By Irene Brown

When Jim Halsell and his crew started training for a mission to the International Space Station, they expected to be setting up equipment and putting away supplies for the first live-aboard residents. Instead, the shuttle team is making a maintenance call to nurse along the fledging outpost until its core module arrives later this summer.

"The new mission is designed basically to extend the life of the space station," said Halsell, who will command NASA's third station-servicing flight.

Not only was Halsell's mission changed three months before the scheduled launch date, but three of his crew members were pulled from the flight and re-assigned to a later mission.

"With the exception of maybe a marriage and family, I don't think there are any closer bonds than those of a shuttle crew," said Halsell, in a preflight interview. "It was certainly disappointing to have them removed. From a psychological point of view, it was a difficult transition to make."

NASA astronaut Ed Lu and Russian cosmonauts Yuri Malenchenko and Boris Morukov are now in training for a shuttle mission slated to fly in August after the station's key component — the Russian-built service module — arrives. Taking their seats aboard Atlantis' upcoming flight are veteran astronauts Susan Helms, James Voss and cosmonaut Yuri Usachev — a trio already preparing to become the second resident space station crew.

The station's most serious ailment is the electrical system in Zarya, the station's foundation. The Russian-built propulsion and power module and an American-made connecting node called Unity, both launched in 1998, are the only two segments of the station in orbit.

The shuttle crew plans to replace four of Zarya's six batteries, along with equipment to charge and discharge the units. They also will install new fire extinguishers and smoke detectors, two of which have failed. Much of the gear is approaching or past its design lifetime.

Before the astronauts open the hatches to begin work inside the space station, Voss and crewmate Jeff Williams are scheduled for a 6½-hour spacewalk to reattach a crane on the outside of the station's hull. The tool, which was delivered a year ago by the last shuttle crew to visit the station, is not properly secured.

Once inside the station, the crew faces a long to-do list. In addition to work on Zarya's batteries, the astronauts will pull out panels and install aluminum boxes that can be used to stow gear. They also plan to replace part of a communications system located in Unity.

Another problem the crew plans to address is Zarya's poor ventilation, which left the last shuttle crew ill.

"We don't have a definitive cause," said station program manager Tommy Holloway, adding that the crew will take steps to improve air circulation and work on ducts throughout the station.

The astronauts are to take air samples for return and analysis on Earth and install fans, carbon dioxide removers and other devices.

"We have the tools in place should the problem reoccur on this flight," said Holloway.

While they work inside Zarya, the astronauts will be wearing headsets and earplugs to protect their hearing. The module is somewhat noisy, about 55 decibels. That's about the same level of noise in a busy office with typewriters. For comparison, a jackhammer registers about 100 decibels.

Work inside the station should take four days, although managers are trying to manage fuel supplies so the astronauts could stay for a fifth day if needed. Before departing the station, the shuttle will boost the outpost to a higher orbit. The station has been losing about 1 to 1½ miles of altitude per week due to increased solar activity, which expands Earth's atmosphere and creates more resistance for spacecraft in low-Earth orbits.