# Beyond Mitigation: New Metrics for Space Sustainability Assessment +
## 1 Introduction  
The rapid proliferation of satellites in Low Earth Orbit (LEO) has raised significant concerns regarding space sustainability, compelling stakeholders to reevaluate current practices and frameworks governing space activities [1, 2]. As global interest in spaceexploitation intensifies, the associated risks--particularly the growing menace of space debris--pose substantial threats to the safety of space activities. There is an urgent necessity for responsible resource management and proactive measures to mitigate environmental impacts, aligning with the ethical imperatives of modern space endeavours. The current paradigm, which predominantly focuses on debris risk mitigation, is insufficient for addressing the complex challenges of sustainable space exploitation. A paradigm shift is needed that prioritizes sustainability from the onset of mission design [3, 4, 5, 6, 7]. In this framework, to enable an optimized constellation/mission design able to trade system performance and impact the long-term sustainability of space activities, the definition of novel KPIs would play a crucial role. Such KPIs could enable a more nuanced understanding of how different design alternatives impact long-term sustainability, allowing for systematic comparisons and informed decision-making. As a matter of fact, there has been some effort in the direction of _measuring_ the impact of constellations/missions in terms of space sustainability, such as the initiative of the Space Sustainability Rating (SSR) [8]. The SSR uses a _tiered scoring system that takes a series of metrics based on models previously published by agencies and academic institutes that serve to quantify and measure sustainability decisions taken by operators, without disclosing confidential mission data and proprietary information_. A rated entity will receive a Tier Score that will determine the rating level between Bronze, Silver, Gold or Platinum. The SSR utilizes a composite indicator of modules. Selected modules take into account the level of harmful physical interference caused by the planned design and mission operations, the ability to detect, identify and track small objects that might be operational but cannot usually be reliably tracked and identified, collision avoidance capabilities, data sharing between operator and various communities, and the adoption of standardization concepts in design and operations where possible. However, the SSR is a certification that a company obtains after submitting their mission, and it is unsuitable for this paper, which provides system designers with a flexible tool to optimize their design, meeting the best trade-off between performance and long-term space sustainability. With this objective, this work introduces a preliminary framework for establishing a novel KPI that takes into account in a compact way the main parameters and design choices that might impact the space sustainability of the designed constellation/mission. A key characteristic of such KPI is that it also takes into account the performance requirements of the specific mission, thus giving higher scores to the design that achieves the best trade-off between performance and impact on space sustainability.  
The paper is organized as follows: Section 2 presents the concept of space sustainability by design; Section 3 introduces the proposed methodology for building a novel KPI that could play an important role in the design phase of any space mission; using the proposed methodology, Section 4 show the comparison of two of the most important current satellite mega-constellations, namely OneWeb and Starlink, for global broadband communications. KPIs are also used to better understand the impact of different design choices on space sustainability; finally, conclusions are drawn in Section 5.