The spacewalks are being conducted to deal with an ammonia leak in one of the station's cooling systems. The spacewalks, scheduled for December 21, 23 and 25, are aimed at replacing a faulty pump module that stopped working on December 11.
The pump’s Flow Control Valve (FCV) regulates the temperature of the ammonia coolant by mixing warm ammonia with cool ammonia from the station’s radiators. During the spacewalks, the astronauts will replace the pump with a spare stowed on an external stowage platform. All three spacewalks are expected to last six and a half hours and will be televised live with a 7:10 am US EST start.