The astronauts onboard the International Space Station spent Sunday preparing for the first spacewalk, which was rescheduled for Monday at 9:35 a.m. EST, due to unidentified health issues with German astronaut Hans Schlegel.

Schlegel was replaced by Stanley Love, who together with Rex Walheim, will perform tasks to prepare the European Space Agency’s Columbus laboratory for installation on the station. Included in these tasks is installing the Power Data Grapple Fixture on Columbus, which will allow the station’s robotic arm to grab the laboratory module and move it from space shuttle Atlantis’ payload bay to the space station’s Harmony module.

In addition, the spacewalkers will detach nitrogen lines and otherwise begin work to remove the Nitrogen Tank Assembly, a part of the station’s thermal control system, from the P1 truss. The assembly needs to be replaced because the nitrogen is running low.

Schlegel and Pilot Alan Poindexter will coordinate the spacewalk activities from inside the orbiting complex, NASA announced.

It is possible that Schelgel will join Rex Walheim in the second spacewalk. However, NASA refused to disclose any details about Schelgel problems.

The European Space Agency said on its website that Schelgel’s condition is “not life- or mission-threatening in any way, but that could affect his efficiency during a spacewalk”.

NASA also said the STS-122 mission was extended one day with a landing planned for the morning of Tuesday, February 19.