Ikaros Spacecraft Successfully Propelled In Space 229
Posted
by
timothy
from the best-place-for-it-really dept.
from the best-place-for-it-really dept.
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."
Sad writing (and summary) (Score:5, Insightful)
Re:Sad writing (and summary) (Score:5, Funny)
Someone should ask the writers why they can stand outside on a summer's day and no be pounded into the pavement.
Re:Sad writing (and summary) (Score:5, Funny)
Someone should ask the writers why they can stand outside on a summer's day and no be pounded into the pavement.
They are not pounded into the ground because reflected light from the pavement pounds back with an upward force.
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Go ahead and joke all you want, I'm not coming out of the basement until it's proven that sunlight won't pound you into the pavement.
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Isn't that a very very small force though?
This is a solar sail. They produce a very small thrust, and will never be used to get into orbit, but they require no fuel. If you want more thrust, build a bigger sail (i.e., raise the area to mass ratio). With a big enough sail (and some time) you could go anywhere in the inner solar system.
Re:Sad writing (and summary) (Score:5, Informative)
The same way sailboats go against the wind.
Set the sail at an angle different from 90 degrees towards the sun. The resulting force can be divided in two components, one pointing outwards to the sun, which is cancelled by the sun's gravitation, and another component perpendicular to the first, which will increase or decrease the spacecraft's orbital velocity.
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My point was that any deep-space explorers are going to need some patience...
Remember that, unlike a rocket, this is a constant acceleration. They've got about 1E-4 N of force converting from the value in TFS (note to Americans: please use metric for anything involving science. Stuff goes badly wrong if you need to stuff your equations full of fudge factors). The press release appears to claim 1-1.2mN, which would be 1E-3 N, so someone possibly made a conversion error somewhere. The craft masses around 300Kg. f=ma, so a = f/m. At 1E-3N / 300Kg = 3.3E-6ms^-2. After a week, the
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(note to Americans: please use metric for anything involving science. Stuff goes badly wrong if you need to stuff your equations full of fudge factors)
It's worse than that. If you don't know what's the difference between a slug and a poundal, use metric. If you do, you already realize why you should use metric.
Re:Sad writing (and summary) (Score:5, Informative)
I did some math and came up with something like 2.1E20 pounds of thrust. It would either far away or (more likely) shattered to pieces with that much thrust. Doing some other math, I come up with about 1.9E-28 pounds of thrust per photon. That seems more realistic to me.
Based on total force of 1.12mN and assuming a static photon count, that looks like an acceleration of 4E-6 m/s^2, so each day it will pick up a velocity of about 0.3 m/s.
Am I getting this correct?
Re:Sad writing (and summary) (Score:5, Interesting)
> Based on total force of 1.12mN and assuming a static photon count, that looks like an acceleration of 4E-6 m/s^2, so each day it will pick up a velocity of about 0.3 m/s.
Yep. ( ((1.12 millinewton) / (700 pounds)) * (1 day) = 0.304767031 m / s )
Re:Sad writing (and summary) (Score:5, Interesting)
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Haha, thanks for clarifying. When I read that I thought "man, that thing must be going pretty dang fast by now!"
Re:Sad writing (and summary) (Score:5, Informative)
the actual press release from the people that *made* the thing. It has better math, as well as a couple fancy graphs. Perhaps this is what should have been posted to
Re:Sad writing (and summary) (Score:5, Funny)
You must be new here...
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So, if the velocity graph has no units, is it safe to
assume the measurement is in "Furlongs Per Fortnight" ??
Anybody have a metric equivalent ?
> http://www.jaxa.jp/press/2010/07/20100709_ikaros_e.html [www.jaxa.jp]
> the actual press release from the people that *made* the thing. It has /. instead of a 3rd party report?
> better math, as well as a couple fancy graphs. Perhaps this is what
> should have been posted to
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What makes the pendantic side of me even more pissed is that they use pounds to measure the force on the sail, and then attempt to use pounds to indicate the mass of the spacecraft. I actually looked at this first and said to myself (just for a second)... 0.0002 pounds isn't nearly enough force to counteract 700 pounds... and in what direction?
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i also did not get together the rest the nubers. 1.2milliNewton is not .11g for a 310kg probe. Its sad its not even possible to figure it out, because the y-axis on the speed plot is not labeled (km/h, m/s, mph/s, foot/second). Its absurd that they usually talk about a 700pound probe with a 3000sqfeet sail and then use SI units for the force. Sadly thats also on the mission web page in that way. I have send supervised students away to redo their report when they showed that style.
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Arthur C Clarke did a much better job of explaining the concept with his "Sunjammer" story that appeared in Boy's Life ca 1964.
What?! Clarke was writing for Boy's Life in 1964? When and why did that lame-ass publication become so lame-ass when I was reading it in the 80s?!
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Re:Sad writing (and summary) (Score:5, Informative)
Dale Colombo. Here it is: http://books.google.com/books?id=s3fIzLrLOq0C&pg=PA77&lpg=PA77&dq=%22the+amplified+boy%22+boy's+life&source=bl&ots=nxBO5ccMW5&sig=QWULOXqhu9lM90w8X37bNDoR2Ks&hl=en&ei=gPhATP2MC4zksQPKjtiJDQ&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBIQ6AEwAA#v=onepage&q&f=false [google.com]
Story starts on page 25.
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Re:Sad writing (and summary) (Score:5, Funny)
No...thank the Holy Google. I am merely an Earthly conduit.
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hmm, scouts. There is something odd about that story...
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Top Speed ? (Score:5, Insightful)
Also, maneuverability, as I just don't see most of those sailing techniques working in a vacuum.
Can't wait for final results
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It appears to be operating well within expectations, looking at the linked website from the Japanese space administration, it's looking to be well within expectations.
But even so, we're talking about a very, very small acceleration effect - if you were on board, you basically wouldn't notice it at all. It's what, 2/10,000 of a pound of thrust, with a 700 pound payload? Since it takes 1 pound of thrust acting on 1 pound of material to equal 1 G [howstuffworks.com], the amount of accelleration on this is something like 2/(10,000
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Nope, tried and true sailing techniques won't work in a vacuum. Neither do solar sails either, so thats not really relevant.
Space is not a vacuum, its just not very dense.
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You cannot adjust the keel vector in a sailboat either, the force is always perpendicular to the direction you are moving. But that's not important, you only need to adjust one of the two vectors to get a resultant vector in any direction you need.
The main difference between ocean sailing and solar sailing is the rudder. Considering it's used for small corrections to compensate for waves and currents, which do not exist in space, solar sailing can be done without it.
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Solar sails don't, and have never been intended to, use "sailing techniques". In that sense "solar sail" is an unfortunate misnomer. Solar sail maneuvers typically take advantage of the fact that changing the sail orientation enable you to direct the resultant force from the solar radiation pressure either along or counter to the orbital velocity vector. Depending on which way you point the sail you either increa
Re:Top Speed ? (Score:5, Insightful)
No, not really. Photons carry several orders of magnitude more momentum than solar wind. The only "practical" way to capture momentum from solar wind is with a magnetic sail [wikipedia.org], since the surface area required (hundreds of square km) would be unfeasible with any physical material.
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The only "practical" way to capture momentum from solar wind is with a magnetic sail, since the surface area required (hundreds of square km) would be unfeasible with any physical material.
Hmm. We can make bigger sails, or we can make the space the photons they're capturing smaller. We really need to figure out how to manipulate space-time and/or gravity.
Re:Top Speed ? (Score:5, Informative)
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At Jupiter's distance, the Sun is well over a million times brighter than Sirius, the brightest star in the Terran sky. Barely distinguishable? Bah.
If you were standing on Jupiter, you would be squished by the gas pressure and not distinguish anything anyway.
Photon pressure wildly, ludicrously off (Score:5, Informative)
The figure of 0.0002 pounds of pressure per photon is off by a vast degree. The Wikipedia article on Solar Sails [wikipedia.org] 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.
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The figure of 0.0002 pounds of pressure per photon is off by a vast degree. The Wikipedia article on Solar Sails [wikipedia.org] 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.
And your use of "pound-feet" is amusingly incorrect. That would be torque. Did you mean pounds-force?
-Taylor
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And your use of "pound-feet" is amusingly incorrect. That would be torque. Did you mean pounds-force? -Taylor
Ups, of course I did, thanks.
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And while I'm confirming my hand-waving stupidity, I'd like to cite http://cubesat.wikidot.com/opticalflux [wikidot.com], which has a quick calculation showing on the order of 2.55453 x 1020 photons.s-1.m-2, so when I cleverly said 'less than a trillionth of that amount', you should read 'less than 1^1020th' of that amount instead.
Fortunately for me, 1^1020 is more than a trillionth, so dividing it out would result in 1/1^1020, which is less than a trillionth. So it kind of works out.
Further idiotic errors (Score:3, Insightful)
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.
Sigh. (Score:3, Insightful)
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
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Watts per day really pisses me off. You even get it on technical blobs like Engadget, who should know better. Then again, they are also stupid enough to think a "$100" phone is much better value than a "$200" phone...
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You're not a journalist are you?
For *your* benefit, Watts per hour is NOT a unit of power or energy. Let me illustrate by analogy with speeds:
Watt: power
Watt-hour: energy (I.e. energy transfered in one our at one Watt)
Knot: speed
Knot-hour: distance (I.e. distance travelled in one our at one knot)
Watts per hour is as nonsensical as knots per hour or MPH per hour....
Don't worry, hardly anyone gets this right.
Also. (Score:3, Insightful)
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
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Just the fact that it has been propelled, at all, is the achievement. It doesn't matter how fast or how far. Kinda like the satellite... didn't really matter that it just beeped. The achievement was that it was up there.
Use scientific units... (Score:4, Insightful)
Use scientific units... (Score:5, Insightful)
Re: (Score:3, Funny)
"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 278.709 square meter-sail and used the energy to speed its way through space. Each photon of light exerts 0.090718474 grams of pressure on the 278.709 square meter-sail, and the steady stream of solar exposure has succeeded in propelling the nearly 317.514659 kilogram-drone."
Better?
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Re: Use scientific units... (Score:4, Informative)
Or you can get the more precise values from the original at http://www.jaxa.jp/projects/sat/ikaros/index_e.html
JAXA uses metric units. The conversion to American units in the article is rounded.
Another fun fact about imperial units that you are probably not aware of, almost all contries have them, just that they differ. The rest of the world changed to metric units partly to get rid of the problem that the length of an inch were different depending on what country you were in.
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LOL "In Brazil, I'm 9 inches!"
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Um, no. Pressure isn't measured in gram
[...]
Pressure is measured in Newton per square meter.
This is almost tragic. So much discussion of the correct unit for pressure, yet nobody seems to realize that the "pressure" described in the article is not a pressure. It is the total force acting on the sail.
So the correct unit is neither pounds, gram nor N/m. It is N.
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Hm. Apparently, not all chars go well with Slashdot. I posted N/m2 with an uplifted 2 (Alt-253), but the 2 was stripped.
Re: Use scientific units... (Score:4, Interesting)
Most people don't know what a N/m^2 is, sure, but a N/m^2 also has the name Pascal (Pa), which a lot of people do know. Even U.S. high schools are pushing students to use Pa for pressure units instead of atmospheres or Torr or the dreaded inches of Hg. In any case, grams times the standard "g" constant still isn't pressure, it's force, and gram is never an SI unit of pressure or force, nor is gram times g.
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No.
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Epic unit fail (Score:5, Insightful)
> 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.
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[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.
This is, in fact, the case -- not all photons exert the same impulse on the sail. However, there are other factors as well -- for one, the sail reflectivity is not uniform across all wavelengths, and so will have different absorption rates throughout the spectrum; for another, the solar spectrum is not uniform either, and emits many more photons at certain wavelengths than at others. This means that, on average, you may get more thrust out of a lower-energy portion of the spectrum than from a higher energy
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I am pedantic, you insensitive clod, and a pound is unit of weight!
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And you even missed the MOST impressive error in that sentence... if each photon exerted that impulse then the sun would have pretty much instantly obliterated the satellite (and all life on earth). That number should be the TOTAL, not PER PHOTON. All in all, an absolutely horrible article and summary.
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A photon is (more or less) a single thing. There are lots of them flying at the sail from the sun, and each one supplies some impulse to the sail.
Imagine you're trying to walk forward through an oncoming swarm of flies. You feel resistance -- that's the force from the flies -- but each fly is just knocking you a little bit back.
If each fly you hit pushed you back with 0.01 pounds of force, then the farther you walked, the more force you'd accumulate (because you've hit more flies), and it would get harder
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Really? What does PSI stand for? Is that not a measurement of pressure? It is to us common folk.
PSI is a unit of pressure to common folk and to physics nerds as well.
Unfortunately for common folk, PSI doesn't stand for "pounds;" it stands for "pounds per square inch." Kind of like your gas guzzler doesn't get 13 miles; it gets 13 miles per gallon or 3.4 miles per liter. But if you said your car was very efficient and got 55 miles, I would tell you that your statement made no sense, not that your number was wrong.
Yes, they should be using a proper unit of energy.
Um, why energy? Assuming that the solar sail is nonrelativistic, then (energy) = 1/2 (m
Wow! (Score:5, Funny)
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"Each photon of light exerts 0.0002 pounds of pressure"
I was knocked over when I read that!
You should turn down the intensity on your monitor and read /. in the dark.
Pound of pressure? (Score:2, Insightful)
Last time I checked a pound was not a unit of pressure. On that note, I wish pounds weren't used to measure anything.
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Its a good start (Score:5, Interesting)
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?
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The problem there is that solar sails (supposedly) aren't that efficient past Mars. They're great for the inner solar system (theoretically, you can tack them like a normal sail, so you can get closer to the sun with one), but the outer solar system? Not a prime candidate for a solar sail. Great for trips to Venus or Mars, though.
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Thats why I suggested using the sun for a gravity slingshot.
Re:Its a good start (Score:5, Informative)
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Am I missing some attempted nuance here?
3000 feet is 914 meters.
On the off chance you're talking about the length of one edge, the sqrt of 914 is 30 meters. So, still, nothing matches 16.
3000 square feet [wolframalpha.com]
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Troglodyte? Who? Me? (Score:5, Funny)
That's why I stay indoors.
Losing weight at night (Score:2)
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.
Sunjammer (Score:3, Interesting)
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.
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2. Wind from the Sun.
Ok, we get it (Score:5, Insightful)
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?
Re:Ok, we get it (Score:4, Funny)
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Also, it's going the wrong direction.
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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 [wikimedia.org]
and
https://secure.wikimedia.org/wikipedia/en/wiki/Solar_sail#H-reversal_sun_flyby_trajectory [wikimedia.org]
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Venus is closer to the Sun then Earth (Score:2)
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...)
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Absolutely. Angle the sail, so that the reflected optical pressure slows its orbit, and it will spiral in towards the Sun quite effectiely, until and unless the pressure suspends it off the Sun's surface without orbital motion (which is pretty darn close and likely to shread the sail), and it will then spiral back out the other way.
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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
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Amusing, yes, but incorrect, since we're talking about an object orbiting the sun. You go further away from the sun by reflecting the light behind you, and get closer by reflecting the light in front of you. In fact, I can't think of any orbital maneuver where you'd want to reflect sunlight back to the sun.
Crazy Eddie (Score:2)
Perhaps if you were coming from another star, after beimg accelerated to a few percent of c by giant lasers.
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Uh... more like you incorrectly thought that equation was valid for calculating acceleration to speeds approaching c, and Google faithfully did the math you told it to.
Not trying to be mean, but Newton need not apply for such a calculation and Google can't exactly be expected to know what you're trying to do. Also, TFS said 0.0002 pounds (not psi) of pressure (so uh, it should be psi but they gave force) per photon (which is just plain wrong, photon momentum = Plank's constant / wavelength, i.e. way small
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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.
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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.
Heh, yep. c stays constant, and instead the wavelength changes. What a bizarre and amazing universe we live in! :)
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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.
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They apparently still do. Every issue of Nature has an ad from them with that slogan. Drives me nuts.