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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.""
...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.
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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.
The amount of force exerted on the sail decreases as a square of the distance (since the amount of light reaching the sail decreases by a square of the distance). We're not talking about 'meaningful acceleration' in anything like our current thinking of space travel -- this isn't "get on this space yacht and a few months down the road you'll get to the other star," but rather "put something on this vessel and several hundred/thousand years from now it'll get to where you wanted it to get."
This isn't about travel.
Either way, the Japanese are trying to make this look cool by saying it's star-faring technology. Probably true, but only because we're not likely to put humans on this thing -- so it's possible we'll do this before we get to Mars, because the expense and risk could be vastly lower.
Sorry, but that is incorrect. There is a design from the late 60s for an Orion starship that could get to Alpha Centauri in 130 years, for the whopping cost of $1 trillion. Thats much faster than a solar sail could ever hope to do.
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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.
The pictures in the article which show the test sail deployed immediately behind the launch vehicle imply the same thing. The following text says that the launch vehicle reentered and splashed down 400 seconds after liftoff. This can only mean that both the LV and the sail experiment were in ballistic flight when the latter was deployed. For a solar sail to work, it would need to be deployed after orbital insertion (or after escaping the magnetosphere.) The article does not mention orbital insertion, nor was there time for this to occur.
There is also a sequel but I will leave that up to you as a project to find out what it is.
They deployed a sail less than two minutes after launch, had it in place less than two minutes, threw it away, deployed a second sail, then less than three minutes later it crashed into the ocean.
Total trip, liftoff to crash-down, less than 7 minutes.
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Just to clarify what people seem to be mistaking, the sail is *not* powered by Solar Wind, it is powered by the light of from the sun. The idea is that each photon of light that reflects off of the surface of the sail transfers a little bit of it's momentum to the sail.
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 ?
Um, no. As you approach the destination star, its light pressure will start to counteract your velocity and slow you down. The "brakes" are built in.
The Mongrel Dogs Who Teach
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!
The article was short on technical details. A lot more info can be found easily on the net, such as here.
To clear up one point, solar sails are not powered by the solar wind, which is a stream of particles. They are powered by light, which exerts several thousand times more force than the solar wind.
The sail is not direction. It is affected by light coming from all directions, but it "blows" in the direction of the prevailing light, which would come from the brightest/closest star. To change direction a solar sail ship must change the angle of the sail in relation to the nearest star.
At the start of a journey the sail would be ahead of the ship, towing the ship behind it. Sometime between stars the ship could use small maneuvering jets or something to flip itself around and put the sail behind it. The increasingly strong light from the destination star would gradually slow it down.
More likely though, the sail would be retracted or jettisoned in mid-journey, when the light from the destination star equalled the light from the original star. This is when the ship would be at its maximum velocity. It would then coast at that speed for the rest of the trip and use the gravity of the destination star or planets to decelerate much more quickly.
Unfortunately, it turns out that the interstellar medium is much thinner, in most places and in particular around the Sun, than Bussard thought. Even if you could somehow persuade protons to fuse in the few nanoseconds while they are passing through your ship at nearly the speed of light (and on average it takes about 15 billion years for any given proton to fuse in the core of the Sun), there just aren't many of them around here.
A beamrider of some kind (leave the engines at home and ship momentum up to the spacecraft in some convenient form) or an antimatter rocket are looking like the best ideas at present.
Steve
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
Using onboard rockets to steer doesn't help you slow down. As you point out, the gravitational pull of the destination star will cause you to accelerate even more. You'll end up heading towards your destination at greater than the escape velocity for that system.
This doesn't help you stop. To do that, you flip yourself around so that the sail is pointing towards the destination, and you use the radiation pressure from that star to kill your velocity. Can't do this if you're already jettisoned it.
And, no, chemical rockets won't work to shed that much velocity. If you get get that much delta-v from chemical rockets, you'd just use chemical rockets to get on your way as well. But that's precisely why you're using a solar sail instead: chemical rockets suck in terms of specific impulse.
You're forgetting the biggest drawback of the Bussard Ramjet... That is the gas collection.
The gas collection mechanism will create such resistance at high velocities that it would jam up and slow the device down a lot.
I believe that there has been some research done which suggest that it would never be able to obtain velocities exceeding 0.1c let alone 'relativistic velocities'.
I think we are more or less stuck on this island Earth, until we can think of something better than Newtonian physics to traverse the gap between the stars... Some revolution akin to Gene Roddenberry's Warp drive or Iain Banks's Exotic Matter drive - something which doesn't require a reaction mass.
OT: Early STTOS was fun - somehow the warp drive sound effects always sound like the London Underground trains...
-- The universe began. Life started on a billion worlds...
-- Except on one where stupidity was there first.
Where wind sails have Bernoulli to work off of to go against the wind, solar sails have Newton. What the gp is saying is you start off in orbit around a star. If you want to get away from that star, you angle your sail +45 degrees, which reflects the light back along your orbit. Thanks to conservation of momentum you gain tangental velocity which propells you in a spiral outwards as you slowly break the sun's gravitational pull. If you want to go towards that star, you angle your sail -45 degrees, reflecting the light forward along your orbit. You lose tangental velocity and the sun's gravitational pull reels you in. You're right, completely different principles are at work, but you get a similar result.
common sense: noun
What those who are ignorant of the subject matter think; usually wrong.
far beyond what solar energy would of provided
It's would have, you blithering idiot.
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.
Regardless of space or the ocean, basic principles of physics apply. Action and reaction.
Uh, that's a resounding negative, Houston. In the ocean, we have this thing called *water* in which one sails. Action: wind pushes against sails from somewhere near the front. Reaction: sail pushes back agains wind and pushes into the water; water pushes back, and ship tends to go forward. In space, there is no dense medium through which one sails. Action: photons from a star push against sails from somewhere near the front. Reaction: ship pushes back and moves further away from the star. You can't "tack" in a vacuum.
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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)
Actually, I was reading up on this a while back.. They'd use the gravity of the star to tack. Well, kinda.
It seems like a good idea, if they want to send something away from Sol for a long duration in one direction. Not too much navigation necessary (or possible).
The one thing I don't see really mentioned is debris in space. You know, micrometeors, and the like. It should make for a nice shreaded solar sail by the time it gets to the edge of the solar system. Hopefully it didn't encounter enough debris hitting it to knock it off course, or stop it all together.
But hey, if they're just looking to find out how fast a solar sail will accelerate away from SOL at Earth's distance, cool. It'd probably make for a faster way to get from Earth to our neighboring (outward) planets, if they can point it in the right direction. 1 degree makes a big difference over a few million miles.
Serious? Seriousness is well above my pay grade.
Nice post, but one detail has to be wrong. If you could use the destination system for gravity braking then you would be able to equally use our own system for gravity acceleration. The only way the destination is more effective is if you actually slam into it (or perform aerobraking).
The speeds involved in inter-stellar are so high that gravity assisted decelleration is probably out of the question. Aero-braking in an atmosphere is certainly not an option. There have been some proposals for braking on interstellar hydrogen I believe (ramjet concept).
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If you accelerate, you move into a higher orbit (and move away from the star). If you decelerate you move into a lower orbit (towards the star). Sideways vectors are used to change the plane of your orbit.
All this acceleration lasts as long as you have light. So even if the sail only gives you 0.001g of acceleration, after three hours that's as much acceleration as a one second, one gee burn... but you have not used any fuel. This can carry on for many months, and at the end of that time you have as much "fuel" as you started with.
I actually agree that, barring powerful lasers to "push" the sail, solar sails are not an interstellar technology; they don't build up enough speed quickly enough. However it's not true that interstellar debris will slow the sail down substantially. The amount of interstellar material is just not enough to affect it. The density of the local interstallar medium is actually around 10^5 atoms per cubic metre. That is an *incredibly* hard vacuum. One hydrogen atom is about 1.67x10^-27 kg; a 1km square sail will hit 10^14 of these for each kilometre of travel. The sail will have to travel roughly 6x10^12km to encounter one kilo of hydrogen. Alpha Centauri is about 4x10^13km away; in getting there you'll encounter about six kilograms of material.
That's simplifying a bit, as matter is much denser than that before you hit the heliopause. On the other hand, once radiation pressure becomes negligible, turn your sail sideways to the interstellar medium and it won't hit *anything*.
Within a solar system, they are an incredibly efficient means of transportation, because they give constant acceleration with no fuel cost. Outside of the solar system, they are much less useful without the aid of lasers boosting your radiation pressure.
Robert Forward's "Rocheworld" (AKA Flight of the Dragonfly) is SF but covers interstellar use of solar sails fairly well. The SF short "The Wind from the Sun" by Arthur C Clarke (I think I have that title right) gives a good overview of use within a solar system.
There is another existing technology that could travel interstellar distances. NASA's Orion project designed a starship propelled by nuclear weapons and a big pusher plate. And yes, the crew can be properly shielded.
Of course what we really should be working on is actual nuclear rockets - controlled nuclear burn instead of explosives. Nuclear gas core rockets are really not beyond present technology, their exhaust is cleaner than the space shuttle's, and they're so powerful you can build big, heavy, safe vehicles.
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Except you're losing thrust faster than the escape velocity decreases. Escape velocity goes as 1/sqrt(r), whereas the light intensity (hence thrust) goes as 1/r^2. Solar sails probably aren't the best way to go interstellar. But then, neither is anything else we can imagine at the moment. sigh
And now, safely buried in the comments because I have limited bandwidth...
Photos of the Uchinoura Space Center, from back when they called it Kagoshima Space Center. (Kagoshima is the prefecture, Uchinoura is the town. Nobody in Japan has heard of Uchinoura, so they called it Kagoshima Space Center until with the increased level of joint projects with a certain American space agency they decided 'KSC' was too easy to confuse with Kennedy Space Center.)
You are asking a textbook question :-)
No, the radiometer works in a different way. Notice in the radiometer that the rotation is clockwise when the black "sail" is exposed on the LEFT. If this were due to light pressure, the pressure would be greater on the RIGHT (reflective) sail and the radiometer would be spinning in the opposite direction.
What happens in the radiometer is the black sail heats up because it absorbs radiant energy. The gas around the sail (the glass bulb is very low pressure but not a perfect vacuum) conducts heat away from the sail, heating and expanding in the process. The expanding gas reacts against the sail, pushing it away.
If the radiometer bulb was evacuated to a perfect vacuum, the radiometer would actually spin the opposite direction because the light pressure effect would dominate.