Slashdot Mirror
Ikaros Spacecraft Successfully Propelled In Space
An anonymous reader writes "Japan's IKAROS spacecraft has already successfully deployed the first solar sail in space, but today it made the only first that really matters: it successfully captured the sun's rays with its 3,000-square-foot sail and used the energy to speed its way through space. Each photon of light exerts 0.0002 pounds of pressure on the 3,000-square-foot sail, and the steady stream of solar exposure has succeeded in propelling the nearly 700-pound drone."
But when they tried to go to the moon they had to have Ed Belbruno save their butts.
stupid writers reported the total force on the sail (1.12mN) = 0.0002 lbf as the per-photon pressure.
As we all know that I Ikaros (engl. Icarus) flew too close to the sun. This is a bad name for a space craft or any other flying device to call it after an pilot who messed up.
Great, now they can see if it's acceleration is anywhere near what proponents and sci-fi writers have been saying for decades. :)
Also, maneuverability, as I just don't see most of those sailing techniques working in a vacuum.
Can't wait for final results
The figure of 0.0002 pounds of pressure per photon is off by a vast degree. The Wikipedia article on Solar Sails cites a figure of 4.57x106 N/m2, or .00000457 Newtons of force ( 0.000001027 pound-feet) against a square meter of sail material given the full flux of the Sun at Earth's orbit. A single photon would provide less than a trillionth of that amount.
- jon
Ganymede, a GPL'ed metadirectory for UNIX
the speed of light / (((.0002 psi) * (3000 (sq ft))) / (700 pounds)) = 7.8485537 years
Aside from the article being wrong about the forces exerted, I hate that last sentence.
"...the steady stream of solar exposure has succeeded in propelling the nearly 700-pound drone."
Well... how fast has it gotten to so far? That's what it sounds like the sentence is going to say, and then it just ends. It bothers me.
-Taylor
Worldwide Military budgets: $2100 billion. Worldwide Space Exploration budgets: $38 billion. Really, world? Really?
Use SI-units for crying out loud. This is a scientific context. Not a grocery list. Also so the rest of the 90% of the world population can understand it.
But I guess it *shouldn't*, since it's using the sun's light to propel itself *away* from it.
Perhaps they should've called it Anti-Ikaros, Not-Ikaros, !Ikaros, etc. :)
Use SI-units for crying out loud. This is a scientific context. Not a grocery list. Also so the rest of the 90% of the world population can understand it..
...giant fighting robots with swords will be propelled by solar sails throughout the solar system!
> Each photon of light exerts 0.0002 pounds of pressure on the 3,000-square-foot sail
C'mon people, can't you even check if what you're saying makes the slightest sense before posting it? There are two impressive errors in that sentence. First, each photon [1] applies some impulse to the sail. Impulse is what you feel pushing you back when someone punches you. It's a one-time effect and is neither a force (impulse per unit time) nor a pressure. Second, a pound might be a unit of force or of mass, depending who you ask, what you're talking about, and how pedantic you are, but it is never a unit of pressure. (If it were, you might say that the Earth's atmosphere weights 14 pounds, a statement that makes no sense at all.)
[1] For the physically inclined, there's a more subtle error, too. The impulse supplied by a photon is related to its momentum, which is a function of wavelength. So, unless something weird's happening in the sail, blue photons supply a larger impulse than red photons.
"Each photon of light exerts 0.0002 pounds of pressure" I was knocked over when I read that!
It's all part of the 'Knots per hour' and 'Watts per day' malaise that all journalists are infected with.
None of them* can use units correctly, leaving us to try to interpret what the scientist, who wrote the notes that were mismassaged into a press release which was misinterpreted by the journalist, was trying to say.
*unjustified absolute. YHBT
Prediction for end of Universe #42: Fencepost error in Quantum_bogosort.cpp
Point the first: 1^1020 = 1.
Point the second: 1/1 = 1, which is greater than a trillionth.
Point the third: The cited article calculates 2.55453 X 10^20, and a trillion is 10^12, so the trillionth guess was only off by 8 orders of magnitude, not 1,020 orders, as I thought when I wrote that.
Point the main: I should not try to show off my math on the Internet.
- jon
Ganymede, a GPL'ed metadirectory for UNIX
Last time I checked a pound was not a unit of pressure. On that note, I wish pounds weren't used to measure anything.
A 3000 square foot sail is about 16 metres across. Imagine what you could do with a sail one kilometre across. To get to Titan: kill your orbit around the sun with your sail. Gravitational slingshot off the sun with a single burn, possibly combining the sail with a solar thermal rocket, then aero-brake in the atmosphere of Saturn, then repeat at Titan. How's that for a fast trip?
http://michaelsmith.id.au
I think the per photon thing has to do with the original Japanese release (i.e. Engrish). Japanese science writers without a great command of English sometimes use "photon" for "photons." Hamamatsu, a Japanese video company, for example, used to have the following slogan: "Photon is our business."
Kampei! Congratulations to the Japanese. This is a very cool accomplishment, and something all of us geeks have been waiting for for a long time.
"Crude and slow, clansman. Your attack was no better than that of a clumsy child."
That's why I stay indoors.
Doesn't it make you feel good to know that our freedoms are protected by politicans, lawyers and journalists.
Perhaps if you were coming from another star, after beimg accelerated to a few percent of c by giant lasers.
OMG, do we need to jump start the sun??? I thought that was just a movie, not a documentary...
Wait a minute... if photons exert that much force and there are millions or billions of them hitting me in full daylight, shouldn't I feel lighter at night??? Granted I'm a Slashdotter who lives in Mom's basement and plays WoW nonstop and doesn't see the light of day much, but still. I should feel so much lighter that I can fly like a vampire.
Once upon a time (about 1962) Arthur C. Clarke wrote a story called Sunjammer. I was fortunate enough to read it in its original publication. I hunted for it for years afterwards to read again, but he had changed the name because it duplicated the name of another unrelated SF story that year. Imaginary points to anyone who can name:
1: The original magazine of publication.
2: The new story name.
I've been in love with the idea of solar sailing, and in fear of the sun's stormy season, ever since.
"It's the height of ridiculousness to say for those 9 lines you get hundreds of millions."
I see this Slashdot article cites a Slashdot article. Holy recursive Batman!
On that note, I wish pounds weren't used to measure anything.
If I had a pound for every time someone misused pound as a unit, I'd be rich!
Ok folks, we get it -- almost every single comment so far has been about the unit error in the article. You noticed how silly it is, and are therefore smart. Can we get past that now and talk about how ridiculously awesome it is that the first-ever solar sail has been successful, and is propelling through the inner solar system by riding photons from the Sun?
Can a solar sail travel towards the Sun? I must be missing something.
(And no, it can't tack, that requires lateral pressure from a fluid medium...)
No sig today...
Surely I must not be the only one who thought that as I saw the photo.
Ah, USA and its imperial units. If you only would use SI system, you could use the effort to something else rather than conversions and basic unit calculations...
You forgot gravitation, never a good thing to forget... In orbit you do not "lack an immersion medium" as you say, you are fully immersed in the gravitational field of the sun and the planets.
To fly towards the sun, kill your tangential orbital velocity with a sail at a 45 degree angle with relation to the sun, then let the sun's gravitation do the rest.
He got rightly modded down for suggesting that "greenies" are technically ignorant. Quite the opposite: I'm a life scientist, and an overwhelming majority of life scientists is environmentally conscious to some degree. That's because we can see exactly how humanity is screwing itself. One needs to be ignorant not to know or care.
That said, I hate inhabitat as much as you do. They give the green cause a bad name. Not only are they a cesspool of ignorance, they're also sensationalist and in every story, they consistently get one or two facts shamefully wrong. Their target audience probably consists of the handful of hippies and new agers that still haven't gotten their feet to the ground after all those years. Slashdot editors should refuse to post stories that link to them.
On a more fundamental level, the whole premise around which the site is built, design will save the world, is woefully optimistic. Design won't save the world. Not even science & technology will, as long as it's in the hands of the same old greedy bastards. Only changes in society, attitude, life style might change the world. Relying on design, science, technology, god,... to save the world is merely an excuse not to work on the change that 's needed.
The figure of 0.0002 pounds of pressure per photon is off by a vast degree.
It's also off by a couple of dimensions. A pound is a unit of force not a unit of pressure. If they are going to use medieval units they could at least get it right and quote it in pounds per square cubit...or even newtons per square metre if they wanted 95% of the planet to understand what they were on about.
Each photon of light exerts 0.090718474 grams of pressure
A gram is a unit of mass, not force and certainly not pressure. In addition the SI unit of mass is the kilogram, not gram. The unit you are looking for is newtons per square metre....although you might have been forgiven for saying just newtons since the summary messed that up and called it a pressure instead of a force.
Electrodynamic Space Thruster
http://www.youtube.com/watch?v=9ScAHXN_kAY
It was just a joke.
Ah ok in that case there is a more serious problem: it was not funny.
Arguing over a trifle like SI units was the core of the joke.
If units are such a trifle then I take it you would have no problem if your employer paid you the same number of cents as they currently pay in dollars....after all surely you wouldn't argue over such a trifle as units would you? It would be a hilarious joke, no?
On my feed reader, this item came up beside some stuff about http://www.fasterthanthewind.org/ where a vehicle made a record for sailing down wind at just shy of 3 times the speed of the wind. They did so by turning the problem on it's side and inside out a bit -- you can read more there if you want to. In any case, I was beginning to wonder if the concepts (not the mechanics, but the energy balances they achieved) could be used to make a solar sailing space craft that travels down the solar wind at some pretty impressive speeds.
Just a thought. I have no idea what that would look like, and the solar wind is pretty fast, so it may not make much sense (unless the system would at least increase the rate of accelleration over the current design of sail for the same surface area.) None the less, I would love to see if someone who understands the physics better than I could take a go at what such a system would look like, and what would have to be changed or discovered to make it happen.
More Caffeine. NOW
A sailboat can travel against the wind because the force which is perpendicular to the wind is pushing the KEEL/daggerboard with an angle of attack against the WATER! The keel is a wing which generates 'lift' upwind. If you want to test this, pull up the daggerboard and the boat will slip sideways against the wind. Without the keel and the water, the boat will not progress into the wind.
If you angle the sail of the spacecraft, you will get a reduced thrust away from the sun, and a force in the horizontal direction (perpendicular to the radius vector). Canting the sail will bump the s/c side to side, and will reduce the thrust, but you can ONLY reduce thrust to Zero! You can't go negative. No braking thrust. ONLY if you "luff" the sail, parallel to the solar wind, will the thrust drop to zero, but then you are coasting UP the gravity well. By that time, you are probably past escape velocity, and will not be seen again. And remember, you didn't remove the initial orbital velocity of Earth, so you 'climb' is really a slowly-increasing spiral. At that distance, adding 10% to your velocity is escape velocity (at earth radius, V0 * sqrt(2)... 41% increase is escape, less farther out.)
disclosure: I'm a degreed aerospace engineer and accomplished sailor.
If you angle the sail of the spacecraft, you will get a reduced thrust away from the sun, and a force in the horizontal direction (perpendicular to the radius vector). Canting the sail will bump the s/c side to side, and will reduce the thrust, but you can ONLY reduce thrust to Zero! You can't go negative. No braking thrust. ONLY if you "luff" the sail, parallel to the solar wind, will the thrust drop to zero, but then you are coasting UP the gravity well. By that time, you are probably past escape velocity, and will not be seen again. And remember, you didn't remove the initial orbital velocity of Earth, so you 'climb' is really a slowly-increasing spiral. At that distance, adding 10% to your velocity is escape velocity (at earth radius, V0 * sqrt(2)... 41% increase is escape, less farther out.)
disclosure: I'm a degreed aerospace engineer and accomplished sailor.
Of course you can slow down. Remember Kepler, the guy who figured out that orbits are ellipses? Angle the sail perpendicular to the orbit on the half of the orbit when the sail is moving towards the sun, this will give you a net braking thrust. On the other half of the orbit, when the sail is moving away from the sun, angle the sail perpendicular to the sun so you get zero thrust. I.e. slow down when going towards the sun and just coast while moving away.
full disclosure: I'm a physicist and a wind surfer.
Oh, I meant parallel to the sun, not perpendicular. I.e. show no surface area to the sun while going away from the sun.
Ikaros is not in an "orbit". It is in a (nearly) constant-thrust trajectory which drops inward from earth toward Venus. As a windsurfer, you can easily imagine a windboard being towed by a jetski at 10 knots. With the wind from your left, you cut loose, catch the breeze, bear left and accelerate to 20 knots, whipping around another jetski passing at 18 knots. But then, you sail by and surf down the coast, leaving the jet skis behind. That is what IKAROS is doing. It has no way to apply the 'brakes' to stop or orbitally insert into Venus. The parents analogy was misinformation. You can't thrust TOWARD the sun any more than a hot air balloon can set a sail and move upwind.