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著者人数: 19名, 形態: カラー図版あり, Number of authors: 19, Physical characteristics: Original contains color illustrations, Accepted: 2021-01-14, 資料番号: PA2110015000

Hazards from the outer space environment either natural (space weather and asteroids) or artificial (space debris and the growing number of satellites launched to orbit) pose a rising risk to space flight activities. The awareness for space sustainability and space safety has seen a continuous increase in recent years and does not stop at the Earth's sphere of influence. The first spacefaring nations start thinking about sustainability in cislunar space and the Martian environment. This work deals with the issue of space debris in Martian orbits in the light of planetary protection. A Mars Sustainability Framework has been developed. This includes a study on the orbital and regulatory environment of Mars, long-term propagation of orbits of artificial objects and the two natural moons, and the analysis of objects evolution and first approaches for collision probability computation. With this work, the issue of space debris beyond Earth orbit is analysed at an early stage.

In recent years, several metrics have been proposed to quantify the impact of a mission on the space debris environment. In our previous work, we introduced the environmental capacity as the number and typology of missions that are compatible with the stable evolution of the debris environment. This concept enables the evaluation of the effectiveness of mitigation guidelines by looking at the use of environmental capacity due to existing missions. The current work will investigate how the same concept can also be applied as a tool during the design of a mission, facilitating the comparison of different mission architectures depending on their overall contribution to the debris environment. First, the paper will present the latest updates to the debris index used as a metric of the environment capacity, including the inclusion of a penalisation for objects with low trackability and an improved model for the break-up probability. Then, several application cases will be discussed to demonstrate its use as design driver. First, the relevance of adopting operational best practices (i.e. collision avoidance and disposal manoeuvres) will be studied for different spacecraft classes as a way of quantifying better than required behaviour. Then, the environmental impact of a mission will be analysed considering the case when the mission is realised through a single large satellite or through a constellation of smaller satellites. Finally, different end-of-life scenarios will be assessed to demonstrate the use of the method to discriminate between various technologies on a case-by-case basis. The adoption of passive de-orbiting systems will be compared to conventional disposal through de-orbit burns or operations at lower altitudes to gain insight into how two different options, both compliant to existing guidelines, differ in terms of the resulting environmental impact. Finally, a scenario with active-debris-removal will be considered, showing how the quantification of the impact on the debris environment can support the selection of parameters such as the rendez-vous altitude, based on the expected success rate of the capture and final disposal phase.

The current debris mitigation guidelines are formulated by considering single objects, so they cannot dynamically account for variations in the space traffic, or for the actual current level of implementation and success of post-mission disposal manoeuvres. To address this limitation, the concept of space traffic management based on the environment capacity was proposed. Following this approach, a mission (single or multiple satellites and launch vehicles) is evaluated, before its launch, not only with respect to existing mitigation guidelines, but also with respect to what can be accommodated by the environment (e.g. not exceeding a defined risk level), considering objects already in orbits and other planned future missions. In this way, similarly to the approach used for frequency allocation in GEO, each mission can use a share of the environment capacity through a process that may define, for example, a maximum capacity consumption per year. In order to implement such a scheme, a measure of the debris environment criticality is required. Different formulations for such a metric have been proposed and, in the present work, the ECOB index (Environmental Consequences of Orbital Breakups) is applied. This work will give a brief overview of the most recent changes in the formulation of the index and will discuss examples of applications to the current environment. Different Post-Mission Disposal success rates and future launch traffic scenarios will be analysed. In this context, the evaluation provided by the debris index will be compared to other formulations available in literature and to the one from other indicators such as number of objects and catastrophic collisions. A comparison with operational data from ESA's collision avoidance service will be also presented. Finally, it will be analysed how such an index can be connected to the concept of the environment capacity and a concept for its inclusion within a space traffic management process will be formulated.

Libration Point Orbits (LPOs) and Highly Elliptical Orbits (HEOs) are often selected for astrophysics and solar terrestrial missions. No guidelines currently exist for their end-of-life disposal. However, as current and future missions are planned to be placed on these orbits, it is a critical aspect to clear these regions at the end of operations to avoid damage to other spacecraft and ensure on-ground safety. This paper presents an analysis of possible disposal strategies for LPO and HEO missions as a result of a European Space Agency study. The dynamical models and the design approach are presented for each disposal option. Five current missions are selected as test cases Herschel, Gaia, SOHO as LPOs, and INTEGRAL and XMM-Newton as HEOs. A trade-off on the disposal options is made considering technical feasibility, as well as the sustainability context.