HOUSTON — German astronaut Hans Schlegel will return to spacewalking duty outside the International Space Station (ISS) Wednesday after sitting out on an earlier excursion due to a medical issue.
A veteran European Space Agency (ESA), astronaut Schlegel will step outside the ISS on his first spacewalk with crewmate Rex Walheim to replace an empty nitrogen tank on the space station?s metallic backbone-like truss.
"I feel really great right now," Schlegel told reporters from the flight deck of NASA's shuttle Atlantis on Tuesday. "I'm, of course, a little bit anxious because tomorrow is going to be my first [spacewalk]."
Mission managers pulled Schlegel, 56, from the first spacewalk of Atlantis' STS-122 mission — Monday's excursion to help attach the station's new European Columbus lab — due to an undisclosed illness. Officials with NASA, the ESA and Schlegel himself declined to discuss the illness in detail citing the need for medical privacy.
But Schlegel stressed Tuesday that he supported the decision to replace him on Monday?s spacewalk to ensure Columbus, which the astronaut and crewmates opened for business yesterday, reached the ISS successfully.
"I didn't go outside, but I helped from inside and that's the most important thing," said Schlegel, who helped choreograph the orbital work with shuttle pilot Alan Poindexter. "We made sure that this [spacewalk] was executed right, successfully and in a good way."
Poindexter will also watch over today's spacewalk, which calls for Schlegel and Walheim to step outside the space station's Quest airlock at 9:35 a.m. EST (1435 GMT). While the outing marks a first Schlegel, it will be Walheim's fourth foray into the blackness of space clad only in a NASA-issue spacesuit.
The shuttle astronauts have a singular goal for today's spacewalk: the replacement of a so-called nitrogen tank assembly used to charge up the ammonia coolant lines that shed excess heat through the station's port side radiators. Installed in 2002, the tank has been depleted of its 80-pound (36-kg) supply of nitrogen and cannot be refilled in orbit.
"The nitrogen tank weighs around 550 pounds (249-kg) and it is roughly the size of a small refrigerator," said Anna Jarvis, NASA's lead spacewalk officer for the STS-122 flight.
To swap out the empty tank, Walheim will ride the space station's Canadarm2 robotic arm down to the shuttle Atlantis' cargo bay to retrieve its replacement. An orbital shell game will then follow to swap out the tanks before Walheim can take the older one back to Atlantis for the return trip home.
"It seems so easy," Walheim said in a NASA interview.
But the work is expected to last more than 6 1/2 hours, primarily because of the need to wrestle with a series of tough bolts, detach and reattach nitrogen lines and move the station's robotic arm periodically to stay in position. Atlantis mission specialist Leland Melvin will oversee the robotic arm work from inside the ISS.
"[It] takes a fair amount of time, and then doing all the connections in back and doing the gymnastics of swapping them around," Walheim said. "These are very large components."
Schlegel will assist Walheim during the spacewalk, which will include a few extra cable-connecting chores leftover from Monday's excursion. If they have additional time, they may also cover a set of trunnions pins aboard the Columbus lab in insulation.
"The only real difference at the beginning," said NASA station flight director Bob Dempsey of the added work. "Otherwise the timeline is pretty much the same."
Commanded by veteran shuttle flyer Stephen Frick, Atlantis' seven-astronaut crew launched Feb. 7 on what is now a 12-day mission to deliver the Columbus lab to the ISS and swap out one member of the station's Expedition 16 crew. The shuttle is scheduled to land on Feb. 19, but could gain an extra 24 hours docked at the station if mission managers decide later today to extend the spaceflight another day.
Today's spacewalk will begin a 9:35 a.m. EST (1535 GMT). NASA is broadcasting Atlantis' STS-122 mission live on NASA TV. Click here for SPACE.com's shuttle mission coverage and NASA TV feed.