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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 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.
----
lake effect weblog
{Network engineer in Chicago--looking for work!}
Since the photons are reflected, the total impulse is delta-p=2*p, so you get twice the momentum of the photon. (A previous poster incorrectly assumed the photons were absorbed. This would burn up the sails, and impart less impulse to the craft)
To answer another question I saw, why aren't ion engines better: Ion engines require a power source on board the craft, and produce little thrust (~10^-4 Newtons). Solar sails can produce large thrusts (~10 Newtons). The article mentions that they acheive several times g accelerations. The ion engine also requires fuel (though not very much).
There is also the "plasma bottle" engine being developed (at JPL, I think). It uses a confined plasma to create a huge pancake shaped magnetic field, which interacts with the solar wind and gets a push from it. This also requires a largeish power source on board to sustain the plasma. It is also a "solar sail" though it uses the solar wind (energetic particles from the sun), not the sun's photons.
Solar Sails realy are a Good Idea. Not only can they use the sun, which is an always-shining abundant power source, but they can also use ground-based power sources. It's a hell of a lot cheaper to build a ground-based power source and a laser than it is to put that same power source in orbit. (At $10,000 a pound to launch stuff) Also by turning your sail WRT the sun you can get thrusts in different directions. This will allow you to use the solar sail to come back from Mars, for instance, just by turning the sail in the right direction. Orbital mechanics is a little counter-intuitive -- pushing "outward" doesn't necessarily get you farther away since both you and the planet are going in a circle, around the sun. You really want to push in the direction of motion around the sun that you're traveling (or against it) to increase/decrease the size of your orbit, and thus move you closer or further from the sun. Pushing directly away from (or towards) the sun will just get you in an elliptic orbit.
--Bob
1^2=1; (-1)^2=1; 1^2=(-1)^2; 1=-1; 1=0.