James Hartfield said that flight control at the Johnson Space Center in Houston concluded that the threat posed by Hurricane Dean, currently looming off the U.S. mainland, could force the shuttle to return a day early.
As a result, Saturday's spacewalk will last no longer than four and a half hours to allow for leeway in the landing schedule.
Hurricane Dean is currently in the Caribbean Sea and is expected to make landfall in the Yucatan Peninsula Wednesday, the same day Endeavour was initially scheduled to complete its 14-day mission to the International Space Station (ISS).
Should the storm hit the Texas coast, the space center would be unable to continue operating, and flight controllers would have to be evacuated to the Kennedy Space Center.
"We'd really like to protect an option to end the mission on Tuesday," said NASA mission management chair LeRoy Cain late Friday.
The curtailed spacewalk may be the least of NASA's worries concerning STS-118.
A gash in Endeavour's protective heat tiles, caused when a piece of insulating foam broke loose on takeoff, set off a scramble at mission control to determine whether in-flight repairs would be needed or whether the shuttle could survive the heat of re-entry as is.
A similar gash in the heat tiles on Space Shuttle Columbia in 2003 allowed super-heated gases to enter the wing on re-entry, and the vehicle disintegrated over Texas, killing its seven-member crew.
If flight control decides to end the mission early, the shuttle would have to undock from the ISS Sunday for a Tuesday landing.
