Here's a thread of today's talk at the European Conference on #SpaceDebris:
Title: The use of stress tests to evaluate space debris mitigation measures.
My co-authors were Emily Wymer and Tara Landmark, both from @unisouthampton

We typically base assessments of potential space debris mitigation measures on "best-case" scenarios (e.g. "Business As Usual"). This works well in general but might not reveal the true robustness of these interventions or any hidden issues.

[graph in previous tweet shows simulation results of future projections of the > 5 cm orbital population for different post-mission disposal options, including the so-called "25-year rule"]

A hidden issue of the "25-year rule" is the increase in collision activity it causes at altitudes below 600 km. This issue has been identified in very long-term projections & in projections that include large constellations. Neither can be considered "Business As Usual"

[graph 1 in previous tweet shows perigee & apogee altitudes of objects involved in collisions over a very long projection period - large proportion of these collisions involve objects on 25-year disposal orbits]

[graph 2 in previous tweet shows collision "density" - # of collisions per cubic km - for a projection involving a large constellation at 1100 km where constellation satellites adhere to the "25-year rule"]

We propose the idea of stress testing; something that is now done routinely in the financial sector to see how robust a financial institution/organisation/instrument is to an economic crisis. Our proposal is based on hypothetical "what if" scenarios involving environmental crises

The "what if" scenario we chose was the near-simultaneous breakup of 50 statistically most-concerning derelict objects in LEO. These objects were identified in a previous study (McKnight, 2020)

[graph in the previous tweet shows the inclination and average altitude of the top 50 statistically most concerning derelict objects in LEO from McKnight et al, 2020]

We followed a simple 2 x 2 design:
Simulation 1: no PMD, no stress
Simulation 2: no PMD, with stress (50 objects breakup)
Simulation 3: PMD @ 90% compliance rate, no stress
Simulation 4: PMD @ 90%, with stress
We measured the change in the # of catastrophic collisions

[graph in the previous tweet shows an example result from the 2 x 2 study, where the difference in the number of catastrophic collisions can be calculated appropriately to measure the effect of the stress test on the post-mission disposal]

The effect of the near-simultaneous breakup of the 50 large derelict objects was to increase the population > 10 cm by 350%. There was no initiation of a catastrophic collision "chain reaction". Post-mission disposal still provided important & substantial benefits

[graph in previous tweet shows the number of objects > 10 cm in the orbital population for the four (2 x 2) simulations conducted. Without PMD the population grows quickly & exponentially w/ or w/o the breakup of 50 objects]

The catastrophic collision rate increases by a factor of 4 immediately after the breakup of the 50 derelict objects but PMD then reduces the rate to the same level observed w/o those breakups (i.e. without the stress test) indicating a generally positive effect.

[graph in the previous tweet shows the cumulative number of catastrophic collisions over the projection period for the 4 simulation cases; the benefits of PMD are revealed through its impact on the collision rate]

Focusing on the performance measure/indicator: above 650 km PMD remains effective and robust to the stress test (the values shown are all < 0 which indicates a beneficial outcome). Below 650 km, PMD produces a small increase in collision activity, as anticipated.

[graph in previous tweet shows the change in the number of catastrophic collisions versus altitude as a result of the implementation of post-mission disposal]

Below 650 km we found PMD to increase collision activity. The stress test appeared to enhance this unfavourable effect by a small amount (likely not to be statistically significant but it is consistent across multiple altitudes)

[the graph in previous tweet shows the change in the number of catastrophic collisions versus altitude as a result of the implementation of post-mission disposal; only altitudes below 650 km are shown]

(1) We have seen that post-mission disposal - the "25-year rule" - increases collision activity by a small amount below 650 km due to the density of disposal orbits crossing that region. The stress test may have enhanced this effect slightly

(2) Nonetheless post-mission disposal remained an effective space debris mitigation measure, especially at altitudes > 650 km, and appeared to be robust to the effects of the stress test (near-simultaneous breakup of 50 large derelict objects)

(3) the effects of the near-simultaneous breakup of the 50 derelict objects were to increase the orbital population by 350% and the catastrophic collision rate by 400%

(4) We propose the introduction of stress testing - with appropriately designed hypothetical scenarios - to fully understand and evaluate the robustness of space debris mitigation measures.

The end.
Thanks for reading this far!