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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."
stupid writers reported the total force on the sail (1.12mN) = 0.0002 lbf as the per-photon pressure.
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
Also, it's going the wrong direction.
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.
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..
> 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.
Not necessarily:
"The craft will spend six months traveling to Venus, and then it will begin a three-year journey to the far side of the Sun." from wikipedia
and
https://secure.wikimedia.org/wikipedia/en/wiki/Solar_sail#H-reversal_sun_flyby_trajectory
"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
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.
It's an interesting example of relativity though, because you're using the speed of light to try to accelerate you to the speed of light - once you understand that the speed of light is always constant, you arrive at the fact that the faster you're going, the less energy the light has. The light "shifts" to the red side of the spectrum.
We've begun implementing Microsoft's latest "developer stack" at work. Now every time someone refers to "TFS", I think "what, Slashdot is on Team Foundation Server too?" Great. Thanks Microsoft.
I feel fantastic, and I'm still alive.
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."
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?
It might not be able to tack, but it can do something like reaching. It's been a decade since I played with the maths for this, so I'm going to be handwaving and relying on my memory for most of this post - someone doing some actual calculation, please feel free to correct me...
There are two forces acting on the craft. One is the force from the sail, the other is from the Sun's gravity. Actually, this is a massive oversimplification, it's really an n-body problem, and at the moment the Earth and Moon's gravity will also be significant factors.
The important thing to remember, however, is that it already has a considerable orbital velocity. It is not going to fall into the Sun, it is going to continue in the Earth's orbit unless some extra force acts on it. By angling the sail away from the Sun, it can make its orbit more eccentric, meaning that perihelion will be closer. As it falls into this eccentric orbit, it will gain velocity. It can then swing around Venus and head out, at which point it will be running (solar wind directly behind it).
Sailing metaphors are actually not very helpful, because inertia doesn't matter much when sailing (except when you come about), but it is incredibly important in orbital mechanics.
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