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High flier: NASA's planned orbiter for Jupiter's moons could revolutionize planetary exploration. Credit: NASA

Daring, scientifically rewarding but fraught with technical and political risk. That's the way many space scientists view a proposed nuclear-powered Jupiter mission that would be the most capable — and probably the most expensive — planetary spacecraft ever launched by NASA.

The Jupiter Icy Moons Orbiter concept was unveiled earlier this month as part of NASA's budget request for 2004. Slated for launch no earlier than 2011, the orbiter is the centrepiece of Project Prometheus, a multibillion-dollar effort to develop nuclear power and propulsion for deep-space exploration. The planned budget for the orbiter alone would exceed $90 million next year. Sources in Congress suggest that the total budget needed to launch the spacecraft could run to $4 billion.

The orbiter's nuclear reactor would power an ion-drive engine: a device that expels ionized gas at high velocities to produce a small but continuous thrust. In weight terms, nuclear-powered thrusters are more fuel-efficient than those that use conventional chemical fuels. This would allow the orbiter to perform manoeuvres such as orbiting three of Jupiter's moons in succession. The orbiter is intended to circle three satellites — Europa, Ganymede and Callisto — for months at a time, seeking evidence of oceans beneath their icy surfaces.

Using a nuclear reactor also greatly increases the on-board power. The Cassini spacecraft, en route to Saturn, has about 875 watts of power on board. A nuclear reactor could produce tens of kilowatts. “This changes the entire regime of science experiments,” says Colleen Hartman, head of the Solar System exploration division at NASA's headquarters in Washington. More powerful ice-penetrating radars could be flown and more data sent back to Earth.

NASA faces many hurdles if these benefits are to be realized. The agency has never launched a space-based nuclear reactor, for example. Keeping the reactor away from the scientific instruments may dictate a different shape for the orbiter, and NASA will have to learn how to stabilize and manoeuvre such a spacecraft. Public protests have accompanied the previous launches of spacecraft carrying small amounts of nuclear material (see Nature 410, 626; 2001), and the Columbia accident has enhanced fears over safety.

NASA will also have to convince Congress that the mission is worth $4 billion. Advocates of planetary exploration say that nuclear propulsion is the best option, and that a similar system could be used on other missions if the orbiter is successful. But Louis Friedman, executive director of the Planetary Society in Pasadena, California, points out that if the project runs into problems, Europa exploration — a high priority for planetary scientists — could take longer than if conventional propulsion were used.