Astronauts James Newman and Michael Massimino began repressurizing the shuttle Columbia's airlock at 8:56 a.m. this morning, officially ending a seven-hour 16-minute spacewalk. The astronauts installed a second solar array on the Hubble Space Telescope and a new reaction wheel to help it move from target to target. Initial tests show both components are healthy and operating normally.

"Today we completed EVA 2, our objectives were all met," said lead flight director Bryan Austin. "From my vantage point as the flight director, when things go well you can sit back (and) the music kind of plays itself. And I think that's really what it felt like. ... It really, really went well."

The stage is now set for the most challenging task of Columbia's mission: Installation of a new power control unit Wednesday, a make-or-break bit of orbital surgery to replace the electronic heart of the space telescope. Before the 160-pound PCU can be replaced - and it wasn't designed to be serviced by spacewalking astronauts - Hubble must be completely shut down for the first time since launch in 1990. While mission managers are confident astronauts John Grunsfeld and Richard Linnehan will be successful, there are no guarantees all of Hubble's complex subsystems will power back up at the end of the day.

"In 1993, we like to say we performed eye surgery on Hubble to correct its poor vision," said project scientist David Leckrone. "And then in servicing mission 3A in 1999, we successfully changed out the central spacecraft computer and a lot of us referred to that as brain surgery. And I guess carrying this analogy to tomorrow's activities, you could say that tomorrow Hubble gets a heart transplant."

"You can think of the PCU as the central pump that circulates electricity through the spacecraft. I don't want to carry this analogy much further than that, but in a sense that does describe our attitude and approach and feelings about what's going to happen tomorrow. The two doctors, John Grunsfeld and Rick Linnehan who are going to be performing this surgery, have studied hard, trained long and hard and are probably the world's most skilled surgeons for this particular kind of operation.

"But despite that, of course, any major surgery entails a certain degree of risk," Leckrone said. "And that will certainly be the case tomorrow, a certain degree of necessary risk to do a necessary job."

It's a necessary job, he said, because as it now stands, Hubble's current PCU cannot provide enough power to operate all of Hubble's instruments at the same time. In fact, an internal "bus bar fault" could cripple the observatory if it gets worse.

Leckrone said a small screw in the PCU that holds a metal strip, or bus bar, in place to carry power from two of Hubble's six batteries into the 160-pound box has backed off slightly - it apparently was too short to begin with - increasing the impedance in that circuit. Should it back away completely, two of Hubble's batteries could overheat and rupture, taking a third battery out with them. That would effectively end Hubble's life, a threat that makes Wednesday's swap out the lesser of two evils.

Grunsfeld and Linnehan originally planned to begin the spacewalk an hour earlier than usual, at 12:27 a.m. Wednesday, but Austin said today it will begin at 1:27 a.m. instead. Columbia is equipped with an internal airlock, making conditions on its lower deck extremely crowded, and the astronauts are simply not able to get everything done in time to begin the spacewalk an hour early. The spacewalk is expected to last at least seven hours.

For their part, Newman and Massimino finished their two major objectives ahead of schedule today and spent nearly an hour on "get-ahead" tasks that will save time during subsequent spacewalks. To prepare for the PCU swap out, they unbolted equipment bay covers to provide access to Hubble's batteries, which must be disconnected Wednesday before the PCU can be removed, and loosened the bolts on the access doors protecting the PCU itself. They also installed thermal shields to protect sensitive equipment once Hubble's power is turned off.

In addition, the spacewalkers installed a sheet of protective insulation over one of the two reaction wheel equipment bays and tested two latches holding the observatory's aft instrument bay doors closed. One latch appeared to be in good shape while the other needed servicing. A second latch was installed to ensure a tight fit.

With two of five planned Hubble servicing spacewalks now complete, Columbia's spacewalkers have accomplished all of their objectives while encountering no major problems. Total EVA time for both spacewalks is 14 hours and 17 minutes.

Today's excursion was the 15th spacewalk in four flights devoted to servicing the space telescope.

Newman, with his fifth spacewalk today, is now second only to astronaut Jerry Ross on NASA's all-time EVA list, with 35 hours and 43 minutes of spacewalk time to date.