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the future debris population. Many of these models are loosely based on population "birth and death" models, which assign the debris population to a number of "bins," each characterizing the number of particles of a given mass range within a given altitude range. The simplest models (e.g., Talent, 1992) use one bin for all masses and altitudes to represent the entire LEO population, but more complex models have used more mass and altitude ranges (Rossi et al., 1993). NASA's EVOLVE model (Reynolds, 1990) can use a variable number of bins, but typically uses 15 mass and 36 altitude ranges. Although these models incorporate a variety of different assumptions, they generally carry out similar procedures to predict the future debris population.
First, the initial debris population (to a certain limiting size or mass) is calculated for each altitude bin, based on measurements or models of the current debris population. Then the orbits of this initial population are propagated into the future (by using either deterministic or statistical methods), in the course of which some objects are removed from or added to the altitude bin as a result of perturbing forces such as atmospheric drag. Predictions of the amount and distribution of new objects launched into orbit, as well as of the results of possible explosions in orbit, are used to add new space objects to the population. When it is determined that a collision will occur, another model is used to determine the effects of the collision, including the creation of new debris. The entire process is then repeated, with the output of the first iteration used as the initial population for the next.
These models are generally useful only in predicting the magnitude and characteristics of the populations of medium-sized and large debris. As discussed in Chapter 3, knowledge of small debris is so limited that it is extremely difficult to estimate the current population, much less project future trends. About the only predictions that can be made about the future small debris population are that
the amount of small debris produced in breakups is likely to increase if the number of collisions grows, because collisions are predicted to produce very large numbers of small debris particles; and
the amount of very small debris (such as the particles expelled by solid rocket motors and the smallest products of breakups and degradation) in orbit may change markedly from year to year, due to the strong effect of perturbing forces on the orbits of these particles. Thus, regardless of the historical total amount of very small debris released into the environment, the population of these particles at any given time in the future will be strongly dependent on the amount produced during the preceding one or two years.