The Hubble Space Telescope has had a long and illustrious run, helping to pin down the age of the universe and pointing the way to the existence of dark energy. But that run may be halted if engineers can’t switch some operations over to backup units.

A formatter on the satellite’s Science Instrument Command and Data Handling (SIC&DH) unit failed last month, and today NASA began the process of switching over operations to the components on a backup formatter, most of which have not been powered up since Hubble’s 1990 launch.

Since the September 27 failure, Hubble has been largely incapacitated, as the SIC&DH component that failed, the Science Data Formatter, handles many key activities, including routing data and commands on board the satellite and relaying science information back to Earth.

Switching over to the redundant side of the SIC&DH, which contains the backup Science Data Formatter, is being performed by ground-based engineers, and NASA expects to have Hubble back in operation on Friday. (Six related backup components will also need to be brought online by that time.) Art Whipple, manager of Hubble’s Systems Management Office at NASA’s Goddard Space Flight Center, expressed confidence to the Associated Press but cautioned that “it's obviously a possibility that things will not come up.”

Late Wednesday afternoon, Susan Hendrix, a spokesperson for Goddard, told that the engineers were “about halfway there.” She said that some indication of the changeover’s success would come around midnight, when the backup units “will be talking to the ground” if all goes well.

NASA will send up a replacement unit when the space shuttle Atlantis launches next year, Hendrix said. The shuttle, which had been slated to depart for a Hubble-servicing sojourn this month, saw its launch date pushed back when the telescope fell silent in September.