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.""

Re:Stellar Pong?

[mrchaotica]

What if it's only reflective on one side?

Solar sail

[Uplore]

What I dont understand is how they intend to protect these massive sails from being shot full of holes by meteorites and space dust as it propels its way through space.

Also, seing as how it is powered by solar wind, what happens when the craft is between 2 or more stars which are all exerting equal force on the sails. With no fuel it is doomed to slow down and be 'blown' around in space.

What Solar Sails Are

[SlashChick]

In case you, like me, didn't know that much about solar sails, there's a great article at How Stuff Works about them: How Solar Sails Will Work. [howstuffworks.com] Looks like a pretty interesting technology!

Physics

[caitsith01]

Anyone care to fill us in on the rate at which the energy received by a surface decreases with distance? I imagine that, given the incredibly weak force applied by light, it would take one HUGE sail to get anything like meaningful acceleration for space travel. Surely be the time you are a few million kilometres from the Sun the amount of force being applied will have dropped off by a huge amount?

Anyway, we should get to Mars and back a few times before we try to get to the stars... baby steps.

Only if you're sending sails to super-bright stars

[Anonymous Coward]

If the destination star has about the same amount of solar wind (or whatever sails use; I forget what exactly) as our sun, the point where it reverses course would be about 1AU from the destination star. I'd say that's close enough to be considered "reaching another star".

Re:Stellar Pong?

[Igmuth]

And alot sooner than you would think... Once you cross the heliopause [wikipedia.org], the solar wind is basically moot.

Good to see

[T.Hobbes]

It's very good to see this branch of space technology getting funding. I'd rather travel in a starship with warp drive, but until then we need some feasable way to get to other stars. There's no reason, in my mind, why we shouldn't send a few of these off to nearby stars with the sole purpose of taking some close-in measurements and somehow getting the data back here (getting the data back would probably be harder than getting the spacecraft there in the first place). The very fact that it would take hundreds or thousands of years for them to get there is the best reason to start sending them now.

Let's all raise a glass of Sake to the engineers behind this project!

Re:Ironically

[Anonymous Coward]

And why don't we just use a fan to move a sailboat? (Hint: think conservation of energy.)

You might be joking, but my dad used to do this all the time when he'd take us sailing if the wind would die down and we were all still out on the lake. He had a big gasoline generator and a 36-inch fan. Worked fine; moved us right along. (True, by "gasoline generator" I mean "all of us kids" and by "36-inch fan" I mean "enough oars for all of us," but, still, it worked just fine.)

Re:Solar sail

[whopis]

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.

Re:What Solar Sails Are

[Cecil]

Wow, that surprised me. I thought they actually used the solar wind to power them, not light. But that is not so. The article says the light produces 9 newtons per square mile (3.5 newtons per km^2) whereas by my calculations, an average strength solar wind stream of 1 proton per cm^3 at 500km/s would only produce about 0.0004 newtons per km^2.

Kind of counterintuitive. I thought the unbelievably small mass of a proton would still outweigh the nearly infinitesimal mass of a photon. But I guess our star puts out enough photons to make it count.

Cool, anyway.

2 questions (Earth altitude, Sol heliosphere exit)

[MMHere]

1. Did they get high enough above Earth to enter the inter-planetary "void," and thus avoid the significant effects of Earth's atmosphere? 100, 230, and 400 seconds after liftoff hardly seem "high enough."

2. What happens to such sails when they cross the heliosphere of a regionally prominent star such as Sol? Is it all chaotic photons and miscellanous radiation in the interstellar "void?" Or are conditions regulated by the nearest stellar bodies?

-- In other words, how would one navigate effectively once the prominent wind from Sol fades and is replaced by other forces? Are you doomed to follow your trajectory mainly established by Sol once you leave its heliosphere, possibly modifed by various minor (uncontrollable) forces from other winds in the void? Can you take advantage of such extra-Solar winds to go where you want?

Just a question...

[ScottZ]

How do you run against the solar wind? What are the appropriate forces to run your 'keel' against when you want to track across a solar system (say, to somewhere useful)?

Anyone got any pointers?

calculations from NASA

[gentoo4ever]

These solar sails are pretty useless. Here http://solarsails.jpl.nasa.gov/introduction/design -construction.html [nasa.gov] 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.

Re:Ironically

[FireFury03]

There is probably some engineering trick to work around this. It might be possible to use mirrors to shine on the opposite side of the sail. Almost surely wouldn't be as fast, but seems like it would be doable.

Interesting idea... you wouldn't be able to carry the mirror with you once you turned around (since the mirror would be producing exactly the opposite force of your solar sail), but you could probably drop it in space pointing in the right direction - the mirror would accellerate backwards because of the light pressure but it would still reflect the light forwards which I guess you could use.

Bullshit!

[logicnazi]

Yes, it keeps accelerating over long distances....but I can make a rocket do the same thing by asymptotically slowing down the rate of fuel burn. A solar sail is doing nothing differnt, while the sail will keep accelerating the accelaration will fall off with the radiation pressure (about 1/r^2).

Personally, I tend to believe things like ion drive are actually much more efficent and likely to work well with stare exploration (ion drive is just a fancy way of saying you shoot very small amounts of mass out the back going very fast. This is important because it means you can get more thrust from the same amount of fuel weight if you have something like a nuclear power source to accelerate the ions).

Re:Bullshit!

[Saluton_Mondo]

Personally, I tend to believe things like ion drive are actually much more efficent

Maybe for satellite operations (e.g. station keeping, etc.), but I think ion drives are unlikely to be used for serious long-distance spaceflight (at least for the transport of humans). There are also many problems with using ion engines in this way: inability to perform ground launch, inability to accelerate quickly, etc., etc.

Not that much time difference

[Anonymous Coward]

to the crew on board, it'd be like
mere hundreds or even tens of years

Not according to general relativity: they need
to accelerate when leaving, and decelerating when
arriving to destination. Decelerating will almost
reverse any time compression from the
relativistic speed...

Re:Stellar Pong?

[Anonymous Coward]

"Perhaps you'd like to explain how jettisoning a solar sail has enough force to slow down the craft."

It was not my intention to suggest that jettsoning the sail would slow the craft. You use the light from the star you're approaching to slow down. Actually in the case of Forward's design, you detach an outer ring of the sail. This outer ring accelerates faster than the ship and inner circle of sail because it doesn't have the mass of the ship to pull. Once ahead of the ship actuators on the ring bend it slightly to focus on the circle still attached to the ship. Now the laser in sol's orbit can be used to slow the craft much faster than using the light of the destination star, making for a faster overall trip.

I though Forward's book was entertaining. I love his aliens.

If I knew I'd get a +5 informative I'd have tried to dredge up my old slashdot profile and password!

solar wind

[N3wsByt3]

Solar sails do not use (or not primarely, exept maybe when close to the star, in the beginning) solar wind to propel itself. It uses the reflection of the sunlight; thus, photons, rather then ions.

It's also not correct that solarsails can't be used to reach other suns, because the sun there gives an oposite force. It's quite trivial, when using adaptive (rotating) solarsails, which have only one higly reflective side, to slow down or accelerate when nearing a solarsystem. And even withing a solarsystem; for an interesting project in that regard, see the planetary society [planetary.org] where they plan to launch the first non-gov solarsail-powered probe.

Re:Stellar Pong?

[Anonymous Coward]

Correction: a square-rigged ship is just as capable of sailing against the wind as a ship with triangular sails. I spent a couple of weeks this summer sailing around with about a dozen other squarerigger enthusiasts in an Åfjordsbåd, an old Norwegian type used for fishing. It can comfortably sail about 60 degrees to the wind, which isn't impressive by modern standards, but keep in mind that this type was developed in the 14. century, as a continuation of the technology of the Viking-ships.
"Workhorses" of the 1800s were in fact usually gaff rigged, like clippers, while the war ships were square rigged. Square rigged ships require rather more crew, but are more maneuvrable, so for navies they were preferable until steam-ships became an option.

The Wind from the Sun

[Insipid Trunculance]

Hasnt anybody read this story by Arthur C. Clarke about an Earth to Moon race in solar sail powered space ships.

A beautiful story with an excellent description of some problems which may exist.Read the story,i will spare the spoilers.

Re:Stellar Pong?

[anorlunda]

Not true that the sail would have to be massive. Some years back I read a proposal for a aluminum sail as big as the earth but weighing only 100 grams. It would only be 2 atoms thick.

Limited directions?

[wikdwarlock]

Can someone in the know answer me this:

Since a solar sail needs light pressure to accelerate, can it only accelerate in a direct line away from a star?

also

Isn't there a problem, once the sail gets far enough from its original star, that pressure from other stars will interfere w/ the path?

Quantum teleportation propulsion

[PIPBoy3000]

I ran across this novel form of propulsion [astrobio.net] the other day that looks promising if it pans out.

The idea is to entangle two cesium atoms, then send one up into space. Back on earth, excite the one that remains and the one in space will do the same. In theory that could be used as an ion drive while keeping the bulk of your engine back on the ground.