The ballistic limit describes the diameter-velocity threshold at which point a specific meteoroid and orbital shield just stops a given projectile. Any slight damage increase would result in shield failure. Pass and failure of a particular shield is generally a predefined spacecraft requirement and is dependent on the criticality of the component it is protecting. Generally, shield failure is deemed to occur when the shield rear wall is visibly perforated or spalled. The ballistic limit for a shield is a function of many parameters including projectile diameter, velocity, impact angle, density, target areal and volume density, and more.

The ballistic limit may not always behave as one may expect. For example, you would expect that shield damage would consistently increase with projectile velocity (the bigger the impact, the greater the damage). In fact, low velocity projectiles (~3 km/s) can cause more damage than faster moving projectile at 7 km/s. This is because low velocity projectiles do not break up and fragment as higher energy projectiles do. Thus, lower velocity projectiles are often capable of penetrating deeper into the shield.

The graph and equation provided illustrate a generic ballistic limit curve for a shield, showing projectile diameter on the vertical axis and impact velocity on the horizontal.

Ballistic limit equations are important because they functionally describe a shield's performance, and they are combined with the space meteoroid and orbital debris environment model to produce an overall risk assessment for actual spacecraft.