NASA declined to discuss the medical problem beyond saying it was not life-threatening, but a European flight controller confirmed Sunday that Schlegel had been ill and wished him a quick recovery.
The European Space Agency's Web site later said the illness was not contagious and probably would not prevent Schlegel from performing his other scheduled spacewalk.
Schlegel, 56, a two-time space flier, sounded OK on Sunday morning when he spoke to Mission Control after waking up to music from fellow German Herbert Gronemeyer.
"Greetings to everybody in America, in Europe and in Germany, and especially of course to my close family and my lovely wife, Heike," he said.
He spent the morning helping his crewmates prepare for the spacewalk and looked fine as he floated around the station.
Schlegel was supposed to venture outside with American Rex Walheim on the first two of three planned spacewalks. The Columbus lab was to have been unloaded from Atlantis and attached to the space station Sunday, with two spacewalkers outside to help.
NASA said Schlegel's shuttle crewmate, American Stanley Love, would take his place. Love trained for the work as a backup and already was assigned to the mission's third spacewalk, along with Walheim.
Schlegel will help guide Love from inside the station during Monday's spacewalk.
After spending much of the morning preparing for Monday's spacewalk, the crew will use cameras and a robotic arm to gather more images of a 1 1/2 inch-by-1 1/2 inch protrusion on one of the many blankets covering Atlantis' right orbital maneuvering system pod, back near the tail. The damage occurred during Thursday's launch and was discovered Friday, flight director Mike Sarafin said.
Space station flight director Ron Spencer said early Sunday that NASA did not know if the blanket was torn or if it was just sticking up a bit.
Engineers were trying to determine whether the damage posed a hazard for re-entry at flight's end. The peeled-up section is smaller than one that required spacewalking repairs to Atlantis in June.
NASA is particularly attentive to the shuttle's thermal shielding, ever since Columbia was destroyed during re-entry in 2003.
The delay in installing the lab and carrying out the first spacewalk caused NASA to add a 12th day to the mission. Yet another day could be added; NASA had hoped to spend an extra day at the space station to help set up Columbus. Atlantis will remain at the orbiting complex until at least next weekend.
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