Sources: Solar array repair spacewalk possible Friday
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: October 31, 2007; Updated at 10:45 a.m.
NASA and contractor engineers worked through the night assessing a variety of options for possible repairs to fix a ripped solar blanket on a partially extended space station array. Station program managers recommended early today, sources said, that NASA delay a fourth spacewalk from Thursday to Friday and devote the excursion to solar array repair work.
Under that scenario, the astronauts would forego any immediate inspection of a contaminated solar array rotary joint, deferring that work to a later mission, and delay a spacewalk by station commander Peggy Whitson and flight engineer Yuri Malenchenko until after the shuttle Discovery departs.
Flight controllers asked Whitson and shuttle commander Pam Melroy to join them for a discussion of the crew's options earlier this morning, but the conversation was "privatized" and not broadcast on NASA's satellite television downlink. Sources said the decision to pursue a solar array repair was now the top priority, but there was no immediate official confirmation from NASA.
At a news conference earlier today, lead spacewalker Scott Parazynski said he and fellow spacewalker Doug Wheelock were ready for whatever NASA managers decided to do.
"I certainly don't have all the data on board yet," he said. "We've taken hundreds of photographs from our windows and from the station assets and folks in Houston are poring over the data trying to figure out exactly what might have happened. My initial take on it was maybe a guidewire that had been frayed earlier might have been the culprit. However, it looks to our eye via binoculars and photos that that guidewire may be intact.
"It really depends on what the root cause is. We have trained quite a bit (in Houston) and there are numerous contingencies we could effect on the solar array wing. Not sure if they're applicable to this situation, however. One of those might be (to) clip the guidewire, if that might be of help. But we'll see what (the ground team concludes)."
Asked how he and Wheelock might gain access to the damage site, which is well away from any point the station's robot arm can easily reach, Parazynski said the array could be retracted far enough to give the repair team access.
Italian astronaut Paolo Nespoli "suggested we just climb up the mast and give it a good shake!," Parazynski joked this morning. "But that might be a little bit too aggressive."
"We actually have pretty good work site access from the bottom," he said. "We have (equipment) that allows us to basically place our foot restraint a little bit higher above the P6 structure. So we can reach about eight to 10 feet, I think, and get up above the level of the blanket box and make some repairs fairly low in. So if there were a need to effect a repair out where the damage occurred, we'd have to retract that array and do the repair close into the lower blanket box."
The Discovery astronauts successfully moved the 17-ton P6 solar array truss segment Tuesday, bolting it to the far left end of the power truss. The first of its two solar array wings, known as P6-2B, extended a full 110 feet as required, but the crew aborted deployment of the second P6-4B wing when one section of hinged blanket slats hung up, possibly due to a guide wire snag. Two seams between adjacent slats pulled open, resulting in separate tears, and the edges of several nearby slats were crumpled. The largest rip measured some two-and-a-half feet long.
"It was a tough situation because with the beta angle that we're at, which is the angle of the sun, the sun was shining directly into our camera views," Melroy said today. "In fact at one point, we actually did stop (the deploy) because we were concerned we had lost our big picture.
"Of course, we're always going to second guess ourselves and there might have been some other things we could have done. But I think we certainly aborted as soon as we saw something that wasn't right. And at the place we had stopped earlier, everything looked nominal so it was only a few more bays. So I think it happened fairly quickly and probably at a moment there was sun in our eyes. As soon as the sun was gone, we were able to stop. I think we did as well as we could under the environmental circumstances."
Eighty percent deployed, the P6-4B array can generate 97 percent of the electricity of a fully extended wing. The station is not yet using power from the torn array, but engineers say tests confirm no major damage to its internal wiring.
The immediate concern is figuring out a way to fully extend the P6-4B wing to provide the necessary structural rigidity. With a partially deployed panel, none of the arrays on the left side of the main power truss can be rotated as required to track the sun without risking additional damage. As a result, the station's left-side solar alpha rotary joint, or SARJ, is locked in place while engineers assess their options.
Adding to NASA's problems, the station's right-side arrays also are locked in place because of unexpected metallic contamination inside the starboard SARJ. Astronauts Scott Parazynski and Doug Wheelock worked earlier today to prepare their spacesuits and tools for a planned spacewalk Thursday to carry out a more detailed inspection of the right-side SARJ to look for the source of the contamination.
The SARJ mechanism features two 10-foot-wide bull gears and two redundant drive motors called drive lock assemblies, or DLAs. Only one DLA is used at a time to drive the outboard bull gear. The mechanism relies on 12 so-called trundle bearings, pressing against the outboard gear race with 1,000 pounds of force, to rotate smoothly.
During a spacewalk Sunday, astronaut Dan Tani removed one of 22 thermal blankets to look inside for any sign of whatever might be causing unusual vibration and power usage. He was surprised to find large amounts of metallic shavings in the joint, a clear sign of some sort of misalignment or other potentially serious problem.
Parazynski and Wheelock originally planned to test a new heat shield repair technique during the mission's fourth spacewalk but that work was deferred to a future mission to clear the way for a more thorough SARJ inspection. It is that work that would be delayed if NASA presses ahead with a solar array repair spacewalk Friday.
As of this writing, no official decisions have been announced. Space station flight director Heather Rarick said late Tuesday engineers needed additional photographs to help assess the solar panel's condition.
"We have some pictures that we've processed and they're not giving us the best insight into what it is at this point," she said late Tuesday. "I think we're asking (for) the crew to take some more pictures (Wednesday)."
Positioned on the far left end of the main power truss, the P6 solar array wings extend beyond the reach of the space station's robot arm. Engineers are studying the possibility of using the shuttle's 50-foot-long heat shield inspection boom to give the station arm additional reach and it may be possible to retract the damaged wing far enough for an astronaut to reach the damaged section.
"What to do next? Not really a clear answer yet," Rarick said late Tuesday. "They're putting ideas on the table. One of the key factors we have to figure out is how we access it, how we get a crew member up there if we do need to do some work, physically at the site as opposed to being able to do it by commands, wither by deploying or retracting. So that's being looked at, that'll take probably a fair amount of time to figure out and put that plan in place.
"Until we know what we think the cause is, maybe until we get some better pictures, I don't think we really have any solid leads on how to fix it yet," she said.
Here is an updated timeline of today's activity (in EDT and mission elapsed time; includes revision G of the NASA television schedule):
EDT........DD...HH...MM...EVENT 10/31/07 12:38 AM...07...13...00...STS/ISS crew wakeup 03:28 AM...07...15...50...Spacesuit resizing 04:13 AM...07...16...35...ESA/Italian Space Agency VIP event 04:28 AM...07...16...50...EVA-4: Tools configured 05:30 AM...07...17...52...ESA/ASI PAO event replay with translation 05:53 AM...07...18...15...EVA-4: Procedures review 06:48 AM...07...19...10...Joint crew meal 07:48 AM...07...20...10...Crew news conference 08:28 AM...07...20...50...Joint crew photo 08:43 AM...07...21...05...EVA-4: Airlock preps 09:00 AM...07...21...22...Crew news conference replay with translation 10:13 AM...07...22...35...EVA-4: Preparation review 11:08 AM...07...23...30...EVA-4: Conference 12:08 PM...08...00...30...EVA-4: Procedure review 01:53 PM...08...02...15...Crew choice video downlink 02:00 PM...08...02...22...Mission status briefing on NASA TV 02:28 PM...08...02...50...EVA-4: Airlock campout/tools configured 03:38 PM...08...04...00...ISS crew sleep begins 04:08 PM...08...04...30...STS crew sleep begins 05:00 PM...08...05...22...Daily video highlight reel on NASA TV 09:30 PM...08...09...52...Flight director update on NASA TV
Station problems aside, the shuttle Discovery is operating smoothly with no major problems. Earlier in the mission, NASA's Mission Management Team concluded the ship's heat shield came through launch in good condition with no major problems. The only open items as of this writing are a half-dozen readings from wing leading edge sensors in place to measure possible strikes by ascent debris micrometeoroids. Similar readings on past flights were attributed to the effects of temperature changes on the shuttle's aluminum structure. In keeping with post-Columbia practice, the wing leading edge indications will be checked out during an inspection after the shuttle undocks from the space station.