The insulation is in the shuttle's rudder speed brake, which is used to slow the spacecraft as it comes in for a landing.
NASA says the missing clip isn't critical for landing. It's used to protect the speed brake from high temperatures during the shuttle's launch.
"Orbiters have come back with those missing. It's just not a factor for entry," Mission Control told the shuttle crew.
Discovery's crew had also reported seeing something sticking out from the left side of the shuttle's rudder. But Mission Control said it was probably just an optical illusion because of the rudder's angle and the lighting.
"There's no worry at all. Discovery is in great shape," shuttle commander Mark Kelly said in media interviews later Friday.
Discovery's crew of seven is returning from the international space station after delivering and installing a new science lab named Kibo, Japanese for hope. Mission Control told the astronauts the weather looked good for landing at 11:15 a.m. EDT Saturday in Florida.
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.
Discovery's heat shield was expected to be given formal clearance for landing Friday afternoon.
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.
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 during a call between the space station's three-man crew and German officials.
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 in October.
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