Cape Canaveral, Florida - Two shuttle Atlantis astronauts were due to start the second spacewalk later Saturday of their ongoing mission to the orbiting International Space Station.

Saturday's tasks for crew members Michael Foreman and Randolph Bresnik outside the shuttle include installing part of a project to monitor ships at sea, and a separate device to measure the electric charge on the space station in its interaction with plasma in orbit.

With only five more flights after the current mission before US space agency NASA is scheduled to retire the shuttle fleet, the shuttle is hauling the most crucial spare parts for future needs that cannot be delivered by Russia's smaller, Soyuz capsules.

Saturday's slated six and a half hours of work includes the installation of the second of two platforms, so-called Express Logistic Carriers, holding spare parts on the outside of the space station.

The third, final spacewalk is scheduled on Monday to install oxygen tanks outside the space station. (dpa)