NASA to Launch Solar Sail

Tag : Back Coating

No, that's incorrect. In space travel, the vehicle is in someorbit, and in the absence of a force other than gravity, it's justgoing to continue in that (typically elliptical) orbit forever. Sayyou're going to Mars, for instance. You needed to match orbits withMars, which means you're in the same nearly circualr orbit aroundthe sun that Mars is in. (Of course you also have to insertyourself into orbit around Mars, and get yourself out of that orbitas well, but let's not worry about that for now.) Once you're readyto leave, you don't just wait for the sun's gravity to pull youdownhill back to Earth. You're in a circular orbit whose radius isgreater than that of the Earth's orbit, so you're not coming backtoward the sun unless you can reduce your velocity.
To understand how you'd really use a solar sail, let's start withthe case where you just want to increase your distance from thesun. Intuitively, you'd think that you'd just orient the sailperpendicular to the sun's rays, and let it thrust you outward.However, that doesn't work, because the thrust from the sunlight isorders of magnitude less than the sun's gravitational force. Doingthat would be sort of like dialing down the strength of the sun'sgravity by some tiny percentage, which would alter your orbit for agiven velocity vector, but only by a tiny amount.
What you actually do is to point your sail at an angle. Thesunlight's thrust then has both a radial component and a tangentialcomponent. The tangential component does mechanical work [lightandmatter.com], because it operates in the same direction asthe motion of the vehicle. That means it increases the vehicle'skinetic energy. The higher-energy orbit takes you farther out awayfrom the sun.
When you want to come back, you do something similar, but you tiltthe sail the opposite way. The tangential component is now in theopposite direction compared to your motion, so it does negativework, reducing your kinetic energy.