NEW YORK — NASA’s Space Shuttle Discovery is closing in on its next scheduled mission to the international space station (ISS) where astronauts plan to deliver a new piece of the outpost and rewire its power grid.

Discovery and its STS-116 crew are currently poised to blast off Dec. 7, about a week earlier than initially planned. The launch will mark NASA’s first night launch in four years. After three consecutive daytime launches in which they were able to monitor the shedding of foam from shuttle external tanks, NASA officials have decided it is acceptable to again launch at night, a capability they consider critical to their ability to complete the ISS by the ti me the orbiter fleet is retired in 2010.

“I am extremely proud of the work our crew did at the Kennedy Space Center,” Wayne Hale, NASA’s shuttle program manager, said of NASA’s Florida-based shuttle spaceport during a Nov. 6 briefing at Johnson Space Center. “I cannot, in the entire history of the shuttle program, remember a time where we advanced the launch date by a week within the last couple of months of preparation.”

Discovery’s speedy turnaround since its last flight — the STS-121 mission conducted in July — cleared the path to launch the STS-116 mission in the first week of December.

Many in the agency are describing STS-116 as the most difficult ISS assembly mission to date. “Every mission says that theirs is the most challenging and complicated to date,” said NASA’s Tony Ceccacci, the mission’s lead shuttle flight director, during the briefing. “Well, that’s no different for STS-116.”

Discovery’s seven-astronaut crew, commanded by veteran astronaut Mark Polansky, is slated to deliver the hardware known as the Port 5 truss segment to the port-side edge of the station’s backbone during the first of three planned spacewalks during the mission. Another of their tasks during those spacewalks is to rewire the orbital laboratory’s power grid.

“We’re really looking forward to getting off on a good start on the mission,” Polansky told reporters at Discovery’s Pad 39B launch site at Kennedy Space Center Nov. 15.

Joining Polansky on the planned 12-day flight is a diverse crew that includes shuttle pilot William Oefelein — Alaska’s first astronaut — and mission specialists Robert Curbeam, Nicholas Patrick, Joan Higginbotham, Sunita Williams and European Space Agency astronaut Christer Fuglesang, Sweden’s first astronaut . While all seven have spent years training for spaceflight, only Polansky and Curbeam, who flew together during NASA’s STS-98 mission to deliver the space station’s U.S. Destiny laboratory, have spent any time in Earth orbit.

“I can’t wait to see the smiles on the faces of five people being on orbit for the first time,” Polansky said, adding that the mission has been a longtime coming for rookies like Fugle sang, who joined the ESA astronaut ranks in 1992. “There are people who have waited a very long time.”

The STS-116 mission will mark NASA’s third shuttle flight of 2006 and the second this year dedicated to ISS construction. It also will be the first mission since the 2003 Columbia accident scheduled to launch at night. NASA’s first three post-Columbia accident missions each launched in daylight to allow a clear view of any debris shed by shuttle external tanks.

But after NASA’s successful summer daylight launches of Discovery and Atlantis during the STS-121 and STS-115 missions, respectively, with acceptable fuel tank performances, the space agency and its astronauts are again ready to press ahead with night space shots. In addition in-orbit heat shield inspections are now standard, lending more confidence to NASA’s ability to detect any shuttle damage that may occur from debris shed during a night launch, mission managers have said.

“For us, we don’t really view it as a really large change,” Polansky said of night launches. “We feel we’ll be able to assess the health of the vehicle before we de orbit and come home.”

The ISS has relied primarily on a pair of solar arrays attached to its mast-like Port 6 (P6) truss, but the arrangement was always meant to be temporary since that segment must be moved in the future. STS-116 spacewalkers will shift the station’s power systems from the P6 arrays to the solar panels on the Port 3/Port 4 truss, which the crew of the STS-115 mission installed in September.

As part of the STS-116 rewiring process, one of the P6 solar arrays must be folded away to allow the newer solar panels to rotate and track the sun. The folding procedure has never been tried before, and both of the P6 solar arrays have been in space since 2000 experiencing drastic temperature changes that range from minus 129 degrees Celsius (-200 degrees Fahrenheit) to plus 93 Celsius every 45 minutes.

There are, however, backup plans in place in case the solar arrays fail to retract automatically.

“They’ve built in a lot of contingency devices into the design,” said Curbeam, the lead STS-116 spacewalker. “We’ll go up there and if it doesn’t retract, we’ll retract it by hand using a pistol grip tool or a cordless drill if that’s necessary. We’ll latch it if that’s necessary — one way or another, we’re going to get it retracted.”

A bigger worry, Curbeam said, is the possibility that some ISS equipment will not work after the rewiring. Equipment has to be turned off prior to rewiring, and then turned back on again. Some of the equipment is vital to the operation of the station and the crew’s life support, so if they don’t function properly after rewiring, the spacewalkers will have to undo everything and do it again.

“That’s my biggest worry, if things don’t power up correctly,” Curbeam said.

Discovery also is ferrying Williams, who will work alongside Curbeam in the third STS-116 spacewalk, to the ISS, where she will relieve Expedition 14 flight engineer Thomas Reiter of the European Space Agency, and take his place as a member of the station’s crew. She will join Expedition 14 commander Michael Lopez-Alegria and flight engineer Mikhail Tyurin, who arrived at the ISS in September, then stay on as part of the station’s Expedition 15 mission.

“I really wish they could all stay with me up there,” Williams said of her STS-116 crewmates. “But we’ll be in touch.”