Shuttle Discovery starts farewell tour just before 4 p.m.

The long goodbye for space shuttle Discovery should finally begin this afternoon.

NASA managers believe they have solved a bedeviling insulation issue that delayed Discovery's flight nearly four months, and weather appears splendid for a launch set to occur at 3:50 p.m.

This will almost certainly be the last flight for what is by far NASA's most iconic orbiter.

In the space agency's darkest days, after the tragic losses of Challenger and Columbia, NASA looked to Discovery for the first post-accident flights.

The vehicle has also soared higher — 384 miles — than any other shuttle, delivered the Hubble Space Telescope to orbit, returned John Glenn to space and made the first shuttle rendezvous with Russia's now-defunct Mir Space Station.

The oldest orbiter at 26-plus years since its first launch, and the fleet leader with 38 flights and 351 days in orbit, it has proved highly reliable.

"Discovery has a reputation for always being the cleanest vehicle," said Pam Melroy, who commanded the vehicle in a late 2007 flight. "It was the one you could always count on, the one that would always launch on time. If there was a problem it was not usually because of the vehicle. It was weather or something else."

In this case it was the large fuel tank strapped to Discovery shortly before launch.

First due to launch on Nov. 1, weather and technical problems delayed Discovery's launch until four days later when engineers found a fueling line leak and, more seriously, cracks in the external tank foam.

Since then engineers have sought to assess the root cause of the cracking problem, conducted a tanking test, and technicians have attached material to stiffen the external tank's foam insulation.

Some of those employees had already been let go as the Michoud, La.-based external tank production line had been closed in anticipation of the end of the shuttle program. Some were called back.

Final delivery

"There was no question about their dedication," said Bill Gerstenmaier, director of NASA's space operations. "They really want to see this vehicle fly."

The shuttle program's end comes as the International Space Station nears completion. Discovery will deliver the station's final U.S. component, essentially a glorified storage closet called a logistics module, as well as cargo that includes an advanced robot called Robonaut 2.

Late last year the station, which would have been impossible but for the shuttle, celebrated 10 years of continuous habitation. And it's never been busier up there.

During the past month Japanese, Russian and European supply vehicles have docked at the station, bringing new crew and supplies.

"When you step back and think about what's happening in space, this is probably the most intense space operations time in our history," Gerstenmaier said.

Heading for Smithsonian

Under NASA's present plan for three more missions before ending the shuttle program, Discovery will be the first orbiter of the shuttle fleet to be retired. So historic is the ship, the Smithsonian easily chose it above Atlantis and Endeavour for display upon retirement.

The shuttle program has lasted so long - three decades - that some people expected it to always be there. That the end is really coming, that today Discovery's last flight will probably begin, hits hard.

For some of Discovery's former astronauts including Melroy and Eileen Collins, NASA's first female commander who led the 2005 return-to-flight mission, today will therefore be a sad day.

"I'll definitely be sad," Collins said. "I know, intellectually, the right thing to do is to stop flying the shuttles and to move on to something new. I also hate to see the program go. It was in my mind extremely successful, despite the accidents. We have learned so much about flying in the environment of space from the shuttle.

"I'll have very, very mixed feelings."

eric.berger@chron.com