Mission Discovery: Spacewalkers Set to Rewire ISS Power Grid (SPACE.com)

15.12.2006 03:48 Space - Source: Yahoo space

HOUSTON--Two Discovery shuttle astronauts will venture outside the International Space Station (ISS) for their second spacewalk this week, this time to rewire part of the orbital laboratory's power grid.

STS-116 spacewalkers Robert Curbeam and Christer Fuglesang are expected to spend six hours overhauling half of the station's power system from a temporary set up into its permanent configuration.

"[I]t is actually quite risky," Curbeam said of the spacewalk in a NASA interview. "But the biggest is risk is starting up the items that have never been operated on the station."

Today's spacewalk is set to begin at 3:12 p.m. EST (2012 GMT) from the Quest airlock at the ISS.

Curbeam and Fuglesang will work primarily on the space station's central S0 truss, where they will route electrical connections, cables and jumpers to plug two portside solar arrays into two of four channels that make up the outpost's primary power grid.

Today's spacewalk will reconfigure the station's power channels 2 and 3, which should be complete after the first 90 minutes, NASA's lead STS-116 spacewalk officer Tricia Mack has said. A Saturday excursion by Curbeam and newly-arrived ISS flight engineer Sunita Williams will reconfigure channels 1 and 4.

To do that, spacewalkers must disconnect and reconnect dozens of stiff cables to fold the Port 3/Port 4 (P3/P4) solar arrays, which were installed at the ISS in September, into the stations primary electrical system.

The spacewalk will also activate two long dormant Main Bus Switching Units (Channels 2 and 3) that direct power where it's needed for the first time four years, and start up a vital pump module for the station's ammonia cooling system that will prevent the power systems from overheating.

If any one task does not go as planned, the astronauts may have to stop their work and back out of their complicated procedures to replace faulty electronics boxes or perform other troubleshooting work, losing precious time, NASA officials have said.

Since 2000, the space station's main power has been provided by solar arrays on the mast-like Port 6 truss, which was always meant to be a temporary system until additional solar wings were installed and plugged into the outpost's main truss. The P3/P4 solar arrays are the first to be plugged into the station's permanent power system, which is vital for the outpost's expansion to accommodate larger astronaut crews and additional international laboratory modules, NASA officials said.

One fear, Curbeam has said, is that he and Fuglesang will perform all their required tasks, unplug the station's power systems from P6 and into the main truss, only to find that nothing happens.

"We don't expect everything to crumple on us like that," he said. "But you just don't know."

John Curry, NASA's lead ISS flight director for Discovery's STS-116 mission, said that half of the space station's systems will shut down during today's spacewalk, dimming the lights for the eight astronauts inside and leaving many non-critical systems without redundant backups.

"A lot of systems go single string in the power down," Curry said.

Primary control computers and life support systems will be safeguarded using a series of jumper cables, he added, but some ISS communications and ventilation systems, for example, will be affected. Astronauts will have to rely on Discovery's systems instead.

Curbeam and Fuglesang will not be the only ones busy during today's spacewalk.

The astronauts will work in close concert with ISS flight controllers on Earth, who will shut down and reactivate station systems when required. Altogether, space station flight controllers expect to issue some 4,500 commands to the ISS between today's spacewalk and its Saturday counterpart. On an average day, flight controllers send about 800 commands to the ISS, NASA officials said.

Once the ISS power system overhaul is complete, Curbeam and Fuglesang still have work to do outside the space station.

The astronauts expected to move two tool carts, known as Crew Equipment Translation Aids (CETA), to get a head start on future ISS construction during NASA's STS-117 shuttle flight next year.

Curbeam and Fuglesang will also install two thermal covers over sensors on the space station's robotic arm and reconfigure an electrical patch panel that routes power to the orbital laboratory's Zenith 1 (Z1) and Russian segments.

"It's going to be a pretty exciting day," Curbeam said.