BOX 8-1 One Model of the Future Debris Environment

The CHAIN program (Rex and Eichler, 1993) uses a combination of analytic and stochastic methods to predict the future number of orbital objects larger than a centimeter in diameter. Figure 8-3 shows the program's logic flow.

First, the current number of orbiting space objects larger than 1 cm is estimated from measurements of the debris environment and is divided into 24 bins with six mass and four altitude ranges. The collision risk (for 21 different types of collisions) within each altitude bin is then calculated by using analytical formulas. A stochastic Monte Carlo method is used to randomly determine the number of collisions in each altitude bin based on these collision risks. Fragments from these collisions are then added to the population, along with newly launched objects and fragments from space object explosions. The effects of orbital decay on all of these objects are then calculated, and the new "initial" population is determined. The analytic formulas are then applied to this new population to calculate collision probabilities for the next iteration, and the entire process is repeated.

This model predicts that a "business as usual" scenario, in which no debris reduction measures are implemented, would result in the population of objects larger than 1 cm increasing to about 250,000 in the next 50 years—not including the effects of collisions. When the effects of collisions are factored in, the further increase to the population is more than 200,000 additional fragments. Figure 8-1 shows the averaged output of this model.

FIGURE 8-3 Logic flow of the CHAIN program. SOURCE: Rex and Eichler, 1993.

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