(CNN) -- Astronauts took part Sunday in the second of three planned spacewalks to replace an old storage tank on the international space station.
Rick Mastracchio and Clayton Anderson removed an old ammonia tank that is part of the space station's cooling system. The tank has to be replaced periodically. The current one had been at the station for eight years.
A new tank will be installed Tuesday on the final spacewalk. The astronauts conducted their first spacewalk on Friday.
Mastracchio and Anderson, along with the new ammonia tank, arrived at the space station Wednesday on space shuttle Discovery. They are part of a seven-person crew that will work with the space station's six residents for more than a week.
Discovery launched Monday from the Kennedy Space Center in Florida.
In addition to replacing the ammonia tank, the shuttle's 13-day mission includes retrieving a Japanese experiment from the station's exterior.
The shuttle is scheduled to return to Earth on April 18 at 8:35 a.m. ET. There are only three shuttle missions remaining before the fleet is retired.