NASA has decided to wait until Friday to conduct a spacewalk to replace a failed ammonia pump module on the International Space Station.

Mission managers, program managers, flight controllers, engineers, astronauts and spacewalk experts made the decision Monday evening after continuing to analyze and refine engineering requirements, and reviewing the results of an underwater practice session.

Expedition 24 astronauts Doug Wheelock and Tracy Caldwell Dyson currently are scheduled to start the repairs on the station's starboard truss Friday. Fellow astronauts Cady Coleman and Suni Williams spent the afternoon in the Johnson Space Center's Neutral Buoyancy Laboratory practicing underwater the tasks needed to restore the cooling loop over the course of two spacewalks.