By Paul Rincon
BBC News science reporter, Houston
Underneath its outer skin, the space shuttle is made from aluminium, much like any ordinary aircraft.
The TPS comprises tiles and other materials
When returning from space, the orbiter hurtles towards Earth at speeds of up to 27,360km/h (17,000mph).
The confrontation with our atmosphere can heat the surface of the vehicle to temperatures of 1,600C (3,000F) - hot enough to melt steel.
So a layer of tiles and other materials is needed to protect the spacecraft and its human occupants.
These are known collectively as the shuttle's thermal protection system (TPS).
The wing of space shuttle Columbia was badly damaged by a suitcase-sized chunk of foam during lift-off.
The foam slipped off the launch system's external tank and breached the orbiter's heatshield panels, letting hot gases into the shuttle when it tried to re-enter the atmosphere 16 days later.
Up front and hot
The STS-114 return-to-flight mission has been testing methods for repairing damage to these panels and other parts of the shuttle's TPS.
Inspections using the 15m-long Orbiter Boom Sensor System (OBSS) have identified six areas on the belly of Discovery with suspected minor damage.
And though no decision has been made to do so, it is precisely these small "dings" or nicks in the heatshield tiles that the STS-114 mission crew have been trained to fix.
Camera and sensor attachments on a boom can look for damage
The main types of materials used in the TPS are reinforced carbon-carbon
(RCC) panels, low and high temperature insulation tiles (or heatshield tiles), felt reusable insulation blankets and fibrous insulation blankets.
RCC panels are made from a dense, charcoal-like material that has been used on tips of ballistic missiles. They are found on areas such as the wing leading edge, the nose cap, and an area just behind the nose cap called the "chin panel".
These are the regions of the orbiter that become hottest during re-entry.
It was the RCC panels on the wing's leading edge that were breached in the Columbia accident. The shuttle's flight history shows that prior to Columbia, the RCC panels were struck by objects but were never completely penetrated.
Discovery's crew have a substance called Noax (short for Non-oxide Adhesive Experimental) that they can use to repair cracks or loss of coating from the RCC panels up to 0.6cm (0.2in) wide and 10cm (4in) long.
There is also an "emittance wash", which is intended primarily to repair relatively shallow cracks.
But the crew of Discovery is not equipped with the capability to repair the kind of damage thought to have been sustained by Columbia.
Large swathes of the orbiter's underside, where inspections by the astronauts have been focused, are covered with High-Temperature Reusable Surface Insulation (HRSI) tiles.
The coated black tiles are located where temperatures can reach as high as 1,260C (2,300F). No two tiles are the same, because they are matched to the curvature of the shuttle.
Gap filler material closes spaces between tiles
Made from silica fibre, there are about 24,000 of these on any one shuttle.
They dissipate surface heat so quickly that a tile can be held by its corners with a bare hand only seconds after removal from an oven, and while the centre of the tile still glows red.
Gaps between tiles are filled with a ceramic material known simply as gap filler.
This fabric can dislodge, as it has done in at least two places on Discovery. These protrusions disturb the aerodynamic flow as the shuttle returns through the atmosphere to Earth.
This means that areas of tile downstream of protruding gap filler can be heated a few hundred degrees more than they otherwise would. This is particularly concerning if tiles in these areas are themselves damaged.
Coated white tiles-known as Low-Temperature Reusable Surface Insulation are designed to insulate the spacecraft from temperatures up to 650C
These were once used extensively, but are now confined mainly to the forward fuselage and parts of the Orbital Manoeuvring System (Oms) pod.
Over the years, tiles have been replaced in many areas on the orbiter by a material known as Flexible Reusable Surface Insulation (FRSI) and Advanced Flexible Reusable Surface Insulation (AFRSI).
FRSI and AFRSI are lighter and less expensive than the conventional tiles and using them has enabled the shuttle to lift heavier payloads to orbit.
This blanketing material covers areas of the shuttle that do not exceed 370 Celsius (700F) during entry or 400C (750F) during ascent.
Discovery has a tile chip on a landing-gear door
These include the fuselage sides and top, the payload bay doors, the tops of the wings, and the Oms pods near the tail.
All the major materials in the shuttle TPS are bonded to the orbiter with an adhesive cement that will withstand temperatures as high as 290C (550F) and as low as -160C (-260F) without losing bond strength.
The nicks and chips investigated by Discovery's crew can be caused by small chunks of foam that come off the external fuel tank during the climb to orbit.
But they can also be damaged by pieces of space junk, aerodynamic forces and micrometeoroids.