A vast array of virtual tunnels that winds around the sun, planets and moons could slash the amount of fuel that spacecraft need to explore our solar system, according to NASA.

The so-called interplanetary superhighway (or interplanetary transport network) would take advantage of the gravitational pull between celestial bodies. In many cases, the competing forces cancel each other out, leaving corridors where ships could travel using little or no fuel.

I highly recommend the American Scientist article linked from this page. Seek out the magazine at your local newsstand.