Nasa orders risky spacewalk to repair heat shield

HOUSTON - Determined not to lose another space shuttle and crew, Nasa has decided to send one of Discovery's astronauts on a risky spacewalk to the ship's fragile underside to smooth protruding fibres in the heat shield.

The task of removing protruding material from the space shuttle that could theoretically generate dangerous heat on re-entry into the Earth's atmosphere will be added to a third previously scheduled spacewalk on Wednesday.

"This is the new Nasa. If we cannot prove that it's safe, than we don't want to go there," deputy shuttle programme manager Wayne Hale said at Mission Control in Houston.

The second spacewalk of the first shuttle mission since the 2003 Columbia disaster was completed on Monday, with Japanese astronaut Soichi Noguchi and Nasa's Steve Robinson replacing a failed gyroscope at the International Space Station.

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

Discovery does not appear to have suffered the sort of damage to its heat shield that caused Columbia to break up over Texas on February 1, 2003, as the shuttle came in to land.

But Nasa engineers do not know enough about how the two bits of material on Discovery could affect thermal and aerodynamic forces as the vehicle plunges at 22 times the speed of sound through the atmosphere on re-entry. They both stick out about 2.5 cm from the smoothly tiled surface of the ship's belly.

"The bottom line is there is large uncertainty because nobody has a very good handle on aerodynamics at that altitude and at those speeds," Hale said.

Columbia was destroyed and its seven astronauts killed because of heat shield damage on the ship's wing caused by foam falling off the fuel tank during launch.

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

But Discovery's tank also shed large pieces of foam, prompting Nasa to postpone future flights until the tank is repaired.

Discovery's heat shield problem was not believed to have been caused by a debris impact, Hale said.

More likely, the adhesive used to glue the thin ceramic-coated material to the ship's metal skin did not bond properly. That may have allowed the fillers, which are tucked between the heat shield tiles, to float out once the shuttle reached the weightlessness of space.

Although the repair plan is relatively straight-forward, astronaut Robinson will need to take extra care not to damage the delicate tiles that surround the protruding fillers.

"There is no situation in space that you can't make worse," Hale said, quoting former astronaut and long-time Nasa safety advocate John Young. "The risk (of additional damage) is going to be mitigated and controlled."

In the shuttle's 24-year history, spacewalking astronauts have never worked on its belly before.

Out of direct sight from the shuttle and space station crews and working solo from a platform on the station's robot arm, Robinson will first try to gently pull out the protruding material. It that fails, he will try to cut it with a modified hacksaw.

- REUTERS