Nasa gives go-ahead for nuclear mission to outer planets

An ambitious and controversial mission to explore the other planets of the solar system using nuclear-powered spacecraft has come a step closer after Nasa gave a giant aerospace company the go-ahead to develop revolutionary new engines.

An ambitious and controversial mission to explore the other planets of the solar system using nuclear-powered spacecraft has come a step closer after Nasa gave a giant aerospace company the go-ahead to develop revolutionary new engines.

The aim is to build an interplanetary space probe powerful enough to fly vast distances and still to have enough power to collect scientific information and send it back to Earth.

Nasa, the US's National Aeronautics and Space Administration, is giving $6m (£4.3m) to the American company Lockheed Martin to investigate the concepts needed to build such a deep-space explorer powered by a miniature nuclear reactor.

Project Prometheus, named after the Greek god who gave fire to humanity, will concentrate on nuclear propulsion because it is considered the only form of power that can meet the mission's requirements.

But critics will question the safety of launching potentially dangerous nuclear material into space, especially after the technical failures that led to shuttle crash earlier this year.

Nasa said an immediate goal of Prometheus was to provide the propulsion to send a spacecraft to the planet-sized moons of Jupiter - Callisto, Ganymede and Europa - which may harbour life beneath their icy surfaces.

Solar-powered instruments would be of little help in analysing the moons and planets of the outer solar system, where the sunlight is weak and ineffectual. Nasa has decided, therefore, that only a nuclear-fission reactor would provide sufficient electricity to probe these worlds.

Existing space probes have reached as far as Jupiter and beyond but they have had to operate on the power equivalent to a few electric lightbulbs, which has severely limited what they could do, where they could go and what they could transmit back to Earth.

A nuclear-powered spacecraft could not only operate powerful radars and other remote-sensing instruments but use its engines to travel more freely instead of relying on the gravitational "sling-shot" technique that limits the trajectory of existing probes.

"Project Prometheus will develop the means to efficiently increase power for spacecraft, thereby fundamentally increasing our capability for Solar System exploration," Nasa said. "Increased power for spacecraft means not only travelling farther or faster, but is also means exploring more efficiently with enormously greater scientific return."

The first candidate for nuclear propulsion, the Jupiter Icy Moons Orbiter ( Jimo), is unlikely to be launched before 2011 because of the time needed to build the spacecraft. However, as a fleet of space probes heads for Mars over the coming two years, the next target is likely to be Europa and its vast ice-covered ocean, confirmed by the space probe Galileo. Last year, the US National Research Council ranked a Europa orbiter as top priority for a "flagship" mission because of the possibility it might harbour life.

Nasa scientists calculate that a nuclear-fission reactor on the Jimo spacecraft would give it a hundred times more power than a comparable space probe powered by solar panels. A reactor for a spacecraft would have to be about 10,000 times smaller than typical scientific reactors used on the ground.

It would also have to incorporate safety enhancement. One feature is for the reactors to remain "cold" - not turned on - until they are well beyond Earth's orbit. Another is to ensure that once the mission is over the spacecraft is sent into a trajectory that takes it well away from Earth.