CAPE CANAVERAL, Florida (CNN) -- Thanks to improving weather conditions, chances are now better than ever that the space shuttle Endeavour will be able to blast off as scheduled early Thursday morning.

By late Wednesday, NASA forecasters said there was only about a 30 percent chance that the mission would have to be scrubbed because of bad weather. Earlier in the day, the chances of an aborted mission were 60 percent.

"Weather officers have reported a sizable improvement," said NASA spokesman Joel Wells.

Wednesday evening, NASA began the final fueling of the shuttle in preparation for liftoff.

Endeavour, carrying a six-person crew, will begin the process of constructing the new International Space Station. The shuttle will carry a module for the station, called Unity, which will be affixed to an already orbiting Russian-made module called Zarya.


Unity module inside the shuttle cargo bay

In order to correctly choreograph its rendezvous with Zarya, Endeavour has only a 10-minute window in which to launch, from 3:54 a.m. to 4:04 a.m. EST (0854 to 0904 GMT).

Should the mission be delayed beyond that window, it would have to be scrubbed and rescheduled for either early Friday or Saturday.

Cloud cover at the Kennedy Space Center is what is imperiling the mission. NASA guidelines mandate that launches cannot occur if the cloud ceiling is below 5,000 feet.

"If clouds and/or showers are north, south, east or west of us, we're good to go. If they're right overhead, then we've got a problem," said shuttle weather officer Ed Priselac.

In addition to hooking Unity with Zarya, astronauts may also try to fix a faulty battery aboard the Russian module. The repair would involve replacing control boxes that prevent one of Zarya's three batteries from working in automatic mode.


NASA animation shows Endeavour preparing to link Unity with Zarya

A final decision on whether to undertake the repair project will not be made until the crew has pressurized Zarya and inspected the faulty control boxes.

The shuttle's robotic arm operator, Nancy Currie, says plucking Zarya from orbit will be a challenge. Currie's job is to delicately move the 25,000-pound module to within four inches and four degrees of alignment from Unity.

After that, spacewalkers Jerry Ross and Jim Newman are scheduled to strap on space tool belts and step out of the shuttle to connect cables, handrails, tethers and antennas. The work assigned to Ross and Newman is scheduled to be done in three spacewalks.

"I think some of the difficulties are in the tight tolerances that we are talking about," Currie said.

Unity will serve as a connector for future space station modules and as a future docking spot for shuttles.

Correspondent Miles O'Brien and Reuters contributed to this report.