Solar array activities added to Saturday spacewalk
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: December 15, 2006
NASA managers late this evening told the Discovery astronauts that if enough time is available at the end of an already-planned spacewalk Saturday, two spacewalkers will be asked to carry out an up-close inspection of a partially retracted solar array to help engineers figure out what might be needed to coax the blankets into full retraction.
"We'll plan to execute EVA-3 tomorrow as published," astronaut Stephen Robinson radioed the astronauts late today. "It includes all the (planned) tasks. When we are done, assuming all the power and thermal reconfiguration goes well as it did during EVA-2, then if we have time, we may do a fairly simple task to do an inspection of this solar array wing.
Flight controllers plan to ask one or both spacewalkers to manually shake the array storage box to loosen a presumably stuck guide wire believed to be preventing a cluster of solar blanket slats from evenly folding up like a pleated blind. No other repair work will be attempted, at least according to the latest planning, but engineers are holding open the possibility of staging a fourth, unplanned spacewalk Monday if necessary.
"Right now, there's a lot of discussions going on as to mechanically what really is the hang up," Robinson said. "We're currently envisioning this as an inspection task. How much and what you could touch and what kind of good it could do is still very much under discussion, so we're not ready to really tell you what that could be. That will probably be developing even during EVA-3 tomorrow. So right now, think of it as going up, taking a really close look and telling us what's really going on."
The primary goal of Saturday's spacewalk, scheduled to begin around 2:42 p.m., is to re-wire two of the space station's four major electrical circuits. Power channels 2 and 3 were successfully activated during a spacewalk Thursday along with one of the station's two independent ammonia cooling loops.
Astronauts Robert Curbeam and newly arrived station astronaut Sunita "Suni" Williams plan to re-wire power channels 1 and 4 during Saturday's spacewalk followed by activation of cooling loop A.
As originally laid out, the spacewalk was expected to last about six hours. But during Thursday's excursion, Curbeam and Swedish astronaut Christer Fuglesang completed the channel 2/3 re-wiring a full hour ahead of schedule. Flight controllers say Saturday's spacewalk could be extended to seven hours if necessary, providing time for the solar array inspection.
The problem with solar wing P6-4B cropped up Wednesday, when the astronauts attempted to retract the blankets as part of work to switch the space station over to its permanent power system.
The P6 array, which features two wings - 2B extending on the right side of the station and 4B on the left - stretches 240 feet from tip to tip. It was mounted on the station six years ago to provide interim power during the initial stages of assembly.
To activate the station's permanent power system, the Discovery astronauts needed to retract the left wing of P6 to clear the way for a newly-installed set of arrays, known as P4, to begin rotating to track the sun. The right wing of P6, the 2B panel, will be retracted next March and in the fall of 2006, P6 will be moved to its permanent position on the left end of the station's main solar array truss next to P4.
But the P6-4B wing refused to cooperate and despite more than six-and-a-half hours of trying, the astronauts were only able to retract the panel about halfway. That was enough to permit the P4 array to rotate as required to track the sun, but not enough to provide the desired long-term structural stability.
Today, the astronauts attempted to shake the stuck slats loose by rotating the array's central mast to set up oscillations in the flexible blanket. German astronaut Thomas Reiter even tried exercising with bungie cords in a bid to set up vibrations in the station structure that might jostle the unruly array. But the efforts had no discernible effects.
NASA managers are reluctant to add a dedicated spacewalk to Discovery's mission because that would force the astronauts to forego a planned heat shield inspection after undocking from the station. In addition, work on the arrays poses a variety of risks because of the unknown nature of the problem, sharp edges, lack of training and the potential for electric shock.
Unlike some shuttle missions, Discovery's flight cannot be extended. Before launch, NASA managers decided to add a day to the flight and the shuttle does not have enough hydrogen and oxygen for its electricity producing fuel cells for any additional extensions beyond the two days NASA always keeps in reserve for bad weather.
While some engineers clearly favor a spacewalk repair, others are opposed, arguing the partially retracted panel is stable in the short term and that it makes more sense to defer retraction until engineers understand the problem - and possible solutions - more thoroughly.
The space station crew could carry out a retraction spacewalk, or the work could be deferred until the next shuttle crew arrives.
However the debate plays out, a decision must be made Saturday one way or the other.
"We have a great view of what's going on with the array up there from the shuttle flight deck," station commander Mike Lopez-Alegria told reporters earlier today. "My perspective looking at it from the inside last time was it doesn't need much coaxing. You've probably heard us use the analogy of trying to fold a map. As you know at times when you're folding a map it's helpful to poke it here and there and I think our approach will be not very different from that, although we'll be poking gently."
For his part, Curbeam said "there's a lot of very delicate pieces to the solar array wing and we want to make sure we don't rip one of those blankets or damage any of the hardware that's up there. ... As always, you want to do no harm first."