Christmas Eve spacewalk attempts to fix faulty pump on ISS [VIDEO] Dec. 24 (UPI) -- Two NASA astronauts are performing a rare spacewalk on Christmas Eve Tuesday, attempting to fix a a faulty ammonia pump on the International Space Station. Flight engineers Rick Mastracchio and Mike Hopkins are conducting the second in a series of spacewalks to fix the faulty pump, which failed December 11. The astronauts began the space walk at 6:53 a.m. EST and should take around six and a half hours to install the new pump. NASA is live streaming the spacewalk and posting updates on Twitter. The astronauts in the first spacewalk on Saturday were able to remove the faulty pump, which is used to regulate the temperature of equipment on the space lab.