This via Bruce Sterling: an experimental rocket engine that runs on methane instead of oxygen/hydrogen or solid fuel.
The point being that methane is cheap and plentiful in the outer solar system, so you wouldn’t have to haul all your fuel up out of earth’s gravity well:
Traveling further out in the solar system, methane becomes even easier to come by. On Saturn’s moon Titan, it is literally raining liquid methane. Titan is dotted with lakes and rivers of methane and other hydrocarbons that could one day serve as fuel depots. Imagine, a methane-powered rocket could allow a robotic probe to land on the surface of Titan, gather geological samples, refill its tanks, and blast off to return those samples to Earth.
Also, it looks cool. The movie was worth firing up the lameness of Windows Media Player’s for OSX. NASA must be spending all that money on cinematographers.