NASA scientists are developing a new ion-propulsion system that could enable spacecraft to reach unheard-of speeds and undertake long-term explorations of planets in the outer solar system.

Dubbed "Herakles," the system would use an ion beam produced from xenon gas to propel the craft to speeds of 200,000 mph, 10 times faster than the top speed of the space shuttle. Because the new technology is much more efficient than conventional propulsion systems, the craft will be able to carry heavier, more-sophisticated scientific equipment.

The propulsion system is being developed through the National Aeronautics and Space Administration's Prometheus Nuclear Systems and Technology program, and by engineers at the Glenn Research Center in Ohio and the Jet Propulsion Laboratory in La Caņada Flintridge, Calif.

A Prometheus craft would be launched by conventional chemical rockets, but once in space, it would produce thrust by extracting ions from the xenon and expelling them into space. Although the thrusting begins slowly, the speed constantly increases over time.

The new system would make the moon, which is about 239,000 miles away from Earth, a short trip.

The craft would be powered by an onboard nuclear reactor.

The first ion thruster was installed on Deep Space 1, which was launched in 1998 and conducted a flyby of comet Borrelly. That system relied on solar energy, which loses power as the craft moves away from the sun.

Herakles and the other elements of a Prometheus craft are scheduled for advanced flight development in 2006.