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US space engineers are working to correct minor problems with the Galileo spacecraft that have ruined observations of Jupiter's icy moon Europa during two of the probe's last three fly-bys. The last of Galileo's close encounters with Europa is scheduled for February.

During the most recent fly-by on 22 November, the spacecraft unexpectedly went into a protective ‘safe’ mode just before passing the moon at a distance of 2,300 kilometres. This prevented it from taking photographs and gathering data.

Galileo project scientist Torrence Johnson of the Jet Propulsion Laboratory in Pasadena, California, says the most likely culprit is increased radiation when the spacecraft's highly elliptical orbit brings it close to the giant planet. Galileo, which left Earth in 1989 and arrived at Jupiter six years later, has already taken 150 per cent of the radiation dose for which it was designed.

Having completed its primary mission last December, the spacecraft was given a two-year extension to gather more data on Europa, study the torus of charged particles associated with Jupiter's satellite Io, and make two close fly-bys of that volcanic moon next autumn.

Europa is a high-priority target for the US space agency NASA, which plans to send a follow-on mission early in the next decade to help decide the question of whether an ocean lies beneath the satellite's icy crust. The problems with Galileo mean that some of the moon's surface will not be photographed at high resolution as planned.

But Johnson says some high-priority observations may be made by rescheduling targets to be viewed during the final fly-by, as was done after a short-circuit from a piece of space debris led to the loss of Europa data from an encounter last July.

Project engineers are working on a software fix that would in effect tell Galileo to ignore circumstances that trigger the protective mode during close encounters. This is not likely to be ready by February, however, so the last fly-by may meet the same problem.

The strategy is also risky because it is akin to disconnecting an alarm. But the extended mission was always planned to be riskier, says Johnson.

Hoping that the spacecraft survives the high-radiation environment around Io next year, project scientists are drawing up a proposal to NASA for another one-year extension. That would allow Galileo to make coordinated observations with the Saturn-bound Cassini spacecraft when it flies through the Jupiter system in December 2000.