Cosmonaut Sergei Krikalev works inside the International Space Station in December 1998. Photo: NASA

NASA originally planned to launch Atlantis on space station assembly mission 2A.2 after arrival of a critical Russian component called the service module. Similar to the core module of the Mir space station, the service module will provide the international space station's primary propulsion and its initial crew quarters.

The original 2A.2 mission called for Atlantis's crew to outfit the new module with various pieces of equipment and to deliver supplies for the first full-time station astronauts, scheduled to arrive later this fall.

But Russian funding problems and recent Proton rocket failures have delayed launch of the service module to mid July. Until it gets there, the Russian-built, NASA-financed Zarya must provide the station's electrical power and the propulsion needed to keep it at a safe altitude.

But Zarya has had its own problems in recent months. The performance of four of its six batteries, which provide power when the station is in Earth's shadow, has degraded to unacceptable levels. Aviation Week & Space Technology magazine reported in its April 17 issue that battery performance was affected by the use of improper recharging and maintenance procedures that were mistakenly implemented by Russian ground controllers.

Zarya also has unexpected environmental problems. The crew of the last shuttle to visit the space station a year ago reported high CO2 levels and unacceptably loud -- 75 db -- background noise from fans and other systems.

The easily refuelable service module will take over much of the load from Zarya once it is attached to the station.

An artist's concept of the Zvezda service module docked to the International Space Station (left most module). Photo: NASA

But given the service module's on-going delays and the on-going degradation of Zarya's electrical system, NASA managers decided in mid February to break Atlantis's 2A.2 service module outfitting mission into two flights.

The first, now known as mission 2A.2a, is dedicated to repairing and refurbishing Zarya. By replacing the batteries and associated electronic gear, Krunichev, the Russian company that built the module, will warranty the spacecraft through the end of the year.

"Is it important? Yes. Could the vehicle make it until August when we expect the arrival of the service module? Yes, probably. But it's a smart thing to go on up there and visit the vehicle now," said Atlantis skipper James Halsell.

The second flight, also aboard Atlantis and known as assembly mission 2A.2b, will be similar to the original 2A.2 service module outfitting flight. It is scheduled for launch one month after the service module gets off the ground.