The new solar array is lifted out of its carrier in the shuttle's payload bay for installation on Hubble. Photo: NASA TV/Spaceflight Now

Grunsfeld will disconnect the old array and Linnehan will temporarily mount it on a cargo carrier in the payload bay. Grunsfeld then will install a diode box needed to ensure power from the arrays flows to Hubble's batteries and not vice versa while Linnehan pulls the new array, folded in half, from its carrier.

Grunsfeld will attach the new solar wing, crank it open and wire it to the diode box. Another cable, known as P601, will be connected to route power to a diode box controller.

The spacewalkers also will do a bit of preparatory work needed before Hubble can be powered down for replacement of the power control unit during the third spacewalk. They will install a light shield over Hubble's star trackers and thermal covers over the Wide Field-Planetary Camera-2 and equipment bays five and 10.

The second solar wing will be installed the next day by Newman and Massimino, who will ride Columbia's robot arm throughout the excursion. Once the +V2 array is in place, the spacewalkers will replace reaction wheel assembly No. 1, engaging three latches and hooking up four cable connectors. The task is expected to take about an hour.

The second new solar array is maneuvered into position for attachment. Photo: NASA TV/Spaceflight Now

Newman and Massimino also will remove bolts from equipment bays two and four, which will be opened for the PCU swap-out during the third spacewalk.

"The PCU is quite a unique task in that to do everything that's needed to be done, to prepare the vehicle to be powered down with some of the thermal covers we're putting on, to do the PCU changeout and then to take off those thermal covers so the vehicle can do science again, that whole long task is about eight hours of EVA time," Austin said.

"And we can't do that all in one EVA. So what we've done is we've found an optimized time on the first two EVAs to go do some of this prepartory work done."

Once Hubble is shut down for the PCU change out, an imaginary "thermal clock" will start ticking. The astronauts must complete the PCU replacement and restore power by the end of the day, or Hubble's critical systems could be damaged by the effects of the normal temperature extremes of space.

While NASA managers typically downplay the drama associated with key events like this one, Anne Kinney, director of astronomy and physics at NASA headquarters, said she was "nervous as hell" about the PCU swap out.

"It's extremely challenging, it was not designed to be taken out," Austin said of the 160-pound black box. "The original one has 36 connectors and several other ground connectors on the bottom."

The PCU measures one foot by two feet by four feet long.

"What makes it difficult is, as you're facing the PCU, those connectors are on the left-hand side, they're not staring right at you, they're on the left face," he said. "And that's on the side that that bay door is hinged. For the suited crewman to reach his hand in there, he's pretty much reaching in there blind.

"I kind of equate it to changing out spark plugs on your car. There's always those spark plugs down there where you sort of can't see real well, you've just got to go down and feel and make sure you're oriented such that you're unscrewing it without a lot of offset force."

Grunsfeld and Linnehan will start EVA-3 an hour earlier than usual because the job is expected to take at least seven-and-a-half hours. While a half hour of contingency time is built into that schedule, the spacewalk could run even longer. But nine hours is the limit on NASA's current spacesuits.

Grunsfeld will begin the spacewalk as the free floater, installing thermal covers on the new solar array diode boxes and deploying thermal shields over equipment bays five and 10 that were put in place by Newman and Massimino. At the same time, Linnehan, riding the robot arm, will disconnect Hubble's six batteries. Three batteries are located in equipment bay two and the other three are in bay three.

Grunsfeld then will deploy the light shields put in place over the telescope's star trackers during the first spacewalk. The astronauts then will open bay four and begin the tedious job of disconnecting and removing the PCU.

After latching the door open and installing a plastic cable holder to keep the connectors in order after they are removed, Linnehan will begin disconnecting the PCU, starting at the top and working down the double rows of cables.

The Power Control Unit, shown here opened up, will be replaced on Columbia's mission. Photo: NASA

With six cables to go, Grunsfeld will take Linnehan's place on the arm, complete the PCU's electrical disconnection and remove it. He then will install the new PCU and make all 36 electrical connections.

"These are not our typical fully EVA- friendly connectors with the big wing tabs on them that make it convenient for an EVA-suited crewman to manipulate," Austin said. "We've worked really hard to have a special tool that we're going to use to get a good grip on these and get through this."

In addition, the left side of the new PCU is canted slightly, giving Grunsfeld slightly better access for re-attaching the electrical cables. Assuming all goes well, the astronauts will reconnect Hubble's six batteries and retract the thermal covers, clearing the way for anxious ground controllers to re-power the observatory.

Austin said mission planners designed all five spacewalks with various "breakout" points to ensure the telescope can be left in a safe configuration overnight in case any given task cannot be completed during a single EVA.

"There is a period of time, though, on EVA-3, the PCU, where it's kind of like open heart surgery except we don't have it on a heart-lung machine, we've just taken the heart out and we've got that period of time before the telescope is even awake again, before it's even viable to be powered through the orbiter," he said.

"So we've got a minimum amount of work we have to do that day once we take the PCU out, get it back in and get some minimum amount of connectors hooked back up so at least we can apply power from the orbiter back through the telescope to revitalize it. So that day will be extensive."

Various contingency plans have been developed to provide enough power to Hubble to keep the observatory alive overnight in case of problems completing the PCU installation. Eleven of the 36 connectors are needed to keep Hubble safe overnight with its batteries still disconnected. At least 23 of the connectors must be hooked up before Hubble can be released from the shuttle.

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