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Japanese Deploy Solar Sail
Chuck1318 writes "The Japanese ISAS (Institute of Space and Astronautical Science) announced the launch and deployment of the first ever large-scale solar sail. In the news release they state "Because it carries no fuel and keeps accelerating over almost unlimited distances, it is the only technology now in existence that can one day take us to the stars.""
I call baloney. Doesn't a solar sail work because of it's high reflectivity? Isn't that high reflectivity only on one side of the sail?
And what's so difficult about retracting the sail? The force on the sail at any given time is so miniscule it's trivial to retract them (as opposed to, say, when you have intense winds blowing on your sailboat's sail).
...it is the only technology now in existence that can one day take us to the stars.
Orion can take us to the stars, and it can be done with today's technology, not something that's just starting to enter the very earliest test phases. But it's nuk-yu-ler, so it doesn't count.
Mod down posts with a "Free Mac Mini/iPod" sig, they're spam!
Actually you want to slow down when you reach the other star. Or else you miss your stop. Once there, you jettison the sale, or use it to fly around the star system.
Read "Flight of the Dragonfy"/"Rocheworld" (they are the same book) by Doctor Robert L. Forward for an informative and entertaining novel using (laser pumped) solar sails.
A perfectly non-reflective surface (i.e. a black surface) would experience half the force that a perfectly reflective surface would. In other words, a black sail will work, but only half as well as a mirrored sail would work.
This is due to conservation of momentum. If a photon is reflected, its momentum p is reversed to be -p. Thus the sail must acquire a momentum 2p to conserve momentum. Whereas if the photon is absorbed, its momentum changes from p to 0, thus the momentum of the sail must increase by p, again to preserve momentum.
The difference in kinetic energy is converted into heat. A black sail heats up. An ideal, perfectly reflective mirrored sail does not heat up at all.
Ever seen a radiometer? or hear about the radiation pressure equation?
This is a common misconception... one that even Maxwell mistakenly believed. Apparently along with the folks at Encyclopedia Britannica as well.
Pay attention to which way a radiometer turns. If it were turning due to radiation pressure, it should act as if a force were pushing on the white side of the plates. Since the white plates reflect the light, there should be twice as much pressure on them as there is on the black plates which absorb the light. (It takes a greater transfer of momentum for something to bounce off of you than for you to catch it... think of the conservation laws).
The problem with the radiometer is that it turns the wrong directions... it acts as if something is pushing on the black side of the plates. And there is... air pressure. The black side will reach a higher temperature than the white side, and then due to the thermal transpiration, the gas near the edges moves from the hot side to the cool side, and in doing so it pushes the blades along.
Radiometers are in a near vacuum, but there is enough air pressure inside to allow this effect to happen.
I'm surprised that no-one else has mentioned this.
:).
The truth is that a solar sail doesn't get you away from the sun by just having the sail aimed straight at the sun. It does it much more trickily than that
What happens is that you orientate the sail at 45% (or something like that) to the sun. That way, a large amount of the force from the sun actually goes to changing your orbital speed, and not just pushing you away from the sun. By orienting the sail so that it increases your orbital speed, you end up making greater size orbits around the sun, until you are far enough away from the sun and you can do some other tricky stuff to leave the solar system.
But, it works the opposite way too. Orient your sail so that you are decreasing your orbital speed. You go slower, and therefore your orbit size decreases, and you start approaching the sun.
Of course another poster queried why you would want to travel to the sun. Good question. But how about Mercury or Venus ?
Unless I am misunderstanding your post, your physics is incorrect here. A modern sailboat, with the triangular sail, angles the sail somewhat into the wind, creating a pressure difference that pulls the sail in the direction of lower pressure. However, most of that pull is not linearly forward, but instead rotationally sideways - the only reason the ship does not immediately flip over is because of the keel, that being the strip of material that runs along the boat's centerline, on the bottom. The keel sticks out beyond the smooth hull, and as the sail tries to make the boat rotate, the keel pushes against the water, and the rotational motion is (partially) converted into fowards linear motion. This system does not work without three elements: sail, keel, and a fluid to flow in, that the keel pushes against. Older, square-rigged ships, like the workhorses from the 1800s back, simply work by having the wind push against the sails from behind, propelling the ship forward. This is how a solar sail works, except with light rather than wind. Check it out here:
w orks
http://en.wikipedia.org/wiki/Sailing#How_sailing_
Cheers!
Same principles apply as in Earth-based ocean sailing - if you angle the sail, you can deflect the particles, thus allowing you to use the solar wind of another star even though you are approaching it rather than leaving it.
I'm not so sure that's the case - when we sail in water, we can either be on a run (the wind directly behind us, as you would expect a solar sail to work) or on a reach (the wind to one side).
On a reach the sail acts more or less like an aeroplane wing because of it's curved surfaces and as well as generating a forward force it generates a lot of lateral force too. The closer to the wind you sail, the greater the proportion of lateral force.
The only reason that's not a big problem for us is that your craft has a centreboard which greatly reduces it's ability to slide sideways, especially at speed - when I'm windsurfing in a reasonable wind, I will be doing about 30-35 knots and can easilly sail upwind with about 300cm^2 of fin area, but I won't be able to go upwind if the wind drops off because my speed will have greatly dropped. In space there is no way to have a centreboard to prevent the lateral forces pushing you sideways since there is nothing for it to react against.
I'm also not sure about the "aerodynamics" of a solar sail - as I described above, a modern sail works very much like an aeroplane wing when reaching and relies on the air have a laminar flow over both sides of the sail. I very much doubt photons are going to have a laminar flow over your sail so the sail isn't going to be anywhere near as efficient for reaching as boat or windsurfing sail. In windsurfing the most efficient point of sailing is on a slightly broad reach - i.e. the wind is coming from one side and slightly behind you, I would expect the most efficient use of a solar sail would be on a run.
http://blog.nexusuk.org
These solar sails are pretty useless. Here http://solarsails.jpl.nasa.gov/introduction/design -construction.html
are calculations from NASA guys. It looks like this
Japanese sail has acceleration of few mm/s^2 and is not able to get out of sun gravitational field (and, of course, the Earth's one). It would take solar sail 100 years to get
to alpha centauri if it had acceleration 10 m/s^2 (table 3 in the above link, there is "-" in the
table for 5 m/s^2 and less , that is it will never get away from sun ).
There was a good idea though to build a huge mirror to focus sunlight on such sail. This would effectivly increase surface area of a sail and
pressure would not drop as square of the distanse from the sun.
For those of you who are - like me - not experts in physics, this technology was featured in the BBC documentary "Space" presented by Sam Neill.
c at ,5,,11,science,831
http://www.imdb.com/title/tt0273608/
http://www.bbcshop.com/invt/bbcdvd1090&bklist=i
One of the chapters discusses how travel to other stars would be possible. As far as I remember there is another technical solution in discussion which would involve nuclear detonations as part of a propulsion system. (I might have confused something there, though)