NASA bumps ISS spacewalk to Saturday

Needs more time to prep cooling pump swap-out

NASA has postponed today's planned spacewalk to replace a failed ammonia pump assembly on the International Space Station, and Tracy Caldwell Dyson and Doug Wheelock will now exit the orbiting outpost tomorrow at 10:55 GMT.

The agency explains: "Teams of flight controllers, engineers, and spacewalk experts have made significant progress in preparing for the first spacewalk, but need an additional day to get ready. The additional time allows for final procedures to be sent late Thursday to the station, giving the crew a full day to review the plans developed by Mission Control."

The pump failure last Saturday night took out half of the ISS's cooling system, although the station's systems are "stable", and its six crew members "are not in any danger".

Swapping out the pump will require two spacewalks - one to physically bolt on a new unit, and a second on Wednesday to attach the electrical and fluid lines.

NASA describes the EVAs as "challenging", since Caldwell Dyson and Wheelock will be "handling ammonia lines at full operating pressure, which makes the lines stiff during reconnection and mating". ®