The previous repair mission to the Hubble Space Telescope was in 1999 when a space shuttle's robot arm did maintenance work.

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The National Aeronautics and Space Administration plans to send the space shuttle Columbia soaring toward the orbiting telescope on Feb. 28 for an elaborate overhaul and upgrade of what has become one of astronomy's most valuable tools.

Over the 12-day mission, Columbia, returning to duty after two years at rest for a major upgrade of its own, is to dock with Hubble 360 miles above the Earth and attach it to a brace in its open cargo bay. There, pairs of astronauts are to conduct five spacewalks to revamp the telescope.

The seven-member crew includes four astronauts trained to take turns venturing out of Columbia to remove outmoded equipment and install new instruments and power devices. Because of the complexity of the tasks and the amount of gear to be changed, including some items not originally designed to be replaced, NASA managers say this will be one of the toughest jobs spacewalking astronauts have ever tackled.

"This is the most technically challenging servicing mission we have done to date," said Dr. Anne Kinney, director of the agency's astronomy and physics division.

Tasks include adding a new primary camera, replacing the telescope's electricity- generating solar arrays, replacing the main power switching controller, replacing a critical pointing device and installing an experimental cooling system in hopes of reviving a dormant instrument.

Since the 12.5-ton observatory was launched in April 1990, NASA has sent missions to Hubble in 1993, 1997 and 1999 to correct problems, update instruments and keep the prized telescope in working order. The next, and last, upgrade mission for Hubble during its planned 20-year life is scheduled for 2004.

Dr. Edward Weiler, NASA's chief of space science, said no one should be complacent about this mission because the first three went well. The latest job at least rivals the difficulty of the first service flight, in which astronauts had to install corrective optics to fix blurred imaging caused by a defect in the telescope's main mirror. "This mission is no cakewalk," he said.

The mission is to complete work started during the 1999 visit, which had been scheduled for 2000 but was moved up because of failures in Hubble's stabilizing gyroscopes that caused the telescope to shut down briefly. During the emergency flight, astronauts installed six new gyroscopes, replaced the main computer and installed a new data recorder and radio transmitter. But because some equipment was not ready in time for the early flight, some duties were put off until the current mission.

Since its launching, Hubble has become one of astronomy's most important telescopes. Working above Earth's atmosphere, which obscures images and absorbs important parts of the light spectrum useful in understanding faraway objects, the $2 billion observatory has given astronomers an unprecedented view of the heavens.

Hubble has revealed star-formation nurseries, evidence of energy- eating black holes and remnants of events that occurred early in the history of the universe. The telescope even captured images of a comet as it smashed into Jupiter.

In addition to its scientific value, scientists said, Hubble has spurred an interest in astronomy by dazzling the public with a continuing series of images of faraway phenomena.

Dr. David Leckrone of NASA's Goddard Space Flight Center, the chief Hubble project scientist, said giving the telescope a new main camera and revitalizing the observatory's principal infrared instrument would be a boon to astronomy.

For example, he said, the new Advanced Camera for Surveys, which will become Hubble's workhorse instrument, will have 10 times as much "discovery potential" as the current main camera because of faster, better detectors and a wider field of view.

"We are going after a lot of data we can't get now," Dr. Leckrone said, "We can't begin to imagine what we will find."

For the promise of Hubble to be realized, the crew members of Columbia will have to complete the mission, which has involved two years of training. NASA's oldest shuttle, revitalized with a strengthened crew cabin and a new cockpit with digital electronics replacing mechanical instruments and vacuum tubes, will carry 6,000 pounds of equipment to be installed on Hubble.

Navy Cmdr. Scott D. Altman, a veteran of two space flights, will command Columbia, flying the shuttle with the assistance of the mission pilot, Air Force Lt. Col. Duane G. Carey, a space rookie. Army Lt. Col. Nancy J. Currie, a veteran of three shuttle missions, will operate the shuttle's robot arm to capture and release Hubble and to assist in the spacewalks.

The remaining four astronauts are to work as two teams dividing the spacewalking duties and hands-on Hubble work on alternate days. Dr. John M. Grunsfeld, whose three previous space flights included the 1999 Hubble service mission, will be in charge of payload operations. Dr. Grunsfeld and Dr. Richard M. Linnehan, who has flown shuttles twice before, will do three of the spacewalks, each planned to last six and a half to seven hours.

The other team to work outside the shuttle consists of Dr. James H. Newman, a veteran of three space flights and four spacewalks, and Dr. Michael J. Massimino, who is going into space for the first time.

During the first spacewalk, Dr. Grunsfeld and Dr. Linnehan are to replace one of Hubble's two solar power arrays and its control mechanism. The current flexible arrays of silicon cells, which roll up and down like window shades, will be replaced by rigid panels of more efficient gallium arsenide cells. The new arrays, supported by fold-out aluminum-lithium frames, are two-thirds as large as the older ones but produce 20 percent more power.

The stiffer, smaller solar panels should be less prone to flexing and vibration because of temperature shifts that can affect sensitive instruments, engineers said, and their smaller size will decrease atmospheric drag that slowly lowers Hubble's orbit.

Dr. Newman and Dr. Massimino, during the second walk, are to switch out the second solar array and replace one of the telescope's four reaction wheel assemblies. The latter devices, which contain spinning flywheels that use rotational momentum to move Hubble from one target to another, hold the telescope steady during observations. One of these assemblies malfunctioned briefly last fall and NASA decided to swap it with a spare.

The third spacewalk is expected to be the most difficult. Astronauts are to replace the power control unit that regulates and distributes power from Hubble's solar panels and storage batteries. Changing the 160- pound unit, which was not designed to be replaced, will require shutting off all of the observatory's power for the first time since its been in orbit and hoping that it can be turned on again. The job also requires the astronauts to use a special tool to disengage and later rejoin 36 closely spaced electrical connectors in a tight enclosure.

Dr. Kinney, NASA's astronomy director, said that she was nervous about turning off Hubble but that the replacement was necessary. Four of the control unit's relays have failed and another fault prevents using two of the telescope's six batteries, engineers said.

During the fourth spacewalk, astronauts are to remove the last of Hubble's original instruments, the now-obsolete Faint Object Camera, and replace it with the Advanced Camera for Surveys. The new instrument, about the size of a large phone booth and weighing 800 pounds, will take over many observing duties from the Wide Field Planetary Camera 2, which will remain on the observatory until replaced by another device in 2004.

Astronauts on this outside shift also will begin installing the experimental cryocooler refrigeration system, which is designed to cool down detectors in the Near Infrared Camera and Multi-Object Spectrometer, an important instrument added to Hubble in 1997 that has been dormant since 1999. Recording data in the infrared part of the light spectrum requires cooling detectors to about minus 352 degrees Fahrenheit. The infrared instrument was initially cooled by a drum of solid nitrogen ice, but an accidental heat leak consumed the coolant.

Engineers at NASA's Goddard center devised the cryocooler, which uses a miniature high-speed turbine to cool and circulate neon gas to chill the detectors. Astronauts on the fourth spacewalk are to install the electronic components of the cooling system.

During the fifth spacewalk, the team will finish installing the cooling system, which requires hooking up several circulator loops and numerous other connections to the infrared instrument. It will probably take two weeks for the new cooler to chill the detectors to working temperatures and to find out if the fix worked, authorities said.

After the new equipment is installed, Columbia is to use its rockets to boost itself and Hubble to a higher altitude, making up for decay in the observatory's orbit since it last got a nudge up from a visiting shuttle. The day after the last spacewalk, the shuttle is to use its robot arm to redeploy Hubble.

Dr. Kinney said some science observations from Hubble should resume about three weeks after the refit and the first test pictures from the new survey camera should be available five weeks after the service mission.