## Ikaros Spacecraft Successfully Propelled In Space 229 229

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."*
## 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.

## Re:Photon pressure wildly, ludicrously off (Score:3, Informative)

## 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, 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:1, Informative)

Not to mention that a pressure is a time average but photons are discrete. An impulse per photon would be the correct quantity to use if you wanted to put it in the article. That impulse is (h/c)*f; h being Planck's constant, c is the speed of light in a vacuum and f is the frequency of the photon.

## 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.

## Re:Top Speed ? (Score:5, Informative)

## 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.

## Re: Use scientific units... (Score:2, Informative)

## Re:Sunjammer (Score:3, Informative)

2. Wind from the Sun.

## Re:Sad writing (and summary) (Score:1, Informative)

0.0002 lb is the number that TFA quotes. If you follow the link to the JAXA release, that number (1.12mN in SI units) seems to be for the entire sail, which is definitely not just catching one photon. Which seems a more reasonable number.

Isn't that a very very small force though? I'd imagine that doing any sort of acceleration with it would take a long, long time. The mass of the system is given by the Wikipedia article [wikipedia.org] is ~300kg, so this thing's acceleration should be about 3.7e-6 m/s^2. The typical speed needed by a rocket to get into orbit is about 17000 mph or roughly 7600 m/s. Realising that that's not necessarily comparing apples to apples, but that's the sort of speed we're used to from space-ships. Thus, if the light of the sun manages to accelerate this thing constantly it should take about 2e9 seconds or 64 and a half years by my rough calculations to get up to speed.

I'll admit though, it's an amazing proof-of-concept.

## Re:Its a good start (Score:5, Informative)

## Re:Top Speed ? (Score:3, Informative)

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 increase or decrease your orbit energy. Increases in orbit energy correspond to increases in orbit radius (or semi-major axis if in a non-circular orbit), while decreases in energy decrease the radius of the orbit. There's no "tacking" in the sense of ocean sailing.

## Re:Sad writing (and summary) (Score:3, Informative)

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

getinto 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:Sigh. (Score:3, Informative)

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.

## 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.

## Re:Sad writing (and summary) (Score:3, Informative)

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, they're going at 2m/s. After two weeks, they're going at 4m/s, and so on.

These numbers seem pretty low, considering that they're aiming to get to Jupiter in two years, but possibly they get a gravity boost of some kind.