(filled with inert helium on orbit and not needed for reentry) provide some internal shielding for subsystem components. The main engine nozzles and the thermal shields mounted around them to protect equipment in the aft bay from radiant heating and low pressure backflow during space shuttle main engine operations provide additional protection.

The body flap is an aluminum structure with no internal systems. It is shielded from the top by the main rocket nozzles and has thick TPS tiles on its lower surface. The vertical tail is an aluminum structure consisting of the primary fin structural box and the moveable rudder/speed brake panels. These are relatively robust structural components with a small exposed area covered by thick TPS tiles and insulation blankets.

The damage caused by a particular impactor depends largely on the location of the impact. Table 2–2 summarizes the damage thresholds for several key components of the orbiter. Calculating the minimum diameter of an impactor that would cause each effect requires making numerous assumptions about impactor composition, shape, and velocity, impact angle, and exact impact location; nevertheless, the table illustrates the range of potential impactors that could damage the orbiter.

The impacts shown in Table 2–2, as well as impacts not included in the table, could cause damage ranging from minor pitting, which would require increased maintenance, to loss of life or loss of the orbiter.

There are a number of different mechanisms by which meteoroid and orbital debris could cause critical failure (i.e., involving loss of life or the orbiter). Any