Goddard's Campion said details about what happened will be forthcoming throughout the day.
Even if the repairs had gone as planned, switching to the backup formatter involves some risk.
The device has been exposed to the same cyclic heat stress that may have caused the failure of the primary side A instrument. Switching to side B would leave Hubble with no backup should that system fail, too.
But the risk would be temporary, as engineers are beginning tests now on a duplicate data formatter, including both a primary and backup system, that has been stored on Earth since Hubble's 1990 launch.
(See a gallery of Hubble's top ten science discoveries to date.)
Hubble scientists are hopeful that the boxy, 135-pound (61-kilogram) instrument will be tested and ready for transport to the space telescope during the next space shuttle servicing mission.
Originally slated for October 10, the Atlantis shuttle mission to Hubble was postponed until at least February after the formatter failed.
Art Whipple, lead mission systems engineer for Hubble at Goddard, said on Monday that each month of the delay costs the Hubble program about U.S. $10 million.
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