Watch | Nasa astronaut’s marathon spacewalk before US record

Nasa's yearlong spaceman, Scott Kelly, is taking his first spacewalk just hours before setting a new U.S. flight record. Kelly and Kjell Lindgren floated out of the International Space Station on Wednesday morning for the six-and-a-half hour spacewalk. They'll grease the station's big robot arm, route cables, remove insulation from an electronic switching unit and cover an antimatter detector.

At early Thursday, Kelly will break the American record for NASA's longest single space shot. That 215-day record — more than seven months — was set in 2007.

Kelly has been living on the orbiting lab since March. He'll remain there until March 2016. Russian Mikhail Kornienko is also part of the one-year experiment, although he will fall shy of the 14-month world record held by a fellow cosmonaut.

The spacewalk was held up slightly after Lindgren inadvertently switched on the water flow in his suit too soon. Mission Control wanted to make certain the suit would be safe, before giving the all-clear. Water is needed to cool the suit; a malfunction two years ago caused a spacewalker's helmet to flood and almost drowned him.

A second spacewalk — to further whittle down a backlog of station chores — will be conducted Nov. 6 by the same two men.