Two Atlantis astronauts have ventured out on the third and final space walk of their shuttle mission to install an external oxygen tank at the International Space Station.

On Monday, the astronauts removed the tank from a newly delivered cargo platform on the station, then used a robotic arm to maneuver it into place. The tank was attached to the NASA air lock, a chamber leading out into the vacuum of space. It was filled with high-pressure oxygen for future spacewalks.

Atlantis and its crew of seven will depart the space station on Wednesday. The shuttle is due to land back at Kennedy Space Center on Friday.