Slashdot Mirror
How Solar Sails Work
Gary writes "You can also learn more about How Solar Sails work.
In mid-April 2001, The Planetary Society, in collaboration with several Russian space organizations, will test launch the deployment mechanism for the first solar-sail mission, Cosmos-1, in a sub-orbital flight.
It is a rounded solar sail that is divided into eight triangular blades with inflatable booms for support.
Maneuvering a solar-sail spacecraft requires balancing two factors: the direction of the solar sail relative to the sun and the orbital speed of the spacecraft."
They also have a good high-level overview of solar sails, and theirs in particular.
Good to see Carl's people still working to promote his dreams. I got all nostalgic reading the site.
Don Negro
Don Negro
Perl 6 will give you the big knob. -- Larry Wall
I think it depends a lot on the mission characteristics. Some propulsions systems are good for some kinds of missions, some for others.
Besides, how are solar sails ever going to become feasible unless someone starts actively working on them?
Um. I think the force imparted by the sunlight is the same. The force imparted just results in a lower acceleration if the mass of the sailcraft is higher.
In case anyone's interested, I worked out some numbers on the force of the light on this thing relative to that of the sun's gravity. Both forces drop as inverse squares of the distance from the sun, so the ratio between them remains constant.
The force due to the light is determined by the area of the sail and the luminosity of the sun (which I looked up), while the force due to gravity is just determined by the mass of the spacecraft and the mass of the sun.
When you work out the numbers using the area and mass quoted in the article, the force of gravity from the sun turns out to be about 200,000 times greater than that of the light.
This means (as the article states) that the best such a spacecraft can do is remain in orbit around the sun, and angle the light in such a way as to gradually spiral out to a higher orbit.
Alas, no interstellar travel with one of these guys, since we can never break out of solar orbit. For a spacecraft with this size solar sail to achieve interstellar travel, it would need to have a mass of less than 0.3 grams! That would require quite an impressive materials breakthrough, I'm afraid.
It's like this, if i can explain it.
The sail gets its acceleration from the inertia imparted on it by bouncing photons, essentially. So imagine a tennis ball bouncing off a wall. Whatever direction you throw the ball, the net force on the wall is directly perpendicular to it - it changed the ball's velocity only in the perpendicular direction. The ball maintained all of its momentum parallel to the wall, but its momentum perpendicular to it changed. Hence, the wall wants to accelerate perpendicularly to its own face.
The same is the case with the solar sail. It will accelerate perpendicularly to its own sails. The magnitude of the acceleration will be greatest when the sails are perpendicular to the "light beams" or however you want to refer to them - that way, the sail absorbs all of the momentum times two of each photon. when at an angle, the sail will only absorb (total momentum) sin (whatever the angle is) times two.
I don't see how the angle of the sail relative to the sun would have any effect on the direction of the vehicle. It's not a sail boat. The difference between this and a sailboat is that a sail has a rudder which has drag against the water to control direction.
With no atmosphere and no water, a solar sail has nothing to drag against to control direction. It can simply be pushed one way: Away from the sun, and then affected by gravitation of other objects. The angle of the sail to the sun just can't have any effect that I can fathom. Maybe I'm wrong.
On a separate though, I wonder if they could use the negative refractive index composite material in making the solar sails, since they are probably still investigating the material to use for the sail?
From yesterday's linked article:
"``negative refraction'' would make possible the construction of a lens capable of focusing light to limits not currently achievable. "
and a related article on the Economist about how negative refractive index material can possibly help make a "perfect lens"
Perhaps they could use the phenomenon of the negative refractive index to make more efficient or more maneuverable sails?
Would be really useful if they could find a material that the sails could withstand damage done by getting hit by meteors, maybe people at NASA could actually put them to extreme uses and send them billions of years in space.
It would've also been nice if included in the article, there was a link to show how astronomers at places like NASA control these things when they're so damn far. I know they would pre-program directions, but what if something was spotted close by, how the heck do they manage to change the course of something millions of miles away to investigate it?
millions of bandwidth bits away
360 degrees of Karma
The sails are reflective (mirror-like), so by angling them you can get sideways thrust -- a combination of the incoming momentum of the sunlight and the reaction from the momentum of the reflected light. So angling the sails at 45 degrees gives you thrust at 45 degrees to the sun, but it cuts the thrust by half. (sin(45) squared -- once for the reduction in light intercepted by angling the sail, once for the off-axis thrust.)
You can't tack like a ship tacking into the wind (where the vector sum of forces on the sails, keel, and rudder gives a net force partially into the wind) because you don't have any equivalent to the keel. But you can use an angled sail to reduce the orbital velocity, so the ship drops sunward. And the best way to go outwards is to use sideways thrust to increase the orbital velocity.
The real problem with solar sails is the very tiny force per square meter that is possible. It should be enough for minor orbital adjustments. If you plan way ahead, it might be possible to use light sails to slowly spiral in or out (like weeks or months to the moon, years or decades to Mars). Or you find a way to augment the thrust -- like building a giant battery of lasers on the moon to provide much more intense light. In a couple of Larry Niven stories, they used lasers to launch an interstellar lightsailer, but to brake at the destination, it had to almost dive into the sun...
You can generate thrust in any direction in the half-sphere centered on the Sun-spacecraft line, with the thrust falling off according to cos^2 theta. So long as your trajectory changes can be performed without a thrust vector aiming away from the Sun, you can (theoretically) perform them with a solar sail. In space, gravity (especially from a third body, like Earth) and inertia allow tack-like maneuvers.
Solar sails use the pressure of the photons, not the plasma wind. There may be some small influence from plasma impingement, but it is just that: small. You should look up the "heliogyro" concept for a Comet Halley rendezvous probe (it was never built, of course). The details will show you where your thinking is faulty.--
Give a man a fish and he eats for a day.
Scientists restrict study to entire physical universe; creationist
--