. We have no real idea how to do that.
at d = 10^16m (1 parsec) you need a 5000 km diamater coherent laser source to focus your beam onto the 1km sail.
2. This is easy to calculate. You have to balance the ability of your sail to radiate away the absorbed energy with its strength and its reflectivity. I believe the best bet was thought to be a tungsten foil. Aluminium is more reflective, but not nearly as strong.
4. see 1.
5. The other win with light is that the momentum transfered to the spaceship does not degrade quickly with the speed of the ship. If you use a slower beam, you get more momentum/energy initially, but as the speed of the ship gets up towards that of the beam it becomes less useful
Vol 1. Chapter 16-4. Read it and get back to me on how crazy the definition I gave is.
Now when you step on a scale, the heat in your body is indeed part of that measurement. As is the air you displace, the thermal effects on the springs, and local variations in gravity. The effects of your body heat are lost in the noise, but they do indeed affect how strongly you are attracted to the Earth, and affect how strongly the Earth is attracted to you.
As for GR, and global conservation laws, you are absolutely and utterly wrong. Proof. GR is consistent with wormholes where things can move backwards in time. At one time the thing isn't there and suddenly it is - straight from the future. Non-conservation.
GR has 2 types of conservation laws of interest. Local conservation laws due to locally looking like SR. And global conservation laws when you place certain types of constraints on the space-time manifold. But without some sort of constraints you cannot even *state* what a global conservation law should look like.
As for my definition of mass, "Killing me", you are completely wrong. It doesn't matter which language you use, you are describing the same basic physics and you come out to the same basic answer.
As for Einstein's theory of special relativity, and Newton, Newton's own statements were nothing like what we are used to dealing with. For instance he said only that for every action there is an equal and opposite reaction along the same line. True. He said that momentum was conserved. True. He and his successors did not take into account that (relativistic) mass was not constant, but if you did then his statement is still inaccurate. When you state that force is equal to the rate of change of momentum, you are right. If you state that momentum is mv, you are still right if m is the relativistic mass. When you state that the rate of change of momentum is ma, you are assuming that mass is not changing. And that is what physicists did before relativity. But that wasn't Newton's statement, and Newton's words remain literally correct if you use relativistic mass and add in the fact that mass changes with velocity.
Oh, finally. If you use relativistic mass as your definition, then relativistic mass is the same thing as energy (albeit usually measured in different units) and therefore it is conserved. Nuclear bombs still work perfectly fine.
Which they darned well better because all it is is a slightly different notation for the same physics. The language you use to discuss why things go "boom" doesn't change the fact that they really do go "boom"!
Regards,
Ben
My usual seat in the cluetrain is at A HREF="http://pub4.ezboard.com/biwethey.ht
After all either way you talk about the energy-momentum tensor, and everyone means the same thing by that.
OTOH using the convention that I was taught (am I dating myself with that comment?) mass means something closer to the classical concept. For instance mass is roughly how much gravitational force you exert on the world around you, and is how much of a tendancy you have to have inertia.
In fact if you use the original definition by Newton, mass is momentum divided by velocity and that figure just happens to be the figure he theorized got plugged into the calculation for gravity. Well in SR that definition clearly gives you relativistic mass, and in GR that clearly is the energy part of the energy-momentum tensor which shows that Newton was on the right track!
So going back to the roots of the idea, it certainly makes a lot of sense.
Cheers,
Ben
My usual seat in the cluetrain is at A HREF="http://pub4.ezboard.com/biwethey.ht
My interest was math. Lemme look at my bookshelf. Here are the physics books I have with anything about relativity.
Feynman Lectures on Physics, vols 1-3, by Feynman
Tensor Calculus, by Spain (I learned GR from this book. My copy is a 3rd edition from 1960.)
Both of them are from late 50's/early 60's and talk about relativistic mass. I have a few other physics books, but none about relativity. Lemme see without moving. A fluid mechanics book, one on group theory and physics, and a biography on Newton. 8 physics books (remember 3 volumes of Feynman), 2 of which are physics/math. Without moving I stopped counting at over 3 times as many math titles.
Until today I really hadn't realized that physicists were not using the concept of relativistic mass. It has been part of every treatment I have had (high-school, Feynman on my own, and GR), and I just took it for granted.
Besides which, the definition of relativistic mass is much closer to the original definition used by Newton. How else can you make sense of questions like asking if the ratio between inertial and gravitational mass is truly 1? (This was obviously one of the questions that Newton's work raised.)
Cheers,
Ben
My usual seat in the cluetrain is at A HREF="http://pub4.ezboard.com/biwethey.ht
If life is easier thinking in terms of rest mass you start using m_0.
It is just a renaming. But it is a renaming with conceptual content. Read The Feynman Lectures on Physics vol 1, chapter 15-9. Even if I get into the habit of never discussing relativistic mass, his point there will always be how I will understand E=mc^2. That mass is equivalent to energy. And unless someone explains otherwise I will always believe that that definition is the one that fits with Newton's original definition.
OTOH it won't be the only time that a key concept gets moved around winding up somewhere completely different. For instance the original definition of "continuous" was roughly what we today mean by "analytic". It is just a little disconcerting to be on the wrong end of such a name change without realizing it.
As for tensors, I had little problem doing it. As long as I didn't mix up what it meant. However I can really say that I didn't understand it until I saw the entire thing in completely different notation in differential geometry. If that sounds interesting to you, I recommend checking out Spivak. His elementary text is Calculus on Manifolds, and his advanced text is A Comprehensive Introduction to Differential Geometry. Both are recommended.
OTOH the difference between covariant and contravariant (or vector fields and forms in the language of differential geometers) is one that is in some sense meaningless to physicists in most contexts. Outside of GR you are unlikely to deal with situations where you don't want to use a coordinate system in which there is an obvious (usually trivial) metric. Differential geometers don't have it so easy. The idea of a metric is not integral to a manifold...
Incidentally have you seen the connection between contra vs co and the concept of a vector space and its dual? If you have not then I suggest looking that up, it is the same relationship except that the linear algebra version is somewhat simpler since there is a lot less going on.
Cheers,
Ben
My usual seat in the cluetrain is at A HREF="http://pub4.ezboard.com/biwethey.ht
Robert L. Forward's book of that title has an interesting section on virtually anything to do with space that geeks could be interested in.
Space sails in particular were more interesting than I thought. Yes, there is a braking system designed using them. And the general idea is that we would set up a laser on Mercury, possibly solar powered, and use that to propel an interstellar craft. The win? The craft does not need to carry its own fuel. That is not a small win.
Anyways if you are interested in space, and are interested in what physicists who do blue sky research for NASA think about, read this book. You won't regret it.
Cheers,
Ben
My usual seat in the cluetrain is at A HREF="http://pub4.ezboard.com/biwethey.ht
1) Lasers have existed for some time and have no problems with focus.
2) That is the potential catch. Even if you reflect as much as you can, the remainder still adds up to a lot of heat. They claim it can work.
3) That kind of technical detail is less important than the proof of concept.
4) For all intents and purposes, the range of a laser is unlimited.
5) "Crisp!" is more like it. This beam would have a reasonable momentum and one heck of a lot of energy. Light is very inefficient in that way. (The only win is that you don't have to carry your energy around.)
6) In answer to another person's question, tears are a smaller deal than you would think. You want the sail itself to be light-weight, and therefore patches of sail would be connected to lines that hold the tension. Tears would degrade the sail over time, but unless you lose major cables, you are OK.
Cheers,
Ben
My usual seat in the cluetrain is at A HREF="http://pub4.ezboard.com/biwethey.ht
Because you may well be mistaken about what is incorrect and what is just a different notation. Take a look at a random decent reference for a second and scroll down to momentum.
You will see that if you call the mass of the object its rest mass, then you need a relativistic correction term. If you call the mass of the object its total mass including the mass of its kinetic energy, Newton's formulation still holds.
It is entirely a question of definition. Here is a random argument for defining the mass one way. OTOH my background is in math. I learned physics from older texts, and from the point of view of a mathematician the concept of relativistic mass is a lot cleaner than carrying around the correction term.
Now when I am explicit about saying that I am using the word mass to mean mass with the relativistic correction (ie with the mass of the kinetic energy counted as mass), what I said becomes both unambiguous and clearly correct.
I can also point out that from the point of view of a purist it is cleaner to think that way. For instance a fraction of your mass is the mass of the heat of your body, which is just kinetic energy. Likewise a fraction of the mass of the Solar System is from the motion of the planets around the Sun. To me the mass of a system of objects should just be the sum of their masses. Using relativistic masses that is true. Using rest masses it doesn't quite add up.
Regards,
Ben
My usual seat in the cluetrain is at A HREF="http://pub4.ezboard.com/biwethey.ht
There are considerable difficulties with the laser. For an interesting and detailed discussion, try this paper. (For people who don't know, the Dr. Forward they refer to is the same Robert L. Forward that writes sci-fi occasionally.)
Cheers,
Ben
My usual seat in the cluetrain is at A HREF="http://pub4.ezboard.com/biwethey.ht
The sails in this story that are being hit with energy are simply being pushed along by the momentum of the particle/waves.
The two technologies are quite different.
It isn't at all clear to me that ion drives are a better idea.
First of all, I consider the danger to be minimal compared to ion drives - The laser would be earth bound, and nothing more than a a large peace of equipment. A laser pointing towards the sky shouldn't scare anyone. However, ion drives, though safe, are on the vessel itself so if they crash, there is potentially - however small - the danger of radiation leakage. There is no such danger with light sails.
This brings me directly to the second advantage - where the propulsion system lies. The fact that the propulsion system remains on earth has several HUGE advantages. First of all, energy on earth is cheep compared to energy in space. Then, fuel doesn't need to be carried, and not even the drive needs to be carried - Actually, only a super light sail is all, meaning the mass can be much lower, making space flight cheaper and easier. Lastly, there is the issue of whethre the laser source even needs to be man made. The sun provides a large source of free energy here... and we don't care much if the acceleration is 0.0001 g, eventually, it'll get somewhere, for in space, friction is entirely negligible.
Light sails work by using the momentum of photons for to speed up. They reflect the light, essentially inverting it's direction and thus get a speed boost. The lasers that form the backbone here are a valid technology in and of themselves, as are the interesting new materials for the sails. As frequently, this space-oriented research is useful whether or not it ever moves a vessel.
maddog42 asks:
Why don't we take useless national defense projects like Star Wars and turn the technology around to propel solar sails with all the high-powered laser and particle cannons that are probably still up in orbit somewheres?
Ha ha. We wish we had particle cannons in space. (You do realize that the Strategic Defense Initiative never led to anything, right? Except the recent National Missile Defense system proposals. And it still isn't deployed.)
Actually, you're closer to the truth than you think. I can't say for certain, but it seems highly likely to me that the experiment relied on advances in laser and microwave technology made under SDI funding. (Indeed, one researcher's home page shows that he's done contract work for Raytheon to "suppress enemy air defense".)
So in a sense, this is beating swords into plowshares.
----
lake effect weblog
{Network engineer in Chicago--looking for work!}
James Benford of Material Sciences and Energy Sciences Laboratories.
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lake effect weblog
{Network engineer in Chicago--looking for work!}
Three questions for you:
1) With the "Plasma bottle", how big and how efficient is this magnetic sail? I thought the whole point of a physical sail is that you didn't have to drag along a power supply. Needing to sustain plasma seems to defeat the purpose.
2) How much solar wind makes it out past Mars?
3) Hitting even a large sail with a ground based laser from thousands or millions of miles away seems like a hell of a shot. Would the atmosphere throw off the aim? Can it be compensated for like the new telescopes do?
Thanks. I dropped out of college before they got to that stuff.
-B
aiming probably isn't that much of a problem. Laser light is coherent, but still the area that gets hit gets larger as distance increases. I think I once read somewhere that a laser pointed at the moon from earth generates a light spot about the size of a football-field (soccer field for those who think football is played with your hands). It's probably safe to say that the sail gets hit over the entire surface rather than a defined spot. the propelling force will be perpendicular to the sail, and the craft can be directed by changing the angle of the sail, much like a sailboat.
But I may be completely wrong...
No one can understand the truth until he drinks of coffee's frothy goodness.
--Sheikh Abd-Al-Kadir, 1587
True.
Momentum is mass*velocity.
I have to disagree on this one. Since photons (light "particles") move with the speed of light c in vacuum (duh), they CANNOT have mass. This is a direct consequence of the Special Theory of Relativity. All particles that move with the speed of light must be massless. Yet photons do have momentum, and therefore your definition of momentum cannot be correct.
Momentum is defined as p=dE/dv, with E=Gmc^2, G=1/(1-(v/c)^2)^0.5 for matter. This results in p=Gmv which is allmost equal to mv for non-relativistic particles.
For light, the definition gives p=d(hc/l)/dc, with h the Planck constant and l the wavelenght (which, btw, relates to the color) of the light. So we have p=h/l.
Good, now we see that we want to use short wavelenghts, since those photons have a lot of momentum.
The mass is dependent on the frequency.
Photons have no mass.
So light coming from a gravity well, or from something receeding from us loses energy, and the photon is "red-shifted".
No. Photons coming from a gravity well (curved spacetime) are not shifted because of their enery loss, because they can't lose energy, enery is conserved -- allways. Gravity itself causes the redshift.
Also, redshift can be caused by objects moving away from the observer, or towards (then, it's blueshift, actually). This is again a result of the General Theory of Relativity.
Photons just travelling throught the cosmos experience redshift, allthought spacetime is rather flat. This is a result of the universe expanding at a steady rate. Ever heard of the cosmic 3 Kelvin background radiation? It was created in a time when no planets or stars were around. The ambient temperature was in the order of a million Kelvin. The universe much much smaller, and much much hotter. Now, 20 or so Billion years later, the universe is bigger, and all the energy in those photons created long ago is spread out over an enourmous volume.
They would want to avoid absorbing photons since that would burn up the sail very fast.
This is not the reason they want to avoid absorbing phonons, IMHO. Yes, the sail would heat up, since it is pretty thin, it would pretty fast at that too. But then again, it's pretty big as well, so it will also irradiate thermal photons (so called black body radiation). So I can't say if it will melt. It certainly won't burn, you need oxygen for that.
But there is a better reason not to absorb photons: less momentum is transferred in comparison to reflection. If the photons moves back to where it came from, the momentum transfer is twice the momentum of the photon, once for `stopping' it, once for getting it going again in the opposite direction.
Energy scales as velocity squared, and light moves pretty fast, so it delivers a *lot* of heat for the momentum.
Again, you're trying to use Newtonian mechanics on a photon. The energy of a photon is simply hc/l. Nothing E=0.5mv^2 about it; m is zero for a photon.
cheers, Alfred.
More to the point is the power levels required for a given amount of thrust. Momentum and energy of massless particles are related by the equation e=pc, so a gigawatt of photons will give you a force of about 3.3 Newtons (= 1e9 watts / 3e8 m/sec). If you have a perfect mirror and you're reflecting the beam straight back, you can double that.
Am I the only person who realizes that this puts Starwisp a lot closer? (I haven't read the whole thread yet, only downloaded through resp. #168.)
--
Ancient Goth: Someone who overthrew the Roman Empire.
Time is Nature's way of keeping everything from happening at once... the bitch.
However, you are right to ask what happens to the momentum. Thing is, momentum is a vector; it has both magnitude and a direction. If the photon is absorbed by the sail, the sail takes on the momentum as well as the energy (where the momentum p equals e/c, where c is the good old speed of light). If the photon is reflected, things get interesting. You have a photon coming in and going out again, with more or less the same quantity of momentum - but a different direction. If the photon bounces directly back toward its source, the reflector absorbs twice the photon's momentum: the difference between the photon moving toward it and zero (e/c), and the difference between zero and the photon moving away in the opposite direction again (another e/c, total 2e/c). If you don't reflect directly back, multiply by cosine theta.
If you look at things in the frame of reference of the source (where the sail may be moving), you have additional complications due to Doppler shifting of the photon frequency (and thus an increase or decrease in its energy and also its momentum). When you're doing numbers on something like this, you have to be real careful about your reference frame or you risk getting complete nonsense. Ain't physics grand?
--
Ancient Goth: Someone who overthrew the Roman Empire.
Time is Nature's way of keeping everything from happening at once... the bitch.
A laser's output is just as susceptible to scattering by dust or Rayleigh effects as anything else. An earth-bound laser will have a fair amount of its output bounced around randomly on its way out into space. However, it is only light.
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Ancient Goth: Someone who overthrew the Roman Empire.
Time is Nature's way of keeping everything from happening at once... the bitch.
Of course, part of the point of this article is that you are not depending on the sun's light - you are using some kind of artificial radiation source, such as microwaves or laser light, which can be focused ono the sail (and hopefully provide more thrust than is provided by "normal" solar radiation). I'm kind of thinking that the solar wind (charged particles, not necessarily just radiation) is still a pretty good source of "push", however.
I can imagine how using light from the sun might work for this (as long as you want to go AWAY from the sun), but I can't see how you could manage to keep a laser aimed at a craft as it travels great distances. I'm sure the craft and the laser source would be moving around at least a tiny bit and would need to correct for that, but by the time the laser emittor can figure out where the craft is, it will have moved (unless they can figure out someway to communicate faster than light between the source and craft).
We are talking a TINY tolorance to get the beam to hit *directly* on the sail (off a little bit in any direction will start it spinning) and once you get a bit of error it gets harder and harder to predict the craft's position.
- Isaac =)
It's not that crazy, haven't you ever seen one of those science fair, solar windmill thingies ? That is running in a vacuum - no wind either.
The concept is sound, and remember in space you have perpetual motion, you only need the sail and some sunlight to start you off, then your flying and you can litterally "surf" on the orbits created by celestial bodies so as to remain in motion.
Imagine, setting off with the sun behind you towards jupiter, by the time you get there even a heavy ship might be traveling at 400 to 500 miles and hour (keep in mind your curve starts low but is an exponential curve) then slingshot around jupiter and come arround at about 7000 mph. From there it's clean sailing all the way to andromeda or whereever, with no extra ennergy. Asuming of course that by then we have someone who can survive the 180 year trip and get to come back.
This tech will not ever put up or restore satelites, but it will be a great, efficient means for space-build craft to explore other planets, galaxies and one-day maybe even star systems.
"Semper in excretum set alta variant"
One of the biggest problems with the solar sail techology is that it's relatively easy to speed the sail up to interstellar speeds (theoretically almost to the speed of light).
The problem is slowing them down again. Especially if you're using some kind of laser or microwave to boost the speed of the sail. Sure, you could make it to Alpha Centauri, but you'd hurtle past it close to speed of light. Maybe you'd have time for a near-relativistic snapshot.
Bob Zubrin's latest book, Entering Space has interesting theory on how you could slow yourself back down again. (Bob's the guy who wrote The Case for Mars).
Essentially you use a different technology called a Magnetic Sail to slow yourself down - it's a thin grid of superconducting wire charged with electricity. The grid interacts with the solar wind coming from your target star, and slows you back down. The faster you're going, the more you slow down.
Fraser Cain
Publisher, Universe Today - http://www.universetoday.com
Why don't we take useless national defense projects like Star Wars and turn the technology around to propel solar sails with all the high-powered laser and particle cannons that are probably still up in orbit somewheres?
Or even more terrifying, that's 625 trillion ergs!
Of course, if you look at it as 17 kilowatt hours, or a couple bucks electricity, it's less worrisome. If you leave a light bulb on for a day, you've just expended enough energy to put that light bulb in space.
Even if you can't use electricity to get off the Earth's surface (and we can't, yet), rocket fuel isn't that much more expensive; you'll be paying $20 a kilogram instead of $1. That'd be $2000 (well, $5000 after you add in life support and overhead costs) for a vacation in orbit; I'd happily buy a ticket.
Instead, getting into orbit costs $5000 to $10000 per kilogram. There are differing ideas about how to reduce this (reduce the "standing army" of ground crew, don't throw away a rocket with each flight, turn around and fly again every few days instead of months), but reducing energy costs isn't among them.
Err, no they don't. The light is absorbed, not reflected. If it was reflected then they'd have to slow down to transfer momentum, which is obviously impossible.
Wrong. If you redirect the light, then by good old conservation of momentum you have to have a transfer of momentum. No slowdown needed. The light is reflected for several reasons:
1) If you absorb it your sail would quickly burn up.
2) By reflecting it the light has a bigger change of momentum, giving more push.
3) By choosing the direction of reflection you get some steering capability.
4) There is a theoretical double-sail configuration for braking, but it only works if you are using reflection. (One of the potential interstellar craft on the drawing board uses this design, see the book "Indistinguishable From Magic" for details.)
Sorry,
Ben
My usual seat in the cluetrain is at A HREF="http://pub4.ezboard.com/biwethey.ht
What I said is absolutely correct in special relativity. The momentum of something is its mass times its velocity. You have to include the mass of its kinetic energy as part of its mass of course, but why should kinetic mass be treated any differently than any other kind of mass? As an extreme example, photons have a mass, but they have have no rest mass.
As for conservation of momentum and energy, the old sci.physics FAQ had an neat section on that pointing out that even stating global conservation laws in general relativity is problematical. And yes, I have heard of the 3K background radiation. I have even observed informed discussions on whether there is a non-conservation of energy going on in that cooling process.
The sail temperature issue is non-trivial. If the mass of the energy making up a photon is m, the energy is m*c*c, and the momentum is m*c. So no, the energy is not 0.5mv^2, it is off by a full factor of 2. Big whoop. The ratio between energy and momentum scales approximately linearly with speed (varying by a full factor of 2), and that is what matters here. Unless you have a very reflective surface, what you can deliver is limited by when your sail will melt.
Incidentally the wavelength is irrelevant to how efficient light sails are. Assuming that you get a good reflection, they are all equally inefficient because of the ratio between energy and momentum that is a basic consequence of SR.
Do you have any other "corrections" for me?
Ben
PS Trivia note. Newton's statement of his three laws is actually completely correct in SR. The (slightly) incorrect f=ma formulation came after.
My usual seat in the cluetrain is at A HREF="http://pub4.ezboard.com/biwethey.ht
I'm guessing that to launch a craft of any reasonale size you would probably need a huge solar sail. Now i've got no evidence whatsoever to back this up but personally i'd imagine huge football pitch sized things.
Actually, most proposals I've heard were for sails many square kilometres (or more) in area.
Thrust from a reflective sail should be on the order of 1 newton per 1.5e8 watts for a perfectly reflective (1 newton per 3.0e8 watts for a perfectly absorbing sail or for a perfect photon drive).
At earth's distance from the sun, the sun provides about 1.0e3 watts per square metre (_roughly_; I don't have the exact value).
This means that to accelerate a 100kg mini-probe at 1 cm/sec/sec (suitable for in-system movement, not launch), using the sun's light, you'd need a circular sail about half a kilometre in diameter.
For heavier probes or probes that still need to accelerate when farther from the sun (interstellar probes) or for probes that need to accelerate more quickly or for probes with sails that are partly transparent, you'll need a bigger sail.
With all the crap floating about in space surely the chance that these would rupture is unacceptably high, given the ery thing material they are made from.
The best way to design these sails is to give them a film-on-mesh structure so that they'll still hold together with holes in them. Matter in interplanetary space is very sparse; they'd last a while. The main junk hazard would be in low Earth orbit, which isn't where your craft would be deployed.
On an inter_stellar_ trip, OTOH, your sail would probably degrade, but an interplanetary trip could probably be managed.
Could someone please explain how exactly a solar sail works, because classical physics is apparently not helping me on this one..
The basic idea as I see it is: photon hits sail, gives part of momentum to sail, sail goes forward. However, photons have no mass (right?), and momentum is mass*speed. So the momentum should be zero... (unless speed is infinite, which it obviously isn't)....
"Interplanetary ship Mars travellor XI to planet earth, come in, come in."
"This is Houston Mars XI, we have detected the problem as well."
"Houston, how do you suppose we get back to earth now? We have already used all 3 of our spare sails, and you brilliant engineers wouldn't let us put on enough fuel to get us back to earth! New superlightweith sails don't require fuel you said! And what if the sail breaks, you have 3 backup sails thats what! They didn't do us any good this time Houston."
"Mars XI we are working on it. Give us some time. We just called in McGuyver to help us build a new sail out of your existing equipment"
cp -R
I'm guessing that to launch a craft of any reasonale size you would probably need a huge solar sail. Now i've got no evidence whatsoever to back this up but personally i'd imagine huge football pitch sized things.
With all the crap floating about in space surely the chance that these would rupture is unacceptably high, given the very thing material they are made from.
It's still a cool idea though.
The interesting thing about /. is how little basic science a lot of the people who think they are qualified to answer your question have.
Anyways, here are the answers:
1. I have no idea how long until they try this in space.
2. Yes, by Newton's third law the source is pushed back. As long as that is a planet, you won't care much.
3.
a) Think ping-pong balls. When you hit them you can change the direction of motion without changing their speed. That is a transfer of momentum.
b) Momentum is mass*velocity. The velocity of light in a vacuum is constant. The mass is dependent on the frequency. So light coming from a gravity well, or from something receeding from us loses energy, and the photon is "red-shifted".
c) They would want to avoid absorbing photons since that would burn up the sail very fast. Energy scales as velocity squared, and light moves pretty fast, so it delivers a *lot* of heat for the momentum.
Cheers,
Ben
My usual seat in the cluetrain is at A HREF="http://pub4.ezboard.com/biwethey.ht
Even at 100% efficiency, according to my calculations, you would need 62.5 million joules of energy per kilogram to reach Space from Earth. That's equivalent, per pound, to taking something that weighs a million pounds and lifting it twenty feet! And it's clear that perfect efficiency can never be reached, in fact, due to entropy, we won't even get close.
But perhaps a society of (probably necessarily genetically engineerd) people could exist in space permanently - always in space stations, and on (or in) asteroids and small moons. If fusion and He-3 ever become a reality, then such a society could live of the minerals in space debree, with the energy from fusion.
For those that are interested, Scientific American had a set of articles concerning new/alternative methods of space propulsion, also mentioning light sails.
While I'm at it, you might want to check out a fantastic (recreational) sf novel - The Reality Dysfunction by Peter F. Hamilton - any sf fan must read this (I think...:-) )!
Quotes are for those that don't know what to say... ;-)
--EMN