Two veteran spacewalkers floated outside the International Space Station on Monday to prepare the orbital outpost for a new module and beef up its living quarters against micrometeorite and debris impacts, officials said.

Dressed in spacesuits, station commander Gennady Padalka and flight engineer Yuri Malenchenko opened the hatch on the station’s Pirs airlock at 11:37 a.m. EDT (1537 GMT) to begin a 6-1/2 hour spacewalk to relocate a construction crane, install debris shields and release a small satellite into orbit.

The spacewalk was delayed about an hour while engineers assessed a leak or leaks between Pirs and Russian segments of the station, a $100 billion laboratory for microgravity experiments and technology testing that flies about 250 miles above Earth.

“It is difficult to determine which compartment is leaking,” a flight controller at the Russian mission control center near Moscow radioed to the cosmonauts, who suited up for their spacewalk at 8:30 a.m. EDT (1230 GMT).

“We’re in no rush,” replied Padalka, a veteran of eight previous spacewalks.

Engineers monitored pressure readings in the modules for several minutes before clearing the cosmonauts to proceed with the spacewalk.

Padalka and Malenchenko, who has made four previous spacewalks, plan to move a hand-operated crane, called Strela-2, from the outside of the Pirs docking module to Zarya, the cornerstone of the station. The expandable 46-foot (14-meter) boom will be needed next year to install a new Russia module to the station, a project of 15 countries.

The United States completed construction of its part of the outpost last year and retired its three space shuttles.

The cosmonauts also plan to release a 20-pound (nine-kg) spherical satellite and install five debris shields to the outside of the Zvezda module, which serves as the crew’s primary living compartment.