Comparing the FCC 5-Year and IADC 25-Year Post-Mission Disposal Rules: A Reduced-Order Stochastic Model of Altitude-Dependent Cascade Risk in LEO

Low Earth orbit (LEO) is becoming increasingly congested, and the risk of collisional cascading has produced several post-mission disposal (PMD) rules. Two stand out: the Inter-Agency Space Debris Coordination Committee (IADC) 25-year rule and the stricter Federal Communications Commission (FCC) 5-year rule. The two set very different deadlines yet are rarely compared directly, so it is unclear at which altitudes and compliance levels the choice between them changes cascade risk. This study uses a reduced-order stochastic source-sink model to compare the two rules across compliance levels and LEO altitude shells. Populations are grounded in a catalog snapshot of 26,628 on-orbit LEO objects, fragment yields follow the NASA Standard Breakup Model, and uncertainty is quantified across 15 independent seeds. Four findings emerge. First, in the rule-binding shells the FCC 5-year rule reaches cascade stability at 12.5 to 15 percentage points lower compliance than the IADC 25-year rule, with the gap widening to 25 to 37 points near the transition zone. Second, below approximately 625 km both rules are redundant for cascade prevention, since natural decay clears fragments faster than they accumulate. Third, below a 75 percent disposal-hardware success rate neither rule prevents cascade at any compliance level, a floor that sits directly against the 90 percent disposal-success thresholds now entering United States and European requirements. Finally, active collision avoidance has almost no effect on cascade risk, because that risk is dominated by the unmaneuverable legacy population. Results are a relative policy comparison and a tracked-population lower bound, not an operational forecast.