With the Faint Object Camera temporarily attached to fixture on side of payload bay, the Advanced Camera for Surveys is installed into Hubble as shown in animation. Photo: NASA TV/Spaceflight Now

Unlike the PCU, the advanced camera was designed to be user friendly and installation should be relatively straight forward. Newman, riding the robot arm, first will remove Hubble's no-longer-used Faint Object Camera, which will be temporarily mounted on the side of the shuttle's cargo bay.

The Faint Object Camera was the last of Hubble's instruments to use COSTAR, the set of corrective optics installed in 1993 that counteracted the spherical aberration of the telescope's primary mirror. COSTAR's optical bench already has been stowed and it will be replaced by another science instrument during the next Hubble servicing mission.

While Newman is moving the FOC to its temporary payload bay mounting point, leaving that portion of Hubble's instrument bay empty, Massimino will partially enter the telescope and install a cooling system cable harness that later will be hooked up to the NICMOS cryocooler system.

Newman then will pull the ACS from its cargo carrier and, with guidance cues from Massimino, slide it into place inside the observatory. After stowing the FOC in the ACS cargo carrier, Newman and Massimino will trade places on the robot arm and install a NICMOS cryocooler electronics module on the floor of the aft bay just in front of the Advanced Camera for Surveys.

After closing the aft shroud doors, the spacewalkers will remove the thermal and light shields installed earlier for the PCU replacement and tighten up the covers over equipment bays two and four.

And that will set the stage for the fifth and final spacewalk of Servicing Mission 3B, installation of the NICMOS cryocooler. While it is the lowest priority task of all five spacewalks, it is one of the most technically challenging.

First, Linnehan, riding the robot arm, will remove the 300-pound cryocooler from its cargo carrier and latch it to the bulkhead floor of Hubble's aft instrument bay directly in front of the NICMOS instrument.

The Nicmos Cooling System will allow Hubble's Near-Infrared Camera to be reactivated. Photo: NASA

The astronauts will trade places and Currie will guide Grunsfeld down into the cargo bay to retrieve the cryocooler radiator. Linnehan, meanwhile, will float into the telescope to hook up various electrical and cooling lines between NICMOS and the cryocooler.

"We're going to put Rick in the telescope with one of the doors closed," Grunsfeld said. "He'll be in basically all the way up to his ankles, replumbing the inside of the telescope to the NICMOS instrument from the cryocooler. That's a point where's he's really all alone by himself inside the telescope."

The astronauts then will mount the 13-foot-long radiator to Hubble's exterior. Linnehan, anchored in a foot restraint, will feed ammonia coolant lines and electrical connectors from the radiator through a vent opening in the telescope's aft bulkhead. Grunsfeld, now working inside the aft instrument bay, will pull the lines through and make final connections.

"Snaking the various coolant lines and electrical connectors through the bottom of the telescope is a task we have no real data on, it's the kind of thing we've never done on Hubble, it was never anticipated when Hubble was designed that we'd be hanging radiators on the outside and plumbing them through the bottom," Grunsfeld said. "So that's a task we're going to find out how hard it is in real time."

Engineers say it will take about a week for NICMOS to fully cool down once the cryocooler is activated. If all goes well, the first test images should be snapped about a month later.

"The NICMOS cooling system works like the refrigerator in your kitchen," said Burch. "It's a closed loop system, it's a mechanical device, it's basically a heat pump. It uses neon gas, it compresses it and then allows it to expand and gets a cooling effect. Then the heat is dumped overboard through the radiator.

"In theory, the NICMOS cooling system, operating in the weightless environment, the life of the NICMOS cooling system could be indefinite."

If it works.

Leckrone said "if we don't get it done, we'll be disappointed; I don't want to say we'll be surprised."

But NICMOS is crucial to the search for type 1A supernovae and other planned studies of the early universe. About 20 percent of Hubble's post-3B observation time has been booked by NICMOS researchers.

Hubble is redeployed into space after the servicing. Photo: NASA TV/Spaceflight Now

NASA managers are protecting an option for staging a sixth spacewalk if problems are encountered in the first five that prevent the astronaut from completing a critical task. While the additional EVA would be used to complete a task, it would not be used to begin one. If the NICMOS cryocooler installation has not been started by the end of the fifth spacewalk, it will not be attempted at all.

"I'm a professional astrophysicist by training and so for this mission, it's a combination of holy grail and pilgrimage to be able to go up to the Hubble Space Telescope," Grunsfeld said.

"Hubble is really our window to the whole universe. We're going up to not only make some improvements to Hubble, but really to make it much better than new, make it much better than it was when it was launched in 1990 as far as spacecraft capability.

"That being said, when we went to the moon the inspirational words were, 'we choose to do these things not because they're easy, but because they're hard.' And this mission is going to be a hard one."

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