An artist's concept of the Unity and Zarya modules linked together -- the current state of the International Space Station. Photo: NASA

As space shuttle Endeavour nears its homecoming on Tuesday, NASA officials have formally approved plans to split the next mission to the International Space Station into two voyages of Atlantis.

The U.S. space agency made the decision last week in the wake of continued Russian delays in launching the next major piece of the $60 billion outpost -- the Zvezda service module that will serve as the initial crew living quarters. Russian officials say Zvezda should be launched between July 8 and 14 from the Baikonur Cosmodrome in Kazakhstan.

Atlantis' upcoming visit to the station was originally scheduled to take place after Zvezda was launched atop a Proton rocket and docked to ISS. The shuttle and crew would arrive shortly thereafter to outfit Zvezda for the first permanent station crew.

Under the new game plan Atlantis will still launch around April 13, but about three months ahead of Zvezda. The early visit will allow astronauts to perform maintenance on the station segments already orbiting Earth.

Veteran shuttle commander Jim Halsell will lead the STS-101 mission, also known as ISS flight 2A.2a. The other crewmembers already assigned to the flight include pilot Scott Horowitz and mission specialists Mary Ellen Weber and Jeffrey Williams.

The rest of the shuttle fliers will be comprised of Jim Voss, Susan Helms and cosmonaut Yuri Usachev. The trio later will serve as the second permanent ISS crew.

Three astronauts originally slated to fly on STS-101 -- Ed Lu and Russians Yuri Malenchenko and Boris Marukov -- have been bumped to the newly created STS-106 mission that will launch after Zvezda links up with ISS. STS-106 also is known as ISS mission 2A.2b.

Joining those three men on Atlantis' planned August 19 launch will be commander Terry Wilcutt, pilot Scott Altman and mission specialists Richard Mastracchio and Dan Burbank.

The main job of STS-101 will be servicing the Zarya module -- the first ISS element launched in November 1998. The work should extend Zarya's certified life to the end of 2000. The astronauts will also act as repairmen to replace a failed battery and install electronics to fix a second battery to restore Zarya's power system to proper working order.

Astronauts Jerry Ross and Jim Newman make a spacewalk outside ISS in December 1998. Photo: NASA

One spacewalk is planned for STS-101 by Williams and Voss. The excursion will prepare the station for arrival of Zvezda and attach the rest of the Strela crane to ISS. The first part was taken to the station during the last shuttle visit last May.

The other time docked to the station will be spent transferring equipment and supplies from the shuttle to ISS.

STS-106 will feature at least one, and possibly two spacewalks depending on how much work is completed on STS-101, to connect cables between Zvezda and the Zarya modules of ISS and move the Strela crane to its final location on the station's exterior. The spacewalkers also will complete "get-ahead" work to free up future missions.

The astronauts will transfer more supplies carried into space aboard Atlantis to outfit the station in preparation for the arrival of the first long-duration crew later in the year.

Beyond this pair of missions, NASA expects to fly two more shuttles to ISS in late 2000. Discovery's STS-92 mission will deliver the Z-1 truss and another docking port to the station in September; Endeavour will bring the first batch of giant solar arrays to ISS in November.