A new spacecraft is being launched in about a year, one designed to travel across the solar system. But instead of using rockets to propel itself, it uses sails. Sails pushed by light.
Starlight carries not only energy but momentum. Comet tails, for example, are the result of light blowing dust off the comet's core. It's not a lot of juice, but it's enough.
The force on a solar sail is gentle, if not feeble, but unlike a rocket, which fires for a few minutes at most, it is constant. Over days and years a big enough sail, say a mile on a side, could reach speeds of hundreds of thousands of miles an hour, fast enough to traverse the solar system in 5 years. Riding the beam from a powerful laser, a sail could even make the journey to another star system in 100 years, that is to say, a human lifespan.
We're pretty damned far off from a person riding the light to whatever planet Avatar is set on, but it's still a pretty neat idea. [NY Times]