The new Reaction Wheel Assembly is prepared for launch at Kennedy Space Center. Photo: NASA

And contrary to what one might assume, installation of the ACS and the NICMOS cryocooler are not the top priorities.

"Establishing the priorities on a Hubble mission is a challenging job in and of itself," said Engelauf, the HST SM-3B mission manager. "You can ask 15 different people and get different perspectives on what's the most important thing to do.

"The unique aspect of the Hubble is that it is a long-term program and an on-going vehicle and as such, you have to protect the health of the facility in order to assure the continued return of the science.

"As the priorities ultimately came out, protecting the health of the facility is the highest priority because you can't get science unless the facility is healthy," Engelauf said. "And then upgrading or advancing its capability to do new and more relevant science is behind that."

The highest priority item on NASA's list is the reaction wheel assembly, one of four gyroscopes that ground controllers speed up or slow down to move the telescope from one target to another.

Last November, gyro speed telemetry from RWA-1 briefly dropped out, the result of an internal electrical problem. While Hubble can operate with just three RWAs, another problem would leave the telescope unable to slew and thus, unable to accomplish any scientific observations.

Playing it safe, mission managers added the RWA swap-out to Columbia's mission at the last minute. And given its critical nature, the RWA is the highest priority of the mission.

The next highest priority is replacement of Hubble's central power control unit, followed by installation of the ACS and the NICMOS cryocooler in that order.

Priorities aside, the most technically complex task is installation of the power control unit followed by the NICMOS cryocooler and the solar arrays. The most straight-forward tasks are installation of the ACS and the reaction wheel assembly.

But the order of the spacewalks was not dictated by either mission priority or technical complexity. Because of the inter-related nature of the upgrades, mission managers were faced with a single "best fit" option.

"It's like a puzzle," said lead flight director Bryan Austin. "In putting the puzzle together, the board is only so big, it fits five EVAs, and the puzzle pieces unfortunately are big and they only fit really in one way. And so that's how the mission has laid out."

The new solar arrays are being installed first, during the first two spacewalks, because associated diode controller boxes will allow ground controllers to better manage the electrical system when Hubble is shut down for the power control unit replacement. RWA-1 will be installed near the end of the second spacewalk, after the +V2 array is installed.

"We need to do the solar arrays first, before we do the PCU, because there are some capabilities we're adding on the new solar arrays - diode boxes, relay controls - that allow the ground to control some relays and safing of the power systems to do the PCU," Austin said.

The PCU installation, certainly the riskiest procedure that will be attempted, is scheduled for the third spacewalk. Only then, with Hubble's most critical systems healthy, will the astronauts proceed to installation of the Advanced Camera for Surveys and the NICMOS cryocooler.

"We have five EVAs, they are very full EVAs, the tasks are long," Austin said. "In a sense, that complexity has made this mission a little more straight forward in how we planned out these five EVAs. They really only fit in one logical order."

Hubble program manager Preston Burch described the overhaul as "an enormous job."

"Two of the five items that they'll be installing, namely the power control unit and the NICMOS cooling system, are outside of the previous Hubble servicing experience base," he said.

"Every minute of EVA time throughout the five EVA days will be precious. To maximize the probability of success, the astronauts on this mission have trained harder than on any previous Hubble servicing mission."

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