The two astronauts switched their spacesuits to internal battery power just past 3:30 p.m. EST (2031 GMT). The spacewalk is expected to run about six hours, and calls for the installation of a new truss segment, the relocation of a grapple fixture and replacement of a malfunctioning camera.
Their primary goal is the installation of a new piece of the ISS — the Port 5 (P5) spacer — to the portside end of the station's main truss.
Small, but vital, the boxy P5 truss is designed to serve as a bridge between the station's Port 3/Port 4 (P3/P4) solar arrays and the Port 6 (P6) solar array element.
The P3/P4 extends off to the station's port side, but the older P6 array reaches upward, mast-like from the orbital laboratory's centerline and will be moved during a later shuttle mission.
"This truss that we're bringing up, the P5 truss, is almost like a spider," said Curbeam, who is the lead spacewalker of STS-116, in a preflight NASA interview. "It allows us to take the P6 truss, which has been up there for several years now ... and put it in its rightful place, if you will."
Curbeam's NASA spacesuit, known as an Extravehicular Mobility Unit, will sport a red stripe and U.S. flag, while that of Fuglesang — who represents the European Space Agency — is all white and carries a Swedish flag in honor of his homeland.
Shuttle pilot William Oefelein will coordinate the spacewalkers' movements from the flight deck of Discovery.
Today's spacewalk will mark the third extravehicular activity (EVA) for Curbeam and the first for Fuglesang. The EVA is also the first of three planned during their STS-116 mission to install P5 and rewire the space station's electrical grid.
A tight squeeze
Discovery astronauts plucked the two-ton P5 truss out of the shuttle's payload bay after docking at the ISS late Monday. They later it to the end of the space station's robotic arm, leaving it in a parked position for today's spacewalk.
Later today, STS-116 mission specialist Joan Higginbotham and newly arrived ISS astronaut Sunita Williams — who rode up to the station aboard Discovery — will ease P5 into position with a delicate touch, relying on Fuglesang to call out distances and directions as the truss comes within inches of space station hardware.
"Picture as if you're helping a friend park in a very tight spot," lead STS-116 spacewalk officer Tricia Mack said of Tuesday's truss installation. "You're going to want to give them a, 'Whoa, back a little to the right, a little to the left.'"
The parking analogy is more than apt since, at 11 feet (3.3 meters) long, the P5 truss is about the length of a compact car, NASA officials said. Built by Boeing, the truss element is also 14 feet (4.5 meters) wide and stands about 13 feet (4.2 meters) high.
Higginbotham said she will be depending on Fuglesang's verbal calls to avoid damaging P5 or the ISS.
"There are very few cameras on that side of the truss," Higginbotham said before flight, adding that she is especially wary of hitting an electronics box on a nearby P3/P4 truss solar array. "We're coming within inches of that box, and it's a box that, you know, if you hit it, you can cause sparks."
Curbeam and Fuglesang will remove a series of launch locks and restraints from P5 before it is in place, then secure it to the ISS via drill-driven bolts.
In addition to installing P5, Curbeam and Fuglesang plan to relocate a grapple fixture on the segment and replace a broken camera elsewhere on the space station's main truss.
For Fuglesang, who is making his first spaceflight as well as his first spacewalks during the STS-116 mission, today's EVA debut is a welcome challenge.
"I'm sure it's going to be fantastic," Fuglesang said in a NASA interview. "But, really, going out, you cannot be more in space than doing a spacewalk."
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