In what amounted to a dramatic contamination scare, Curbeam found himself in a cloud of frozen ammonia Saturday after shuttle robot arm operator Marsha Ivins pulled off the tricky installation of the $1.4 billion U.S. Destiny lab at the station.

And in a space-to-ground interview Tuesday, Curbeam said the startling spray of poisonous refrigerant raised the possibility that a crucial station construction mission might have been taking a serious turn for the worse.

"Let's just say it got my attention," Curbeam said just before he and crewmate Tom Jones started preparing for another spacewalk that will begin at 10:18 a.m. EST (15:18 GMT) Wednesday.

His first thought: That the crew of Atlantis might be unable to carry out the time-critical activation of Destiny's cooling system, which uses ammonia to keep sensitive computer and electronics gear within the lab at proper operating temperatures.

In that case, emergency action would have been needed to keep the station's first science lab thermally conditioned until the all-important cooling system could be rigged up and put into operation.

The next thing to cross his mind: The possibility that Ivins would have to stow the 16-ton lab back in the shuttle's cargo bay and carry out the precision installation - which called for the hefty science center to be flipped 180 degrees - a second time.

"Of course, my first reaction was: "Are we going to have to try to do something with this lab to keep it cool overnight?" Curbeam said. "And two, is Marsha going to be upset because she's going to have to put this thing back in the payload bay?"

A common refrigerant, ammonia is a colorless gas with a pungent, suffocating odor that can be a severe irritant to the eyes, nose, throat and lungs. The National Institute for Occupational Health and Safety considers it dangerous - and even fatal - when exposure levels are high.

Curbeam was sprayed as he tried to hook up one of the lab's four coolant lines, and the resulting snowstorm of crystals prompted an unprecedented NASA decontamination effort.

The reason: Any ammonia crystals that adhered to his protective spacesuit might have sublimated when Curbeam reentered the shuttle, turning into poisonous gas.

"I really didn't have too many concerns about personal safety, because the suit is extremely protective," Curbeam said. "The people I was really worried about were the people in [Atlantis] because I didn't want to bring any of it back in and hurt anybody else. So that was probably my biggest concern."

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In a bid to get rid of any stuck crystals, mission managers had Jones wipe down Curbeam's suit with a large paint-type brush. Curbeam also was told to remain in harsh sunlight for an extended period of time to "bake out" any crystals that might have been missed.

"We'd been briefed enough about ammonia that I knew that it should sublimate off fairly quickly," Curbeam said. "So I knew I had plenty of time to bake out, sit in the sun, and hopefully all the ammonia would be gone."

As an extra precaution, though, the spacewalkers were told to sit in Atlantis' airlock while humid air was cycled through it so that any toxic gas would be whisked into the shuttle's atmospheric scrubbers.

What's more, the astronauts on the shuttle's flight deck donned protective rubber gas marks until an all clear was given.

"Luckily it all worked out," Curbeam said.

With the contamination scare and a second spacewalk now history, Curbeam and Jones aim to stow a spare communications antenna on the exterior of the international outpost during a planned five-hour spacewalk Wednesday.

The pair also plans to carry out a crisis management drill to see if a spacewalker could haul an incapacitated crewmate back into the shuttle's airlock for medical attention in an emergency.

And they'll scale to the upper levels of the 17-story station to take photographs of a metal latching bar that failed to snap firmly in place when the outpost's American-made solar wings were deployed during a December shuttle mission.

The excursion will follow a relatively relaxing day for the shuttle astronauts, who took a half-day off Tuesday before hauling the outpost into a higher orbit.

Firing the shuttle's nose-and-tail steering thrusters, Atlantis commander Ken Cockrell and pilot Mark Polansky boosted the 112-ton station up to an orbit some 230 miles (368 kilometers) above Earth - an increase in altitude of about four miles (6.4 kilometers).

Now larger in terms of habitable space than NASA's 1970s Skylab complex or Russia's Mir station, the international outpost dips earthward due to atmospheric drag and periodically is reboosted to keep it in its intended operational orbit.

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Station skipper Bill Shepherd, meanwhile, wrestled with a balky carbon dioxide scrubber housed within Destiny's refrigerator-sized air revitalization system.

The scrubber failed to operate properly after its activation. Ground controllers, consequently, shut the machine down and had Shepherd disconnect a vacuum hose and various power and data cables while engineers began troubleshooting the problem.

Scrubbers within the station's Russian-made crew quarters are being used to remove carbon dioxide from the outpost's atmosphere.

"There's no real rush to come to a resolution on this problem," NASA station flight director Andy Algate said. "The normal CO2 removal system for this timeframe is up and operating in the Russian segment - just as it was prior to the lab docking."

Flight directors were more concerned with passing yet another key milestone in the operation of the station.

Four American-made gyroscopes took over control of the station for the first time early Tuesday - a move made possible by the activation of associated computers in the Destiny lab.

Delivered by a visiting shuttle crew last October, the domed-shaped devices will greatly reduce the amount of rocket propellant needed to keep the station properly positioned in orbit.

Fewer thruster firings also will reduce station vibrations that could spoil future science experiments that require a quiescent environment.

"We've reached another benchmark," Mission Control told Shepherd after the gyroscopes were spun up and placed in operation.

"Another step for the Federation," the station skipper replied with a reference to the TV show Star Trek.

Atlantis remains scheduled to depart the station Friday after a final farewell and hatch-closing ceremony set 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.