The Atlantis returned from its four-day secret space mission more bruised than normal, but the damage is not severe enough to delay its next scheduled launch, NASA says.

Up to 175 thermal tiles were heavily damaged, apparently during liftoff, possibly by something loose hitting the orbiter, said Conrad Nagel, Atlantis' processing director."The damage comparatively is significant, but it's not a major concern," he said Wednesday. "We have to understand why we had this damage and keep it from happening again."

Nagel said the damage would be easy to fix and that it would not affect Atlantis' next flight, scheduled for launch April 28 with a spacecraft that is to be dispatched to the planet Venus. Overall, he said, Atlantis returned from its classified military mission on Tuesday in "very clean" shape.

There were only a few minor technical problems during the flight, he said. One entailed a minor malfunction in the shuttle's air conditioning system, causing about a gallon of water to leak into the cabin. The astronauts used a vacuum cleaner to clean it up, Nagel said.

During the flight, the five astronauts inspected and assessed the battered tiles, using a television camera on the end of the ship's 50-foot robot arm, Nagel said.

The shuttle is covered with about 28,000 tiles made of various materials designed to protect it from atmospheric re-entry heat. He said the tile damage posed no threat to the shuttle's return to Earth.

Nagel said the heaviest damage was concentrated in a strip 3 to 4 feet long "down the right hand side, along the line of the fuselage."

One tile in the nose was missing and "it looked like it was popped off by the impact of something," he said.

There was speculation here that a part from the solid fuel booster rockets, the external tank or the orbiter might have fallen off and hit the craft. Nagel said he did not want to speculate.

A small fiberglass panel was missing from one of Atlantis' maneuvering engines, but this probably did not cause the problem, officials said.

Nagel made his comments in an interview from Edwards Air Force Base, Calif., where he leads a team that is inspecting Atlantis and preparing it for return to the Kennedy Space Center here Tuesday atop a Boeing 747 airliner.

He said Atlantis' tile damage was about twice that suffered by Discovery, which made the first post-Challenger shuttle flight two months ago.