4.       The most critical satellites and debris, because they are the most numerous and they orbit in a limited space, are those in Earth orbits. Starting from the most distant orbits to the closest, the strategies adopted by ESA are the following:

• • • ·         The satellites located at the Lagrange points of the Earth-Sun system (distance to Earth of about 1 200 000 km) do not pose particular difficulties, although they are in an area of Space whose demand increases: at the end of their life they can free this area by being moved to a solar orbit from which the risk of return to Earth is almost null (see slides).
    • Satellites in very eccentric orbits (up to more than 100 000 km of apogee): orbit maneuvers are applied to make end-of-life satellites re-enter the atmosphere at safe latitudes about two decades later
    • Closer to Earth (36 000 km), geostationary satellites are located in very congested orbits. The maneuvers to make these satellites re-enter in Earth’s atmosphere at the end of their life would be too costly. They are therefore moved to “parking” orbits to free up space and avoid the risk of collisions with active satellites. The same strategy is applied to the upper stages of launchers. The sustainability of this strategy raises however questions quite similar to those that arise on low orbits (see next)
    • Satellite constellations for terrestrial navigation (about 20 000 km – 25 000 km) are at altitudes that make it very difficult to move them towards parking orbits. Fortunately, to date (2011) no incident (collision or explosion) has ever disturbed the use of this category of orbits
    • Low Earth orbits (1000 km to a few thousands km) are those in which both the congestion of satellites in circulation and the accumulation of debris and launch vehicle fragments are most critical: they concentrate two thirds of the objects launched into space from Earth. The situation is even worse since it is in these orbits that mega constellations of satellites such as Starlink have recently been accumulating.

To ensure the sustainability of the operations at these low orbits, the control of debris and/or inactive satellites is of utmost importance.

Various actions arepossible :

1. When satellites are active, monitor their environment and perform the necessary maneuvers to avoid collisions with drifting debris or other satellites
2. Passivation of launchers stages: depleting residual fuel, venting pressure tanks and discharging batteries
3. Reduction of the number of inactive satellites to avoid cascading collisions that could hinder the use of these orbits. To this end, a duration of less than 25 years in orbit after the end of the mission is recommended

At these altitudes, the most effective solution to reduce the number of inactive satellitest is to move them towards lower orbits where the increase in the density of the atmosphere will slow them down progressively, and drag them into an atmospheric re-entry during which they will be destroyed

This last phase is critical because most of the time the moment and the position of the beginning of re-entry are not controlled. It is then necessary to master sufficiently well the physical phenomena of re-entry, and the mechanisms of destruction of the satellites and their components, in order to guarantee  the absence of risk for the populations with an extremely high probability.

The ATD3 Workshop focuses on the issues that need to be addressed to provide this guarantee:

• mastering the space environment in which the objects in orbit evolve,
• physical and numerical modeling of the atmospheric re-entry of space objects (satellites, launchers…),
• design of satellites allowing to control the destruction process during re-entry.

References

[1]  https://www.esa.int/Safety_Security/Space_Debris/Space_debris_by_the_numbers , issue of 4 April 2022

[2] : ESA’S ANNUAL SPACE ENVIRONMENT REPORT – GEN-DB-LOG-00288-OPS-SD – 22 April 2022 https://www.sdo.esoc.esa.int/environment_report/Space_Environment_Report_latest.pdf