Hello Jennifer,

Solar Sails are a novel way of using sunlight to push spacecraft. When light bounces of the reflective surface of an object it gives a little push to the object. When the light reflects back off the surface the object gets and additional push (this is called momentum transfer where the object gains momentum at the expense of the light). A solar sail works by being a giant reflector so that it can intercept as much sunlight as possible. Designs for solar sails have sails as large as 500m across. The problem is that the momentum transfer from the sunlight is very small, so not only does the sail have to be very large but also very very light. A solar sail can be as thin as 3 microns across about 1/10th the diameter of a human hair. Imagine a large mirror 500m across and only 3 microns thick. Such a sail can get enough push from sunlight to move it around the solar system. The closer to the Sun it gets the faster it can go.

You can probably see the problem here: how do you launch somethign so large and climsy into space. This is one of the major technical difficulties to be overcome by the engineers. Fortunately, there are several ideas out there. All of them essentially rely on folding up the sail so that it can be stowed in the launch vehicle and then using various techniques to unfold them in space. You can check out some of the ideas at http://www.ugcs.caltech.e...diedrich/solarsails/

To "sail" the spacecraft you only need to change the angle of the sail with respect to the Sun. This allows the push to act in different directions and so the sailcraft can move around. All of this can be controlled by computers and don't need an astronaut to control them. The unique thing about solar sails is that they don't need to carry any fuel and can keep going as long as the sail holds up. There are several ideas for using solar sails to look at the poles of the Sun, to return samples from planets like Mercury and Mars, and even to visit nearby stars.

http://www.ugcs.caltech.e...diedrich/solarsails/