Crew members aboard the International Space Station have returned to the American segment they were forced evacuate early Wednesday due to fear of a possible ammonia leak.
NASA astronauts Barry Wilmore, commander of the current mission, and Terry Virts, a flight engineer, wore black and pink protective masks as they opened the hatch and tested the air inside the cabin.
The tests turned up "no indications of any ammonia," according to NASA.
Ground crews became concerned that the deadly gas may have escaped from the heat exchange system that helps cool the space station about 1 a.m. PST. Mike Suffredini, NASA's program manager for the ISS, said the water level in one of the coolant loops was "off scale" – which could have been a sign of an ammonia leak.
That prompted the first of two evacuations from the U.S. segment of the space station, which has the ammonia system mounted on its exterior , to the Russian segment, which includes the service module.
The astronauts returned after getting an all-clear, but then the ground crew detected a change in cabin pressure.
"If you're leaking ammonia into the water loop and it eventually finds its way into the cabin, then you would expect the cabin pressure to go up," Suffredini said. The evacuation procedure was then repeated.
It now appears that the pressure rose as a result of various actions taken during the first evacuation, not because of any leak, Suffredini said.
The initial problem was traced to a faulty card inside one of the space station's computer relay systems. After rebooting the computer equipment in question, the error message cleared and the relay box returned to "good operating condition," NASA said.
Mission managers at Johnson Space Center in Houston had informed the crew several hours earlier that the ammonia system was probably fine.
"Everything's looking pretty normal right now," spacecraft communicator James Kelly told Wilmore, who has been in command of the space station since November. "It's becoming a stronger case that this was a false indication, which is great news."
"Outstanding news," Wilmore responded. "We'll be ready to do whatever you need us to do when the time comes."
The returning astronauts were surely welcomed by the colony of fruit flies that was left behind in the lab.
The insects were scheduled to be fed on Wednesday, Suffredini said, but the evacuations forced a change in plans.
"We have to have healthy fruit flies," he said.
The flies, members of the species Drosophila melanogaster, joined the ISS in September and are scheduled to remain in orbit until March. Researchers led by Sharmila Bhattacharya of the NASA Ames Research Center in Moffett Field, Calif., are using the flies to learn more about the ways that astronauts may become more vulnerable to disease-causing pathogens in space.