The ORDEM96 model relies largely on internal NASA verification and validation to ensure that it operates as intended. Because it is primarily an empirical curve fit of data, NASA compares new data with ORDEM96 predictions. If the two correlate well, no changes are made. If sufficient new data indicate that the model is incorrectly predicting the environment, the model may be modified. ORDEM96 underwent an international peer review before it was released but has not been subject to formal verification and validation (NASA, 1996).
Because ORDEM96 is empirically derived, it can be modified whenever a significant breakup occurs. Before each shuttle mission, an evaluation is made of the effects of recent breakups. The effects of breakups that might affect the orbiter’s environment are added to the output of ORDEM96 for predictions of the debris environment for the mission. The breakup of a Pegasus rocket upper stage in June 1996, which produced several hundred fragments detectable by the SSN, is an example (Johnson, 1997).
The meteoroid model used by the shuttle program consists of a flux model (Grün et al., 1985) and a velocity model (Erickson, 1968; Kessler, 1969). Both are well accepted and widely used. The effects of normal, annual meteor showers are incorporated into the model, but rare meteor storms that occur when the Earth passes through a particularly dense portion of a comet dust trail are not. NASA, however, evaluates threats from meteor showers and storms before every shuttle mission and has delayed two missions to avoid potential hazards from meteor showers. NASA does not plan to fly the shuttle during future meteor storms. NASA is currently developing a new meteoroid model that includes the background environment as well as the effects of meteor showers and meteor storms.
The primary tool for preflight risk assessment and damage prediction from meteoroids and orbital debris is the BUMPER computer code. This code has been used since 1990 to assess the risks to the orbiter from meteoroids and orbital debris. BUMPER’s configuration is controlled by the Space Shuttle Requirements Control Board. The NASA Johnson Space Center Space and Life Sciences Directorate maintains the model and determines when updates are warranted (Christiansen, 1997). Recent updates have included new failure criteria and the incorporation of ORDEM96 (Zhang and Prior, 1996).
BUMPER employs a finite element model to represent the geometry of the orbiter and various mission components. This model contains more than 25,000 elements and includes the effects of shadowing some orbiter elements by others. On average, each element in the model measures 25 cm on a side. The size of the elements varies with location on the orbiter: the areas most vulnerable to critical penetrations are modeled using the smallest elements. The model divides the orbiter into 57 different regions (excluding payloads) to describe different materials, configurations, and failure criteria. BUMPER’S finite element model library