Spacewalk No. 5 ends
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: March 22, 2008
Astronauts Robert Behnken and Michael Foreman staged a successful six-hour two-minute spacewalk Saturday, mounting the shuttle Endeavour's heat-shield inspection boom on the station, deploying an experiment package and carrying out a critical inspection of a stalled solar array positioning mechanism. It's still not clear what is causing internal contamination, but an impact from orbital debris does not appear to be the root cause.
"You were just fabulous out there today," spacewalk coordinator Richard Linnehan radioed as the spacewalk ended. "I can't say enough. Thanks for making everyone look good."
Thank you, Rick," Foreman said.
"It was an absolutely fantastic EVA today, fellas, I'm just happy to have had a chance to play along," added space station flight engineer Garrett Reisman, who helped Linnehan direct the operation. "Thanks for letting me join in."
It was the 109th spacewalk devoted to station assembly and maintenance since construction began in 1998 and the fifth and final excursion for Endeavour's crew. Total station EVA assembly time now stands at 687 hours and 11 minutes. Total spacewalk time for Endeavour's crew stands at 33 hours and 28 minutes.
During today's outing, Behnken and Foreman bolted Endeavour's heat shield inspection boom to the station's solar power truss for use by the next shuttle assembly crew. The shuttle Discovery is scheduled for launch May 25 to deliver Japan's huge Kibo research module to the station, a payload so large there's no room in the shuttle's cargo bay for an inspection boom.
The astronauts also deployed a materials science experiment that could not be attached earlier and installed thermal covers over fittings on a newly installed Japanese logistics module.
"We now have the OBSS transferred and installed over on station," said lead station Flight Director Dana Weigel. "It has power to both of its heater strings, so it'll be nice and toasty when flight 1J goes up to retrieve it at the end of May. We also went back to install the MISSE-6 payloads we had problems with before. We were able to get them installed. Bob had the same problems with the pit pins he had before, but a hammer helped persuade them in place and now they're secure on station."
The astronauts plan to take time off Sunday before making final preparations for undocking Monday evening and landing back at the Kennedy Space Center on Wednesday. Weigel said engineers assessing data from laser scans of the shuttle's nose cap and wing leading edge panels Friday, a "late inspection" to look for signs of micrometeoroid impact damage, found no problems of any significance.
"Over on the orbiter side, the ground teams have completed the analysis for all of the late inspection imagery that we took yesterday and the TPS (thermal protection system) has been cleared for entry," she said. "Tomorrow is flight day 14, and it's a certainly well-deserved off-duty day for the crew."
Trouble with the starboard Solar Alpha Rotary Joint, or SARJ, has proven especially troubling for station engineers. The lab complex is equipped with two massive SARJs, one on each side of the station's main solar power truss, that use 10-foot-wide motor-driven gears to turn outboard solar arrays like giant paddle wheels to track the sun. Last fall, engineers discovered extensive internal metallic contamination in the right-side SARJ indicative of a major problem of some sort. To minimize additional damage, flight controllers no longer run that starboard SARJ in "auto-track" mode, although it is occasionally repositioned to improve power generation.
During previous spacewalks, astronauts removed 17 of 22 thermal covers around the circumference of the joint to look for signs of micrometeoroid impact damage or any other issues that might explain the problem. In addition, one of the 12 trundle bearing assemblies positioned around the race ring was removed and returned to Earth for analysis, along with samples of the metallic contamination.
During the Endeavour crew's fifth and final spacewalk Saturday, Forman removed the final five thermal covers and looked inside. So far, nothing obvious has emerged. But Weigel said today's inspection should let engineers close at least one branch of the fault tree.
"One of the key things that we learned during the EVA today was that we don't have any obvious MMOD (micrometeoroid debris) hit through the covers," Weigel said. "They're aluminum plates with beta cloth over them. We're looking to see if we have an MMOD hit through them and if that's a source of the debris. That's a big help for us, that kind of narrows down one of the chains of the fault tree."
Engineers are assessing a variety of potential repair options, ranging from cleaning up the debris and resuming normal operation to moving the 12 trundle bearing assemblies to a redundant, outboard gear/race ring. But the latter is a last-resort option, requiring several spacewalks, and mission managers do not want to take that step without understanding the cause of the current problem.
The right-side SARJ can still be used to periodically move its solar panels to improve power generation and an analysis of the station's electrical demand, even with the addition of Japan's Kibo research module in May, shows near-normal operation is possible without putting the SARJ in auto track.
"In terms of when decisions need to be made from a power standpoint, we're actually doing pretty well," Weigel said. "We can get by for quite a few more flights. The program, though, was interested in kind of coming up with more of a game plan by the end of March, just so we can figure out when we want to start attacking the different pieces of this puzzle. And there are a lot of different options we have.
"One is to try to clean it up and live with using the current race ring as is. And that's something they're going to go and talk about now that we've finished the rest of the inspection and we understand we do have uniform debris around the ring. We also have experience now with cleaning. I think with those pieces, we'll be able to figure out if we want to go clean and use it as is. I'm not exactly sure when we'll talk about if we want to go to the outboard ops. If we have a good solution that lets us stay on the inboard or the current race ring, then we're not as anxious to get to the outboard because we don't have as much redundancy when we run in that configuration. So I think the first decision point is going to be the end of March.
"Our current power predictions show that we're good for the next couple of flights without auto tracking, so we can continue to do what we've been doing, which is positioning the solar arrays strategically. For certain power contingencies, we could get into cases where we'd want to auto track for a while so it's desirable to get into a configuration where we could do it if we wanted to. Technically, we can auto track right now, it's just that we're putting the hardware at higher risk by doing that. So we really want to make sure we understand what's going on and clean it up as much as we can before we commit to using it in auto-track mode."
One of the items Foreman inspected was a small blemish on the race ring that appeared to be either a small divot or perhaps an accumulation of debris. Flight controllers could not tell whether the blemish represented a bump or a depression. Foreman's inspection was not totally conclusive, but Weigel said it appears to be a small divot. It's not yet clear whether there's any relationship between the blemish and the overall SARJ problem.