CAPE CANAVERAL, Fla. - Four or five flakes of toxic ammonia dripped from a cooling-line cap yesterday but apparently did not touch two U.S. astronauts conducting the first of three space walks planned outside the International Space Station over nine days.

The leak occurred late in the walk, as astronauts Michael Lopez-Alegria and Sunita Williams disconnected and prepared to stow away two fluid lines that had been connected to an ammonia reservoir outside the station.

Ammonia, which can cause contamination, was a big concern since the toxic substance leaked out of a cooling line onto astronaut Robert Curbeam's spacesuit when he performed a similar task in 2001.

Mission Control told the spacewalkers they would have to remain in their spacesuits once they entered the station's airlock to make sure no ammonia was on their suit that could contaminate the orbiting lab.

"We don't think we have contamination, but that's probably for the lawyers," Lopez-Alegria said.

Replied Mission Control: "We're going to follow the path of safety."

During yesterday's space walk 220 miles above Earth, the pair successfully switched coolant lines from a temporary cooling system to a permanent one and secured a thermal cover around an obsolete radiator that Mission Control retracted by remote control. Lopez-Alegria made electrical connections for a new system that will allow power from the station to be shared with a docked shuttle.

The astronauts will perform identical tasks during a second space walk Sunday. They will take a third walk together next Thursday to jettison thermal blankets.

U.S. astronauts have never tried three space walks in such a short time without a space shuttle docked at the station. If the space walks go as planned, Lopez-Alegria will have two weeks to rest before going on a fourth space walk Feb. 22.

See and read more from NASA on the space mission via http:// go.philly.com/spacestation EndText