NASA switched to backup power for the main camera on the Hubble Space Telescope on Friday and expected to know soon if the change had revived the disabled device.

The switchover, lasting several hours, will likely clear up the problem, but it is possible the camera still won't work, said Ed Ruitberg, deputy associate director of the Astrophysics Division at the Goddard Space Flight Center in Greenbelt.

"In that case we will learn a lot more about the problem and the potential solution," he said.

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Ground controllers received indications June 19 that voltage readings on the power supply for the Advanced Camera for Surveys were outside of their acceptable range, causing the instrument to shut down. Other instruments on the orbiting telescope have continued to operate.

A NASA board that met Thursday to determine how to handle the camera's power problem decided unanimously to switch to the redundant power supply, Ruitberg said, describing it as the best and safest next step.

If the power switch works, the camera could resume its observations Sunday night.

The ACS camera, installed by a space shuttle crew in March 2002, increased Hubble's vision 10 times and has given the clearest pictures yet of galaxies forming in the very early universe.

The instrument consists of three electronic cameras, filters and dispersers that detect light from the ultraviolet to the near infrared.

The power supply problem affected two of the three detectors.

If the switchover doesn't work, engineers would begin looking at each of the three detectors individually to try to isolate the problem, Ruitberg said.

Dave Leckrone, Hubble senior project scientist at Goddard, said the component that engineers believe is at fault "might be comparable to something like on your laptop computer, you have an adapter you plug into the wall and it provides the correct voltage to your power charger on your lap top. It's something like that power adapter."

One bright spot stemming from the outage was that it would allow engineers to recalibrate the ultraviolet detector to make it more sensitive, Leckrone said.

The instrument was developed jointly by Goddard, Johns Hopkins University, Ball Aerospace in Boulder, Colo., and the Baltimore-based Space Telescope Science Institute, which coordinates use of the orbiting observatory.

Jennifer Wiseman, Hubble program scientist at NASA headquarters, said the ongoing uncertainty over whether another shuttle servicing mission will be sent to the space telescope has forced Hubble scientists to work through how they would deal with such problems from the ground.

"They've already sort of thought through this problem ahead of time, so that makes it very efficient for what we're doing," Wiseman said.