(Updates with shuttle cleared, debris analysis)

By Irene Klotz

CAPE CANAVERAL, Fla., June 13 (Reuters) - NASA on Friday cleared space shuttle Discovery for a landing attempt on Saturday after determining that a piece of floating debris and an apparent bump on its tail would not pose problems during re-entry into Earth's atmosphere.

Shuttle commander Mark Kelly and his crewmates checked Discovery's landing systems and packed gear for landing after a successful mission to deliver a $1 billion Japanese laboratory to the International Space Station.

Touchdown was scheduled for 11:15 a.m. EDT (1515 GMT) at the Kennedy Space Center in Florida.

"The weather's looking good," astronaut Terry Virts radioed to Kelly from Mission Control in Houston.

Just after Kelly and pilot Ken Ham test-fired their ship's steering jets, the crew reported seeing an object floating out of the ship's cargo bay.

They grabbed cameras to photograph the object, which was about 1 to 1 1/2 feet (30 to 45 cm) in diameter and glinted in the sunlight as it flew away.

Imagery experts later determined the object was one of three spring-like clips that protect the back of the rudder during launch.

"It's just not a factor for entry," Virts said. "We're still pressing ahead."

NASA engineers also studied what the crew reported as slight bump in the shuttle's vertical tail fin, an aerodynamic surface used to help guide the shuttle through the atmosphere for landing.

The apparent protuberance initially was believed to be a bit of thermal insulation. But later analysis showed it wasn't a bump at all.

"With the lighting we have on here it looked like a little bit of a concern," Kelly said during an inflight interview.

The shuttle has been in orbit for two weeks to install Japan's bus-sized Kibo laboratory onto the space station, resupply the outpost and bring home returning station flight engineer Garrett Reisman, who has been in orbit for three months.

He was replaced by NASA astronaut Greg Chamitoff.

Reisman's last full day in space began with a wake-up song selected by his wife, "Baby Won't You Please Come Home," by Louis Prima and Keely Smith.

"A special good morning to Simone, my favorite earthling," Reisman radioed to his wife. "Get ready, doll face: Discovery's coming home."

If weather or technical issues delay landing, the ship has enough supplies to remain in orbit until Tuesday, said NASA's deputy shuttle program manager LeRoy Cain.

NASA's next mission is an October servicing call to the Hubble Space Telescope. The U.S. space agency plans 10 more missions before retiring the shuttle fleet in 2010.

The shuttles have been flying since 1981. Discovery's mission is the program's 123rd. (Editing by Jim Loney and Mohammad Zargham)

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