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 Telecommunications Union (ITU) internationally, to ensure that the activities of operators (including radio astronomers) do not interfere with each other. In this framework, orbital space, and the activities that involve looking through it, are already implicitly considered “environment”.

The optical sky as an environment

We can use the same approach/framework for the optical/infrared sky. It is not necessary to be in space to be interacting with it. The sky constitutes the working environment for astronomy and stargazing, and this inescapably includes orbital space: it can be argued that astronomers carry out space activities in the sense conveyed in Articles IX and XI of the Outer Space Treaty of 1967 [13]. The sky environment has also important cultural significance and has inspired strong traditions around the world since the beginning of human history, such as Maori New Year being associated with the heliacal rising of the Pleiades, or Indigenous Polynesian's star-based navigation [16].

Orbital space as an environment

Within orbital space itself, the Outer Space Treaty [13] and Liability Convention [14] recognise that the activities of each operator have potential consequences for other operators. However, there is a growing sense worldwide that we should be explicitly considering the sustainability of space activities, and considering orbital space as an environment. For example, the recent G7 summit issued a statement on space sustainability and the World Economic Forum has partnered with the European Space Agency and the University of Texas at Austin, amongst others, to develop a Space Sustainability Rating. Establishing the principle that space activities are subject to environmental laws such as NEPA would be a key step in translating such international good wishes into concrete action.

Many human activities have a locally constrained environmental impact. Most space activities are however inherently global. A satellite may launch from California, but an hour later it is flying over France. Conversely, a Chinese or Russian system will soon appear in the sky over the USA. A coordinated international approach is therefore crucial, but this has to start with each sovereign state recognising its global responsibility. Again, there is a close similarity with other environmental issues such as climate change, or plastic in the sea.

Cumulative effects and emergent behaviour

The incremental impact of any single proposal for a satellite constellation may be relatively modest, but if all such proposals are allowed because their individual impact is deemed to be modest, the cumulative effect could nevertheless be extremely serious. Furthermore, because of complex interdependencies, the emergent behaviour is not a simple addition and is extremely hard to predict. This is also the case in other environmental issues, such as climate change, and it is widely accepted that environmental assessments need to carefully account for such emergent behaviour. Similar principles should apply to orbital space. Much like other ecosystems, orbital space has a finite "carrying capacity" for traffic. This limit has yet to be globally defined, but it should be evident that if everyone freely populates orbital space without a jointly managed system, this orbital carrying capacity is likely to become saturated, making specific orbital "highways" useless for the safe conduct of space operations and activities. In fact, we should be motivated to define a Space Traffic Footprint,