"It's too early to tell right now what the threat is," KSC spokesman Joel Wells said Tuesday after NASAs top shuttle program managers finished a daylong meeting to review their readiness to fly.

"We understand why that one tile fell off and now we want to make sure that none of Endeavours tiles fall off for the same reason," Wells said.

Made from sand and consisting mostly of air, thousands of ceramic tiles are glued to the shuttles skin to protect each of NASAs space planes from the intense heat caused by friction while reentering Earths atmosphere.

Most of the shuttles tiles are essentially square, but some that cover more rounded areas of the shuttle such as near the wing tips and around the nose have a more complex shape and are glued to the shuttle using different materials and procedures.

It was one of these more complex tiles that fell off Discovery before it landed at KSC -- apparently because the glue used was the wrong consistency -- and its the more complex tiles on Endeavour that engineers need to confirm were properly installed before allowing launch, Wells said.

The inspection work mostly involves a check of photos and paperwork that documented the installation procedure. Wells said he didnt know how many of these tiles had yet to be examined. A couple of tiles are already scheduled to be replaced on Endeavours nose.

Should the need to replace additional tiles appear, the launch could be delayed by as much as a week.

Replacing a tile already installed on a shuttle takes between seven and ten days, Wells said.

For now, liftoff is set for January 31 between 12:47 p.m. Eastern Standard Time and 2:49 p.m. EST (17:47 and 19:49 Greenwich Mean Time). An on-time launch would lead to a February 11 landing back in Florida at 4:55 p.m. EST (21:55 GMT).

These last minute inspections and schedule threats are a reprise of what happened to shuttle Discovery before its launch last December.

Originally set to fly last year on October 14, a variety of problems kept delaying the mission to repair the Hubble Space Telescope until NASA managers finally thought they were ready to go and announced an official launch date of December 6.

But as final inspections continued new problems were found that had to be fixed before the agency could safely fly, so the official launch date slipped to December 9, then to the 11th, and then again to December 16. Discovery finally launched December 19, after waiting out bad weather.

Like Discovery, Endeavour's flight was delayed to allow additional safety inspections of its wiring. The mission was originally targeted to launch on September 16, 1999.

Despite the wait, Endeavour commander Kevin Kregel said he and his crew have put the extra training time to good use and will be ready to fly when the shuttle is.

"NASA management has shown that their Number One concern is safety," Kregel said last week while training at KSC. "Theres nothing more important than safety, and we certainly dont want to go until we have a good vehicle."

Should this potential problem with the tiles, other last-minute technical glitches or even bad weather delay Endeavours flight beyond February 1, NASAs next opportunity to launch the shuttles Earth mapping mission would not come until February 8 at the earliest, Wells said.

The reason: A pair of commercial satellite-delivery missions now are scheduled to launch nearby from the Air Force's Cape Canaveral Air Station in early February.

A Lockheed Martin Atlas rocket is scheduled to launch a commercial communications satellite February 3 and also has the next day booked for a backup launch opportunity.

Then a Boeing Delta rocket is scheduled to launch four Globalstar cellular telephone satellites on February 6, and also has the 7th booked as a backup date.

It typically takes 36 to 48 hours to reset rocket tracking and range safety systems at launch support ground stations between flights from Floridas Space Coast, so a Delta launch on February 6 could clear the way for a shuttle launch two days later on the 8th.

Meanwhile, launch of an Air Force Atlas rocket carrying a military communications satellite from Cape Canaveral Air Station remains on schedule with a launch window set for Thursday January 20 or Friday January 21, between 7:15 p.m. and 8:39 p.m. EST (00:15 and 01:39 GMT Friday and Saturday).

There is a 60 percent chance of favorable weather for the launch, 45th Space Wing spokesman Ken Warren said Tuesday. Clouds and gusting winds might be a problem.