Astronauts from shuttle Endeavour have stored a 50ft (15m) inspection device at the International Space Station (ISS), for use by the next shuttle.
Saturday's spacewalk lasted six hours
The astronauts attached the robotic arm during a fifth and final spacewalk before they return to Earth.
Discovery, which is due to fly to the space station in May, will not have room to carry its own alongside the Kibo laboratory.
The boom carries instruments used to inspect the ship for damage.
Its use has become standard procedure since the Columbia accident in 2003.
Endeavour astronauts Michael Foreman and Robert Behnken spent six hours on Saturday's walk.
After fixing the Orbiter Boom Sensor System, the astronauts inspected a jammed rotating joint and attached a science experiment to Europe's Columbus lab.
The Endeavour astronauts have already tested a heat-shield tile-repair kit and installed the new Japanese Logistics Module - a storage facility for the Japan Aerospace Exploration Agency's Kibo laboratory.
The ISS has also taken delivery of the Dextre two-armed robot which will be used to do external maintenance on the platform.
Endeavour will have spent 16 days at the space station, the longest shuttle visit yet.