Nasa may send astronauts into belly of shuttle

HOUSTON - Nasa is considering sending two astronauts on an unprecedented spacewalk to the belly of space shuttle Discovery to smooth out rough spots in the ship's heat shield.

Nasa managers are concerned that two protruding bits of material known as "gap fillers" could overheat Discovery as it plunges through Earth's atmosphere for landing.

Heat shield damage was responsible for the loss of Columbia and its seven astronauts during Nasa's last shuttle mission.

"My immediate knee-jerk reaction was that we could live with this," said deputy shuttle programme manager Wayne Hale. "On the other hand, if the [spacewalk repair] plan is relatively simple, why worry? Why would you not just go take care of it?"

Thousands of gap fillers are tucked between the black heat-resistant ceramic tiles on the shuttle's belly, which experiences temperatures of about 1370 deg C as the shuttle re-enters the atmosphere.

Two gap fillers on Discovery are sticking up about 2.5cm from the tiles, a protrusion that could trigger a 15 per cent to 25 per cent temperature spike, said orbiter projects manager Steve Poulos.

Nasa has successfully landed space shuttles 112 times previously, despite sometimes extensive heat shield damage.

"The Columbia accident made us realise that we had been playing Russian roulette with the shuttle crews," Hale said.

Columbia's damage was caused by a chunk of foam insulation that broke off the ship's fuel tank during launch and smashed into the ship's wing.

Nasa subsequently adopted new procedures, spent US$1 billion ($1.48 billion) on safety upgrades and built equipment to inspect the shuttle while it is in orbit. With new imaging instruments, engineers now know - for the first time in the programme's 24-year history - precise details of the shuttle's condition in space.

Analysis of the gap fillers' effect on shuttle re-entry heating, as well as possible repair plans, were expected to be presented to mission managers today.

The protruding parts of Discovery's heat shield are not the result of debris impacts. Poulos said the adhesive that bonds the filler to the ship's metal skin likely weakened.

Nasa is working to improve how the gap fillers are attached to the shuttle to avoid future problems.

The agency also has had to go back and readdress the primary cause of the Columbia accident: debris falling off the fuel tank and striking the ship.

Several pieces of insulation fell off Discovery's tank during launch last week, prompting Nasa to again ground the fleet for tank repairs. Nasa has extended Discovery's mission by one day and rescheduled landing for August 8.

If the crew is asked to remove or trim gap fillers, the work could be added to a previously scheduled spacewalk on Thursday. Managers also could order a fourth spacewalk on Saturday. The second spacewalk, which aims to replace a failed space station gyroscope, was scheduled to begin today.

- REUTERS