Astronauts on board the US space shuttle Discovery have reported seeing an object drifting away from the craft.
Shuttle crew said the object was 30-45cm (1-1.5 feet) long.
Nasa engineers are trying to identify it from pictures taken by astronauts.
A Nasa official said it was not unusual for objects to become dislodged from the shuttle or its payload bay.
Nasa engineers are also trying to identify a bump sticking out from the shuttle's tail rudder.
Discovery has been on a mission to the International Space Station (ISS) to install the Japanese-built Kibo laboratory.
The shuttle has already detached from the ISS and is conducting safety checks before it is due to re-enter the atmosphere and land in Florida on Saturday.
Chunk of ice?
"After completing a standard day-before-landing test of the shuttle steering jets, the crew indicated they had seen a 1-1.5 foot-long rectangular object floating away from the shuttle from behind the rear portion of the right wing," Nasa said in a statement.
"Shortly afterwards, the crew described what they called a 'bump' on the left side trailing edge of Discovery's rudder."
Objects have previously broken off the shuttle, but have turned out to be chunks of ice or harmless items from the payload bay.
Engineers have speculated that the "bump" could be a small piece of thermal insulation protruding from the rudder.
Neither issue is expected to delay the shuttle's landing, planned for 1115 Florida time (1515 GMT) on Saturday.
(BBC)
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