Coated black ceramic tiles (known as High-Temperature Reusable Surface Insulation) cover many areas of the shuttle. They are found on its lower surface; in areas around the forward windows; on its upper body flap; at the base heat shield; on the "eyeballs" at the front of the Orbital Maneuvering System (OMS) pods; and on the leading and trailing edges of the vertical stabilizer and the rudder speed brake. Black tiles are located where temperatures on the outside of the shuttle are most extreme.

Coated white tiles (known as Low-Temperature Reusable Surface Insulation) are designed to insulate the spacecraft from temperatures up to 1,200 degrees F. Originally used extensively, those tiles are now replaced in most areas by Flexible Insulation Blankets. White tiles are still used, however, on the upper surface of the forward fuselage above the crew windows and on some parts of the OMS pods.

Before the shuttle’s thermal protection system was incorporated into its final design, the heat tiles were put to a difficult test. In 1975, simulating extremely high temperatures the orbiters would encounter on reentry, a torch was put to the tiles. The tests were successful.

Heat tiles have been damaged, without causing problems, on prior shuttle missions. (For example, one of the starboard pods protecting Endeavour’s orbital maneuvering system shows damage on 10 October 1994.) And wreckage of the Challenger, retrieved from the Atlantic Ocean, still had thermal tiles intact despite its horrific explosion.

The small (six-by-six-inch) tiles are easily repaired or replaced while the shuttle is on the ground. A technician, for example, worked on tiles underneath Columbia as she was readied for her STS-28 mission. Crew members from that same mission inspected those tiles on 13 August 1989, after the mission was safely completed.

Repairing damaged heat tiles in space, however, is risky. Even if an orbiter is docked at  the International Space Station, astronauts would have to make repairs during a space walk. The issue became acute during Endeavour's August 2007 flight (STS-118) when a baseball-sized piece of foam broke loose  from the external fuel tank at 58 seconds into the mission. Falling from a bracket,  the debris  ultimately struck two tiles  on the orbiter's underbelly.

To determine whether there was any damage to the shuttle, Endeavour's crew executed a pitch maneuver  as the orbiter approached the space station. That process enabled personnel inside the space station to take pictures of the shuttle's underside.

When NASA's flight managers saw an apparent gash, they ordered Endeavour's astronauts to determine its dimensions, including the depth.  Using a laser-tipped inspection boom,  crew members were able to confirm the gash  measured 3½ by 2 inches (9 by 5 centimeters), leaving a small area of Endeavour with no heat-resisting tile protection. The key issue for NASA, to be determined before the shuttle undocks  from the space station, is to balance the risk of a space-walking repair versus the risk of a re-entry without full heat-tile protection.

In a picture that seems eerie, given the events of February 1st, 2003, a black heat tile floats outside Columbia’s window as she traveled on Mission 61-C. The photograph was taken just sixteen days before the Challenger disaster.