CAPE CANAVERAL, Fla. (AP) -- A rookie astronaut who was a late fill-in for NASA's current space station crew is benefiting yet again from the medical misfortunes of others.

Donald Pettit will substitute for a Russian cosmonaut during a spacewalk next week outside the international space station, NASA said Tuesday.

Pettit will venture outside Jan. 15 with the space station's commander, Kenneth Bowersox, to perform a variety of maintenance work.

Nikolai Budarin, a veteran of Russia's former Mir space station, was supposed to go out with Bowersox during the six-hour spacewalk, originally scheduled for last month. But U.S. flight surgeons opposed Budarin's involvement because of undisclosed medical concerns, and the spacewalk was delayed.

NASA spokesman Kyle Herring said the concerns are based on medical data gathered after the three men moved into the space station in November. The decision to pull Budarin from the spacewalk -- the only one planned for the men's four-month expedition -- does not affect his other duties in orbit, Herring said.

Spacewalking can be exhausting work, and astronauts and cosmonauts need to be in top physical condition.

It will be the first spacewalk not only for Pettit but also for Bowersox, a former space shuttle pilot. The two will work on the space station's newest girder, installed in November.

Pettit, 47, a chemical engineer, originally was a backup for the space station. NASA moved him ahead last summer to replace Donald Thomas, who was removed because of doctors' concerns about his radiation exposure in space.

Pettit, Bowersox and Budarin will remain aboard the space station until March. They are the sixth crew to inhabit the orbiting complex.