原文：
CAPE CANAVERAL, Fla. — NASA is considering whether an emergency spacewalk by the crew of space shuttle Discovery is needed to clear a stuck valve in a critical cooling loop at the International Space Station, officials said Wednesday. Mission Control would have to add a 15th day to the shuttle flight in order to squeeze in the spacewalk, which would be the fourth for this mission. Flight director Ron Spencer said repeated attempts from the ground to open the nitrogen valve by remote control have failed. Unless flight controllers somehow succeed, a spacewalk will be required — either by Discovery's astronauts or by the space station crew after the shuttle leaves. Discovery currently is set to depart Saturday. Mission managers hope to make a decision by Thursday on whether shuttle astronauts should tackle the repair job. The nitrogen valve failed to open Tuesday after spacewalkers Rick Mastracchio and Clayton Anderson hooked up a new ammonia tank. Nitrogen is needed to pressurize the ammonia coolant. For now, the space station is being cooled properly. But the valve needs to be opened by about late April, when a period of intense sunlight on the complex heats it more than usual, at least on one side. Ammonia expands with heat even more than water, Spencer noted. As a precaution against thermal-enduced damage, half of the station's electronics likely would have to be turned off. Spencer said it's uncertain whether the station could support all six full-time residents, if that were to happen. A spacewalk would involve replacing the entire nitrogen tank assembly, a job performed by previous crews. Two spares happen to be on board. NASA is concerned, however, about asking Mastracchio and Anderson to attempt it. That's because they already have performed three spacewalks and did not train for this particular situation, Spencer said. On the other hand, replacing the nitrogen tank would be similar to the hookup of the ammonia tank. "We want to make sure we get it right before we go out there," Spencer told reporters. "We're still sharpening our pencils on what the options are." It's possible NASA may be able to operate with the stuck valve into May, Spencer said. That would give flight controllers — and the station crew — more time to prepare for the spacewalk repairs. Engineers are perplexed because the valve worked fine earlier in the mission. Normally, however, the valve is not closed all the way, like it is now. On Wednesday, flight controllers tried heating it up and making it as cold as possible while flying on the nighttime side of Earth. But it would not budge. Discovery's astronauts are willing to take a stab at it. During a news conference earlier in the day, Mastracchio said both he and Anderson were feeling fine and would be happy to go out again if necessary. The two had to struggle with stiff bolts on each of the three spacewalks to replace the ammonia tank. "It may have seemed like we were working hard," Mastracchio told reporters. "But actually as we were struggling with those bolts, we were just doing a lot more thinking than we were actually working." "Today, Clay and I feel fine. And if necessary, we'll go out for future spacewalks, but hopefully, none of those will be required."
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译文：美国宇航局考虑紧急太空行走修复阀门
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