NASA's Galileo spacecraft soars above Jupiter's volcanic moon Io in this artist's impression. Photo: NASA/JPL

How should the highly successful voyage of NASA's Galileo spacecraft to Jupiter end? More importantly, where should controllers guide the probe to avoid polluting the jovian system?

These questions were posed to a team of scientists at the National Academy of Sciences' Space Studies Board's Committee on Planetary and Lunar Exploration (COMPLEX), at the direction of NASA.

Despite Galileo's general spaceworthiness, it is unrealistic to assume that it will remain both controllable and scientifically useful for the indefinite future," the panel write in a recently released report. "It is, therefore, prudent to begin planning for the most scientifically productive use of the spacecraft's remaining life and to make provision for its safe disposal."

To avoid the contamination of potential life-growing areas such as the jovian moon Europa, NASA is trying to investigate ways to bring the mission to a safe end. The only real option -- send Galileo probe crashing into Jupiter or one of its other moons.

The COMPLEX team ruled out the option of using gravity to eject Galileo from orbit around Jupiter, sending the craft into a heliocentric orbit because of uncertainty where the nuclear-laden satellite might ultimately go.

Such an option, the panel said, might require a launch-safety review similar to the one ordered before Galileo was sent aloft by a space shuttle 11 years ago.

"The reason for this is the very small, but nonzero, chance of eventual impact with Earth. The anticipated cost of such a review is so great -- in excess of Galileo's current annual operations budget of some $7 million -- that NASA has no option but to dispose of the spacecraft within the jovian system."

An active volcanic eruption on Io seen February 22. Photo: NASA/JPL

After being launched aboard the space shuttle in 1989, Galileo arrived at Jupiter in December of 1995 to begin a planned two-year mission to study the surroundings of our solar system's largest planet. After the first two years were successfully complete, NASA scientists opted to extend the mission another two years through 1999 to provide opportunities for additional studies of the moons Europa and Io.

The data returned to Earth from those two years left scientists even more curious than before about the jovian environment. So, once again, NASA extended the mission for at least another year to provide even more extensive studies of the moons and to temporarily join up in jovian orbit with the Cassini space probe in late 2000 to provide more research of Jupiter's magnetosphere. Cassini is bound for a 2004 rendezvous with Saturn.

Even through five years of being subjected to the dense radiation around Jupiter and its moons, Galileo continues to function quite normally. No major failures of any essential spacecraft systems have occurred.

Lately, however, scientists have built evidence that a possible ocean might be hidden under the icy surface of Europa. This leads NASA to consider proposals for guiding the probe into Jupiter or one of the planet's other moons to protect the possibility of life in the theoretical ocean on Europa.

Contamination is such a significant threat because micro-organisms from Earth that could still be clinging to Galileo.

Europa's icy surface from Galileo. Photo: NASA

The panel looked at the probabilities of life on each of the four major jovian moons: Io, Europa, Callisto and Ganymede). COMPLEX concluded that, even though there are only slight chances of life on both Callisto and Ganymede, Io and Jupiter itself are the prime candidates as the final destination for Galileo.

Chances of life there are almost nil on Io because of its ceaseless volcanic activity. But in the end, Jupiter was chosen as the most likely target.

Four possible trajectories were considered, all of which would end up with an impact on Jupiter. The COMPLEX board then made a recommendation on a preferred trajectory.

This route would take Galileo on several flybys of Io to try to gain even more data on this most geologically-active body in the solar system. A possibility of one flyby of the much smaller asteroid-like inner moon Amalthea is also available. This trajectory would culminate with an impact on Jupiter in either December 2002, September 2003 or January 2005.