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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."
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
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?
You can never go home again... but I guess you can shop there.
http://www.jaxa.jp/press/2010/07/20100709_ikaros_e.html
/. instead of a 3rd party report?
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
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Barely distinguishable? Jupiter is only 5 times Earth's distance from the Sun. Outside Earth's atmosphere, solar insolation averages around 1370 watts per square meter. At Jupiter's orbital distance, it's about 50 watts per sq meter. That's a huge amount of power. 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.
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.
The problem is that bleeding off energy with a solar sail isn't like just jumping onto a sunward elliptical orbit. You're likely to spiral in towards the sun, rather than zip around it. More importantly, a "slingshot" takes advantage of planetary motion relative to the sun to achieve a large trajectory change: a "slingshot" around the sun won't do anything except get you onto the outbound leg of the trajectory you're already on (i.e. it won't help you get further out from the sun than you were already going anyway). You'd probably be better off conserving the energy you already have, and using the sail to spiral out into a higher orbit.
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.