A 59-year-old Russian cosmonaut became the world's oldest spacewalker Friday, joining a much younger cosmonaut's son for maintenance work outside the International Space Station.

Pavel Vinogradov, a cosmonaut for two decades, claimed the honor as he emerged from the hatch with Roman Romanenko. But he inadvertently added to the booming population of space junk when he lost his grip on an experiment tray that he was retrieving toward the end of the 6 1/2-hour spacewalk.

The lost aluminum panel -- 18 inches by 12 inches and about 6 1/2 pounds -- contained metal samples. Scientists wanted to see how the samples had fared after a year out in the vacuum of space.

Otherwise, the spacewalk had gone well, with the spacewalkers installing new science equipment and replacing a navigation device needed for the June arrival of a European cargo ship.

Collecting the experiment tray was Vinogradov's last task outside.

The tray drifted toward the solar panels of the main Russian space station compartment, called Zvezda, Russian for Star. Flight controllers did not believe it struck anything, and the object was not thought to pose a safety hazard in the hours and days ahead.

"That's unfortunate," someone radioed in Russian.

Another panel of similar experiments will be collected on a future spacewalk.

This is the first of eight spacewalks to be conducted this year, most of them by Russians. Two will be led by NASA this summer.

Until Friday, the oldest spacewalker was retired NASA astronaut Story Musgrave, who was 58 when he helped fix the Hubble Space Telescope in 1993.

Romanenko, 41, is a second-generation spaceman who's following in his father's bootsteps. Retired cosmonaut Yuri Romanenko performed spacewalks back in the 1970s and 1980s. This is the son's first experience out in the vacuum of space.

Vinogradov made his seventh spacewalk; he ventured into a dark, ruptured chamber at Russia's old Mir space station in 1997 following a cargo ship collision. He arrived late last month for a six-month stay at the space station; he'll turn 60 aboard the orbiting complex in August.

The spacewalkers joked as they toiled 260 miles above the planet.

"I'm afraid of the darkness," one of them said in Russian as the space station passed over the night side of Earth.

Russian flight controllers outside Moscow oversaw Friday's action. The four other space station residents monitored the activity from inside; Canadian commander Chris Hadfield drew the short straw and had to work on a balky toilet.