Mission scientists conducted the test to see if astronauts spending time in lower air pressure could adapt more quickly to conditions inside their spacesuits, which is equivalent to venturing suddenly into higher altitudes on Earth.

To prevent altitude sickness, climbers typically spend time at intermediate altitudes to purge nitrogen from their bloodstreams, which in excess amounts can bring on the painful condition known to deep sea divers as "the bends."

On the ISS, the standard procedure had been for crew members to breathe pure oxygen prior to donning spacesuits. The astronauts slept in the airlock to see if the process could be simplified and accelerated.

Although the experiment had been planned to last nearly three more hours, when the alarm sounded controllers decided to move the two U.S. astronauts back to the normal-pressure portion of the station as a precaution. McArthur and Williams completed their scheduled sleep periods there. So far, controllers have not determined why the alarm sounded.

"We did collect a lot of information about the test," Kylie Clem, a spokeswoman at NASA's Johnson Space Center, told SpaceDaily.com, but she added that mission scientists will discuss whether to repeat the pressurization test, or whether the data collected was sufficient to validate the experiment.

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