The space community loosely divides objects in space into ‘active satellites’, ‘dead and leftovers’ such as derelict satellites and rocket stages, and ‘debris’, resulting from fragmentations, explosions and collisions. As described above, debris can in turn be divided into ‘tracked debris’, down to 10 cm size, and smaller ‘untracked’ debris, which can only be estimated. The number of debris objects grows faster with time than the leftover population, and specific events like the Iridium–Kosmos collision in 2009, and anti-satellite weapons tests like those conducted by China, the USA, India and Russia in 2007, 2008, 2009, 2019, and 2021 respectively, can cause large leaps (Fig.1).
A concern arising from these trends is that certain orbital highways will exceed their carrying capacity, rendering them unusable. This saturation would become manifest when our decisions and actions can no longer prevent the loss, disruption, or degradation of space operations, services and activities. When we launch dozens of satellites every few weeks, we remove the environment’s ability to inform us of the unintended consequences of our actions and we cannot predict what the dynamic equilibrium state actually is. To wit, it clouds our decision intelligence.
Classifying collisions
We can roughly classify collisions into minor, disabling and disrupting or lethal. Anything 1 mm in size or larger can cause minor damage, such as perforating a solar array. This can include natural micro-meteoroids as well as satellite debris. A piece of debris 10 cm in size will have a mass of about 1 kg, and if it is moving at 10 km/s (typical for relative velocities in LEO), it can completely destroy an active satellite [4]. Between these extremes, a 1 cm piece of debris is capable of disabling an active satellite [4]. Note that pieces as small as 1cm are not currently tracked, and even very small pieces of debris or micrometeoroids can cause damage, as seen in the recent case of an impact on the Canadian robotic arm of the International Space Station.
Risk of disruptive (lethal) collisions
Without avoidance methods, the current debris density means there will on average be one collision per satellite every 50 years in LEO, with a piece of debris that is 10 cm or larger [4]. However, large objects are tracked and orbital elements made publicly available, so that potential collisions can be predicted and actively avoided. ‘Conjunctions’, where one satellite passes within a few km of another, happen many times every year, but so far only one major accidental collision has taken place. The presence of large constellations will increasingly put any avoidance manoeuvring system under severe stress, with some close calls summarised and analysed in [3, 4].
Risk of disabling collisions
Calculating the likelihood of disabling damage by debris with size > 1 cm to active satellites is a complex problem requiring many physical variables. Note that disabling a satellite leaves an uncontrolled derelict that may then be a danger to other spacecraft. Simplified modelling of the possible future is given in the recent comprehensive JASON report [4]. This includes allowance for continually de-orbiting satellites at the end of an assumed 5 year lifetime. For a target population of 10,000 active satellites, debris grows only slowly but we can expect about 300 disabling collisions within the next 30 years (Figure 6a). For a target population of
