Spacewalkers restore steering system

Two spacewalking astronauts replaced a broken steering device on the International Space Station yesterday and prepared for a possible new and untested venture to space shuttle Discovery's belly to fix the ship's heat shield.

During the second of three spacewalks planned during their mission, astronauts Soichi Noguchi and Steve Robinson removed one of the station's four gyroscopes and replaced it with a new one.

The gyroscopes keep the outpost properly positioned in space without using the station's limited supply of propellant for rocket thruster burns.

After unbolting the old gyroscope, Mr Robinson pulled it out and waited for Mr Noguchi, riding on the station's robot arm, to get into position to carry it away.

"It's not easy for me to hold this rascal," Mr Robinson said, clasping the 300-kilogramme mass.

Then, noting Mr Noguchi perched on the arm, he joked, "You've got a ticket to ride."

Mr Noguchi deposited the gyroscope onto a holding rack while the new gyroscope was hauled into place on the station's truss. The failed instrument was packed into the shuttle's cargo bay for engineers to analyse after the ship's return to Earth.

Before returning to the shuttle's airlock, the astronauts prepared some tools that may be needed for an unscheduled heat shield repair during the third spacewalk tomorrow, or possibly a fourth outing on Friday.

Nasa managers are trying to decide if Discovery's heat shield needs work before the shuttle attempts Nasa's first landing since the destruction of Columbia and the deaths of its seven astronauts on February 1, 2003.

Columbia had been struck by a piece of falling foam insulation during launch. The impact damaged heat panels on the ship's wing and as the shuttle blasted through the atmosphere for landing, superheated gases breached the ship's wing, triggering the breakup of the vehicle over Texas.

After the accident, Nasa adopted new procedures, spent $1 billion on safety upgrades and built equipment to inspect the shuttle while it is in orbit.

Debris fell from Discovery's tank as well, prompting Nasa to ground its shuttle fleet, halting plans to resume space station servicing and construction until additional repairs are made.

New in-flight inspections mandated after the Columbia disaster showed two bits of material protruding from the shuttle's belly, which could cause overheating during the atmospheric re-entry.

Managers were expected to decide late yesterday whether to have the spacewalkers venture to Discovery's belly to remove or trim the two protruding strips, known as gap fillers.

Thousands of fillers are placed between the shuttle's heat-resistant tiles to assure a smooth aerodynamic flow as the ship plunges through the atmosphere at 20 times the speed of sound.

Nasa was concerned the protrusions could change aerodynamics and increase heat on the shuttle by as much as 25 per cent when it re-enters Earth's atmosphere for landing, scheduled for August 8.