Preparations are on schedule for NASA's first nighttime space shuttle launch in four years as the space agency readies Discovery for a mission to the international space station, managers at Cape Canaveral said Wednesday.

NASA plans to launch Discovery at 9:35 p.m. on Dec. 7 for the third shuttle flight of the year and the fourth since the Columbia disaster killed seven astronauts in 2003.

The agency required the three launches after the Columbia accident to be in daylight so clear images could be taken of the shuttle's external fuel tank in case foam falls off. Foam breaking off the tank and striking Columbia's wing at liftoff caused the damage that led to the disaster.

"There were really no dissenting opinions on the night launch," said Bill Gerstenmaier, NASA's associate administrator.

Nighttime launches necessary

The space agency needs to start launching shuttles at night to take advantage of more launch opportunities in order to finish space station construction by 2010, when the shuttle program ends. The external tanks had acceptable levels of foam loss during the last two liftoffs, officials said.

During the launch, NASA managers believe radar will be sufficient to spot any pieces falling from Discovery's tank and that two in-flight inspections would detect any damage.

NASA engineers feel they have learned so much about the foam that the space agency has reduced the risk of the external tank from "catastrophic probable" to a "strong hazard," said Wayne Hale, shuttle program manager.

"It's not nearly as bad as we thought it was because we now know more — through the use of cameras and the other sensors we have flown — about how foam comes off or doesn't come off the tank," Hale said.

Rewiring to be done

In Houston, engineers evaluated a potential problem with a unit that controls power to an enormous joint that rotates solar arrays at the space station. A circuit breaker opened this week during a software test on the device, which ensures that the panels follow the sun to provide electricity.

During Discovery's visit to the space station, half of the outpost's U.S. segment will be powered down for two spacewalks while shuttle astronauts rewire it. NASA officials want to make sure the rotary joint's two power sources are working at that time.

Asked if the problem could delay the launch, Gerstenmaier said: "I don't know. … There are lots of things that need to be analyzed."

Another problem popped up Wednesday when a thruster used to boost the space station to a higher altitude so it can more easily dock with the shuttle stopped working after two minutes. Another attempt to boost the space station will be made Friday.

New year computer issue

If the launch does not happen on Dec. 7, NASA can keep trying through Dec. 17. After that, the agency will re-evaluate its options and may call it quits until mid-January.

NASA wants Discovery back from its 12-day mission by New Year's Eve because shuttle computers are not designed to make the change from the 365th day of the old year to the first day of the new year while in flight.

The space agency has figured out a solution for the New Year's problem, but managers are reluctant to try it since it has not been thoroughly tested. If the space shuttle is not back on the ground during the change into the new year, NASA officials want it docked to the space station and not flying.