Evaluating the short and medium term impact of space activities in low Earth orbit

The evolution of cataloged orbital debris in low Earth orbit (LEO) over the last quarter of century was analyzed in detail, to gather insights on the development of space activities, on the effectiveness of the debris mitigation measures recommended in the meantime, and on the environmental impact of fragmentations, in particular collisions, both intentional and accidental. The main conclusion was that the observed evolution matched on the whole the predictions of the unmitigated business-as-usual scenarios simulated twenty years ago, and that the benefits caused by the progressive worldwide adoption of mitigation measures were unfortunately offset by a couple of catastrophic collisions and prolonged weak solar activity. Nevertheless, and despite the dramatic increase of launched satellites since the mid-2010s, nowhere have the signs of more than linear increases been revealed so far, aside from a few sizable, but circumscribed, fragmentation events. Even though above 700 km the debris population may be intrinsically unstable in the long run, the situation can still be managed and controlled. Therefore, even though the overall picture has worsened during the last 25 years, nothing of irremediable has been done so far. This does not mean that extreme care is not required in planning and conducting new space activities from now on, especially in a phase of increased and ever more rapid exploitation of space, to ensure its long-term sustainability and full utilization. In order to assess the sustainability of space activities, especially in the short and medium term, that is over the next 10-30 years, several environmental criticality indexes have been introduced and discussed, estimating their current values in LEO, as well as their upper limits - sometimes considered tolerable, others not - associated with specific scenarios of debris growth. They could provide simple tools for evaluating the relative and absolute impact on the debris environment, either in LEO as a whole or in specific altitude shells, of new spacecraft deployments and operations, as in the case of mega-constellations of satellites.