The astronauts reported to Mission Control that they observed a rectangular object, about 1 to 11/2 feet long, floating away from the tail of the shuttle around the right wing. They also reported seeing something sticking out from the left side of the shuttle's rudder.
"We see a little bump kind of sticking out," shuttle pilot Ken Ham said.
Initial speculation by engineers was that the floating object could be something from the payload bay or a piece of ice. They said the protrusion might be a small piece of thermal insulation sticking out.
The astronauts sent down video and photographs for engineers to review.
"We'll take a look," Mission Control said.
On Thursday, the shuttle's heat shield was given a preliminary thumbs up for the return trip to Earth after engineers finished scrutinizing all the images of it on the wing and nose, which were collected Wednesday with a laser-tipped inspection boom.
"Looks like you have a clean orbiter," Mission Control radioed the shuttle astronauts.
Discovery's heat shield was expected to be given formal clearance for landing during the mission management team's meeting Friday afternoon. Discovery is scheduled to land at 11:15 a.m. EDT Saturday in Florida.
The thermal survey -- an exhaustive search for damage -- was conducted later than usual because the astronauts had to wait until they got to the space station to retrieve their inspection pole. There wasn't enough room aboard Discovery for the pole at liftoff because the Japanese lab the shuttle delivered to the space station had taken up nearly all the room in its payload bay.
The inspection is one of the safety measures put in place by NASA after the 2003 Columbia accident. Columbia was destroyed during re-entry as a result of a gashed wing.
Discovery's crew of seven installed the new lab named Kibo, Japanese for hope, to the space station.
Besides delivering the new lab, the shuttle also dropped off Gregory Chamitoff, the station's newest crew member. He traded places with Garrett Reisman, who lived on the station for three months. Chamitoff will stay on the station for six months.
"We had a very successful shuttle mission, with the Japanese module attached. It's a very big facility now," Chamitoff told German President Horst Kohler on Friday morning during a call between the space station's three-man crew and German officials.
The 37-foot lab, about the size of a bus, is the biggest room at the space station. Kibo also has a storage closet and a 33-foot robotic arm. A final section -- a "porch" for exterior experiments -- and a second, smaller robotic arm will be delivered next year.
Discovery also brought a pump that fixed the space station's malfunctioning toilet. The problem had forced the station's crew to flush manually with extra water several times a day.
On Friday, NASA continued investigating what caused extensive damage at the launch pad used to shoot Discovery into orbit two weeks ago.
About 5,300 bricks flew off the pad during the May 31 launch, exposing a thick concrete wall underneath. The pad was built for the Apollo moon shots, and the bricks might not have adhered properly to the wall of the flame trench when they were installed in the 1960s, said LeRoy Cain, chairman of the mission management team.
The flyaway bricks posed no danger to Discovery, but NASA wants
to fix the flame trench -- designed to deflect the exhaust of the
booster rockets -- so it does not get worse. Cain said he's
confident it will be repaired in time for the next shuttle flight