US astronaut Michael Foale and his Russian colleague Alexander Kaleri last week observed "a 20-centimetre long strip of soft material" which was floating in space, a NASA representative in Russia, Sergei Puzanov, told AFP.

"For the moment. It's not clear what it is. US and Russian experts are studying photos sent by the crew to Earth to try and determine its origin," he said, adding that it posed "no danger to the station or its crew members."

It could be be a piece of the station's insulation or a strip used to attach some technical equipment to the outside of the station, according to Russian experts quoted by the Interfax news agency.

"It is possible that we will never manage to find out what it was," a spokeman for Russian mission control outside Moscow, Valery Lyndin, told Interfax.

According to Lyndin, there was a similar incident on the now scrapped Russian space station Mir.

The cosmonauts saw a "shining object" which they filmed but experts who studied these images were "unable to establish what the object was," he added.

SPACE.WIRE