HOUSTON: Two astronauts from the US space shuttle Discovery early on Monday embarked on their second space walk of the mission in a bid to replace a faulty gyroscope aboard the International Space Station. The walk took place as the Discovery crew awaited NASA’s decision on Monday on whether the space shuttle is safe to return to Earth.

Donning bulky space suits, Japanese astronaut Soichi Noguchi and American Steve Robinson moved into Discovery’s airlock and switched on their autonomous electrical batteries, a procedure that formally marked the beginning of the walk at 3:42 am (0842 GMT).

“All right! Let’s get started,” radioed one of them as they began making their way into open space.

As opposed to Saturday’s walk when the astronauts juggled several tasks, the main purpose of their excursion on Monday is to replace the gyroscope, one of four devices that help keep the station in predetermined orbit.

In a slight adjustment to the programme, Noguchi and Robinson also tested the exit hatch after they moved into the cargo bay. The change was prompted by some “difficulties” the astronauts experienced while closing the hatch after their spacewalk on Saturday, according to US space officials.

The nature of the difficulties was not specified.

After finishing with the hatch, the pair made their way to a station truss where they began unscrewing bolts holding the defective gyroscope.

Noguchi will then ride the stations robot arm, taking the failed device to Discovery’s cargo bay, and ride back to the truss with a new one, the officials said.

The mechanical arm is being operated by Discovery pilot Jim Kelly and mission specialist Wendy Lawrence.

After the gyroscope is installed, station flight controllers will power it up and check it out. The whole operation is expected to take about six and a half hours. A total of three walks are planned for the current shuttle flight.

Meanwhile, other astronauts will continue transferring equipment and supplies between Discovery and the station.

While engineers and mission managers have given Discovery’s outer coating of thermal protection tiles the all-clear, they are still reviewing a problem with protruding gap fillers between the tiles on the bottom of the spacecraft, officials said.

The ceramic fibre fillers are used to keep hot gas from flowing into gaps between the tiles.—AFP