About the size of a city bus, the crew module is equipped with small thrusters that are used to help keep the station properly oriented as it circles high above the planet.

Powered by toxic rocket fuel, the thrusters spew out exhaust when fired, leaving behind hazardous residues that could adhere to a spacewalker's spacesuit.

The concern is that the soot-like substance could inadvertently be brought back inside the station where it could sublimate, potentially exposing crews to noxious fumes.

"Anytime you're dealing with fuels or oxidizers or materials like that, you're always concerned about bringing it back into the station on a spacesuit," said Daryl Schuck, an engineer in NASA's spacewalk projects office at Johnson Space Center in Houston.

What's more, exhaust from the steering thrusters could foul sensitive external science experiments, spread contaminants on outpost windows or discolor the outer hull of the complex.

Consequently, Russian space officials have called upon Onufrienko and Bursch to install deflectors on each of six attitude control thrusters that ring the rear end of the Russian-built Zvezda, or Star, crew module.

"It's in their best interest to try to limit the amount of spread of this residue such that it doesn't get tracked around and isn't brought into the crew's environment inside," Schuck said.

Custom-made to fit around the thrusters, the metal shields are designed to either contain exhaust residue or deflect the black-and-brown substance out into open space, Schuck said.

Special precautions will be taken by the spacewalkers to make certain that none of the residue will make its way back into the station.

Inside the station, the residue could create fishy fumes that potentially would be an eye irritant. Station air scrubbers are capable of ridding the outpost of any noxious gas, and special clean-up procedures already have been put in place. But Onufrienko and Busch nevertheless will towel their suits off prior to reentering the complex.

The installation of the deflectors is just one of several chores the pair aims to complete during the planned 5-hour, 44-minute excursion.

A Russian contamination monitor that has been collecting thruster exhaust residue since November will be swapped out, and an external science package will be set up to gather evidence of natural low-energy heavy nuclei of solar or galactic origin.

A second amateur radio antenna also will be hooked up outside the crew module. An initial antenna was installed during a spacewalk earlier this month, and a total of three more originally were to have been mounted during Friday's sortie.

The installation of the final two, however, is being deferred until later this year because Russian planners determined that Onufrienko and Bursch would not have enough time to complete that work along with their other tasks.

The spacewalk will be the second of three that will be carried out by the current outpost crew.

Tentatively set for Feb. 20, the third will be staged from the $164 million U.S. Quest airlock by Bursch and Walz. One of the prime goals: Carrying out a dress rehearsal for a quartet of spacewalks to be conducted out of the same airlock -- which has only been used once to date -- by a visiting shuttle crew in April.

The two NASA astronauts also will perform some preparatory work for the April excursions, which will be key to the delivery and installation of the central segment of a station truss that eventually will extend 356 feet (108 meters) from end to end.

Once completed, the truss will serve as a mounting platform for four U.S. electrical power towers. It also will be equipped with attachment points for European and Japanese science labs that are to be hauled up to the station around 2004 or 2005.

Launched Dec. 5 aboard shuttle Endeavour, the current station crew originally had planned to carry out four spacewalks, the last two of which were to have been performed after shuttle Atlantis departs the station in mid-April.

Astronauts on that shuttle flight, however, now will carry out the work that had been planned for one of those sorties, and the other tasks will be carried out during the Feb. 20 walk.

Onufrienko, Bursch and Walz remain scheduled to return to Earth in mid-May aboard shuttle Endeavour.