Added Curbeam: "There's definitely ammonia coming out - ice crystals all over the place."

A common refrigerant, ammonia is critical to shedding heat generated by electronics gear inside the lab. Two ammonia coolant loops circulate between the lab and radiators located near the station's American-made solar wings, which stretch 240 feet (73 meters) from tip to tip.

A colorless gas with a pungent, suffocating odor, ammonia also is deadly. The National Institute for Occupational Health and Safety considers it dangerous - and even fatal - when exposure levels are high.

Working in the deadly vacuum of space, Curbeam and Jones both were wearing protective spacesuits when the ammonia leak cropped up.

The prime concern, consequently, was the possibility that ammonia might have adhered to Curbeam's suit, presenting the possibility that residual crystals could sublimate when the astronauts reentered the shuttle, turning into poisonous gas.

Curbeam, meanwhile, was virtually certain that the leaking line showered his suit with the toxic coolant.

"Although I had it pointed away from me, I still got ammonia on myself," the rookie spacewalker told shuttle crewmates. "I can't see any crystals, but I know it sprayed this way."

As a precaution, Jones was asked to wipe down Curbeam's bulky suit with a large paint-type brush originally designed to remove icy crystals of the toxic rocket fuel hydrazine.

"Let me ask a practical question here - what if I don't see anything to brush?" Jones asked.

"That's a good question, but we would like you to brush the suit anyway," astronaut Gerhardt Thiele replied from NASA's Mission Control Center in Houston, Texas. "The crystals are fairly small on a white suit, so you might not see them."

"I'm going to brush my hands off myself, Gerhardt, because I know they were definitely exposed," Curbeam added.

Jones didn't spot any crystals coming off Curbeam's suit during the brush-down, but the situation is the very one that shuttle skipper Ken Cockrell worried about when contemplating "what-if scenarios" before the flight.

An ammonia leak, he noted in an interview with SPACE.com, would present two problems. First, icy ammonia crystals could clog a coolant line connector, preventing a spacewalker from hooking up a critical station system.

The other thorny issue?

"What do you do about this ammonia leak, and how do you get it off them before they come back in the cabin?" Cockrell asked.

"I'll be very anxious to hear the condition of those [connectors]...to see of any little sparkles of ammonia ice come floating out," he added. "That'll be my first concern."

~

Mission controllers were ready with an elaborate answer to the question: The suit brush-down was just the first of three precautionary decontamination moves ordered by flight directors.

Curbeam also was told to remain in harsh sunlight for an extended period of time in a bid to "bake-out" any residual crystals that might have been missed.

What's more, the spacewalkers waited in the shuttle's airlock while it was repressurized, then depressurized, and finally repressurized again - moves also meant to negate the possibility of poisonous gasses being released within Atlantis.

"That is to encourage evaporation of the ammonia crystals," said NASA flight commentator John Ira Petty. "But again, all this is being done as a precaution. The consensus [among flight controllers] is that few if any such crystals would remain after exposure to the vacuum of space."

Inside Atlantis, meanwhile, Cockrell, pilot Mark Polansky and mission specialist Marsha Ivins were ordered to put on protective masks, but no signs of poisonous gas were apparent the spacewalkers prepared to doff their suits on the shuttle's middeck.

"We cannot detect any contamination. The worst smell we've had the last 20 minutes has been the inside of the magic masks," Cockrell said.

"That's what we expected," Thiele replied.

The five Atlantis astronauts, in fact, stood ready to meet up with the station's resident crew to begin the activation of the lab's critical electrical power, cooling and computer systems.

If all goes well the joined shuttle-station crew will enter Destiny for the first time Sunday morning.

The contamination scare followed a textbook operation to attach the 16-ton lab to the station's American-made Unity module, a pressurized passageway to other parts of the outpost.

Wielding a Canadian-built robot arm, Ivins first moved a shuttle docking port from Unity so it could be temporarily stowed on an outpost truss. That cleared the way for Destiny, the station's first science research facility.

The veteran astronaut then hoisted the hefty lab from the shuttle's cargo bay and flipped it 180 degrees so it could be properly positioned on the end of the Unity module.

Deftly done, the job was deemed crucial to NASA's $60 billion station construction project. No further assembly would have been possible if the lab for some reason could not be put in place.

The toxic fuel leak cropped up 20 minutes later as Curbeam was trying to hook up the first of four ammonia lines outside the station. The spacewalker moved quickly to cap the leak, plugging the line into its proper receptacle.

Flight directors then had Curbeam and Jones finish hooking up nine electrical power lines while they mulled over the situation. The spacewalkers ultimately were given a green light to go back to work on the coolant lines - a job that came off without additional problems.

The subsequent decontamination measures, meanwhile, stretched a planned 6.5-hour spacewalk into a 7-hour, 34-minute excursion.

Curbeam and Jones are scheduled to make two more spacewalks during the shuttle's weeklong stay at the station, which now is occupied by U.S. astronaut Bill Shepherd and two cosmonaut colleagues - Yuri Gidzenko and Sergei Krikalev.

The spacewalkers on Monday are scheduled to move the stowed shuttle docking port to the forward end of the lab, where it will serve as a parking place for future construction crews.

The primary job on the third spacewalk - which is scheduled for Wednesday - will be to stow a spare communications antenna outside the outpost.

Atlantis is scheduled to depart the station Friday after a final farewell and hatch-closing ceremony now scheduled for 7:18 a.m. EST (12:18 GMT) that day. The shuttle and its crew are due to land at Kennedy Space Center at 12:52 p.m. EST (17:52 GMT) Sunday, Feb. 18.