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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.
as for photons losing momentum and thus dropping to sub-C speeds, this doesn't happen because the photons are absorbed by the sail; most of them don't reflect. As for the few that do reflect, If they lose any energy it will only change their frequency (possibly also their wavelength?) thus changing their color.
I do not believe that photons (i.e. light) can travel at any speed other than C...
I wish /. would give me the contacts of trolls and flamers... I'd whet my knife on their intestines.
I think we're forgetting physics again (common issue on slashdot ;). Assume Craft A, in the front with sails up ready to move on to its adventure in space. Assume Craft B, equiped with laser or microwave source, used to propel Craft A.
When Craft B fires its laser or microwave source at Craft A, Craft B will start drifting backwards (newton's third law) which could ultimately doom it into the sun if positioned correctly. If, however, the two crafts are connected, they won't go anywhere since Craft A has to first overcome the force from Craft B going backwards, resulting in no force in either direction.
We come up with the best ideas for perpetual motion yet we overlook the basic things
-DK.-
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.
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?
They are trying this because a) sailcraft in principle can have a greater delta-v potental than just about any other thrust mechanism, including ion drives, and because b) once the technology matures, sailcraft will be dirt cheap.
Point a) holds because of the limited power generation capability of any probe. A sail has a *huge* collection area and directly converts any incoming beamed power to thrust. While thrust per unit power is low, your sail is big enough to handle a lot of light, and your ground-based laser won't run out of fuel. Leave it on for a few months, and your sail-and-probe-package ship will be moving faster than any probe based on chemical or photovoltaic power, and will be far lighter than any fusion-powered craft (which can't even be built yet).
Hold the laser on for a few years instead of a few months, and you can bring the probe up to sufficient speed to reach another star in our lifetimes. The only thing that can possibly match that is a Bussard ramjet, which is even harder to build than an ordinary fusion ship (we'll manage it eventually, but why not start sending probes now?).
Point b) holds because the sail is very light and is made of a common substance (carbon fiber). It's expensive to manufacture now, but this will improve as engineering techniques get better.
As this is the most promising current technology for interstellar probes, I would most certainly argue that it's worth spending money on.
Assume Craft B is in an orbit stable enough to to counteract the thrust, at least over the long term. (Easy enough to do, depending on the orbit.) And/or have Craft B counteract the force on itself with an equal and opposite impulse. Yeah, that'd cut down overall effeciency of the system, but that's not much of an issue, as Craft B could be solar-powered, and you'd still get the major bennie of not needing to carry (much) fuel on Craft A.
allanj asks:
:-)
I was reading through the NASA press release and read that they characterized their sail material as 'stiff'.... 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.
Well, of course, this is just a feasibility demonstrator. It's nothing like a vehicle yet.
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
Actually, using an ion engine to brake isn't a bad idea. Even an ion engine can't carry enough fuel to cross the distances a lightsail could, but it would be ideal for a short burst of comparatively strong thrust to pull off an orbit insertion for reasonably size gas giants, or possibly eventually another star.
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.
The trouble is we don't know yet what WILL be easier to solve with other technologies. ALmost all of them involve solving some sort of fuel-mass-acceleration equation. The beauty of a sail is that you keep your propulsion system at home.
It's not inconceivable that building a remotely useful interstellar probe would involve a large percentage of the resources of a given industrial society, say, ours. Anything we can do to make that process easier is good.
People have been talking about different tech to reach orbit for half a century, but you know what? We're still using plain old rockets that von Braun himself would understand. The only real advances we've made that have turned out to be practical are ones of efficiency.
I wouldn't get too concerned, anyway. It's not like the universe is going away anytime soon. We've got a lot of time to play around and find the one that works.
----
lake effect weblog
{Network engineer in Chicago--looking for work!}
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.
----
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
I think I have to disagree with one point here... the comment that they haven't been screwing up the planetary probe program
I like NASA. They give me money. So don't get me wrong on this - but in my opinion, they did screw something up. Goldin's motto has been 'faster, better, cheaper' and they have been messing that up, and not because of Congress - it's because of the way NASA's structured. They spend far too much time on development and far too little time on testing and quality control. This is understandable - NASA pushes for the 'best' approach (development/planning) and then, when the design is ready, they push for the 'faster' portion and often times, they push a bit too hard.
What needs to be done is more than congressional fixing - a bit of restructuring is necessary on NASA's end, too. Less beauracracy in the way things are developed, and more money spend on the actual creation/testing portion of the process.
Note that 'development' and 'research' are not the same things - when I say development, I mean the portion of a project where it is decided what is going to be done, before actually attempting to do it.
After seeing a few NASA projects go through the development/planning->design->production stage, I think I have to agree with those who say that the "faster" portion should never touch the "design" segment.
Incidentally, the MCO and MPL losses are delaying Mars exploration, about two years, I think, for reevaluation. This is a good thing. One loss is an "oops, oh well." Two losses are not mistakes - they indicate that something is not quite going according to plan.
The question is whether Newtons's laws effect it at all. Do quantum mechanics come in to play here, and if they do, do they have the same type of effect as Newton's third law.
In other words, do the photons just appear out of the laser or are they "propelled" out of the laser, if they are "propelled" then it would seem newton's law applies, but if the energy in the laser creates the photons and points them in the right direction, then it would seem that Newton's laws wouldn't matter.
That was a long sentence.
cp -R
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.
--
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"
Bruce Perens?!?! Is the Debian contract speculative fiction now?
I think it was more likely to have been Arthur C. Clarke in 'Wind from the Sun', proving once and for all that NASA have done no real research for years, preferring instead to read SF and watch old movies...I wonder when they'll get round to launching a Twin Ion Engine craft...
-- Bah weep grah nah weep nini bong
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.
Agreed. The hardest part about quantum mechanics is that in order to grasp the ideas behind it you need to unlearn a lot of classical ideas that you get taught beforehand. This is fine if you never do any advanced physics but when you're going on to higher study it just makes it harder.
Just look at the model of the atom and the conceptual revisions it goes through at different stages of education - solar system model, Bohr model and then finally the full quantum model. And the same applies to a lot of other concepts too.
Quite frankly physics teaching sucks before you get to university. It's dull and waaay too focused on things like mechanics.
And don't even get me started on action principles and when they get taught :)
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Jon E. Erikson
Jon Erikson, IT guru
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
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?
... just like this light-sail one.
... because we have an LEO vehicle. Talk about your circular justifications. I watch the manned program like the next geek, but it's a lot of money to spend on something that doesn't accomplish much besides ... itself.
Because they are not just the pack-up-your-satellite-and-go-places agency, they are the aerospace research & development agency.
Ion drives are clearly a much more practical propulsion for certain kinds of space probes, and will continue to be so for the future. Of course, this is only because NASA spent twenty years perfecting the technology with demonstrator projects
Light sails will also have their uses in the future. They won't have immediate practical applications, true, mainly because it doesn't make a lot of sense to power a lightsail by shooting your laser through a planetary atmosphere. You'll want to put it on the Moon, or a stray asteroid you can park someplace handy like L5. Then you can use light sails for applications where getting there fustest-with-the-mostest isn't the top priority -- say, ferrying stuff around the asteroid belt.
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.
Fine, YOU go tell Congress. I have some problems with their priorities as well. The fact is the budget of NASA is set legislatively, and has as much to do with politics foreign (Russian participation in ISS) and domestic (ISS hardware manufacture in 40+ states) as the pure goal of git-up-and-go-somewheres.
Right now the planetary probe program is something I'd defend against the common perception that they've been screwing up. (This stuff is hard. It doesn't always work.) I hope the MCO and MPL losses don't unduly delay continued Mars exploration, and NASA is doing everything it can to drum up interest in the quest for life in places like Europa.
If you want to criticize a waste of money, hit on the ISS and the shuttle program. Now more than ever those are all about getting into LEO and doing things
What NASA needs to do is MORE cutting-edge research like light sails, not less. Choose your targets better.
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