Countdown continues for shuttle launch

Shuttle Discovery is being prepped for launch

By Amanda Barnett
CNN

(CNN) -- Technicians on Tuesday were fueling the space shuttle Discovery for launch on a mission to deliver a new crew to the international space station and to bring home the current crew.

"We're right where we want to be on schedule and the team's ready to launch," NASA Test Director Pete Nickolenko said at a briefing on Tuesday.

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, or SRBs.

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 launch window at 5:38 p.m. Nickolenko said the window extends about 10 minutes.

At the T-9 minute mark, the exact location of the orbiting 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.

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The system aims the rocket nozzle during liftoff.

Mission managers huddled in meetings and talked by phone 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.

Discovery commander Scott Horowitz prepares for a training flight at Kennedy Space Center

"The program completed reviews on the SRB hydraulic power units and has decided for us to fly as is," said Nickolenko.

The weather also appears to be cooperating.

"Conditions still look promising for Thursday's launch," said Ed Priselac, the shuttle weather officer.

Priselac said there is a 30 percent chance of showers or thunderstorms at launch time.

Special delivery for Alpha

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

New space station commander Frank Culbertson arrives at Kennedy Space Center

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 Tuesday, August 21 at 1:17 p.m. EDT.