Astronauts on the space shuttle Discovery spotted an unidentified object floating behind the craft, as well as a bump on the shuttle rudder, the US space agency said on Friday.

"After completing a standard day-before-landing test of the shuttle steering jets, the crew indicated they had seen a one-foot- to 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, a day before the shuttle's scheduled landing.

"Shortly afterwards, the crew described what they called a 'bump' on the left side trailing edge of Discovery's rudder," it said. NASA experts back on Earth were studying images and video of the object and the bump while the crew prepares for Saturday's landing, it said.

Any object floating away from the shuttle raises concerns that a part of the spacecraft has broken off but previously the objects turned out to be ice or harmless items from the payload bay.

The Discovery successfully undocked from the International Space Station Wednesday at 1142 GMT ahead of its return to Earth set for Saturday at the Kennedy Space Center in Cape Canaveral, Florida, after 14 days in space.

The undocking, carried out on schedule, took place 344 kilometers above the South Pacific east of Australia. It capped a 10-day building mission that expanded the station with the addition of Japan's bus-sized Kibo lab.

The shuttle brought a new crew member to the station, Greg Chamitoff, who replaced fellow American astronaut Garrett Reisman, who is returning to Earth aboard Discovery after a three-month stint at the outpost.

The shuttle delivered, opened and outfitted the 11.2-meter long, 4.4-meter wide Kibo module last week. The mission also included three spacewalks.

NASA, which hopes to complete construction of the station by 2010, considers the station a central part of space exploration ambitions, allowing scientists to study the effects of microgravity on humans.