The International Space Station’s two Russian astronauts began spacewalking work Wednesday to prepare for next year’s arrival of a long-delayed lab, but had to scrap another chore because of a stubborn bolt.

Spacewalkers Sergey Ryzhikov and Sergey Kud-Sverchkov — dubbed Sergey 1 and Sergey 2 by flight controllers — left four Americans and one Japanese inside. The space station population grew to seven late Monday with the arrival of a SpaceX capsule, making the company’s second astronaut flight.

Ryzhikov, the station’s commander, and Kud-Sverchkov spent nearly two hours doing extra leak checks before exiting Russia’s Poisk air lock. The 11-year-old compartment had never been used before by spacewalkers.

“Congratulations. You are out!” Russian Mission Control told the first-time spacewalkers after the lengthy testing.

The astronauts immediately encountered a difficult bolt that prevented them from replacing a fluid-flow regulator on Russia’s oldest compartment — Zarya, Russian for Dawn. A NASA commentator said the old regulator was still working on Zarya — the first piece of the space station launched in 1998 _ and that the swap would be deferred to a later spacewalk.

Ryzhikov and Kud-Sverchkov had better luck relocating a cable, the first step in decommissioning Russia’s old Pirs spacewalking compartment to make room for the new lab. By then, the spacewalk was past the four-hour mark, and flight controllers near Moscow urged the two to rest: “It’s been intense.”

The Pirs module will be removed and junked next year to make room for the research lab Nauka — Russian for “science.” Several Russian-directed spacewalks will be required to deal with all this. The plan calls for attaching a cargo ship to Pirs in order to guide it to a fiery reentry.

The new 22-ton lab — stretching 13 metres long — is so big that it will be launched from Kazakhstan by a powerful Proton rocket. Once at the orbiting outpost, it will double as an air lock and docking port.

The science lab’s launch — nearly a decade late because of a long string of repairs — is targeted for sometime early next year.

Ryzhikov and Kud-Sverchkov arrived with an American last month on a Soyuz capsule. Like the newly arrived SpaceX crew, the Russian-launched astronauts will remain on board until spring.

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The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Department of Science Education. The AP is solely responsible for all content.

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