With increasing space activities, a new and unexpected hazard started to emerge: space debris. In almost 50 years of space activities more than 4800 launches have placed some 6000 satellites into orbit, of which only a minor fraction of about 1000 are still operational today. Besides this large amount of intact space hardware, with a total mass of about 6000 tonnes, several additional objects are known to orbit the Earth. More than 16,000 in total are regularly tracked by the US Space Surveillance Network and maintained in their catalogue, which covers objects larger than approximately 5cm to 10cm in low Earth orbit (LEO) and 30cm to 1m at geostationary altitudes (GEO).

Only 6% of the catalogued orbit population are operational spacecraft, while 33% can be attributed to decommissioned satellites, spent upper stages, and mission related objects (launch adapters, lens covers, etc.). The remaining 61% originate from more than 200 on-orbit fragmentations which have been recorded since 1961. Except for a few collisions (less than 10 accidental and intentional events), the majority of the 200 break-ups were explosions of spacecraft and upper stages.

Collisions might be the prime source for new objects in the mid-term future. With the current rate of catastrophic collisions (of approx. 1 in 5 years) and the current level of implementation of mitigation measures one can expect the rate to increase. A consequence of this might be a rising share of feedback collisions (i.e. collisions involving fragments from previous collisions) which are a sign of an inset of a chain reaction (Kessler syndrome). Recent studies using environment evolution models have suggested that the current status of the environment in LEO might be already unstable, meaning that there is a high likelihood for an increase of the number of objects even if no further object is actively added to the environment.

This has raised the question of additional counter measures apart from the currently suggested mitigation actions. Environment remediation via the active removal (through a dedicated mission with e.g. robotic capture and retrieval means) will be the only possible way to prevent an uncontrolled growth. This technique requires an intelligent selection of the removal targets, meaning that objects which:

- Have a high probability of collision
- and for which the long-term consequences on the environment of such a collision are the most severe.

Both points shall be tackled in this study using a systematic and parametric simulation approach.

In addition to this, ESA’s role in this context shall also be considered. ESA has not contributed to this problem. This is mainly due to the fact that a considerable share of the missions so far went into non-Earth-bound orbits. In order to maintain the currently low contributions to the pollution of near-Earth space, a close look at ESA’s assets in orbit and to those that are in the planning phase needs to be taken.

An internal study found that, being stationed or transiting densely populated regions ESA's currently 55 (operational and non-operational) and 37 upper-stages in Earth-bound orbits generate a non-negligible collision risk. Depending on the impactor, the resulting combined mass and the type of hit (central, lateral,...) and in particular depending on the target orbit, different debris clouds and different cloud evolution can be expected.

In the light of the discussion above, ESA’s on-orbit risk potential and long-term consequences shall be analysed in parallel and along with the identified approach.

The objectives of this study shall be:

• To review the existing work on break-up models, environment predictions and analysis of past collision events
• To select representative objects in terms of mass and orbit region
• To setup the required simulation environment
• To apply a fragmentation model under a limited number of assumptions on collision geometry and impactor mass, orbit and epoch
• To analyse the evolution of the fragments, their lifetimes and the history of the spatial density they generate
• To describe the environment evolution and effects with a simple analytical approach to be calibrated with the simulation results
• To analyse the time dependent flux on the most frequently used operational orbits
• To analyse the impact of the events on the evolution of the future environment
• To propose a reasonable scheme to quantify the environmental criticality of a spacecraft based on these results.

The activity is carried out by a consortium constituted by IFAC/CNR (prime contractor), ISTI/CNR and University of Southampton (UK).