Finally, breakup models are a major source of uncertainty in any predictive model of the future debris environment. As discussed in Chapter 2, breakup models are based on a very limited set of data, and models of both explosions and collisions contain large uncertainties. Models of collisions, in particular, are supported by data from only two in-space collisions and a few ground tests conducted at velocities lower than typical LEO collision velocities. It is not at all certain how well current collision models
incorporate the effects of different space object configurations, different spacecraft materials, and different impact geometries on the results of a collision;
approximate the threshold size (in terms of mass and/or energy) of debris that can cause space objects of a given size to break up; or
estimate the distribution of the size and mass of particles produced in collisions (although the limited ground and space tests that have been conducted indicate that current models are fairly accurate at predicting the amount of large debris produced in a collision).
Models of the future debris population often deal with these uncertainties by treating them as variables. For example, a model of the future population can be run with the rate at which rocket bodies explode in the future set equal to current levels, and then run again with the rate set to zero, to predict the effects of implementing rocket body passivation measures on the future growth of the space object population. Treating these uncertainties as variables does not reduce the overall uncertainty in the model, of course, but it does serve to clarify the dependence of the model's results on each variable.
If the only additions to the future debris population were rocket bodies, nonfunctional spacecraft, mission-related debris, and the products of explosions and surface deterioration, the space object population would likely continue its roughly linear growth. Implementation of measures to reduce the number of explosions of spacecraft and rocket bodies, and to limit the amount of mission-related debris released as a result of spacecraft deployment and operations, might result in a slower rate of growth, just as changes in future launch patterns could result in a faster rate of growth. Collisions between space objects, however, threaten to add a potentially large and exponentially growing number of new objects to this population.
The probability that a collision will occur in any particular orbital region increases with roughly the square of the number of objects in that