Two cosmonauts orbiting the Earth in Russia's ageing Mir space station started what was planned to be a 5,5 hour space walk on Friday, a spokesman for mission control said.

Sergei Zalyotin and Alexander Kaleri, who returned to Mir last month to turn the lights back on after it was left empty for 223 days, were expected to conduct an experiment on sealing minute cracks in the hull with specially designed glue. The cracks, which appeared when the station collided with a cargo craft in June 1997, have caused constant air leakage from the cabin, forcing the crew to use more precious energy to maintain normal pressure inside. Several teams of cosmonauts have tried to locate the puncture but have failed. Mission control has said that Friday's experiment could be useful. Washington wants Russia to dump Mir to concentrate on building the new International Space Station which is already behind schedule. U.S. officials suspect Moscow's meagre resources are being depleted by attempts to keep Mir alive. The accident-prone, 14-year-old station, originally designed to serve for just five years, was due to be scrapped earlier this year but a $30 million cash injection from foreign backers kept it aloft. Mir is now to stay in orbit until at least August.