Shuttle Atlantis and a crew of five astronauts had been tentatively scheduled to launch June 14 on a mission to deliver a specially designed airlock to the International Space Station. Atlantis last visited the ISS in February of 2001.

The launch now faces a delay until at least June 16 but could be pushed back to June 21 or later if NASA is forced to move the flight behind a previously scheduled Lockheed Martin Atlas rocket mission now scheduled for liftoff June 19.

"We know June 14 is not a viable launch date at this time," Kennedy Space Center (KSC) spokesman Joel Wells said Wednesday. "We do expect a delay of a few days."

Heres the situation:

Fresh off a mission to deliver the $1.4 billion U.S. Destiny science laboratory to the international station, Atlantis landed Feb. 20 at Edwards Air Force Base in California after being detoured there to avoid bad weather at KSC.

The $2 billion spaceship, however, was exposed to four days of pouring rain while technicians prepared to ferry Atlantis back to Florida from the Mojave Desert military base, which is not equipped with protective shuttle hangars.

"It was a very strange occurrence," Wells said. "It was very unexpected to experience that kind of rain out there."

The rain soaked at least 500 of the more than 20,000 thermal tiles that protect the spaceship and its crew from the intense heat experienced during atmospheric reentry.

The concern now: Wet tiles exposed to frigid temperatures in orbit could come loose or even fall off, breaching the ships thermal protection system.

Located on the belly of the shuttle near the leading edge of its right wing and its main landing gear doors, the soaked tiles in recent weeks had been "baked" with high-intensity heat lamps in a bid to dry them out.

Most in fact had been cleared for flight. But a more recent analysis showed that many of the custom-made tiles are still too wet to fly safely.

Plans to move Atlantis from its hangar to the KSC Vehicle Assembly Building this week were held up while engineers set out to determine why moisture has reappeared in tiles that had been cleared for flight.

The process of "baking" the tiles, meanwhile, is continuing in a shuttle hangar here at KSC.

Wells said the shuttles move to the 52-story assembly building likely would be delayed until Sunday at the earliest. In that case, Atlantis probably would not be rolled out to its oceanside launch pad until Friday, May 25.

At least two to three weeks of prep work will remain once Atlantis reaches the pad.

Mission managers will be reviewing the situation over the next several days in an effort to establish a new target launch date.

Led by veteran astronaut Steve Lindsey, the Atlantis crew plans to deliver NASAs so-called joint airlock to the international station. Shaped like a giant genie bottle, the airtight chamber will serve as a staging area for spacewalking construction and maintenance work at the outpost.