Orbital Debris and Sustainability of Space Operations

The orbital particle environment around the Earth is dominated by man-made space objects, except for a limited particle size regime below 1 mm, where meteoroids provide a significant contribution, or may even prevail in some orbit regions. The mass of man-made objects in Earth orbits is on the order of 6,300 t, of which more than 99% is concentrated in trackable, cataloged objects larger than typically 10 cm. The mass of meteoroids within the regime of Earth orbits is only on the order of 2–3 t, with most probable sizes around 200 μm. As a consequence of their size spectrum and associated mass man-made space objects, in contrast with meteoroids, represent a considerable risk potential for space assets in Earth orbits. To assess related risk levels a good understanding of the space debris environment is essential, both at catalog sizes and sub-catalog sizes. The derivation process and the key elements of today’s debris environment models will be outlined, and results in terms of spatial densities and impact flux levels will be sketched for those orbit regions that are most relevant for space applications.

To cope with the existing space debris environment spacecraft can actively mitigate the risk of collisions with large-size, trackable space objects through evasive maneuvers. Alternatively, or in addition, the risk of mission-critical impacts by non-trackable objects can be reduced through shielding, in combination with protective arrangements of critical spacecraft subsystems. With a view on the future debris environment international consensus has been reached on a core set of space debris mitigation measures. These measures, which will be explained in more detail hereafter, are suited to reduce the debris growth rate. However, even if they are rigorously applied they are found to be inadequate to stabilize the debris environment. Long-term debris environment projections indicate that even a complete halt of launch activities cannot prevent the onset of a collisional run-away situation in some LEO altitude regimes. The only way of controlling this progressive increase of catastrophic collisions is through space debris environment remediation, with active mass removal, focused on retired spacecraft and spent orbital stages.