Teams of NASA engineers prepared details of an operation that may be needed to remove two strands of gap filler on Discovery's belly while Japanese astronaut Soichi Noguchi and American Steve Robinson went on a seven hour and 14 minute mission into space to replace a faulty gyroscope.

Noguchi rode a 15.25 metre (50 feet) long robotic arm on the space station to get to one of the four gyroscopes that steer the station and keep it on its pre-determined orbit.

Donning bulky space suits for the second time on this mission, Noguchi and Robinson went out into space at 0842 GMT.

Noguchi was attached to the robotic arm to collect the broken gyroscope, take it back to Discovery's cargo bay, and ride back with a new one. "Oh, the view is priceless," he said as he moved toward the gyroscope. "I can see the moon."

"You've got a ticket to ride," Robinson joked.

NASA said the walk had been a complete success. But as the astronauts worked and joked, NASA engineers were divided over what to do about protruding gap fillers between the tiles on the underside of Discovery.

Noguchi and Robinson may have to spend their next spacewalk on Wednesday carrying out emergency repairs to the shuttle's belly to make it safe for the return.

The ceramic fiber fillers are used to keep hot gas from flowing into gaps between the thermal outer protection tiles.

While shuttles have flown with the protruding strands in the past, both in this case are sticking out further than the quarter of an inchcentimetre) limit generally considered safe.

Some experts fear the strands could become super-heated when Discovery re-enters the Earth's atmosphere on August 8.

Engineers are considering whether the fiber fillers should be cut or just folded back in-between the tiles.

"We have teams of people working on that," said NASA spacewalk manager Cindy Begley.

She said the most likely operation would be to cut off the strands, but officials emphasised that no final decision has been taken.

A piece of foam came off the Columbia shuttle and damaged a wing, which led to superheated gases getting into the shuttle and causing it to break up on February 1, 2003, killing all seven astronauts aboard.

NASA is otherwise confident that Discovery can come back safely but was to give its official verdict on Monday.

"With the exception of the gap filler, I think we are pretty much ready to declare the (shuttle) is safe for reentry," said shuttle program official Steve Poulos on Sunday.

"We have a very clean orbiter," said Wayne Hale, head of the shuttle program management team.

NASA's chief administrator admitted Sunday that the US space agency had "goofed" on key safety checks prior to Discovery's launch last Tuesday.

"Our judgment at the time was that it was okay. As everyone has said without any attempt to hide it ... we goofed on that one," Michael Griffin said on US television.

Discovery suffered several problems including the loss of a sizable chunk of insulating foam from Discovery's external fuel tank just minutes into the launch.

All rights reserved. copyright 2018 Agence France-Presse. Sections of the information displayed on this page (dispatches, photographs, logos) are protected by intellectual property rights owned by Agence France-Presse. As a consequence, you may not copy, reproduce, modify, transmit, publish, display or in any way commercially exploit any of the content of this section without the prior written consent of Agence France-Presse.