CAPE CANAVERAL, Fla. (AP) The countdown clock to the launch of Discovery ticked away Tuesday as a promising weather forecast gave NASA officials hope that the space shuttle would lift off on schedule Thursday night.
The chances of favorable weather for liftoff Thursday were put at 80 percent.
"We're ready to go fly,'' said John Curry, NASA's lead flight director for the international space station.
Engineers reported to their consoles as the 43-hour countdown clock began at 11 p.m. EST Monday. There are several built-in holds during the countdown to the launch set for 9:35 p.m. EST on Thursday.
Hours before the countdown began, 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. An effort to do that was aborted last week after only three minutes because of a software problem. The boost will help Discovery dock with the space lab on the third day of the flight.
"It was a flawless firing,'' said NASA spokeswoman Lynnette Madison.
Meanwhile in Houston, NASA worked on a software problem that had caused a breaker to open on a circuit to the motor that causes giant solar arrays to rotate at the space station in the direction of the sun. The solar arrays will generate power to 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.
Discovery's liftoff will be NASA's first night shuttle 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.