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Last Updated: Tuesday, 23 August 2005, 17:03 GMT 18:03 UK
The damage to Discovery
Nasa has grounded its space shuttle fleet after Discovery sustained damage in several places during its mission, despite the wave of modifications made after the Columbia accident.

PAL RAMP FOAM LOSS

The two Pal [Protuberance Air Load] ramps act as aerodynamic covers to the various cables and air lines running up the side of the tank. They are covered with insulating foam, like the rest of the tank, to prevent the formation of ice when it is filled with freezing liquid hydrogen fuel.

During Discovery's launch, a 0.5m long chunk of foam weighing about 450g broke away from one of the Pal ramps on the external fuel tank.

Although it caused no damage, a similar foam-loss incident is believed to have directly caused the Columbia accident.

Preventing foam and ice breaking off from the tank during launch was one of the key recommendations of the investigation into the loss of Columbia.

DAMAGED GAP FILLER

On 3 August, astronaut Stephen Robinson made a six-hour spacewalk to remove two loose "gap fillers" that were sticking out between heatshield tiles near Discovery's nose section.

The pieces of ceramic fabric, which act like a grouting for the tiles, were probably shaken out during the vibrations of launch, Nasa said.

Nasa ordered their removal amid fears that protrusions on the otherwise smooth belly of the shuttle could disturb the air flow during re-entry, causing turbulence that raises temperatures on heatshield tiles downstream.

Chipped tiles were also detected close to Discovery's landing gear doors, but it was judged that these would not endanger the spacecraft during re-entry.

DAMAGED THERMAL BLANKET

Shortly after removal of the loose gap fillers and days before Discovery was due to return to Earth, mission controllers noticed that a thermal insulation blanket had billowed out just under the commander's window.

Despite some concerns that the blanket could come loose during re-entry, engineers gave the shuttle a clean bill of health and the spacecraft made a successful descent and landing on 9 August.





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