The weather forecast for the planned liftoff of Discovery was downgraded slightly Tuesday but still remained favorable for the first night space shuttle launch in more than four years.
Concerns about clouds over the Kennedy Space Center at the launch time of 9:35 p.m. EST Thursday caused forecasters to reduce the chances of favorable weather to 70 percent from 80 percent.
Strong wind was expected on Friday and Saturday, diminishing the chances of good launch weather for those days to 40 percent.
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"The first day is the best day weatherwise," said Kathy Winters, shuttle weather officer.
Weather will improve early next week. NASA has four launch opportunities over five days, if need be, to start the 12-day mission.
The space agency likely won't attempt to launch past Dec. 17, since flight controllers want Discovery on the ground before the new year.
Shuttle computers aren't designed to make the change from the 365th day of the old year to the first day of the new year while in flight. A potential solution to the problem hasn't been thoroughly tested.
However, NASA officials haven't ruled out going past Dec. 17 if they run out of launch opportunities.
"It's not a show stopper," NASA test director Steven Payne said. "It can be done."
Technicians on Tuesday checked out Discovery's flight systems and planned to fuel power cells aboard the shuttle.
"We have no issues of consequence," Payne said.
The launch countdown clock was started late Monday.
NASA hopes to dock Discovery with the international space station.
In order to help that procedure, Russian flight controllers boosted the space station's orbit by firing the engine on a supply vehicle docked to the space lab for 23 minutes Monday.
An effort to do that last week was aborted after only three minutes because of a software problem.
"It was a flawless firing," said NASA spokeswoman Lynnette Madison.
In Houston, NASA worked on a software problem that had caused a breaker to open on a circuit to the motor that rotates the space station's giant solar arrays in the direction of the sun. The solar arrays will generate power for the space station after Discovery's mission.
NASA planned to test the software fix on Tuesday.
During their 12-day mission, Discovery's seven astronauts planned to rewire the space station, deliver a 2-ton addition and replace one of the space station's three crew members.
This is the first planned night launch in four years. NASA required daylight liftoffs for the three flights after the 2003 Columbia accident to make sure the agency could get good photos of the external fuel tank. Foam breaking off the tank at liftoff caused the damage that killed Columbia's seven astronauts.