Kenneth Bowersox and Donald Pettit are set to leave the rotating space station at 4:25 pm (1225 GMT) for a six-hour spacewalk, NASA's Moscow representative Sergei Puzanov said, quoted by ITAR-TASS news agency.

The third member of the current ISS crew, Russian cosmonaut Nikolai Budarin, will not take part, he said.

Russian space sources told the news agency that the spacewalk "will probably be the last this year at the International Space Station."

Russia had warned that it would have to reduce its participation in the 16-nation project if its cash-strapped space program did not receive additional external funding.

The United States, which along with Russia is the main participant in the program, has refused to provide additional funding and the Russian government said last week it would earmark an additional 1.2 billion rubles (38 million dollars, 35 million euros) in budgetary funds to the space program over the next six months.

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

Cosmonaut Yuri Malenchenko and US astronaut Edward Lu are due to blast off for the rotating space station on April 26 aboard a Russian-made Soyuz rocket.

Bowersox, Pettit and 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.

Bowersox and Pettit are due to replace lamps on an external trolley, part of the remote control to operate a mechanical arm and parts of the heating system for the US module Destiny that is attached to the ISS, said Valery Lyndin, spokesman for Russian mission control.

They are also due to change the configuration of the electricity supply of one of the rotating station's gyrodynes, a stabilizing device that keeps it from spinning out of control, he said, quoted by ITAR-TASS.

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