NASA begins the countdown today for the launch of the space shuttle Discovery, the culmination of nearly 21/2 years of safety improvements and management changes since the Columbia tragedy grounded the shuttle fleet and triggered a national bout of soul-searching over the future of the United States' human spaceflight program.

With Hurricane Dennis expected by NASA to be far enough west and barring another spate of bad weather or last-minute mechanical glitches, Discovery will lift off Launch Pad 39B at the Kennedy Space Center on Wednesday afternoon carrying seven astronauts, nearly 15 tons of cargo, and the hopes of thousands of technicians and engineers who have striven to rectify the mistakes that doomed Columbia. The three-day countdown begins at 6 p.m. Eastern time.

Shuttle planners have done big things. They redesigned the external fuel tank, built a new boom sensor to look under the wings and enlisted spy satellites to take pictures of the shuttle in flight. And they have done small things. They have equipped the crew of the international space station with cameras for taking snapshots of the shuttle's underbelly. And they placed a putty knife in the repair kit to give astronauts a shot at fixing cracks in the heat shielding.

Space aficionados inside and outside NASA agree that the agency has dramatically reduced the risk of another Columbia-style catastrophe, but it has not reduced it to zero, nor has it made the shuttle "safe."

Safe, for an experimental vehicle that has flown only 113 times, is not an option, and Discovery's mission is a test flight -- the final exam on how good a job NASA has done. "We feel the main way we can get smarter at this point is to go fly," NASA's John F. Muratore said.

Discovery will be away 13 days, spending most of that time at the international space station. Astronauts will replace a gyroscope, install a stowage platform for replacement parts, unload 29,725 pounds of equipment and supplies, and pick up 25,121 pounds of mostly trash and junk that have accumulated aboard the station since the last shuttle visit, in 2002.

But the mission is anything but routine.

Discovery's flight is not only the first since the Columbia accident but also the first since President Bush announced a plan to return humans to the moon by 2020 and eventually to send them to Mars.

The first step is to finish construction of the space station, and NASA wants to do that by 2010, retire the shuttle, replace it with a next-generation spaceship and move on to the moon. That timetable would be impossible if the shuttles could not fly.

Discovery will also seek to reaffirm the United States' status as the world's preeminent space-faring nation, a status that the new NASA administrator, Michael D. Griffin, says ought to be "beyond debate," even as the country reinforces one of the most effective international scientific collaborations ever undertaken.

Once in orbit, Discovery will scan itself for any launch damage with a new extendable sensing boom built in Canada. Most of its cargo will be stowed in a 21-foot-long carrying case called Raffaello, built by the Italian Space Agency. The mission's lead spacewalker is astronaut Soichi Noguchi of the Japan Aerospace Exploration Agency. And the shuttle will be greeted by space station commander Sergei Krikalev, a Russian cosmonaut.

Finally, for all of the participants and, perhaps, for all of humankind, Discovery will resume the quest to fulfill what mission commander Eileen Collins calls "my most exciting dream -- that in my lifetime I could see people walking on Mars."

Still, the reason Discovery's launch will be the most watched shuttle mission in years has relatively little to do with the grand themes of space travel and much to do with the legacy of a flight gone wrong. Columbia went down because of hardware and management failures that NASA could and, perhaps, should have known about long ago.

So are they fixed?

"It's still my studied opinion that a golden BB could get us -- we are going to fly with some risk, and to characterize it otherwise would be inappropriate," N. Wayne Hale Jr., the deputy space shuttle manager, said in a briefing this spring. "I believe it's going to be safe to fly, but I don't believe we've driven the risk to zero."

In late 2003, the independent Columbia Accident Investigation Board made 15 "return to flight" recommendations to NASA, covering such things as strengthening the shuttle's damage resistance and making sure that two people sign off on important repairs and maintenance jobs.

Last month, the NASA-appointed Return to Flight Task Group ruled that the agency had failed to fulfill three of the recommendations -- for orbiter hardening, onboard repair and debris shedding by the shuttle's external fuel tank.

But both retired Navy Adm. Harold W. Gehman Jr., head of the investigation board, and retired Air Force Col. Richard O. Covey, co-chairman of the task group and a former astronaut, praised NASA's efforts and gave their respective blessings for the upcoming mission.

"If I were a young flying person," Covey said, "I would not have a concern."

Gehman cautioned, however, that nothing NASA does will fix the shuttle's central flaw. "We have to replace this vehicle as soon as possible," Gehman said. "I'm sure it's safer than before, but by any definition, it's not safe."

The most obvious Achilles' heel is the orbiter's heat shielding -- ceramic tiles on its belly and hard, brittle "reinforced carbon-carbon" sheathing on its nose cone and its wings' leading edges. Columbia disintegrated in the searing heat of reentry because a 1.67-pound chunk of insulating foam from its external fuel tank broke off during launch and punched a hole in the left wing's carbon-carbon.

"The thermal protection system was designed to withstand temperature, not impacts," noted the shuttle impact test director, Justin H. Kerr, and there is no fixing this oversight, because, as NASA engineers are wont to say, "the shuttle is what it is." To change the heat shielding, NASA would have to trade the shuttle itself for something else. That will not happen for at least five years.

NASA's focus since Columbia has been to minimize the amount and size of debris coming off the torpedo-shaped, 154-foot external tank during launch. Two of three Discovery launch postponements have come because engineers needed to make tank modifications or complete tests of how impacts of insulating foam or ice shed from the tank during launch would affect the shuttle.

Engineers have redesigned the tank to all but eliminate the possibility it will shed pieces of foam of the size that brought Columbia down. They have also retrained foam-spraying technicians so they minimize the number of "voids" in the insulation, a key cause of foam loss during launch.

"We have identified 155 sources of debris and modeled 160 million cases," said Muratore, NASA's shuttle systems engineering and integration manager. "We've basically kept all the NASA supercomputers running for the last year and a half."

To detect any damage to the craft or other problems during the launch, Discovery, unlike Columbia, will be videotaped, filmed, photographed and scanned from stem to stern, topside and underside, for at least the mission's first three days. It took the investigation board 10 months to find the damage that destroyed Columbia. This time, engineers expect to have a "first take" on a potential catastrophe within 24 hours.

The Ground Camera Ascent Imagery System features 107 separate devices, including "Quick Look" digital video cameras, ship-mounted radars, 70mm film cameras for "big sky" views, and infrared cameras mounted in airplanes flying racetrack routes over the ocean on each side of the launch pad. The carbon-carbon on the shuttle's wings has been outfitted with impact sensors.

Once aloft, the orbiter will undergo detailed safety inspections. On the mission's second day, the shuttle crew will deploy the new, 50-foot inspection boom, whose laser imaging system and television cameras will examine the wings' leading edges and the nose cone.

The next day, Collins will back-flip the shuttle as it approaches the space station so Krikalev and American space station flight engineer John Phillips can photograph the belly of the orbiter. Extra surveys may be conducted with the new boom on the mission's fourth day. And spy satellites will also image the shuttle at some point.

"It's truly an immense amount of data, and it's in large measure things we have never done before," said John Shannon, shuttle flight operations and integration manager. "I really have a high level of confidence that if we have any degree of damage, we'll be able to see it and understand it."

And -- maybe -- do something about it.

No part of Discovery's mission is more truly a test flight than the experiments the crew will perform with repair techniques. Onboard repair is a discipline that did not even exist before the Columbia disaster.

"I've worked this area for a very long time," said Discovery astronaut Charles J. Camarda, a materials scientist with 30 years of experience at NASA. "We never thought we'd be able to repair this vehicle in space. It's a very daunting problem."

Discovery will carry five repair kits. Two techniques, including one that uses putty knives to smooth goo into cracks in the carbon-carbon, will be tested during a spacewalk, and another will be tested inside the orbiter.

None will be used on the shuttle itself. "You wouldn't put your faith in [onboard repair] because it hasn't been tested," said Andrew Thomas, the mission's most experienced astronaut, with nearly six months in space.

Still, the experiments must be done because repairs "depend on how materials behave in a vacuum, zero gravity and under a variety of temperatures," said Discovery spacewalker Stephen Robinson. "To get it all at once, you have to be in space."

Damage, should it occur, is likely to present an agonizing dilemma. A repair carries the risk that imperfectly applied goo or caulk will leave beads protruding from a crack or hole -- potential hotspots during reentry. The cure could be worse than the disease.

The alternative will be to abandon Discovery and use the space station as a "safe haven" for the astronauts until the shuttle Atlantis can be launched on a rescue mission. This is a desperate measure and "frankly, the last thing we want," Hale said.

But, he added, "we have a plan," something that did not exist when Columbia was circling Earth with a hole in its left wing. "We are not going to be in the position of saying to the American public, 'There's nothing we can do,' " Hale said. "This is a competency test for us as an agency."

The crew members of Discovery are, from left, Andrew Thomas, Charles Camarda, Wendy Lawrence, commander Eileen Collins, Stephen Robinson, Soichi Noguchi of Japan and pilot James Kelly. Their launch is set for Wednesday.Kennedy Space Center workers prepare the shuttle Discovery's redesigned external fuel tank for launch.