Careful to avoid toxic fluid leaks, two U.S. astronauts Wednesday successfully hooked up coolant lines at the international space station during the first of three spacewalks over the next nine days.

Michael Lopez-Alegria and Sunita Williams removed a series of thermal covers, known as "booties," that covered the connections and used some elbow grease to undo some stubborn valves connecting the ammonia lines.

"The booty integrity is bootylicious," Lopez-Alegria said in a light moment during the tedious work in a tight corner outside the space station.

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Ammonia was a big concern since the toxic substance leaked out of a cooling line in 2001 when astronaut Robert Curbeam was performing a similar task.

Lopez-Alegria and Williams did not face any of the same problems as Curbeam did with ammonia, but had crib sheets attached to the cuffs of their spacesuits with instructions on what to do should they come in contact with the toxic substance.

Those steps included using heat from tools or the sun to bake off ammonia on a spacesuit and flushing the air out of the airlock to prevent contamination when they came back into the station.

The astronauts will perform identical tasks during their second spacewalk set for Sunday. The astronauts will take a third U.S. spacewalk together on Thursday to jettison thermal blankets.

U.S. astronauts have never attempted three spacewalks in such a short time without a space shuttle docked at the station.

If the spacewalks go as planned, Lopez-Alegria will have two weeks to rest before going on a fourth spacewalk set for Feb. 22.

During Wednesday's spacewalk 220 miles above Earth, the astronauts switched coolant lines from a temporary cooling system to a permanent one.

Lopez-Alegria also made electrical connections for a new system that will allow power from the station to be shared with a docked shuttle so that the shuttles can stay longer at the space lab.

At the start of the spacewalk, Lopez-Alegria was impressed with the weather he saw around Earth 220 miles below.

"Beautiful," he said. "Just a little bit of a circular storm down there with the clouds."

It's not uncommon for U.S. astronauts to conduct three spacewalks over a short period of time during a visit by a space shuttle to the station — four were carried out during Discovery's trip in December.

The extra astronauts are handy for helping the spacewalkers prepare their spacesuits and the airlock, and for operating robotic arms on the station and shuttle to provide camera views or move the spacewalkers.

Conducting a spacewalk with just a three-man crew at the space station is a much tougher task. The difficulty is magnified when trying three in such a short period of time — a major reason it has never been done before.

Unlike spacewalks with a docked shuttle, the coordinator during the spacewalk was at Mission Control in Houston instead of at the space station.

NASA officials said they scheduled the spacewalks so closely together because the first two are almost identical in nature, and time and energy could be saved on preparation.

Timing also played a role, as the U.S. spacewalks were scheduled to take place between the last shuttle visit in December — when the cooling system was turned on — and the next scheduled visit in mid-March.

Lopez-Alegria and Williams will have three days to rest between each spacewalk, although Mission Control could give them more time if they need it.

"These two crew members are extremely fit and we don't think there is any reason for concern that they would have undue physical exhaustion," Laws said.

Because Lopez-Alegria has been at the space station since September, the spacewalkers haven't trained together in the practice pool at the Johnson Space Center since last July. So Mission Control has been giving them extra time to study up and practice computer simulations.

"It's a refresher in every sense of the word in what they last practiced in Houston," said Derek Hassmann, spacewalk flight director.