An Orbital House of Cards: Frequent Megaconstellation Close Conjunctions

TL;DR

The paper tackles the rising risk to the orbital environment from megaconstellations by introducing the CRASH Clock, a Key Environmental Indicator that estimates the time to a potential catastrophic collision under no-manoeuvre conditions. It combines RSOs density distributions, collision cross sections, and typical relative speeds across LEO to compute a global collision timescale, validated against direct N-body simulations. The analysis shows a dramatic increase in stress since 2018, with a June 2025 value of $\tau_{\rm col}=5.5$ days (vs $164$ days in 2018) and a 24-hour collision probability around $17\%$, driven largely by dense megaconstellation shells such as Starlink at $\sim550$ km. The CRASH Clock is proposed as a flexible, policy-relevant KEI to monitor orbital stress and guide risk mitigation, acknowledging that it is not a fixed threshold for runaways but a tool to inform safer constellation design and space-traffic management.

Abstract

The number of objects in orbit is rapidly increasing, primarily driven by the launch of megaconstellations, an approach to satellite constellation design that involves large numbers of satellites paired with their rapid launch and disposal. While satellites provide many benefits to society, their use comes with challenges, including the growth of space debris, collisions, ground casualty risks, optical and radio-spectrum pollution, and the alteration of Earth's upper atmosphere through rocket emissions and reentry ablation. There is potential for current or planned actions in orbit to cause serious degradation of the orbital environment or lead to catastrophic outcomes, highlighting the urgent need to find better ways to quantify stress on the orbital environment. Here we propose a new metric, the CRASH Clock, that measures such stress in terms of the timescale for a possible catastrophic collision to occur if there are no satellite manoeuvres or there is a severe loss in situational awareness. Our calculations show the CRASH Clock is currently 5.5 days, which suggests there is limited time to recover from a wide-spread disruptive event, such as a solar storm. This is in stark contrast to the pre-megaconstellation era: in 2018, the CRASH Clock was 164 days.

Figures (2)

• Figure 1: Orbit-averaged volume density distribution of RSOs by classification (left) and origin (right) as of 25 June 2025. Left: 'Payload' refers to active and defunct satellites, 'Tracked Debris' only those debris pieces that are reliably tracked in the debris catalogue, excluding abandoned rocket bodies and defunct satellites. Right: Densities of active constellations and megaconstellations, as well as tracked debris, debris from the 2009 Iridium-Cosmos collision, and debris from the 2007 Fengyun 1C ASAT test.
• Figure 2: Average time between conjunctions $< 1 \, {\rm km}$ as a function of altitude for the 2018 (left) and 2025 (right) RSO populations. The dips around $500 \, {\rm km}$ correspond to the peaks in RSO density at the same altitude presented in Figure \ref{['fig:density']} due to Starlink. Our analytic calculation is shown in dark blue, with the simulations overlaid in red. The simulation duration was one month.