The Columbia astronauts are gearing up for a fifth and final spacewalk to attach an experimental ultra low-temperature refrigerator to the Hubble Space Telescope in a bid to revive a dormant infrared camera-spectrometer that ran out of nitrogen ice coolant in 1999.

The Near Infrared Camera and Multi-Object Spectrometer, or NICMOS, was designed to operate at near absolute zero to detect the faint heat of stars and galaxies in the remotest reaches of space and time.

Installed during the second Hubble servicing mission in 1997, NICMOS was victim of an internal "thermal short" that caused its nitrogen ice dewar to come in contact with surrounding structure. As a result, the instrument's nitrogen ice coolant sublimated away faster than expected, leaving NICMOS dormant after just two-and-a-half years.

To revive the instrument, astronauts John Grunsfeld and Richard Linnehan will install an experimental "cryocooler" that uses neon gas and three tiny turbines spinning at an astonishing 400,000 rpm to chill NICMOS to about 75 degrees above absolute zero.

The spacewalk is scheduled to begin around 3:27 a.m. Friday and take six-and-a-half to seven hours to complete. Because of the experimental nature of the refrigerator, the repair job is the lowest priority task of Columbia's mission. But it is one of the most technically complex.

Grunsfeld and Linnehan will have to work deep inside the telescope's lower instrument bay to connect the cryocooler to NICMOS and attach a 13-foot-long radiator to the side of the telescope to dissipate the unwanted heat. Cables and coolant lines from the radiator to the cryocooler will be snaked through a small vent opening in the telescope's aft bulkhead.

"We believe we now have in hand a new technology developed jointly by NASA and the Air Force, which gives us the first really good shot at a reliable, mechanical cooler in space on an infrared instrument," said David Leckrone, Hubble project scientist at the Goddard Space Flight Center.

"So a very important objective of Servicing Mission 3B is, as an experiment, to try this new technology and see if we can bring the NICMOS instrument back from the dead."

To kick off Friday's spacewalk, Linnehan, riding on the end of Columbia's 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 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 Hubble program manager Preston 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 servicing mission 3B observation time has been booked by NICMOS researchers.