Captain Talgat Musabayev and colleague Nikolai Budarin returned to Mir after successfully completing the latest in a series of space walks.

Their latest mission had been to remove an old thruster engine in an operation to keep the ageing station aligned with the sun.

Marathon six-hour space walk

The two Russians crawled along a 14-metre support boom on the 12-year-old station from where they detached the engine and pushed off into space.

The engine will go into a deteriorating orbit and burn up in the Earth's upper atmosphere.

Speaking to a tense mission control as he watched the engine float away, Capt Musabayev said: "That's it, it's gone."

Out of fuel

During a previous space walk, the engine ran out of fuel and the cosmonauts were forced to rush back into the station to activate a back-up booster to restore the station's alignment.

Orientation engines are not rechargeable and must be replaced when they run out of fuel.

Mission control gave the go-ahead for the planned six-hour operation at 1.55pm Moscow time (0955 GMT).

Despite the obvious tension surrounding such a delicate operation, the two cosmonauts managed to take a swipe at the technicians back home.

After being told not to rush and take a rest, cosmonaut Budarin sardonically replied: "When we return to Earth we'll put you in a spacesuit so you can rest in it for some eight hours."

After an hour of careful crawling, the cosmonauts reached the engine as Mir circled high above the Sahara desert.

The third Mir crew member, Nasa astronaut Andrew Thomas, stayed inside the station to film his colleagues' progress.

Next space walks

Mir: troubled past

Two more space walks are planned over the next fortnight when the cosmonauts will install the new orientation engine.

But the accident-prone space station which became the focus of high drama last year has enjoyed a relatively trouble free few months.

Russian officials want to man the station at least until next year when the planned international space station could be ready to welcome its first crew.

Disasters which have befallen the space station have gone from the extremely serious to the faintly comic.

At one point there were genuine fears for the lives of the crew, which included British-born astronaut Michael Foales, after a mid-space collision with the station's own supply ship.

In March the crew experienced a minor crisis when were forced to cancel a space walk after they broke three spanners trying to loosen a stubborn hatch.