Two U.S. astronauts conducted an emergency spacewalk Saturday to inspect and replace a device believed to be the source of an ammonialeak in the orbiting laboratory's power system, U.S. space agency NASA said.

　　Astronauts Chris Cassidy and Tom Marshburn successfully installed a new pump controller box, which contains the mechanical systems that drive the cooling functions, after removing the old one suspected of leaking flakes of frozen ammoniacoolant two days earlier.

　　"Mission Control ran the new pump while the spacewalkers watched for any ammonia snowflakes, but so far there have been no new signs of a leak," NASA said in a statement.

　　"Long-term monitoring of the pump will be required to determine whether the pump replacement has fixed the leak," it said.

　　The spacewalk, the third for both Cassidy and Marshburn, who conducted two spacewalks together in July 2009, began at 8:44 a.m. EDT (1244 GMT). It lasted five and a half hours, an hour less than expected.

　　"Back inside!" tweeted Marshburn after the spacewalk. "Hopeful that our work resolves the leak. What a great way to spend one of my last days in space!"

　　Marshburn has been in space since December and is set to return to Earth Monday. Cassidy is a new arrival, aboard the outpost for just around 40 days.

　　The spacewalk was hastily planned after space station crew members alerted NASA about the leak late Thursday morning when they spotted "small white flakes" of frozen ammonia floating near the pump box.

　　相关新闻：美国航天局11日发布消息说，国际空间站内的两名宇航员当天出舱，维修一套出现泄漏的液氨冷却系统，但是否已经修好还需长期观察。

　　美国航天局在一份声明中说，国际空间站的两名美国宇航员汤姆?马什伯恩和克里斯?卡西迪当天早些时候出舱，实施了总共5个半小时的太空行走。他们将可能存在泄漏问题的一个液氨泵拆除，换上一个备用泵。新安装的备用泵启动后，未再发现液氨泄漏。

　　声明强调，更换液氨泵是否意味着解决了此前的泄漏问题，还需要长期观察。

　　国际空间站宇航员9日报告“看见空间站外有微小白色片状物飘浮”，经进一步分析是液氨冷却剂渗出，导致该问题的直接原因可能是站外的一个液氨泵泄漏，这一设备此前曾发生过泄漏。美国航天局因此紧急安排了这次太空行走。(新华网)