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The US space agency NASA may repair the Hubble Space Telescope earlier than scheduled, to prevent a failure in its guidance system that would temporarily halt scientific observations.

If the plan goes ahead, an astronaut crew will replace the telescope's gyroscopes in October, rather than waiting until the planned ‘servicing mission’ in June next year.

The same crew would then return a year later to install a new scientific camera and to perform other repairs scheduled for the June 2000 mission. An emergency rescue is needed following the partial failure in January of one of the telescope's six gyroscopes, which keep it orientated in space. Hubble needs three gyroscopes to function normally, with the rest as backups.

But, with two having already failed completely, an electrical malfunction caused a third gyro to behave erratically, and it was shut down by engineers at NASA's Goddard Space Flight Center in Maryland.

According to David Leckrone, the Hubble project scientist at Goddard, there is no way to predict when a similar failure might occur in another gyro. A fourth loss would automatically put Hubble into a ‘safe’ mode, where it would remain protected but stop observations.

Hubble managers endorse the October mission, but NASA's space shuttle and science offices have yet to agree to the plan.

Delays with the Russian ‘service module’ have left shuttle managers juggling their launch schedule. The module was to have been delivered to the space station in September, but now may not fly until November or later. These Russian schedule slips left the shuttle with no mission this autumn, allowing the Hubble rescue.

NASA science managers have a different concern. Splitting the repair mission would give twice the opportunity to damage or contaminate the telescope.

But, says Leckrone, the planned June 2000 repair, which will install an Advance Camera for Surveys and cryogenic cooling system, and update computers, “was shaping up to be quite complex, ” with six space walks. Spreading the work over two missions makes it more manageable, he says.

NASA administrator Daniel Goldin told a congressional committee last week that NASA wants to decide soon. Leckrone says it may be a “couple of weeks” before the agency can commit to the plan.