(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. CNN
