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The next Shuttle mission was scheduled for 14 October, but a computer failure aboard Hubble has forced NASA to delay it until February while a replacement part is made ready and astronauts are trained to carry out the repairs. NASA says that, if necessary, the flight could even be pushed back to April 2009.

The mission was, coincidentally, originally intended to send astronauts to the telescope for overhaul work - the first human visit since 2002.

The problem occurred when the science data formatter, which takes information from the telescope and

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prepares it for transmission back to earth, "totally failed".

"Shortly after 8pm on Saturday, 27 September, the telescope's spacecraft computer issued commands to save the payload computer and science instruments when errors were detected within the Science Data Formatter. An attempt to reset the formatter and obtain a dump of the payload computer's memory was unsuccessful," says a NASA statement.

The unit has an exact duplicate, but switching to that one will require that several other instruments are reconfigured.

"A transition to the redundant Side B should restore full functionality to the science instruments and operations," continues the statement, while admitting that the transfer is "complex" and has never been attempted before except for on Earth prior to launch.

The necessary repairs will require a two-hour spacewalk, and may mean that other, lower-priority work is bumped from the schedule.

Two new scientific instruments will also be added to the telescope, allowing new experiments to be carried out. The upgrades are expected to add another five years to the 18-year-old Hubble's life expectancy.