CAPE CANAVERAL, Fla. -- The two space station astronauts installed antennae and released a baby Sputnik during a spacewalk yesterday, completing the work just before the orbiting outpost drifted and rolled slightly because of overloaded gyroscopes.
The astronauts were in no danger, and the slow roll -- which lasted just 17 minutes -- was not unexpected. In fact, the National Aeronautics and Space Administration said the space station held steady longer than anticipated.
For reasons that are not entirely clear to NASA, the space station has tended to drift during spacewalks over the past year or so. The space agency thought the problem might be even worse this time, because one of the gyroscopes that keep the orbiting outpost stable and pointed in the right direction stopped working two weeks ago.
But the space station held steady until the very end of the 4¼-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 so-called free drift by immediately 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 antennae for a new type of cargo carrier due to fly next year. They also released a 1-foot-long, 11-pound satellite called Nanosputnik, designed for experimental maneuvering by ground controllers.
During the spacewalk, they left the space station 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 happened 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 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 Discovery to the space station in mid-May.
Technicians had trouble aligning the shuttle and its transporter yesterday 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 today.