NASA working around station gyro failure

CAPE CANAVERAL, Fla., June 8 (UPI) -- A piece of equipment that helps keep the space station properly positioned in orbit failed on Saturday, but backup gear will keep the outpost stable until a replacement can be launched sometime next year, NASA officials said.

The failure occurred just as the shuttle Endeavour crew was finishing installing a cargo module onto the station that is filled with equipment, supplies and scientific gear for the new resident crew.

Station astronaut Carl Walz, who will be returning to Earth along with his two crewmates after six months in space, reported hearing a disconcerting noise from inside the station's Unity connecting node shortly before 11 a.m. ET.

"We're hearing a pretty loud noise, kind of a growling ... from inside the node," Walz told flight directors at the Johnson Space Center in Houston.

Engineers already had been watching suspicious voltage hikes and high temperature readings from one of the station's 800-pound gyroscopes. The station has Control Moment Gyroscopes, or CMGs, to keep the outpost properly oriented and stable in weightlessness. Each gyro has a huge flywheel that spins at speeds of up to 6,600 rpm to create a stable vector.

The station needs at least two gyroscopes to maintain a stable position, although it also is equipped with steering rockets that can be used as a backup system. The station has only a limited supply of fuel, however, so the jets cannot be used long-term.

"Losing a CMG is a big deal," said flight director Paul Hill. "It's a major component, but from a risk perspective right now, we're in good shape ... We can still lose one more CMG and still hold attitude on the station side and minimize the amount of propellant we use."

Engineers do not yet know why a bearing assembly inside the wheel broke, but Hill says it probably would be at least six- to nine months before a replacement can be flown to the outpost. The next few shuttle missions to the station are fully booked, with no room to fit the half-ton gyroscope assembly and its support equipment in the spaceship cargo bays.

"We're going to do the best we can to get the next CMG ready to fly and into an orbiter and get it changed out," Hill said.

The station's four gyroscopes are located inside an exterior truss segment on the Unity node. The truss was installed in October 2000. Preliminary camera surveys of the truss showed no visible signs of a gyroscope breakdown, such as floating fragments.

The equipment failure is not expected to significantly impact shuttle Endeavour's ongoing mission at the station. One minor technical issue under assessment was how to best position the station so that a platform slated to be installed Monday on the station's robotic crane can be kept in sunlight to prevent damage from cold temperatures.

NASA had planned to slightly tilt the station's orbit to keep the equipment out of the shadow of the shuttle's nose, but planned the maneuver using all four of the station's gyroscopes.