NASA engineers hope to begin reviving the ailing Hubble Space Telescope Wednesday with a days-long switch to a backup system after a hardware failure cut off its ability to transmit images back to Earth last month.

The switch will take at least two days, with engineers working around the clock in Hubble's operations center at NASA's Goddard Space Flight Center to rouse the backup system from nearly two decades of slumber.

NASA is confident the fix will resuscitate Hubble's science instruments by Friday, but there is always the chance of an unexpected glitch during the process, Hubble managers said.

"It is obviously a possibility that things will not come up," Art Whipple, chief of NASA's Hubble systems management office at Goddard in Greenbelt, Md., told reporters in a Tuesday teleconference.

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But NASA's history of booting up systems in space after years of inactivity is encouraging, and weeks of testing and analysis on Earth show Hubble to be ready for reactivation, he added.

"We have very good confidence that this will work," Whipple said.

Hubble has been unable to send the majority of its science data and images to Earth since Sept. 27, when a data relay channel failed after 18 years in action.

The failed channel, known as Side A of a device called a Control Unit/Science Data Formatter in Hubble's Data Handling Unit, has been the primary path for the telescope's observations since it launched in April 1990. Hubble does have a backup channel, Side B. But that channel has sat dormant since before the telescope launched.

Switching the space telescope's systems over to that path requires the tricky transition of six other backup systems, five of which have also been powered off since 1990, Hubble managers have said. Engineers have spent the past few weeks exhaustively reviewing the lengthy process in the hope of ensuring a successful reactivation.

Aside from the failed relay channel, the space telescope is in good health. One instrument — a fine guidance sensor that can communicate with Earth independently — is still able to conduct astrometry observations by tracking target stars, Hubble managers said.

If for some reason, Hubble's switch to Side B fails, the telescope could continue its astrometry studies until a shuttle repair mission slated for next year, said John Morse, director of NASA's astrophysics division.

Rebooting Hubble

A team of up to 50 Hubble engineers and flight controllers are expected to report for duty between 6:00-7:00 a.m. EDT (1000-1100 GMT) to begin the space telescope's resurrection. But the bulk of the repair comes around midday, when they will bring direct Hubble to boot up its Side B backup relay channel.

"The most critical part of the commanding will happen between 8:30 a.m. and 11:00 a.m. Eastern time," Whipple said. "We expect the full flow of science to resume Friday morning."

A few dozen computer commands should handle Hubble's backup system activation, though it will take about 200 or so computer commands to move the telescope in and out of a protective "safe mode" to make the switch, said Whipple.

He likened the process as similar to an IT professional setting up routers for an office computer network.

Some instruments could begin beaming down data as early as late Thursday evening, Whipple added.

The recent Hubble failure prompted NASA to hold off on launching the space shuttle Atlantis and seven astronauts to the space telescope this month on the fifth and final service call on the orbital observatory.

The mission has been pushed to no earlier than February 2009, with engineers at Goddard hoping to add a spare data formatter unit to replace the broken Side A channel.

"The [replacement] unit will start testing next week," Whipple said, adding that a series of stress tests to check the spare's health will likely last for months.

But, he added, there is ample room in the shuttle Atlantis' cargo bay to hold the spare, and weight margins to launch the unit to Hubble next year.

Shuttle mission managers are still studying how adding the relatively straightforward repair tasks will impact the already packed servicing flight schedule, Morse added.

Commanded by veteran spaceflyer Scott Altman, Atlantis' Hubble servicing crew plans to perform five back-to-back spacewalks to add new cameras, replace old batteries and gyroscopes, add docking equipment and upgrade the telescope's guidance system.

The astronauts will also attempt to fix a vital spectrometer and Hubble's main survey camera — equipment never designed to be repaired in orbit.

"Perhaps one of the ironies here is how much effort will go into bring the Hubble telescope back to its normal productivity," Morse said. "And that's everyone's goal is to see wonderful science coming back from the telescope again."

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