Russian cosmonaut Yury Malenchenko and US astronaut Edward Lu are due to blast off for the rotating space station on Saturday next week aboard a Russian-made Soyuz rocket.

Alexander Kalery of Russia and American Michael Foale have also been training as a replacement crew at the Star City astronaut training centre outside Moscow.

"One of the two crews trained in the ISS model and the other in the Soyuz. They will swap places tomorrow. This is a test which will be evaluated by a state commission" to confirm all four astronauts are ready, a spokeswoman for Star City, Galina Nikolayevna, told AFP.

The two crews are to fly on Sunday to the Baikonur cosmodrome in Kazakhstan, where the launch will take place.

After the US Columbia space shuttle disintegrated on February 1, killing all seven crew, NASA halted all planned flights to the ISS and grounded the remaining three space shuttles.

Russia's spacecraft are now the only means of transporting crew and supplying the ISS.

Two US astronauts and one Russian cosmonaut arrived on the ISS for a four-month mission in early December.

Kenneth Bowersox, Donald Pettit and Nikolai Budarin were due to return to earth in March, but their mission was extended following the uncertainty triggered by the Columbia disaster in which the seven astronauts aboard the US shuttle died as it disintegrated upon re-entry.

Moscow admitted for the first time this month that it would have to fund extra flights to the ISS following the US decision to ground its shuttle program.

The Russian government decided to earmark an additional 1.2 billion rubles (38 million dollars, 35 million euros) in budgetary funds to the space programme over the next six months.

In part to cut down on spending, future missions to the ISS will comprise two, instead of three, astronauts.

SPACE.WIRE