Bracing for Hubble's 'heart transplant'

'Any major surgery entails a certain degree of risk'

Operating at the end of Columbia's 50-foot robotic arm, spacewalkers on Tuesday installed Hubble's second of two new solar "arrays," or panels, designed to boost its energy by some 20 percent.

Editor's note: The third of five Hubble spacewalks -- a sensitive one in which mission specialists John Grunsfeld and Richard Linnehan are to replace the telescope's "PCU," or power control unit -- is to begin at roughly 1:30 a.m. EST Wednesday and last for some seven hours.

(CNN) -- Medical metaphors were everywhere as NASA prepared to mount Wednesday's third of five spacewalks at the orbiting shuttle Columbia on its Hubble-repair mission.

"In 1993," program scientist Dave Leckrone said in a briefing, "we like to say we performed 'eye surgery' on Hubble to correct its poor vision." Then, in a 1999 mission, NASA changed out part of the Hubble's gyroscope hardware, "and a lot of us," said Leckrone, "referred to that as 'brain surgery.'

"And I guess carrying this analogy to tomorrow's activity," he said, "we could say that tomorrow Hubble gets a 'heart transplant.' You can think of the 'PCU' (power control unit) as the central pump that circulates electricity through the spacecraft. ... Any major surgery entails a certain degree of risk."

That risk comes in powering down the Hubble, something never done before, to replace that power control unit -- which was never meant to be replaced.

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And starting at about 1:30 a.m. EST on Wednesday, mission specialists John Grunsfeld and Richard Linnehan are to make a complex spacewalk to change out Hubble's "heart," giving it a new power control unit. (Grunsfeld will be wearing solid red stripes on his spacesuit; Linnehan's suit is all-white, no stripes.)

The big question is whether the Hubble will come back on with its new power unit installed.

Hubble has a screw loose

Thermal blanketing has been applied to help keep the craft warm while it's dark for the first time since its 1990 launch. Tuesday, mission specialists Jim Newman and Mike Massimino installed a blanket on Bay 6 of the telescope -- along with door-stop extensions on Bay 5 and foot restraints for Grunsfeld and Linnehan to use Wednesday.

And once the new PCU is in place, the four-story orbiting observatory won't be switched back on fully at once, either. Leckrone said in Tuesday's briefing that the protocol calls for a gradual powering-up of the various systems and instruments on Hubble.

It promises to be a nerve-wracking time for Hubble specialists and NASA officials -- not only the 12-year-old Hubble and its future operation are at stake, but so are $172 million in new equipment, including the new solar arrays installed by astronauts in Monday and Tuesday's spacewalks.

"As with any beloved relative," Leckrone said, "you're worried about sending them in for bypass surgery or even a heart transplant. But you realize that the risk of not doing it is severe. And in this case, the risk of not changing out the PCU is severe. So that's why we're doing it."

The problem is tiny but potentially disruptive. One of the screws that holds down a piece of metal in the current power unit is loose and making the flow of power erratic. NASA wants to install new instruments on the Hubble -- new instruments it won't be able to run all at once as long as the screw's interference persists.

Furthermore, that wobbly screw becomes a loose cannon aboard the Hubble. Hopes for years more service from the telescope could be dashed if the screw became completely detached and broke off all operation of the observatory.

Extending his medical metaphors, Leckrone said, "The consequences of not doing something about this problem are much more profound. ... Do you want to just sort of ignore something that's serious and hope it goes away and doesn't get worse? Or do you want to go to the doctor or the hospital and take care of it?"

Prescribed procedures

Mission specialists Jim Newman and Mike Massimino spent seven hours, 16 minutes, in Tuesday's second of a scheduled five spacewalks at the Hubble Space Telescope.

Here's a quick rundown of what mission specialists Grunsfeld and Linnehan expect to do in Wednesday's pivotal spacewalk at the Hubble, which is latched into Columbia's cargo bay.