It seems that crew on International Space Station will spent time before Christmas not on the wrapping presents.
NASA announced on 18 December 2015 about necessity of spacewalk performed on ISS. It is not planned EVA - after last spacewalk for repair cooling system of the ISS (which took place on 6 November, 2015 - described here), NASA claimed that it was last extravehicular activity on 2015. Unfortunately technical problems with robotic arm appeared on 16 December 2015, when arm (called also Canadarm2 - arm with rail system is called Mobile Servicing System) was controlled by operators on Earth. Problem refers to long for 108 m rail, which allows moving arm through whole length of the station with speed up to 1.5 m/s. On the rail there are points for latching arm and stabilizing it during operating. During moving on , just about 10 centimeters before latch placed in the center of the station, platform with arm stopped on rail. Attempts for moving platform to latch for secure position failed.
Arm is very important part of the ISS - it is utilized during docking of cargo spacecrafts, supports astronauts during spacewalks, allows to be used as vehicle for moving around the station. It is utilized for storing tools, accessories and instruments utilizing during EVA. Commander Scott Kelly and Flight Engineer Tim Kopra will have time until Monday or Tuesday. On 23rd December 2015 docking next Progress with cargo is scheduled (Pirs module remains free for the first time since 166 days, after Progress 60 filled with trashes separated today on 02:35 UTC and started to move toward the Earth to burn in the atmosphere). Canadarm2 must be operational to grab Progress and move it to docking port of the Pirs module, so Scott Kelly and Tim Kopra, with remaining four crew members, must start Monday spacewalk with good plan of repairing Canadarm2. NASA is predicting EVA time on Monday for 3.5 hours.