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The Hubble Space Telescope during a previous servicing mission. Columbia's upcoming flight will be the fourth servicing call to the orbiting observatory. Photo: NASA

Taking a break from space station assembly work, NASA is gearing up for the next Hubble Space Telescope servicing mission, a high-stakes five-spacewalk flight in February to install new solar arrays, a critical power switching unit and a new camera to extend the observatory's scientific reach.

Spacewalking astronauts also will attach a high-tech cooling system to the base of the telescope to revive an infrared camera/spectrometer that's been out of action since January 1999 when the nitrogen ice in its original cryogenic cooling system ran out.

From an engineering point of view, the new solar arrays - smaller, stiffer and more powerful than the ones they are replacing - and the new electrical power control unit, or PCU, will give Hubble a new lease on life, resolving current shortcomings and restoring the observatory to near-perfect health.

But to install the 160-pound power control unit, which regulates and distributes the electricity from the solar arrays to the rest of the observatory, ground controllers will have to completely shut Hubble down for the first time since launch in 1990.

Should an emergency develop that might force the shuttle crew to depart before the swap-out is complete, Hubble would not survive. While the odds of that grim scenario are extremely remote, the PCU replacement is "extremely challenging," says lead flight director Bryan Austin.

"It was not designed to be taken out," he told Astronomy Now. "It has 36 connectors and several other ground connectors on the bottom. Those connectors are on the left-hand side, they're not staring right at you. For the suited crewman to reach his hand in there, he's pretty much reaching in there blind."

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

While flight controllers expect surprises along the way, they are confident the astronauts will get the job done, giving Hubble an upgraded power system that should keep it operating through 2010, when NASA plans to terminate operations.

From a scientific perspective, the new Advanced Camera for Surveys, or ACS, and the revived Near Infrared Camera and Multi-Object Spectrometer - NICMOS - will keep Hubble competitive with larger ground-based observatories for years to come. And then some.

"We're going to put on a new camera, the Advanced Camera for Surveys, which has two times the field of view and five times the sensitivity of the current Wide-Field Planetary Camera 2," said David Leckrone, Hubble program scientist.

"It will be about 10 times more efficient, which means in a given amount of observing time it'll go fainter by quite a bit and also cover a broader field of view with better resolution. So it's going to be a gang busters camera and it will become the work horse camera, supplanting WFPC."

As for restoring NICMOS to good health, "the near infrared capability on Hubble is really fundamental to our mission," Leckrone said. "That extra factor of two in wavelength, between one micron and two microns, really does make a big difference in what people can do."

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

NASA originally intended to launch the third in a series of Hubble servicing missions in June 2000. But by February of 1999, three of the gyroscopes that help lock the telescope on astronomical targets had failed because of subtle age-related breakdowns.

The three remaining gyros were the bare minimum necessary for scientific observations. Justifiably worried that one more failure would put Hubble into electronic hibernation, NASA managers decided to break the third Hubble servicing mission into two parts and to launch the first, known as Servicing Mission 3A, in October 1999.

As it turned out, work to repair shuttle wiring problems, replace a main engine and repair a crushed hydrogen line pushed Hubble Servicing Mission 3A to mid December.

But once in orbit, the astronauts accomplished virtually all of their objectives, installing six new gyroscopes, a new fine guidance sensor, an upgraded flight computer, a new solid-state data recorder, voltage regulators to improve battery performance and a new S-band radio transmitter.

If all goes well, Columbia will blast off on Hubble Servicing Mission 3B at the end of February. The planned launch date of February 14 was scrapped after NASA managers decided to replace a suspect reaction wheel assembly, or RWA, during the second of five spacewalks. The extra work forced the mission to be delayed until February 28. The 64-minute launch window opens at 6:48:14 a.m. EST that day.

On board Columbia will be commander Scott Altman, pilot Duane Carey, robot arm operator Nancy Currie and spacewalkers John Grunsfeld (a veteran of servicing mission 3A), Richard Linnehan, James Newman and Michael Massimino.

The seven STS-109 astronauts in the formal crew portrait. From the left are astronauts Massimino, Linnehan, Carey, Altman, Currie, Grunsfeld and Newman. Photo: NASA

The shuttle will catch up with the space telescope two days after liftoff. Currie, operating the ship's 50-foot-long robot arm, will then pluck it out of open space and mount it vertically on a lazy Susan-type rotating work bench at the back of Columbia's cargo bay.

A motorized shuttle power cable will be plugged in and the telescope's two solar arrays will be commanded to roll up like window shades.

The day after that, the spacewalks begin.

The astronauts will work in alternating two-man teams, with Grunsfeld and Linnehan carrying out the first, third and fifth spacewalks to install one solar array, the new PCU and the NICMOS coolant system. Newman and Massimino will install the second solar array and the Advanced Camera for Surveys on the second and fourth spacewalks.

Engineers test the hinges on one of the new solar array wings to be installed on Hubble. Photo: NASA

The new solar arrays will generate 5,270 watts of power, 670 watts more than the arrays they are replacing. They weigh more - 640 pounds per wing vs 339 pounds for the old panels - but they are much smaller: 23 feet long compared to 40 feet for the old arrays.

The additional power output of the new array will enable four scientific instruments to operate simultaneously for the first time.

Once the old arrays are removed, installation of the new panels should be fairly straight forward. The spacewalkers also will install electronic gear that will allow engineers on the ground to control the arrays' electrical output. The relays will be used to "safe" the spacecraft before installation of the PCU on the third spacewalk.

The PCU changeout is expected to take at least seven hours, 30 minutes longer than NASA normally plans for spacewalks. But with 36 difficult connectors to unplug and reconnect, Grunsfeld and Linnehan will need all the time they can get.

"We're bookkeeping just a couple of minutes for connector mate, connector demate," said Austin. "So multiply that times 36, that's a lot of time. And if you're off by 30 seconds times 36 of those, that's about 15 minutes or so. Things can add up real easily if you start to run into problems."

The Advanced Camera for Surveys will be installed in place of the telescope's Faint Object Camera, the last instrument to use the COSTAR suite of corrective mirrors installed in 1993 to correct the telescope's spherical aberration.

Technicians ready the Advanced Camera for Surveys for launch. Photo: NASA

The ACS and the rest of the observatory's more current instruments have their own corrective mirrors. COSTAR itself will be removed during Servicing Mission 4 in August 2004 and replaced with yet another high-tech instrument.

The infrared-sensitive NICMOS was installed in February 1997. But it used up its nitrogen ice coolant faster than expected and was shut down in 1999. The new cooling system uses neon gas, an external radiator and a small, 400,000-rpm turbine to carry heat away from the instrument.

The new "cryocooler" will chill the infrared detectors in NICMOS to minus 334 degrees Fahrenheit, slightly higher than it's original operating temperature but cold enough to do valuable science. If all goes well, NICMOS will remain operational for at least five years.