[aimhigh]

Here's what I found about the tiles:
http://www.howstuffworks.com/question308.htm

The space shuttles are protected by special silica tiles. Silica (SiO2) is an incredible insulator. It is possible to hold a space shuttle tile by the edge and then heat up the center of the tile with a blow torch. The tile insulates so well that no heat makes it out to the edges. This page discusses the tiles:

Aerobraking tiles are produced from amorphous silica fibers which are pressed and sintered, with the resulting tile having as much as 93% porosity (i.e., very lightweight) and low thermal expansion, low thermal conductivity (e.g., the well known pictures of someone holding a Space Shuttle tile by the corners when the center is red hot), and good thermal shock properties. This process can be readily performed in space when we can produce silica of the required purity.

And from another site:
http://www.personal.psu.edu/faculty/r/8/r81/055/space_shuttle/shuttle.html

Shuttle orbiters use a system of 30,000 tiles made of a silica compound that does not ablate, but does rapidly radiate heat away from the orbiter. These tiles can be repaired in space. Major disadvantages are fragility (tiles easily damaged before launch and by orbital debris -- lots of tile damage due to debris since anti-satellite tests in mid-80's) and complexity (many people needed to manually attach tiles to orbiter in a tedious and time-consuming process, and to inspect them all before launch).

It's the easily damaged part that is of interest here. Note they can also be repaired. It would seem logical that every flight would include a space walk to ensure the integrity of the tiles.

[XBob]

I used to work on the shuttle, and the various types of tiles are indeed amazing, and the insulating ability is amazing, but they are also amazingly fragile.

Repairing them, well, as each of the 30,000+ tiles is made individually, and is made in a different mold, and is hand fitted and glued onto the shuttle using a very special process, and re-manufacturing them has to be done at Rockwell in California, delaying the launch 'repairing' them was tried, though infrequently, depending on their position and the damage to them. But repairs were not normally made when I was there, 10+ years ago.

[XBob]

The external tank is a whole different story, and is sort of like a gigantic thin, 2 liter soda bottle, with a thin film of insulation, and the insulation is sprayed on each tank where it is manufactured in Mississippi, before it is loaded on the covered barge which carries them, one at a time, to Kennedy Space Center.

[ELS]

It would seem logical that every flight would include a space walk to ensure the integrity of the tiles.

One of the NASA managers at the press conference today said that the astronauts cannot access the tile side of the shuttle during space walks.

[Concerned]

AIMHIGH WROTE: "Here's what I found about the tiles: http://www.howstuffworks.com/question308.htm "

AIMHIGH CONTINUED: "The space shuttles are protected by special silica tiles. Silica (SiO2) is an incredible insulator. It is possible to hold a space shuttle tile by the edge and then heat up the center of the tile with a blow torch. The tile insulates so well that no heat makes it out to the edges. This page discusses the tiles: "

AIMHIGH CONTINUED: "Aerobraking tiles are produced from amorphous silica fibers which are pressed and sintered, with the resulting tile having as much as 93% porosity (i.e., very lightweight) and low thermal expansion, low thermal conductivity (e.g., the well known pictures of someone holding a Space Shuttle tile by the corners when the center is red hot), and good thermal shock properties. This process can be readily performed in space when we can produce silica of the required purity."

AIMHIGH CONTINUED: "And from another site:
http://www.personal.psu.edu/faculty/r/8/r81/055/space_shuttle/shuttle.html "

AIMHIGH CONTINUED: "Shuttle orbiters use a system of 30,000 tiles made of a silica compound that does not ablate, but does rapidly radiate heat away from the orbiter. These tiles can be repaired in space. Major disadvantages are fragility (tiles easily damaged before launch and by orbital debris -- lots of tile damage due to debris since anti-satellite tests in mid-80's) and complexity (many people needed to manually attach tiles to orbiter in a tedious and time-consuming process, and to inspect them all before launch)." [AND BEFORE REENTRY!!!]

AIMHIGH CONTINUED: "It's the easily damaged part that is of interest here. Note they can also be repaired. It would seem logical that every flight would include a space walk to ensure the integrity of the tiles."

BINGO!!! Great info.