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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.
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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