But the space station held steady until the very end of the 4 1/4-hour spacewalk, when it went into a partial, slow-motion cartwheel. The drift lasted far less than the three hours expected.
Flight controllers could have prevented this "free drift" by firing the station's thrusters, but waited to do so until the spacewalkers were out of the way, rather than risk contaminating their spacesuits with toxic rocket fuel.
Right after the spacewalk, one of the two good gyroscopes exhibited a brief but unusually strong vibration. Engineers were keeping close watch over the big spinning wheel, which appeared to be working fine later in the day. Besides the gyroscope that shut down two weeks ago, another broke three years ago.
Laboring 220 miles above Earth, Commander Leroy Chiao and his Russian crewmate, Salizhan Sharipov, plugged in four antennas for a new type of cargo carrier due to fly next year.
They also released by hand a one-foot-long, 11-pound satellite called Nanosputnik, designed for experimental maneuvering by ground controllers.
Sharipov let go of Nanosputnik on the count of two as Chiao photographed the event. "Off it goes," Sharipov said as the satellite floated away with a spin.
During the spacewalk, the space station was empty. With the shuttle fleet grounded since the 2003 Columbia catastrophe, the space station has been home to only two astronauts at a time, instead of the usual three.
Chiao and Sharipov hustled through their work and wrapped everything up more than an hour early, despite extra safety precautions. NASA and the Russian Space Agency instituted the extra measures to avoid a repeat of the problem that occurred during the men's spacewalk in January. Because of a miscommunication during that outing, Chiao got too close to the firing thrusters. This time, the thrusters, which fire automatically when the space station tips out of balance, were disabled for the astronauts' safety.
Engineers have yet to identify the mysterious force that causes the space station to tilt during spacewalks. The space station needs to point in the right direction so that its solar panels continue generating electricity and certain components do not become overheated from exposure to the sun.
The spacewalkers ignored the recent problem that knocked out the gyroscope; visiting shuttle astronauts will tackle that repair job in two months.
The two space station residents have spent the past several weeks dealing with an assortment of breakdowns, including an oxygen generator that still is not working. Over the weekend, they replaced a pump panel that is part of a critical cooling system.
NASA hopes to launch the shuttle Discovery to the space station in mid-May. Technicians had trouble aligning the shuttle and its transporter Monday for the big move from the hangar to the Vehicle Assembly Building, where the booster rockets and redesigned fuel tank are attached. The move was rescheduled for Tuesday.
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