## 2 Literature Review
### LCA and System Boundary  
Life cycle assessment (LCA) is a tool used to evaluate the environmental impacts and resource utilization throughout a product's life cycle, encompassing stages from raw material acquisition through production and use phases to waste management. The methodological development in LCA has been robust, and it is widely applied in practice [37]. This life cycle encompasses a series of interconnected stages, beginning with the acquisition or generation of raw materials from natural resources and concluding with the final disposal of the segment [38]. It is widely used across many industries as a tool to compare two products and their environmental footprints, allowing the selection of the product with the lowest environmental impact. To perform LCA, a systemic perspective is needed to support decisions that have effects on the sustainability of policies, production systems and services, i.e., the environmental, social, and economic spheres in which the concept of sustainability is articulated. This is because measures aimed at achieving one specific goal (like reducing greenhouse gases) might involve negative consequences that were not considered in the first instance, whether they be different impact types (i.e., triggering unexpected environmental, social, or economic mechanisms) upon different geographical areas or stages of the value chain of a product or service (i.e., the life cycle). Including all these aspects in the decision-making process to avoid so-called \"burden shifting\" is a great advantage that the application of life cycle thinking brings to the development of policies, products, and services in today's globalized context [39]. ISO standards 14040 and 14044 deal with LCA:  
1. ISO 14040:2006 Environmental management--Life cycle assessment--Principles and framework; this provides a clear overview of the practice, applications, and limitations of LCA to a broad range of potential users and stakeholders, including those with a limited knowledge of life cycle assessment;
2. ISO 14044:2006 Environmental management--Life cycle assessment--Requirements and guidelines; this is designed for the preparation of, conduct of, and critical review of, life cycle inventory analysis. It also provides guidance on the impact assessment phase of LCA and on the interpretation of LCA results as well as the nature and quality of the data collected [7]. When it comes to \"life cycle assessment\" or \"analysis\", several of NASA studies do not match with the ISO standards discussed above. They cover only economic costs, omitting the environmental impacts [9]. Therefore, NASA studies were excluded from our research. However, based on the generic ISO standards that drive LCA studies in any sector, ESA has developed a space-specific LCA that can be used for space missions. Space System Life Cycle Assessment guidelines aim to establish the methodological rules on how to correctly perform space-specific LCAs. It can be applied on two levels directly [10]:  
Level 1: System level (Space system/launch segment/ground segment/space segment). Figure 1 below represents the entire system boundary of a space mission. The ESA handbook is formatted in a way that impact assessment in terms of midpoint indicators can be calculated for the entire space mission or even for individual segments without taking into account other segments. Level 2: Equipment/component/material/process. ESA LCA handbook is used to perform the analysis in this paper. For the analysis, we focus on LEO constellation satellites (space segment from Level 1) and on Level 2 (equipment/component/material/process), considering that the LCI from the ESA database includes subsystem -level and component -level mass budget data. Figure 2 below shows the Level 2 classification on C + D (detailed definition + qualification and production). In this paper, we focus solely on the space segment. The calculations are further limited to the C + D phase, encompassing the production of platform, payload and propellant.