Second Endeavour spacewalk under way

Two Endeavour astronauts have begun what is expected to be the mission's most challenging spacewalk, the second of four.

Drew Feustel and Mike Fincke opened the International Space Station's Quest airlock hatch at 2:03 a.m. EDT and switched their spacesuits to battery power at 2:05 a.m. to officially begin the spacewalk 10 minutes ahead of schedule.

The station was flying over northern Kazakhstan.  

"This is an important one for the longevity of the station, for the power and the cooling, so lets get started," said Greg Chamitoff, who is choreographing the spacewalk from inside Endeavour.

The primary tasks of the six-and-a-half-hour outing are to top off a slightly leaking radiator cooling loop at the far left end of the station's football field-length structural truss and to lubricate a joint that rotates left side solar array wings.

The pair will finish work Feustel and Greg Chamitoff began Friday to reroute cables so ammonia coolant can flow from a tank near the center of the truss out to the leaking loop.

The leak is described as nearly imperceptible, and might not become an issue for 18 months or two years, but now was a good time to take advantage of a shuttle crew to refill it. About five pounds of ammonia will be pumped into the loop.

Spacewalkers have been challenged in the past working with quick disconnect valves that have been stick and leaked ammonia, a toxic substance that can't be carried back inside the station where it could poison the air.

The work on the left Solar Alpha Rotary Joint, or SARJ, is preventative maintenance. In 2008, the starboard joint was successfully lubricated after gears were found to be grinding against themselves, so the goal is to prevent that type of grinding on the port side.

Other tasks include lubricating an attach mechanism and installing a camera cover on the Canadian-built Dextre robot that sits outside the station. And handles will be stowed that would be needed if a radiator needed replacement.

The spacewalk is the 157th supporting space station assembly and maintenance.

It's the fifth for Feustel, whose has logged 27 hours and 17 minutes of prior spacewalking time, more than 20 hours of that working on the Hubble Space Telescope. He's wearing a suit with solid red stripes, has a radio call sign of "EV-1" and will show No. 18 on his helmet camera.

The spacewalk is Fincke's seventh, the first six totaling 26 hours and 12 minutes all performed in Russian Orlan spacesuits. His suit is solid white, he's called "EV-2" and by his nickname of "Spanky," and his helmet camera shows No. 20.

Check out highlights of today's action in this NASA TV schedule and take a look at the Flight Day 7 Execute Package of notes sent to the crew.