Astronaut Michael Foale and Alexander Kalari began a planned five-and-a-half hour spacewalk today, leaving the international space station vacant in the first such outing since crew sizes were reduced in the wake of the Columbia shuttle disaster.

The spacewalk began at 4:17 p.m. EST as the space station sailed high above the northwest Pacific Ocean. The goal of the excursion is to replace experiments mounted on the station's hull and to carry out a visual inspection of the outpost. This is the 52nd spacewalk devoted to station assembly and maintenance, the first since April 8, 2003. Going into this spacewealk, or EVA, U.S., Russian, French and Canadian astronauts had logged 318 hours and 37 minutes of suited work time.

Foale and Kaleri, known by his nickname "Sasha," have seven earlier spacewalks to their credit.

"Sasha and I have both performed spacewalks before, on previous flights, and I have also done a spacewalk in a Russian suit on board tyhe space station Mir," Foale said last month. Today's spacewalk is, "in our minds, anyuway, pretty much a routine or regular, if any spacewalk can be, EVA to replace some science experiments from Japan and from Europe and Russia on the outside surface of the service module and to bring in some experiments that were placed out there already by previous crews for return to Earth eventually.

"This EVA is different for the international space station ... because we will not have a third person on board the space station watching over the station systems while Sasha and I are outside in our suits," he said. "But really, my take on this is not one of alarm but one of this is something we all must get used to. Just as the Apollo astronauts, when a pair of them would land on the lunar surface and venture out of their lunar module, many miles even on their rover, just the two of them in their suits with no one back in the lunar module, I think we will be in no different a situation. In fact, we will be in a much better and safer situation, less risky. To that extent, I feel we can conduct this EVA safely."

The station normally is staffed with three crew members, one of which remains inside during spacewalks to monitor station systems and to assist in case of problems that might affect the safety of the astronauts outside. But following the Columbia disaster last year, NASA and the Russian space agency were forced to reduce the station crew size to two until shuttles can resume flights.

Some at NASA initially questioned the wisdom of conducting spacewalks with just two station crew members, but those concerns were resolved last year. For their part, Russian cosmonauts routinely conducted such outings on the old Mir space station.

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