Gerstenmaier said station managers prepared for a scenario in which there would be no shuttle flights this year. Despite that, he said the grounding of future shuttle missions opens the station to risk.
"We are still susceptible to some large failure of some component that can only be delivered by the shuttle that we don't have a like spare on station," he said.
Among the tasks Discovery's astronauts performed for the station was the reconfiguring of a gyroscope, which failed in March. Four gyroscopes, each weighing 660 pounds, are intended to steer the station, but only two have been working in recent months.
On Monday, during their second spacewalk, Discovery astronauts Stephen Robinson and Soichi Noguchi, planned to replace the other failed gyroscope, which has not worked since 2002.
The pair planned to go over procedures for installing the gyroscope on Sunday with astronaut Andrew Thomas, who was set to direct Monday's replacement.
As they began spacewalk preparations Sunday, Robinson informed Mission Control's Stephen Frick that things might proceed slower than anticipated.
"If you saw what our mid-deck looks like right now, you'd see why," Robinson said as his colleagues transferred items between the shuttle and station. "It's a high traffic zone."
"I'm afraid it would be a little too frightening," Frick responded with a chuckle.
