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Last Updated: Tuesday, 9 December, 2003, 12:30 GMT
Power probe looks to Jovian moons
By Richard Black
BBC science correspondent, in San Francisco

Jimo, Nasa
Jimo would need more power than any previous probe
Scientists have been giving details of a proposed US mission to the moons of Jupiter that may possibly support life - Europa, Callisto and Ganymede.

The huge probe, which would visit each in turn to study sub-surface oceans, will need to be powered by a nuclear reactor and this may be controversial.

The US space agency calls the concept craft the Jupiter Icy Moons obiter.

Its design and mission were described at the annual meeting of the American Geophysical Union in San Francisco.

Energy of life

The Galileo mission that came to an end earlier this year produced tantalising hints that Europa, Ganymede and Callisto might have oceans of liquid water below their ice-crusted surfaces.

Where there is water there could be life, and exploring that is the principal motivation behind the proposed Jupiter Icy Moons Orbiter (Jimo).

There's as much science on this probe as everything that's been done so far on Mars
Louise Prockter, Johns Hopkins University
The team of scientists which the US space agency (Nasa) commissioned to work out some of the mission details was co-chaired by Ronald Greeley, from Arizona State University.

"Our primary theme is to see if there is liquid water beneath the icy crust," he told BBC News Online.

"The second consideration is, is the right chemistry there? We'll be able to determine what the composition of the ice is.

"And then the third consideration is the presence of an energy source; all life requires energy."

Giant planet, giant returns

One of the unusual requirements of the probe is that it will have to jump from moon to moon. This, Nasa says, will require far more energy than for any previous craft, and the only feasible power source is an on-board nuclear fission reactor to drive an electric propulsion system.

This would also allow the use of instruments which normally cannot be deployed in space because they need too much energy.

The Galileo probe's last journey

As a comparison, the Cassini probe, currently en route to Saturn, has a small plutonium unit, with a total power output of 900 Watts for the craft's entire suite of instruments.

Jimo, on the other hand, would have a kilowatt of power available for each individual instrument.

But with images of the space shuttle debris still fresh in Americans' minds, and remembering the fuss when Cassini executed a flyby of Earth on the way to its destination - it is unclear how the idea of putting a nuclear reactor on top of a launch rocket will play with the public.

There are also other significant hurdles to be overcome before Jimo becomes a reality, not least in funding.

But if it gets the go-ahead, which will take several years, the mission will provide scientific data from space at a rate never before attained, and may answer one of the biggest questions in all the cosmos: is there life out there?

Louise Prockter, from Johns Hopkins University and Greeley's co-chair, said: "There's as much science on this probe as everything that's been done so far on Mars. And the returns from it will be as great."

Jimo, Nasa
If funding is found, the mission would launch in 2012 or later
It will investigate the history and make-up of Jupiter's moons
The design of safe nuclear power systems for space is a key aim

Long wait for Jupiter return
22 Sep 03  |  Science/Nature
Galileo ends in blaze of glory
21 Sep 03  |  Science/Nature
In pictures: Galileo's voyage
21 Sep 03  |  Photo Gallery
Galileo: Space success story
20 Sep 03  |  Science/Nature
Europa's ice crust probed
18 May 03  |  Science/Nature

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