A wet start

The two telescope surgeons got off to a tardy start after a cooling system valve within Grunsfeld's $12 million spacesuit failed, triggering a water leak and a two-hour delay.

STS-109

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Soaked suit parts were swapped out before Grunsfeld and his colleague started their work at 3:28 a.m. EST (0828 GMT). The shuttle and Hubble were cruising 362 miles (579 kilometers) above the west coast of Africa at the time.

"A kind of late but hopefully powerful start," said Grunsfeld.

Mounted atop a cargo bay work platform, the 13-ton telescope loomed four stories tall, its electrical heart hidden inside an equipment bay about a third of the way up the side of the observatory.

Beamed back to earth via satellite, live images from the astronauts' "helmet-cams" provided an up-close, vicarious view of the surgical work.

Weighing in at 160 pounds, the old power-switching unit - one of the few Hubble parts not designed for replacement -- looked like an oversized household circuit breaker box with 36 electrical cables and their connectors jutting from it.

The size and shape of a small office refrigerator, the power box was surrounded by bulky wire bundles, making the connectors inherently hard to reach.

And the connectors were spaced so tightly that the spacewalkers had to employ a specially designed wrench - rather than their gloved hands - in their bid to disconnect them.

Staring into the equipment bay, Linnehan said, "The cables are pretty thick, John."

"Yeah, I can see that," Grunsfeld replied.

But one by one, the two astronauts disengaged the connectors, temporarily fastening them to a cable caddie equipped with small rubber loops.

Some of the loops broke, forcing the spacewalkers to tuck a few connectors behind the nearest wire harnesses. And some of the cables were stiff, making it difficult to unfasten the connectors. Yet the painstaking removal of Hubble's electrical heart largely went by the book.

"Nice work, guys," astronaut Michael Massimino said from inside Columbia's crew cabin.

The surgical installation of the new Power Control Unit was just as tedious, but it also went relatively smoothly. The astronauts put the new switching station in place, and then one by one, the 36 connectors were hooked up to it.

A subsequent "aliveness test" confirmed that the new unit had been rigged up properly.

"We have a heartbeat," Runco called up from Mission Control.

"That's good news," shuttle mission specialist Jim Newman replied.

What followed was a lengthy effort to sequentially power up and test Hubble's various scientific instruments and control systems.

The observatory's wide-field planetary camera came back on line first, and then a more extensive test of the new Power Control Unit got under way.

Electricity from Hubble's two new solar wings - which were installed by spacewalkers Monday and Tuesday - then started flowing to the observatory's main computer brain as well as the telescope's power-producing arrays.

Hubble's other science instruments and spacecraft control systems also were being gradually brought back to life, and no serious problems were expected.

Grunsfeld and Linnehan spent six-hours and 48-minutes working outside Columbia, including the time needed to clean up after surgery. The spacewalk was the third of five planned for the $172 million Hubble house call.

Newman and Massimino will set out on the next one about 3:30 a.m. EST (0830 GMT) Thursday. Their prime goal: To outfit Hubble with the $76 million Advanced Camera for Surveys, which is the most powerful planetary camera ever launched into orbit.

Grunsfeld and Newman will carry out the final excursion Friday, aiming to outfit Hubble with an experimental cooling system that would resuscitate an infrared camera and spectrometer that has been dead since 1999.

If all goes well, the renovated Hubble telescope will be cast back into orbit Saturday before the shuttle crew heads off on a three-day trip back to Earth. Landing here at NASA's Kennedy Space Center remains scheduled for 4:35 a.m. EST (0935 GMT) March 12.