Spacewalk No. 1 begins
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: July 8, 2006
Astronauts Piers Sellers and Mike Fossum, floating in the space station's Quest airlock module, switched their spacesuits to internal battery power at 9:17 a.m. today to officially kick off the first of three planned spacewalks during shuttle Discovery's mission.
The goals of today's six-and-a-half-hour excursion are to begin the repair of a stalled transporter on the station's solar array truss and to test the stability of a 50-foot-long boom attached to the end of the shuttle's robot arm for possible use as a work platform.
This is the 66th spacewalk devoted to space station assembly and maintenance since construction began in 1998. Going into today's spacewalk, 57 U.S., Russian, French, Japanese and Canadian astronauts had logged 390 hours and 54 minutes of EVA time working on the station. Sellers participated in three space station spacewalks in 2002 totaling 19 hours and 41 minutes. Fossum is making his first EVA and thus pushes the number of station spacewalkers to 58.
The first task today is to restore the station's stalled crane transporter platform to limited operation in preparation for a complex repair job during the crew's second spacewalk Monday.
The transporter is designed to creep along rails on the front face of the station's unfinished solar array truss, carrying the station's Canadian-built robot arm from one work station to another. At each work station, the transporter and arm can be locked down to provide stability. The long truss eventually will sport four huge sun-tracking solar array panels that will generate the electrical energy needed to support a six-member crew and a full suite of scientific experiments.
But the outer segments of the truss cannot be built without the station's robot arm and the arm cannot be moved from point to point unless the transporter is operational and has redundant power, video and data cables.
As the transporter moves along the truss it plays out or rolls up ribbon-like power and data cables. To provide redundancy, two trailing umbilical system - TUS - reel assemblies play out and rewind separate cables. The transporter was launched with powerful cable cutters in devices on the transporter itself, called interface umbilical assemblies, in case of a jam in either TUS reel that might otherwise strand the work platform.
On Dec. 16, TUS cable No. 1 on the Earth-facing, or nadir, side of the transporter was severed when the cable cutter in the nadir interface umbilical assembly suddenly fired for no apparent reason, slamming down with 960 pounds of force. Engineers still do not know why the spring-driven cutter fired.
But the incident left the mobile transporter with just one set of power and data cables and NASA flight rules forbid its movement along the truss unless full redundancy is available. The concern is that a second failure could leave the transporter stranded between work sites and unable to be safely latched down. That, in turn, would pose a risk during shuttle dockings or other events when unwanted movement could prove dangerous.
"Basically, it killed itself just before Christmas," Sellers said in an interview. "It's a complicated task. If we don't get this thing fixed, we can't move this truck that moves up and down the front face of the station and we can't continue with assembly. So we absolutely have to get it fixed before the next mission."
Earlier this year, worried that the cable cutter on the upward facing, or zenith, side of the transporter might fire, Expedition 12 commander William McArthur and flight engineer Valery Tokarev attempted to disable the zenith cable cutter during an already planned spacewalk. But they were unable to drive in a "safing" bolt and rather than leave the cable in place, they removed it from the cutting mechanism and tied it off. That had the effect of stranding the mobile transporter at work site 4.
Sellers and Fossum will attempt to fix the zenith cable system today by inserting a blade blocker designed to prevent any damage to the cable even if the cutter fires later. If the blade blocker fits properly, they will re-insert the zenith cable in the interface umbilical assembly and thus restore the transporter to single-cable operation.
That's important because the crew needs to move the transporter from work site 4 to work site 5 to get the access needed to repair the nadir cable system during the second spacewalk Mondaay.
Once the cable blocker is in place, Sellers and Fossum will press ahead with the boom stability tests.