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Countdown under way for Discovery

Space shuttle Discovery
Space shuttle Discovery  

(CNN) -- The countdown is under way for the launch of space shuttle Discovery on a mission to ferry a new crew to the international space station and to bring home the current crew.

Mission managers gave the go ahead Monday to start the clock ticking toward liftoff no earlier than 5:38 p.m. EDT on Thursday after resolving concerns over one of Discovery's two solid rocket boosters.

The countdown started at 5 p.m. at the T-43 hour mark. It includes 29 hours and 32 minutes of built-in holds leading to the opening of Thursday's five-minute launch window at 5:38 p.m.

At the T-9 minute mark, the exact location of the orbiting international space station will be determined and the launch director will decide the precise launch time for Discovery.

Booster worries resolved

Last week, NASA discovered cracks in a fuel injector stem on the hydraulic steering system of a booster rocket last used in 1997. The cracked system was manufactured at the same time as Discovery's left booster, one of two tall candlestick shaped rockets that straddle the shuttle's large, mustard-colored external fuel tank.

The system aims the rocket nozzle during liftoff.

NASA mission managers huddled in meetings and talked by phone on Monday to decide whether to replace the system on Discovery, a move that would have delayed the launch by several days.

The verdict -- the shuttle is safe to fly and does not require any repairs.

Special delivery for Alpha

The main goal for this shuttle mission is to take up a relief crew for Alpha's current occupants: commander Yury Usachev and engineers Susan Helms and Jim Voss.

They will be replaced by commander Frank Culbertson and engineers Vladimir Nikolaevich Dezhurov and Mikhail Tyurin, the third crew for the space station.

The shuttle crew includes commander Scott Horowitz, pilot Rick Sturckow, and mission specialists Patrick Forrester and Daniel Barry. Forrester and Barry are slated to conduct two spacewalks to install equipment on the space station and ready it for future construction missions.

Discovery is scheduled to land back at Kennedy Space Center on August 21 at 1:17 p.m. EDT.

• NASA Human Spaceflight
• The Space Shuttle Clickable Map
• Oceanography from the Space Shuttle
• NASA Space Shuttle Virtual Tour
• Boeing: Space Shuttle Homepage
• Earth from Space

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