Validation and Verification of the long-term dynamic evolution of non-operational satellites in LEO to enable Active Debris Removal missions
Authors:   ABSpaceConsulting, La Baule-Escoublac, France, Benoit Alain , Soares Tiago , Pereira Vasco , Conings Vincent , Padilla Estefania , Melone Enrico , Kruse Martin , Offterdinger Peter , Oelze Holger 
Published: January 2024

ESA under the umbrella of the Clean Space Initiative has promoted complementary activities in the area of Active Debris Removal (ADR). First efforts were devoted to the design of an ADR vehicle which would perform the rendezvous and capture with the orbital debris. It has become clear that the removal of unprepared satellites was extremely risky and complex. One of the main challenges driving the complexity of the rendezvous and capture of a debris is that its angular rates can build-up when the satellite becomes non-operational. Observations of several non-operational LEO satellites often show angular rates above 2 deg/s. The prediction and estimation of the angular rates and attitude of non-controlled satellites to be captured is therefore crucial for the design of the chaser and to confirm the feasibility of these critical operations.

This paper proposes preliminary guidelines for the verification of the Passive Magnetic Detumbling (PMD) performance of a satellite in Low Earth Orbit which would have been unable to perform the required end-of-life functions (e.g. failure to perform controlled re-entry due to loss of mission). Such satellite is assumed to have been prepared for Design for Removal (D4R) during its development phase and to be equipped in particular with a Magnetic Detumbling System (e.g. automatic short circuiting of Magnetic Torquers), 2D Navigation Aids including features to support ground-tracking and attitude reconstruction, 3D Navigation Aids to support precise pose and attitude determination for the last phase of the capture and a Mechanical Capture Interface to allow its capture with a robotic gripper.

To set up an adequate simulator and to master the simulation process, it is necessary to first acquire a good understanding of the impact of the different disturbances and of the Magnetic Damping System on the long-term dynamic of non-controlled satellites in LEO.

Although simulations will tackle all dynamic configurations including tumbling at low angular rates, the spinning configuration deserves special investigations and analytical models based on spin-averaged and orbit-averaged torques and spinner dynamics have been developed by ESA and ABSpaceConsulting. They are very helpful to interpret the complex behaviour shown by the simulations, identify S/C driving parameters and representative initial conditions, and sweep parameters accordingly. This is why a quick theoretical survey of the impact of Gravity Gradient, Solar Radiation Pressure (YORP effect), S/C Residual Magnetic Dipole, atmospheric drag, Eddy currents and of course Magnetic Detumbling System is first proposed in this paper, highlighting which effects will guide the simulation exercise.

Due to the complexity of the YORP effect created by Solar Radiation pressure and thermal infrared reemission, together with the low authority of current Passive Magnetic Damping Systems (short-circuited Magnetic Torquers and Eddy currents), the verification process to assess the PMD (Passive Magnetic Detumbling) performance will mainly rely on extensive simulation campaigns performed on a dedicated High-Fidelity simulator.

Guidelines related to the set-up of a representative PMD High-Fidelity simulator are proposed in the paper. This covers a list of specific features which need to be represented (or not) in the satellite dynamics and the space environment simulation. Having to perform open-loop (no AOCS!) long duration simulations, numerical integration methods require special attention to avoid artefacts and wrong conclusions.

A list of S/C and orbit parameters driving the PMD performance will then be proposed. The interest of an analytical framework to quickly sweep parameters, perform sensitiveness assessments and correlate future High-Fidelity simulations will be explained.

Dedicated analyses related to YORP effect for an asymmetric S/C with a single, laterally deployed Solar Array will be suggested (impact of the thermo-optical properties of the front and back faces, of the misalignments of the Solar Array Drive axis with respect to the S/C principal axes …) together with mitigation actions.

The paper will then provide selected examples of precursor simulations with their interpretation, covering a variety of initial conditions for Copernicus-like missions.

Another strategic verification step, a post-launch one, will be the attitude reconstruction from the ground to be performed in-orbit, in order to assess GO/NO GO before a specific ADR mission. The current progress of R/D studies related to attitude reconstruction using the innovative navigation aids will be presented.

At Hardware level, i.e. short-circuited Magnetic Torquers rotating in a weak magnetic field, a major validation milestone was the confirmation by tests that the magnetic core was indeed excited at the very low regimes of tumbling-induced currents. Then a method to obtain the magnetic tensor from measurement data was designed by ZARM Technik AG and successfully applied in 2022. Since the expected low inductions in the magnetic torquer are too small to be measured conventionally a three-step method was proposed. A multiple number of magnetic torquers were examined by ZARM Technik AG, permitting to correlate the analytical prediction of inductance (effective permeability is a critical parameter), resistance and consequently the magnetic tensor. The optimisation theory of Magnetic Torquers for PMD, still respecting the operational requirements, has also been successfully tested on a dedicated prototype.

Preliminary conclusions and recommendations are finally proposed.