Depressurization and smoke alarms woke the shuttle and station crews for a second consecutive night on Friday. Flight controllers quickly determined they were false alarms, but the station’s ventilation system automatically shut down, prompting NASA to cancel spacewalk preparations inside a low-pressure chamber on the station.

Astronauts Michael Foreman and Randy Bresnik were camping out in the station’s Quest airlock to purge their bodies from nitrogen in advance of Saturday’s spacewalk, a planned 6.5-hour outing to install antennas and a cargo mount to the outside of the station. When the fans shut down, NASA decided that it would take too much time to restart the nitrogen purge by lowering pressure in the airlock and switched to a backup method. Before donning their spacesuits, Foreman and Bresnik will exercise vigorously on stationary bikes while breathing pure oxygen. Purging nitrogen from their bodies will prevent a dangerous condition known as the bends.

NASA also planned to clip about 30 minutes from the spacewalk, the second of three planned during Atlantis’ week-long stay at the station. The spacewalk also was to be delayed by about an hour.

Atlantis arrived at the station on Wednesday with 30,000 pounds of spare pumps, gyroscopes, tanks and other gear too big to ride on other spacecraft. NASA is stocking the outpost in hopes of keeping it operational for five- to 10 years or longer after the shuttles’ retirement next year. Russian, European and Japanese cargo ships will continue to keep the station supplied with food, fuel and water. Crewmembers will fly to and from the outpost on Russian Soyuz capsules. 

Atlantis is due back at the Kennedy Space Center on Friday.