Astronauts on the space shuttle Discovery spotted an unidentified object floating behind the craft as well as a bump on the shuttle rudder on Friday but neither was cause for concern, NASA said.

After carrying out routine testing the day before Discovery is due to land back on Earth on Saturday, "the crew indicated they had seen a 1-1.5-foot (30-45 centimeter) long rectangular object floating away from the shuttle from behind the rear portion of the right wing," the US space agency said.

"Shortly afterwards, the crew described what they called a 'bump' on the left side trailing edge of Discovery's rudder," it said in a statement.

NASA experts back on Earth studied images and video of both the object and the bump but concluded that they posed no risk and Discovery was "ship-shape" for Saturday's landing at the Kennedy Space Center in Cape Canaveral, Florida.

The object could be one of three clips from inside the rudder speed brake, William Jeffs, a spokesman at Johnson Space Center in Houston, Texas, told AFP, adding that the loss of one "would not be a concern for entry or landing."

Mission control said on NASA TV that a bump spotted on the shuttle's rudder was also a "typical protrusion" and "is of no concern."

There was the chance that any object floating away from the shuttle could be a part of the spacecraft that had broken off.

"The crew was told a short time ago that there are no concerns for entry and landing, that those clips only serve to protect the speed brake from the flow of heating during ascent," mission control said.

"So with Discovery declared ship-shape for its homecoming to the Kennedy Space Center tomorrow and with the weather appearing very favorable, we press ahead on this day before landing."

Discovery commander Mark Kelly, speaking on NASA TV from the shuttle, admitted the seven-person crew had initial concerns when they saw the object floating past them but were reassured by news that it was a clip.

"We've seen these things come off before," he said, adding: "It is no worry at all, it has no effect on re-entry and Discovery is in great shape.

"We have some stuff to do tonight like put up the seats, and get out our suits and the parachutes on the seats... and then we'll be ready to go tomorrow morning."

After 14 days in space, Discovery undocked from the International Space Station (ISS) on Wednesday about 344 kilometers (214 miles) above the South Pacific east of Australia, and began preparing for its return to Earth.

One of the shuttle's main purposes was to deliver, open and outfit the 11.2-meter long, 4.4-meter wide main module of the Japanese Kibo laboratory. The mission also included three spacewalks.

In addition, Discovery brought a new crew member to the ISS, Greg Chamitoff, who replaced fellow American astronaut Garrett Reisman.

Reisman is returning to Earth after a three-month stint at the outpost, and told NASA TV on Friday that he was primarily looking forward to "being back home, sleeping in my own bed -- and using my own toilet."

The only toilet at the International Space Station began to fail just before Discovery's launch, and spare parts were sent up on the shuttle to repair it.

Reisman said the spacewalk was one of the best things he had done while at the ISS, and also described the incredible views from space, in particular one of the United States at night as a weather front moved across the country.

"The thunderstorms had this electric blue light like a disco or something, and around it was the warm glow of all the city lights. It was absolutely spectacular," he said.

NASA hopes to complete construction of the space station by 2010, viewing it as a central part of space exploration ambitions because it allows scientists to study the effects of microgravity on humans.