NASA Flight Engineers Rick Mastracchio and Mike Hopkins are working outside the orbiting International Space Station today (Dec. 21) to fix a bad pump.

They are doing the first in a series of spacewalks to replace a degraded ammonia pump module with one of the station's two external cooling loops that keeps both internal and external equipment cool, according to NASA.

During the spacewalk, which began at 7:01 a.m. EST and is to last about six and a half hours, the pair are to disconnect from the pump module from the station’s external thermal control system to prepare it for its removal and replacement during a second spacewalk on Monday (Dec. 23). If necessary, a third spacewalk is slated for Christmas day (Dec. 25) to finish the installation.

Hopkins is working at the center of the station's Starboard 1 truss segment while Mastacchio is attached to a foot restraint at the end of the station’s 57-foot robotic arm with Japanese Flight Engineer Koichi Wakata, the robotics operator for the spacewalks, flying Mastracchio to the site.

Mastracchio and Hopkins received help on the spacewalk procedures from NASA astronauts Doug Wheelock and Tracy Caldwell-Dyson, who replaced the ammonia pump at the same location during three spacewalks in August 2010. Earlier this week, Wheelock and Caldwell-Dyson spent time in a partial gravity simulator at the Johnson Space Center in Houston to create a training video.

This is Mastracchio's seventh spacewalk and Hopkins' first on the 175th spacewalk mission for the International Space Station. A Waterbury, Conn., native, Mastracchio, 53, is on his second visit to the space station and has also been on two Space Shuttle missions. Formerly of Lake of the Ozarks, Mo., and a University of Illinois graduate who's also a colonel in the U.S. Air Force, Hopkins, 44, is on his first space station mission.

Three Russian flight engineers, Mikhail Tyurin, Sergey Ryazansky and Oleg Kotov, are also part of the six-member space station crew which is on its current mission from November through March 2014.

The space station, including its large solar arrays, spans the area of a football field and weighs 924,739 pounds (462 tons). The complex has more livable space than a six-bedroom house, with two bathrooms, a gymnasium and a 360-degree bay window. The station has been in orbit with human occupation continuously since Nov. 2, 2000, according to NASA.

More details about the spacewalks to fix the pump module from NASA:

Watch the space walk live: