This story has been updated at 6:12 a.m. EDT.
CAPE CANAVERAL, Fla. ? NASA technicians have begun filling the space shuttle Discovery's enormous external fuel tank in preparation for her final launch later today.
The roughly three-hour long fueling process, called tanking, began at 5:58 a.m. EDT (0958 GMT). It involves filling the tank with more than 500,000 gallons of liquid oxygen and liquid hydrogen. This fuel will power Discovery's three main engines during liftoff and ascent into orbit.
After nearly a week of delays, NASA is readying Discovery for the flight that will cap her storied 26-year spaceflying career. Discovery, NASA's oldest space shuttle still flying, is poised to launch today (Nov. 5) at 3:04 p.m. EDT (1904 GMT) from the seaside Launch Pad 39A here at NASA's Kennedy Space Center.
The mission has been delayed since Monday (Nov. 1) due to minor last-minute hiccups and stormy weather that drenched the Florida spaceport yesterday.
Current weather forecasts, however, show a significant improvement for today's launch. Estimates are projecting a 60 percent chance of acceptable conditions, with the main concern being high winds in the area.
Top mission managers will meet today at 5 a.m. EDT (0900 GMT) to reassess the weather situation.
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 final spaceflight with this launch, and will be the first of NASA's remaining three shuttles to be retired. [GRAPHIC: NASA's Space Shuttle ? From Top to Bottom]
Discovery will fly an 11-day supply mission to the International Space Station to deliver a humanoid robot helper for the station crew and a new storage room for the orbiting lab. The intrepid orbiter and her behind-the-scenes engineering team have already faced down a series of setbacks, including a pair of gas leaks, an electrical glitch and, most recently, uncooperative weather.
If bad weather prevents Discovery's launch again today, NASA could try again on Saturday. 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 a veteran team 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 finale in space before being retired along with the rest of NASA's shuttle fleet in 2011.
"It's a remarkable team that gets her ready to fly," Payne said about the many people behind Discovery's final voyage. "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."
- GRAPHIC: NASA's Space Shuttle ? From Top to Bottom
- Gallery: Building Space Shuttle Discovery
- Video ? Space Shuttle Discovery: A Retrospective, Part 2, Part 3
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.