Astronauts venture outside for space station work

HOUSTON, Dec 13 (Reuters) Two space shuttle astronauts ventured into open space to begin the first of three spacewalks to upgrade the power system on the International Space Station.

Robert Curbeam, who is among NASA's most experienced spacewalkers, and his partner, rookie astronaut Christer Fuglesang of Sweden, opened the hatch on the station's Quest airlock at 3:31 pm EST (0201 IST).

The primary task of the spacewalk is to hook up a new piece of the station's girder-like backbone. That will prepare for spacewalks later this week to rewire the station's electrical system.

The men planned to remove locks that were needed to snap the segment into the shuttle's cargo bay for launch. After that, it will be up to their crewmates aboard the space station who are remotely operating a robotic crane, to carefully lower the truss into place.

Clearances will be tight. The truss will pass as close as 2 inches to one of the station's solar arrays. NASA typically likes a couple of feet between equipment on the move.

The 11-million dollar, two-tonne truss will add 11 feet to the space station's metal backbone, taking it to 180 feet in length. It will be more than 250 feet long when finished.

Once the new beam was in position, Curbeam and Fuglesang were to tighten bolts, securing the truss into place.

Two more spacewalks are planned to rewire the station so it can use a new set of solar power arrays delivered in September and two more sets awaiting launch.

The increased power is needed for laboratory modules built by Europe and Japan, which are expected to be attached to the station beginning next year.

The seven astronauts on Discovery blasted off from the Kennedy Space Center in Florida on Saturday night and docked at the space station on Monday. They joined station commander Michael Lopez-Alegria and crew Mikhail Tyurin of Russia and Thomas Reiter of Germany.

Discovery astronaut Sunita Williams is replacing Reiter on the space station and will stay until June. Reiter, after six months on the station, returns to Florida aboard the shuttle on December. 21.

REUTERS DKS PM0432