Speaker
Description
The introduction of a $\ge90\%$ Probability of Successful Disposal (PSD) requirement by the European Space Agency marks a significant advancement in support of space debris mitigation and Clean Space objectives. At the same time, it raises important questions regarding its consistent interpretation and practical implementation. While the requirement is formally defined in ESSB-ST-U-007, several aspects remain open when translated into system engineering practices and reliability modelling.
One key issue concerns the definition of mission time within the reliability assessment. The current wording suggests coverage up to the end of disposal operations, potentially encompassing all mission phases from launch onwards. However, established reliability engineering practices typically treat launch separately due to differences in qualification approaches and responsibility boundaries. This distinction may become less clear for New Space missions, where heritage is limited and qualification strategies are evolving. Further clarification is needed regarding the scope of PSD. In particular, questions arise as to whether consumables (e.g., residual propellant), passivation measures, and accidental break-up mechanisms should be explicitly included in the modelling. Additionally, emerging considerations such as dark and quiet sky constraints prompt discussion on whether these should be incorporated within PSD or treated as complementary requirements. The timing and criteria for disposal decision-making introduce further complexity. It remains questionable to what extent factors such as accumulated degradation, loss of redundancy, in-flight anomalies, or operation beyond the nominal design lifetime should be reflected in PSD assessments at the point when disposal is initiated (agreed lifetime or lifetime extension). From a modelling perspective, exponential reliability models are widely used and remain appropriate for systems with a constant failure rate. However, their applicability may be limited in scenarios involving critical items and extended mission phases or life extensions, where increasing failure rates over time should be accounted for (e.g., wear-out, mechanical, and propulsion items). Finally, the role of software in enabling successful disposal, including autonomy and fault management, raises the question of how software-related failures should be represented within the PSD framework.
Rather than proposing definitive interpretations, this contribution identifies key areas requiring clarification and encourages a structured exchange within the community. The objective is to support a consistent, technically sound, and operationally practicable understanding of PSD aligned with both engineering realities and Clean Space ambitions.