A planned spacewalk Thursday by new space station commander Kenneth Bowersox and flight engineer Nikolai Budarin has been postponed until January because of a potential medical issue involving one of the lab's crew members. A news conference to discuss the goals of the spacewalk, the 50th devoted to station construction and the 25th to be staged without a shuttle present, was called off Monday evening.

Bowersox, Budarin and Expedition 6 science officer Donald Pettit arrived at the space station Nov. 25 after launch aboard the shuttle Endeavour. They replaced Expedition 5 commander Valery Korzun, Sergei Treschev and science officer Peggy Whitson, who returned to Earth aboard Endeavour on Dec. 7.

During Endeavour's mission, a new solar array truss segment was attached to the lab complex during three spacewalks by shuttle astronauts John Herrington and Michael Lopez-Alegria. Bowersox and Budarin planned to stage a spacewalk Thursday to finish readying the truss for ongoing assembly work next year.

A NASA spokeswoman at the Johnson Space Center in Houston said today the spacewalk was called off after the "potential of a medical issue" cropped up during routine health checks in orbit. While no details were provided, she said it can take a while for crew members to fully adapt to the weightless environment of space.

Because there is no real hurry to conduct this spacewalk - and because the unidentified medical issue is not considered serious - mission managers simply decided to postpone it until after the first of the year.

NASA officials, citing medical privacy issues, do not discuss details of an astronaut's health or even identify the astronaut in question unless the problem poses an impact to a mission. In this case, the spokeswoman said, there was no pressing need to conduct the spacewalk on time and no major mission impact is anticipated.