4th spacewalk approved
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: December 16, 2006
NASA's Mission Management Team today decided to add a fourth spacewalk to Discovery's mission in a bid to fully retract an unruly solar array on the international space station. The decision was announced during a spacewalk by Robert Curbeam and Sunita Williams to finish re-wiring the orbital outpost.
"We are go for EVA-4 on flight day 10," astronaut Stephen Robinson radioed from Houston.
Curbeam and Williams plan to inspect the partially retracted P6-4B array later today and to possibly shake its storage box in a bid to free a presumably hung up guide wire preventing the blankets from folding smoothly.
Assuming that doesn't resolve the issue, Curbeam and Swedish astronaut Christer Fuglesang will carry out a fourth spacewalk Monday to attempt some sort of repair.
Under that scenario, Discovery would undock from the station Tuesday and the astronauts would carry out a final heat shield inspection Wednesday. After packing up Thursday, landing at the Kennedy Space Center would be expected around 3:56 p.m. Friday.
Curbeam and Williams successfully wrapped up a complex re-wiring job today, routing solar array electricity through two main bus switching units, transformers and other equipment making up channels 1 and 4 of the space station's electrical power system. Flight controllers then successfully activated a critical pump to circulate ammonia coolant through cold plates and radiators to keep the electrical components from overheating.
Curbeam and Fuglesang wired in electrical power system channels 2 and 3 during a spacewalk Thursday. Today's work completed the station's switch over from interim to permanent power, a major milestone that clears the way for future crews to build out the station's main solar array truss and, eventually, to add new modules.
Building out the main power truss requires astronauts to fold up and move the P6 solar arrays, which provided interim power to the station. P6 eventually will be moved to the far left side of the power truss where it will joint a set of arrays known as P4 that were attached in September.
The Discovery astronauts attempted to retract the left wing of the P6 array, known as P6-4B, on Wednesday but they were only able to pull it in about half way. Several of the slats in the folding blankets failed to fold evenly, stopping the process in its tracks. Repeated attempts to free a presumably jammed guide wire were unsuccessful.
Engineers initially said the array could be safely left as is, that it had enough structural rigidity in its partially retracted state to withstand shuttle and Russian Progress supply ship dockings and undockings. But there were questions about whether it was up to the jarring from Soyuz crew capsule dockings at a port closest to the array.
NASA managers said earlier an additional spacewalk to deal with the array would force the astronauts to forego a planned final heat shield inspection after undocking because the shuttle only has enough hydrogen and oxygen for its electricity producing fuel cells to stay in orbit until Saturday at the latest.
The original flight plan called for a landing Thursday at the Kennedy Space Center, preserving two weather contingency days. NASA seldom cuts into the weather reserve, prompting speculation that an additional spacewalk would rule out the late heat shield inspection.
NASA went into Discovery's flight with an understanding that a post-undocking heat shield checkout could be taken off the table if the astronauts had problems with the space station electrical work that might require an additional spacewalk.
But during today's Mission Management Team meeting, senior agency managers concluded an additional spacewalk would, in fact, be required to deal with the recalcitrant solar array.
The repair work could have been deferred to the station's three-person crew, but spacewalks use the buddy system and a single astronaut would be required to stay inside and operate the retraction system as well as the space station's big robot arm.
In the end, the MMT, chaired by John Shannon, decided to add a spacewalk to Discovery's mission to resolve the matter sooner rather than later. The P6-2A wing, currently fully extended to the right side of the station, is scheduled to be retracted in March and engineers would like to have experience with P6-4B before attempting a second retraction that might result in the same problem.
The P6 array, with both wings fully retracted, is scheduled to be unbolted and moved to the far left end of the station's solar array truss in September.
Along with adding another spacewalk to Discovery's mission, the MMT also decided to preserve the final heat shield inspection, giving up one of the shuttle's two weather contingency days.
With only one weather contingency day beyond that, NASA likely will activate both of its backup landing sites at Edwards Air Force Base, Calif., and White Sands, N.M., on Friday. Touchdown at the Kennedy Space Center, weather permitting, is scheduled for 3:56 p.m.