(CNN) -- Astronauts have begun a spacewalk to fix a broken part of the International Space Station's cooling system, NASA said.
The seven-hour spacewalk focuses on removing and replacing an ammonia pump that failed a week ago, setting off warning alarms and shutting down part of the cooling system.
The pump module weighs 780 pounds and is 5.5 feet long, 4 feet wide and 3 feet tall, NASA said.
A second spacewalk is planned Wednesday to connect the fluid ammonia lines, NASA said.
Two astronauts are participating in Saturday's spacewalk. It will be the first for NASA astronaut Tracy Caldwell Dyson.
Six people -- three Americans and three Russians -- are on the International Space Station.
The station's crew members are conducting more than 100 ongoing experiments in biology, physical sciences, technology development, and Earth and space sciences, according to NASA.
NASA says that without controls, the temperature of the orbiting station's sun-facing side would soar to 250 degrees (121 degrees Celsius), while thermometers on the dark side would plunge to minus 250 degrees Fahrenheit (-157 degrees Celsius).