Posted on 11/10/2009 6:58:18 AM PST by green iguana
About a year from now, if all goes well, a box about the size of a loaf of bread will pop out of a rocket some 500 miles above the Earth. There in the vacuum it will unfurl four triangular sails as shiny as moonlight and only barely more substantial. Then it will slowly rise on a sunbeam and move across the stars.
...
In principle, a solar sail can do anything a regular sail can do, like tacking.
(Excerpt) Read more at nytimes.com ...
But it's the last sentence I posted that I'm hoping someone smarter than me can explain. A sailboat can tack into the wind because of the hull, keel and rudder of the boat pushing against the resistance of the water, with the keel and rudder setting the direction of travel.
In the vacuum of space, what provides the resistance to tack against? The only thing I can think of is cosmic rays, and that just doesn't seem enough. And if it is cosmic rays or something else, what steers the sail?
Exactly what I thought of when I read that sentence.
I though scientific writers would be better vetted.
If the ship deploys its sails so as to slow itself down, it will move into an orbit closer to the sun (the source of the wind) - i.e it can ‘tack’.
Once outside of the sun’s gravitational field it’s a different matter I guess. But at that distance the solar pressure is going to be unimportant anyway.
I don’t think it’s really taking at all, but the effect is the same. You end up moving into a lower orbit - i.e. closer to the sun, the source of your “wind”. If you were in a circular orbit around the sun at some point you’d use the sail to move into a higher elliptical orbit, one with a higher aphelion and a lower perihelion. As you’re then nearing your perihelion you slow your craft down, if you wanted you could trim it off to another circular orbit, like the one you were in originally, but lower.
Thanks - that seems to make sense. It looks like the sail relies on the energy from the initial launch into orbit to get it moving correctly around the sun so that it can tack. If it’s not already orbiting the sun, that doesn’t appear to work.
Of course, being launched from Earth, it already has that orbiting the sun thing going...
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