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STORY WRITTEN FOR CBS NEWS & USED WITH PERMISSION

Two Russian cosmonauts stepped outside the International Space Station Wednesday, sent birthday greetings to former cosmonaut Alexey Leonov, the first man to walk in space, and then carried out a full slate of maintenance work.

Expedition 59 commander Oleg Kononenko, making his fifth spacewalk, and crewmate Alexey Ovchinin, making his first, opened the hatch of the Pirs airlock compartment at 11:42 a.m. EDT to officially kick off what turned out to be a six-hour one-minute excursion, the year’s fourth spacewalk and the first by the Russians.

A few minutes after floating outside, the cosmonauts paused to send down birthday greetings to Leonov, who turns 85 on Thursday. The Russian space agency Roscosmos said on its web page the spacewalk was dedicated to Leonov, “the man who first took a step into the unknown and found himself one-on-one with unlimited outer space.”

Kononenko and Ovchinin wore placards on their spacesuit backpacks, one saying “Leonov No. 1” and the other “Happy Birthday, Alexey Arkhipovich.”

Leonov chalked up one of Russia’s major space triumphs on March 18, 1965, when he ventured outside his Voskhod 2 spacecraft to carry out history’s first spacewalk.

Connected to the Voskhod by an 18-foot-long tether, Leonov spent 12 minutes floating outside before struggling to get back inside his spacecraft. In the vacuum of space, his suit had ballooned to the point that it would not fit through the hatch. After opening a valve to bleed off pressure, Leonov finally managed to squeeze back inside.

NASA astronaut Ed White became the first American spacewalker three months later.

Now an iconic figure in the international space community, Leonov flew in space a second time in 1975 as Russian spacecraft commander of the Apollo-Soyuz Test Project, the historic orbital linkup of a NASA Apollo capsule and a Russian Soyuz in a mission that symbolized the era of detente.

“It just so happens we are performing a spacewalk the day before the birthday of Alexey Leonov, the first man to ever don a spacesuit and venture out of his spacecraft’s airlock, taking a step into the star-lit abyss, into the vastness of space,” Kononenko said, floating just outside the Pirs module.

Showing off a tethered portrait of Leonov they had carried outside, the cosmonauts took turns sending down birthday greetings.

“Mr. Leonov, please accept our heartfelt birthday wishes,” one said in translated remarks. “You’re with us here and now in outer space and for the entire duration of this spacewalk. Stay in great cosmonaut-worthy health, we hope you continue to be happy, inspired and truly capable of many great new accomplishments. Happy birthday.”

Kononenko and Ovchinin then began working through a checklist of maintenance work that called for installation of handrails, retrieval of materials science space exposure experiments, window cleaning and work to jettison no-longer-needed experiment hardware.

The work went smoothly and the cosmonauts returned to the Pirs airlock compartment a few minutes ahead of schedule, wrapping up the outing at 5:43 p.m. This was the 217th spacewalk devoted to station assembly and maintenance since construction began in 1998 and the 53rd carried out by Russian cosmonauts.

Total ISS spacewalk time now stands at 1,360 hours and 54 minutes, or 56.7 days.