Washington

US planetary scientists are urging NASA to make a conventional mission to Jupiter's moon Europa a top priority.

The call has been spurred by increasing doubts surrounding the agency's plan to send a nuclear-powered craft to Europa and two of Jupiter's other satellites, Ganymede and Callisto.

The troubled Jupiter Icy Moons Orbiter (JIMO) is to be NASA's first nuclear-powered mission, but its launch date has already slipped from 2011 to 2015, largely because of the novel technology involved.

Sensing trouble, a scientific advisory group for JIMO last month wrote to Andrew Dantzler, NASA's chief of Solar System exploration, to advise the agency to consider a conventionally powered Europa mission “if major slips beyond an expected JIMO launch in the 2015 time frame occur”.

Along with Mars, Europa is seen as the most likely place in the Solar System to find extraterrestrial life. Robert Pappalardo, a planetary scientist at the University of Colorado in Boulder, is among some 80 scientists who have signed a white paper, attached to the JIMO advisory group's report, emphasizing the importance of Europa exploration. “We want to remind NASA that with or without JIMO, Europa is a top priority,” Pappalardo says.

NASA surprised many scientists in 2003 by proposing JIMO as the first mission under its Project Prometheus nuclear-propulsion programme.

But JIMO has run into such problems (see Nature 431, 113; 2004) that NASA has ordered a comprehensive review of Prometheus, which should be complete in April. One option under consideration is to defer the Jupiter probe and begin the project with a smaller, more manageable nuclear mission, perhaps to our own Moon or to an asteroid.

No one is ready to declare JIMO dead just yet, but NASA is realizing that reaching Jupiter is “quite a big first step” for its nuclear programme, says Dantzler. He adds that he has already commissioned an informal study into a conventional mission to Europa. “I'm not putting a lot of money into it yet,” he says. “But if it does come down to a conventional Europa mission, I want to hit the ground with the wheels already turning.”