First spacewalk of mission in underway tonight
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: March 13, 2008
Floating in the Quest airlock module, astronauts Richard Linnehan and Garrett Reisman switched their spacesuits to battery power at 9:18 p.m. to officially begin a planned six-and-a-half-hour spacewalk. The goals of the excursion are to prepare a Japanese storage module for attachment to the space station and to begin the assembly of a Canadian maintenance robot known as Dextre.
This is the 105th spacewalk devoted to station assembly and maintenance since construction began in 1998, the sixth so far this year and the first of five planned by the shuttle Endeavour's crew. For identification purposes, Linnehan, call sign EV-1, is wearing a suit with solid red stripes around the legs. Reisman, call sign EV-2, is wearing a suit with diagonal stripes.
Reisman is a rookie making his first spacewalk. Linnehan, a veterinarian before becoming an astronaut, completed three spacewalks as part of the most recent crew to service the Hubble Space Telescope in 2002.
After exiting the airlock, Linnehan and Reisman will make their way to Endeavour's cargo bay where they first will remove eight thermal blankets protecting the Japanese logistics module's common berthing mechanism. Moving to the front of the module, Reisman will unplug an electrical cable that supplied shuttle power to internal heaters.
"Our job is to prepare the JLP for installation on the space station," Reisman said in a NASA interview. "There are covers, kind of blankets that keep it warm in the coldness of space, that need to be removed before it gets installed. So we have to take off those covers and blankets. Then we also have to unplug (an) electrical cable that is used to keep it warm while it's inside the payload bay and that cable needs to be unplugged before it's lifted out, installed on the space station."
Once that is completed, Japanese astronaut Takao Doi, operating Endeavour's robot arm, will pull the module out of the cargo bay and begin the slow process of moving it to the upward-facing port of the Harmony connecting module on the front of the station. Linnehan and Reisman, meanwhile, will move up to the front of the station's power truss, make their way to the mobile base system to begin assembling Dextre.
"The business end of Dextre if you like, Dextre's hands, also called the ORU tool changeout mechanisms (OTCMs), are equipped with force moment sensors," said Daniel Rey, Dextre integration manager for the Canadian Space Agency. "What makes Dextre able to perform these human scale tasks is the sense of touch it has and the automatic control that allows it to compensate for sets of forces, or moments, so that despite the crews commands, since they have no direct view and no direct force feedback except what's on their graphical display, Dextre will be able to prevent something from jamming."
Replacing a failed space station component, he said, is similar to pulling out and re-inserting a drawer.
"An insertion or an extraction is quite a delicate task, it's like trying to remove an old drawer from a chest that maybe is a bit damp and it requires that sense of touch that Dextre has built into it," Rey said. "Other features of the hand are the grippers ... and in the middle of the grippers, you notice there's a socket that can be extended identical to what you'd find on the EVA crew member's pistol grip tool. So it's a seven-sixteenths socket and that extends to unbolt an ORU."
When fully assembled and attached to the station's robot arm, Dextre can be positioned next to a failed component. Using one of its hands to lock onto the station to provide rigidity, the other hand's grippers can lock onto the component, the socket wrench can unbolt it, the unit can be withdrawn and a replacement inserted.
"There's a choreography that goes on with Canadarm2 to place Dextre in front of the work site," Rey said. "One arm will then be used to advance and stabilize the system. Since it's at the very end of a large, flexible arm 17 meters long, it's required to stabilize to avoid too many oscillations for the delicate maintenance tasks. So once the stabilization arm has acquired the stabilization fixture, the other arm is advanced.
"We're looking forward to being used on orbit," Rey said. "We have every reason to believe that Dextre will meet and exceed its specifications the same as Canadarm2 did.
But first, it has to be assembled. Dextre's two arms are mounted on either side of the Spacelab pallet with the robot's central torso in the center. After setting up their tools, Reisman will mount a foot restraint on the pallet while Linnehan clips his boots into a foot restraint on the end of the station's robot arm. He then will release two clamps to free the first OTCM, which will be attached to its arm with four mechanical fasteners and two electrical cables.
To provide plenty of clearance for Doi, who will still be maneuvering the Japanese module into place using the shuttle's robot arm, Linnehan will get off the station arm and the second OTCM will be install while he and Reisman are free floating.
If all goes well, the two assembled arms will be attached to the torso during a second spacewalk Saturday.