Replacing the 230-pound BCDU will take about three to three and a half hours. Because the activity is relatively quick—spacewalks usually span five to eight hours— the two astronauts will perform a few other tasks on their spacewalk. They will be folding back a section of the Multilayer Insulation (MLI), a protective thermal blanket surrounding the station. By folding back a section, the astronauts are going to uncover a robotic arm expansion that can make future repairs. They will also be installing a piece of hardware that supports an upcoming experimental platform that the station’s European scientists plan to establish next year.