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updated 11/5/2010 1:48:10 AM ET 2010-11-05T05:48:10

CAPE CANAVERAL, Fla. NASA is preparing the space shuttle Discovery to launch on one final mission today (Nov. 4) to end its 26-year spaceflying career. But the weather is not on the shuttle's side.

Discovery, NASA's workhorse orbiter, is due to blast off at 3:29 p.m. EDT (1929 GMT) from the seaside Launch Pad 39A here at Kennedy Space Center. The mission has already been delayed several days due to last-minute glitches, and faces an 80 percent chance of bad weather for today's launch attempt.

Still, with NASA's shuttle fleet set to retire next year, today's launch attempt will be the beginning of the end for Discovery. The shuttle is making its 39th and last spaceflight with this launch and is the first of NASA's three shuttles to be retired. [ GRAPHIC: NASA's Space Shuttle From Top to Bottom ]

"There's a lot of excitement in the air," NASA test director Scott Payne said in a preflight briefing.

Discovery is NASA's oldest space shuttle currently in service, and will embark on one final mission to haul critical spare parts to the station, as well as a storage room module and a humanoid robot to assist the crew of the orbiting laboratory.

Discovery's historic final liftoff has been delayed several days due to leaky helium and nitrogen seals that were found in the shuttle's aft engine section, and a last-minute electrical anomaly in a backup computer controller in one of Discovery's main engines.

Engineering teams were able to address the problems in time for Discovery to make its final ascent to the International Space Station later today.

The weather, however, is iffy for launch. Current forecasts predict a mere 20 percent chance of favorable conditions in the area, with the biggest threats being showers and thunderstorms near the launch site.

If bad weather prevents Discovery's launch today, NASA could try again on Friday. The space agency has until Sunday, Nov. 7 and possibly Monday to fly Discovery before its current launch window closes.

The shuttle's final six-astronaut STS-133 crew is made up of commander Steve Lindsey, pilot Eric Boe, and mission specialists Alvin Drew, Tim Kopra, Michael Barratt and Nicole Stott.

Discovery's 11-day mission will include two spacewalks, during which mission specialists Drew and Kopra will perform maintenance tasks on the exterior of the station.

Discovery's STS-133 mission will be the orbiter's final voyage into space before being retired along with the rest of the NASA's shuttle fleet next year.

"It's a remarkable team that gets her ready to fly," Payne said. "They put their heart and soul into this one. They know it's her last flight. She's now poised to take to the skies, and when she goes, she's going to take a little bit of every one of us with her."

Follow SPACE.com Staff Writer Denise Chow on Twitter @denisechow as she covers Discovery's final space voyage from Cape Canaveral, Fla. Click here for mission updates, new stories and a link to NASA's live webcast coverage.

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