Added mission commander Scott Altman: "It's really an awesome responsibility when you think about it."

Buckled into couches in Columbia's crew cabin, NASA's fourth Hubble servicing crew blasted off at 6:22 a.m. EST (1122 GMT) as the storied observatory cruised 362 miles (579 kilometers) above the Gulf of Mexico west of Sarasota, Florida.

Running a day late because of near-freezing weather, the shuttle sliced its way through puffy clouds and arced out over the Atlantic Ocean as Hubble passed directly over Kennedy Space Center.

"We wish you good luck on this very important mission to the Hubble Space Telescope, and you all have fun up there," NASA Launch Director Mike Leinbach told the crew minutes before launch.

"Hubble's up there ready for us, and we're ready to go to work," Altman replied.

Fresh off a $100 million overhaul, Columbia set sail on the heels of a countdown that had been put on hold late Wednesday. Forecasters at that time predicted that temperatures would drop to 38 degrees Fahrenheit the next morning -- or right at NASA's safety limit, considering predicted wind speeds and humidity levels.

NASA's coldest launch-time temperature for a shuttle flight came on Jan. 28, 1986, when Challenger took off in 36 degrees after temperatures had dropped well below freezing overnight.

A leak of hot gasses from the shuttle's right-hand solid rocket booster doomed that flight, triggering an explosion that destroyed the $2 billion spaceship and killed seven astronauts.

Investigators later found that the frigid temperatures had compromised the effectiveness of O-ring seals designed to keep hot gasses from escaping shuttle booster rocket casings.

The temperature at the time of launch Friday was appreciably warmer -- about 55 degrees -- enabling NASA to send Columbia and its crew on their way.

NASA's second post-Sept. 11 shuttle launch, meanwhile, came amid tight security. A "no-fly zone" was put in place around NASA's coastal Florida spaceport to keep intruder aircraft away from the fully fueled shuttle.

Air Force F-15 fighter jets were ready to serve as interceptors, and waters off the coast of Cape Canaveral were off-limits to mariners. No security breaches, however, were immediately reported before Columbia and its crew began a two-day Hubble chase.

Joining Altman and Linnehan onboard: pilot Duane Carey, robot arm operator Nancy Currie and spacewalkers John Grunsfeld, James Newman and Michael Massimino.

If all goes as planned, the crew will rendezvous with Hubble Sunday. Wielding the shuttle's 50-foot robot arm, Currie will snatch the 13-ton observatory out of orbit and place it atop a Lazy Susan-like work platform in Columbia's cargo bay.

With the four-story telescope towering above the shuttle, the observatory's golden solar arrays then will be commanded to roll up like bamboo window shades.

A day later, a pair of alternating two-man teams will set out to equip Hubble with a fresh set of solar wings, install a new planetary camera, resuscitate a dead science instrument, replace a faulty pointing control device and overhaul the observatory's electrical system.

Lead spacewalker Grunsfeld and Linnehan will perform the first, third and fifth spacewalks. Newman and Massimino will carry out the second and fourth.

The first big job: Replacing the telescope's 10-year-old solar wings, a task that will take place over the course of two spacewalking excursions.

Fixed to Hubble during a 1993 mission primarily aimed at correcting a flaw in the telescope's primary mirror, the European Space Agency arrays have degraded over the past decade and now produce only 65 percent of original power.

The new solar wings will generate 5,270 watts of electrical power, or about 20 to 25 percent more than the old panels. Designed to fold open rather than unfurl, the rigid arrays each weigh about 300 pounds more than the panels they are replacing, but they are much smaller: 23 feet (7 meters) long compared to 40 feet (12 meters).

The addition power output will enable scientists for the first time to operate all four of Hubble's science instruments simultaneously, a capability that is expected to speed the rate at which the telescope can crank out astronomical discoveries.

The old arrays are to be stowed on a carrier pallet in Columbia's cargo bay and then returned to Earth. Unless, that is, they can't be rolled up for some reason, and in that case, the wings will be tossed overboard.

"In the event we have any problem and the arrays cannot get fully retracted, there is no other way out," said NASA lead flight director Bryan Austin. "Then we must jettison them."

One of the telescope's original solar arrays failed to roll up during the 1993 repair mission and had to be discarded. Ultimately, it burned up during an atmospheric reentry.

The astronauts also will replace a faulty pointing control device on the second spacewalk before setting out on the riskiest chore of their mission.

In what amounts to major surgery on the electrical heart of Hubble, Grunsfeld and Linnehan will attempt to replace the telescope's crucial but flawed power distribution system, which has been plagued by circuitry problems during recent years.

Considered the heart of Hubble's electrical system, the Power Control Unit - or PCU - routes electricity from the telescope's solar arrays to science instruments, control systems and batteries.

One of the few Hubble components not designed for replacement, the 160-pound (72-kilogram) power switching station looks like a household breaker box with a rat's nest of heavy cabling and electrical connectors hooked up to it.

The replacement plan calls for the telescope to be completely powered down for the first time since its launch in 1990 - a precaution taken to make sure the astronauts are not shocked, or electrocuted, on the job.

The astronauts will attempt to pry three dozen closely spaced connectors off the old unit, put a new switching station in place and then reconnect the 36 cables. The telescope, meanwhile, must be powered back up within about eight hours so that its sensitive instruments and control systems don't freeze up or overheat.

A failed swap-out would leave Hubble a powerless piece of space junk - a fact that is not lost on project managers and the scientific community at large.

"We fully anticipate that everything will work fine," said NASA space science chief Edward Weiler. "But it is a risk we've never faced before."

The top scientific priorities of the mission will be tackled on the fourth and fifth spacewalks.

Newman and Massimino will take on the first of those, removing the telescope's Faint Object Camera and replacing it with an advanced planetary camera the size of a refrigerator.

The $75 million Advanced Camera for Surveys is said to be 10 times more powerful - and three to four times faster - than its predecessor. The team that built it at Johns Hopkins University said if the camera was set up in Washington, D.C., it could discern two fireflies six feet (1.8 meters) apart in Tokyo rather than see a single merged light.

"It's a Cadillac compared to a Volkswagon," said Johns Hopkins astronomer and team leader Holland Ford.

The final foray will involve an attempt to revive Hubble sightless infrared eyes - a $110 million instrument known as the Near Infrared Camera and Multi-Object Spectrometer, or NICMOS.

Once capable of peering through dark masses of interstellar gas and dust to witness the birth of planets, stars and galaxies, the NICMOS instrument was installed in 1997 but died two years later when its cooling system shut down prematurely.

Grunsfeld and Linnehan aim to install a new experimental cooler inside the aft shroud of the telescope. A 13-foot-long (3.9-meter-long) radiator will be fixed to the outer hull of Hubble, and then electrical and coolant lines will be routed to the 300-pound (135-kilogram) cooler through a small vent valve on the bottom of the spacecraft.

"That's kind of like installing an external air conditioner in a house for the first time," Grunsfeld said, adding "nobody's done anything like this on Hubble."

All of the spacewalking work must be wrapped up within a finite amount of time - five to six days at most - making the mission one of the most demanding ever attempted by NASA.

"This will be by far the most challenging, ambitious servicing mission to date," said Preston Burch, who manages the Hubble project from NASA's Goddard Space Flight Center in Greenbelt, Md. "Most of the tasks are very lengthy and complex. They will challenge the endurance and stamina of the astronauts."

But if it all works out, Burch and other agency officials say that Hubble will be poised for a new era of rapid-fire discovery, one which promises to shed unprecedented light on the age, evolution and fate of the universe.

"The astronauts have an enormous job ahead of them. Every minute of time throughout the five spacewalking days will be precious," Burch said. "But when (the mission) is completed, Hubble will be at the peak of its powers of scientific discovery."

"When we're done," Grunsfeld added, "Hubble will be significantly better than it's ever been, and we'll have new capabilities that I think will boggle the imaginations of astronomers and people all over planet Earth."

Launched April 24, 1990, Hubble already has been visited by three shuttle crews - in 1993, 1997 and 1999, respectively. NASA plans one more servicing trip to Hubble in 2004, a mission during which the telescope will be outfitted with a final pair of advanced science instruments.

The observatory - which will represent a $7 billion taxpayer investment by the time it is retired in 2010 - is to be returned to Earth that year. Its likely final resting place: The Smithsonian Institution's National Air and Space Museum.

Columbia and its crew are due to land here at NASA's shuttle runway March 12.