Shuttle Endeavour won't be ready to launch before January 31. Photo: NASA

The first shuttle flight of 2000 might be delayed after new concerns were raised Tuesday with the workmanship on NASA's $2 billion spaceplanes.

NASA officials met Tuesday to clear shuttle Endeavour for launch this month on 11-day mission to generate a three-dimensional topographic map of Earth's surface.

But a problem found when sistership Discovery landed on December 27 is now threatening to scuttle the planned January 31 launch date for the first human spaceflight of the new millennium.

After Discovery rolled to a stop on the runway following its successful mission to repair the Hubble Space Telescope, technicans found a thermal protection tile was missing from the shuttle's right inboard elevon.

All the shuttles are covered with thousands of tiles to protect the spacecraft and astronauts from the fiery reentry into Earth's atmosphere.

An investigation into Discovery's lost tile revealed the so-called "complex tile" measuring 9 by 4 1/2 inches was not properly attached to the shuttle in the first place, NASA spokesman Joel Wells said.

NASA says the padding put between the shuttle's skin and tile was not the appropriate thickness, allowing the tile to de-bond and fall off sometime between entry and landing.

The tile in question is more difficult to install, giving it the "complex" label, and is not like the cookie-cutter variety that covers other parts of the shuttle.

As a result of the investigation's findings, space agency officials have ordered a paperwork check to see if similar tiles on Endeavour were installed correctly.

Officials expect to know by week's end if there are any concerns with Endeavour's tiles, Wells said.

It is not yet known how long Endeavour's flight might be delayed if any tiles need to be replaced, Wells said.

"First we have find out which tiles are complex. Then we have to check Endeavour to make sure they are installed right."

The SRTM radar cargo fills Endeavour's payload bay. Photo: NASA

For now, Endeavour remains slated for launch no earlier than January 31 at 12:47 p.m. EST (1747 GMT), the opening of a two-hour and two-minute window.

But if Endeavour is sidelined for more than two days, NASA will have to wait until February 8 or 9 to fly the shuttle because of other commercial rocket launches planned at Cape Canaveral.

The U.S. Air Force-controlled Range needs 48 hours between launches of different spacecraft to reconfigure its tracking and computer systems.

Wells said no other serious issues are standing in the way of Endeavour's mission, which was delayed from September due to a fleet-wide wiring inspection ordered after Columbia suffered a short-circuit moments after liftoff last July.

Efforts to launch Discovery late last year run into repeated problems involving damaged wiring, a broken drill bit lodged inside one of the main engines and replacement of a crushed fuel line aboard the shuttle that was probably caused by a worker's misstep.

"NASA management has shown that their No. 1 concern is safety," Endeavour commander Kevin Kregel told reporters last week. "There's nothing more important than safety, and we certainly don't want to go until we have a good vehicle."

After reaching an orbit about 126 nautical miles above Earth, the Endeavour and its six crew astronauts from the U.S., Germany and Japan will extend a 200-foot long antenna mast from the shuttle's payload bay.

The mast and another radar antenna that fills the payload bay will together map 80 percent of the planet's surface.

"The team has done a thorough job preparing Endeavour for flight, and we're ready to start this new year as successfully as we ended the last year," shuttle program chief Ron Dittemore said in a statement.