Two astronauts made the first spacewalk on Sunday outside China’s new orbital station to work on setting up a 15-metre-long robotic arm.

Liu Boming and Tang Hongbo were shown by state television climbing out of the airlock as Earth rolled past below them. The third crew member, commander Nie Haisheng, stayed inside.

The astronauts arrived at the orbital station on June 17 for a three-month mission aboard China’s third orbital station, part of an ambitious space programme that landed a robot rover on Mars in May. Their mission comes as the ruling Communist Party celebrates the 100th anniversary of its founding.

The station’s first module, Tianhe, or Heavenly Harmony, was launched on April 29. That was followed by an automated spacecraft with food and fuel. Liu, Nie and Tang arrived on June 17 aboard a Shenzhou capsule.

On Sunday, Liu and Tang were completing the installation of a robotic arm that will be used to assemble the rest of the station, according to state media. State televison said their spacesuits are designed to allow them to work in the vacuum of space for up to six hours if needed.

The space agency plans a total of 11 launches through the end of next year to add two more modules to the 70-ton station.

Liu is a veteran of the Shenzhou 7 mission in 2008, during which Zhai Zhigang made China’s first spacewalk. Nie is on his third trip into space while Liu is making his first. All are military pilots.