Discovery's seven astronauts put on bright orange spacesuits and strapped into the space shuttle for a practice countdown Thursday, three weeks before their scheduled launch to the international space station.

The clock stopped with four seconds left. A short time later, the astronauts practiced an escape from the spacecraft by crawling through the shuttle hatch. In pairs, placing their hands on the shoulders of the astronaut in front of them, they scurried across the launch tower to basket lifts that could whisk them to the ground in an emergency.

"We got the astronauts in the vehicle, let them get hands-on with a real-life space shuttle vehicle," NASA spokesman Bruce Buckingham said.

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The practice capped four days of drills at the Kennedy Space Center. The crew, which has five spaceflight rookies, was to return to Houston on Thursday.

The launch is scheduled for the evening of Dec. 7. It will be the third shuttle launch of the year, the fourth since the Columbia disaster in 2003 and the first night launch in four years.

Discovery's astronauts plan to rewire the international space station during their 12-day mission. The shuttle also will deliver an $11 million addition and rotate out a crew member for a six-month stay at the space lab.

Also Thursday, NASA managers in Houston approved a change to a launch requirement involving fuel tank sensors designed to prevent the main engines from running too long or not long enough during the climb to space.

The move allows a countdown to continue even if only three of the tanks' four engine cutoff sensors are working properly during fueling of the tank. NASA previously required launch controllers to wait 24 hours to examine the problem.

All three launches since Columbia have been delayed because of fuel tank sensor problems.

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