
Design and Guidance of a Multi-Active Debris Removal Mission
of Surrey is another project that demonstrated various debris removal methods, including harpoon and net capture [9].
The CleanSpace-1 mission by the European Space Agency (ESA) aims to deorbit a 112 kg upper stage of the Vega
rocket 2.
While individual removals are essential stepping stones towards ADR implementation, a deployment on a larger scale,
targeting more objects, might be necessary [
8
,
10
]. In order to make it financially feasible, each ADR Servicer might
need to remove more than one object and use mass-efficient low-thrust electric propulsion (EP). This combination of
long, EP-based transfers and complex vehicle paths rendezvousing with multiple moving targets presents a difficult
optimization challenge that has to be addressed at the design stage of ADR missions. Due to the large number of
potential ADR targets to be visited, transfers between consecutive mission orbits need to be analyzed quickly to enable
design iteration and parametric studies.
This paper introduces a novel multi-ADR removal mission concept that involves a two-spacecraft system. On request,
the system is able to provide contact-based debris removal through a rendezvous and deorbit process. One spacecraft
- called the Servicer - is reused for multiple debris, allowing the mission costs to remain low. The other spacecraft
- the Shepherd - performs coupled reentry with the debris, so the reentry process can be controlled adequately, thus
complying with the ground-casualty risk requirements. The majority of the mission utilizes electric propulsion. This
paper is dedicated to discussing the proposed mission in detail and developing a mission design tool to simulate
multi-ADR tours accurately and efficiently.
To this end, a preliminary mission design tool (PMDT) is developed to optimize both fuel consumption and time of
flight of multi-target missions while taking the effect of
J2
, eclipses, and duty ratio into account. PMDT utilizes
J2
to
achieve RAAN changes, in order to reduce the fuel consumption of the mission.
The PMDT extends the traditional Edelbaum method by introducing the contribution of drag and duty ratio. Then drift
orbits are used for matching RAAN when required, as discussed in [
11
]. Lastly, the altitude and inclination of the drift
orbits are optimized to obtain either time or fuel optimal trajectories. The sequence of targets can also be treated as an
optimization variable in the PMDT, however, it was treated as a constant for the examples given in this study.
Our approach shares similarities with the Multidisciplinary desigN Electric Tug tool (MAGNETO) developed in [
12
]
as well as the work by Viavattene et al. in [
13
]. However, it takes the presented models further by taking duty ratios
into account and a more accurate description of eclipses and drag. Furthermore, the tool considers mission-specific
constraints and uses an optimizer to perform rapid design iterations and parametric studies of the proposed multi-ADR
mission.
Three guidance laws are introduced to assess the accuracy of the models adopted in the PMDT. Ruggiero et al. [
14
]
developed a series of closed-loop guidance laws based on the Gauss form of Lagrange Planetary laws. Locoche [
15
]
developed a guidance law based on Lyapunov feedback control known as the
∆v
-Law to supplement preliminary
mission design tools. Finally, Petropoulos [16] developed one of the most versatile and well-known control laws - the
Q-Law - which is also based on Lyapunov control. These three approaches are here used to optimally track the transfers
computed by the PMDT, thus validating its key assumptions and providing a possible way to fly the missions.
The remaining sections of the paper are organized as follows. Firstly, the mission concept of operations is presented.
Then, the design of PMDT is discussed and is used to generate optimal debris removal trajectories at high computational
speeds. Thirdly, guidance schemes implemented on the PMDT outcomes are discussed. The results section provides
an example trajectory optimization solution for both a time and mass optimal multi-ADR mission. Lastly, the paper’s
outcomes are summarized, and conclusions are drawn regarding the method’s usefulness.
2 Concept of Operations of the Multi-ADR Mission
The proposed multi-ADR mission architecture is shown in Figure 1, where two spacecraft are involved in the debris
removal process. A Servicer is used to approach and rendezvous with the debris. Once rendezvoused, the Servicer
brings the object down to a low altitude orbit (
≈350
km). The debris is then handed over to a Reentry Shepherd, which
docks with the debris and performs a controlled reentry on its behalf. Controlled reentry reduces the casualty risk
posed by removing the debris, which is desirable because the ADR targets are, by definition, large and thus contain
components likely to survive the reentry. The Servicer shall be reused for several debris removals, while each Reentry
Shepherd can only be used once as it burns up while deorbiting the debris.
The proposed mission architecture can perform multi-ADR services significantly cheaper than those that use coupled
deorbiting and controlled reentry systems. When the deorbiting and reentry functionality are installed on a single
spacecraft, it cannot be reused, which leads to higher mission costs. Furthermore, depending on the debris features and
2https://www.esa.int/Space_Safety/ClearSpace-1
2