Shuttle inspection boom to be anchored on the station
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: March 22, 2008
Astronauts Robert Behnken and Michael Foreman are preparing for a fifth and final spacewalk, a six-and-a-half hour excursion this evening to mount the shuttle Endeavour's 50-foot-long heat shield inspection boom on the space station for use by the next station assembly crew. The astronauts also plan to install an experiment package they were unable to attach earlier and inspect the station's right-side solar array rotary joint in an ongoing effort to determine what might be causing internal interference and contamination.
This will be the 109th spacewalk devoted to station assembly and maintenance since construction began in 1998, the 10th so far this year and the third each for Behnken and Foreman. The spacewalk is scheduled to begin around 5:23 p.m. when the astronauts switch their spacesuits to battery power. For identification, Behnken, call sign EV-1, will be wearing a suit with no markings. Forman, EV-2, will be wearing a suit with broken red stripes around the legs.
The primary goal of the spacewalk is to install the shuttle's 50-foot-long orbiter boom sensor system, or OBSS, on the station for use by the next shuttle crew in late May. The goal of that flight is to deliver Japan's huge Kibo lab module to the space station.
"That mission is very full, Discovery's payload bay is carrying up the very large Japanese pressurized module and there just simply wasn't enough room to carry up the boom as well," said station Flight Director Ginger Kerrick. "So a long time ago, the program decided to go ahead and leave it on board. We're going to be keeping it powered and keeping a very close watch on it and we promise it will be ready to support that mission when they arrive in May."
The boom will be attached to brackets on the front face of the station's solar power truss.
While the spacewalkers are setting up their tools and running a 30-foot-long power cord down the truss, the shuttle's robot arm will hand the sensor boom to the station arm. As soon as possible, Behnken will plug the power cable into the boom to activate internal heaters to keep the boom's laser scanner and camera warm. The arm then will release the boom so Behnken and Foreman can bolt it in place.
With the OBSS mounted on the station, Behnken and Foreman will split up. Behnken will float to Endeavour's cargo bay, retrieve the briefcase-size MISSE-6 experiment package, and make his way to the Columbus lab module. The astronauts made an initial attempt to attach the materials exposure package to a mounting plate on the outboard side of Columbus during the crew's third spacewalk last Monday. But they were unable to insert locking pins in the mounting bracket. This time around, Behnken has smaller pins and cable ties.
While Behnken focuses on MISSE-6, Foreman will make his way to the right side of the station's main power truss to inspect the starboard solar alpha rotary joint, one of two massive motor-driven joints that turn outboard solar panels to track the sun.
Last fall, engineers became concerned about high vibration levels and power usage and ordered an inspection. To their dismay, spacewalking astronauts reported metal shavings covering the interior of the bearing race ring and damage to the ring itself. One of 12 bearing assemblies later was removed and returned to Earth for analysis, along with samples of the contamination. Engineers are still not sure what is causing the damage and the joint is no longer allowed to "auto track" the sun.
Engineers are considering a plan to remove all 12 bearing assemblies and move them to an identical race ring that is available as a backup. But the work would require multiple spacewalks and flight planners don't want to take that last-resort step until they have a better idea of what might be wrong.
Behnken and Foreman originally planned to replace the bearing assembly that was removed earlier. But that task was deferred to give Behnken time to install the higher-priority MISSE package. Instead, Foreman will focus on inspection only, removing five thermal covers to examine areas of the race ring that have not yet been assessed.
Kerrick said the absence of one bearing assembly is not expected to affect flight controllers' ability to reposition the joint as required.
"The joint has an inner ring and an outer ring and evenly spaced along those rings are 12 trundle bearing assemblies," Kerrick said. "Their primary function is to hold the two rings together and provide a rolling surface for the rings that will allow the joint to rotate. Will it be a problem that we're not installing it? The short answer is no. Right before we removed the trundle bearing back in December, we had our engineering teams go off and assess whether or not we would be good operating the SARJ with 11 of 12 of those trundle bearings installed and they declared that we would be. So while we would prefer to be in our nominal config with a total of 12, it doesn't impede our operations any."
The SARJ is protected by 22 thermal blankets. Astronauts have examined the mechanism under 17 of those blankets and Foreman will inspect under the final five during this evening's spacewalk.
"The signature we were seeing was indicative of a resistance in the rotation of the joint," Kerrick said. "So the things we want to look for, we want to inspect all the mechanical components to make sure they're not wearing unevenly or somehow breaking up and causing this resistance. We would also want to investigate the potential for having a debris impact.
"So what we have been doing in a series of EVAs leading up to this (is) opening covers, because all those mechanical components are underneath thermal covers. So you open up the cover and inspect the mechanical components and then you also inspect the cover for potential debris strikes. So we've done pieces of that and for this EVA, there are five covers left - there's 22 covers total in the SARJ - so five left to go. And then we're going to go off and re-inspect one particular area."
Pictures taken during a January spacewalk showed a small area on the race ring that might be a pit or depression.
"We couldn't tell if it was damage or if it was buildup of material," Kerrick said. "So we want the crew to take a second look there because the answer to that question will help us narrow down the scope of potential problems that could be causing this."
Assuming an on-time start, the spacewalk will end around 11:53 p.m. Here is an updated timeline of today's activity (in EDT and mission elapsed time; includes revision J of the NASA television schedule):
EDT........DD...HH...MM...EVENT 12:28 PM...11...10...00...Crew wakeup 01:08 PM...11...10...40...EVA-5: 14.7 psi repress/hygiene break 01:58 PM...11...11...30...EVA-5: Airlock depress to 10.2 psi 02:28 PM...11...12...00...EVA-5: Campout EVA preps 02:28 PM...11...12...00...Station arm (SSRMS) EVA-5 setup 03:48 PM...11...13...20...EVA-5: Spacesuit purge 04:03 PM...11...13...35...EVA-5: Spacesuit prebreathe 04:53 PM...11...14...25...EVA-5: Crew lock depressurization 05:23 PM...11...14...55...EVA-5: Spacesuits to battery power 05:28 PM...11...15...00...EVA-5: Airlock egress 05:33 PM...11...15...05...SSRMS grapples inspection boom (OBSS) 05:48 PM...11...15...20...EVA-5: Setup 06:03 PM...11...15...35...Shuttle arm (SRMS) releases OBSS 06:08 PM...11...15...40...EVA-5: OBSS KAU install 06:18 PM...11...15...50...OBSS handoff to spacewalkers 07:33 PM...11...17...05...Crew meals begin 07:43 PM...11...17...15...EVA-5: OBSS stow 08:58 PM...11...18...30...EVA-5 (EV1): MISSE experiment package install 08:58 PM...11...18...30...EVA-5 (EV2): Starboard SARJ inspection 11:08 PM...11...20...40...EVA-5: Cleanup 11:33 PM...11...21...05...EVA-5 Airlock ingress 11:53 PM...11...21...25...EVA-5: Airlock repressurization 03/23/08 12:08 AM...11...21...40...Post EVA spacesuit servicing 12:28 AM...11...22...00...SRMS powerdown 01:30 AM...11...23...02...Mission status briefing on NASA TV 03:58 AM...12...01...30...ISS crew sleep begins 04:28 AM...12...02...00...STS crew sleep begins 05:00 AM...12...02...32...Daily video highlights reel on NASA TV 09:30 AM...12...07...02...Flight director update on NASA TV 12:28 PM...12...10...00...Crew wakeup