Although predicting the future debris environment is very difficult, a new NASA study demonstrates the effectiveness of the 25-year Post Mission Disposal (PMD) rule currently in place in limiting the future debris population in low Earth orbit (LEO).
“Controlling the growth of the orbital debris population is a high priority for NASA, the United States, and the major space-faring nations of the world to preserve near-Earth space for future generations,” according to the NASA-JSC Orbital Debris Mitigation webpage.
Using the NASA LEGEND model, J. -C. Liou, an orbital debris mitigation expert at NASA’s Johnson Space Center, studied the actual number of artificial LEO objects larger than 10 cm recorded in the past 55 years (1957-2011). Liou then used the model to project the debris population and number of catastrophic collisions over the next 200-year period (2011-2210) using five scenarios, each corresponding to a set percent success rate of the 25-year PMD rule. “A catastrophic collision occurs when the ratio of impact energy to target mass exceeds 40 J/g,” as defined in the study, “…whereas a non-catastrophic collision only results in damage to the target and generates a small amount of debris.”
The projection simulation assumed an 8-year lifetime for spacecraft. Over the projection period, no station keeping and no collision avoidance maneuvers were implemented. Also, only man-made objects 10 cm and above were included in collision consideration. And no explosions were allowed for spacecraft and rocket bodies after 2011.
Liou’s study found that even at a 95% compliance rate with the 25-year PMD rule, on average, 26 catastrophic and 19 non-catastrophic collisions can be expected in the next 200 years. In case the rule is not implemented over the next 200 years, on average, 71 catastrophic collisions and 76 non-catastrophic collisions are expected to occur.
Since the 1990s when the development of mitigation measures began, the effectiveness of PMD has been demonstrated and documented. PMD is the first and the most cost-effective defense against future population growth.
Watch a trajectory simulation of the debris-generating 2009 collision of Iridium 33 and Cosmos 2251 below: