Mechanics of Space Debris Removal: A Review

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Below is the evaluation based on originality, clarity, structure, methodology, academic writing quality, use of references, and scientific impact.

Comments for author File: Comments.pdf

Author Response

Reviewer 1:

Final Evaluation of the Article "Mechanics of Space Debris Removal: A Review"

Mohammad Bigdeli, Rajat Srivastava, and Michele Scaraggi

Below is the evaluation based on originality, clarity, structure, methodology, academic writing quality, use of references, and scientific impact. 

 About Abstract and Introduction: The article’s motivation could be more clearly articulated in the introduction. While both the abstract and introduction emphasize the necessity of space debris removal, they do not explicitly identify the specific gap in the existing literature that this paper seeks to address. Clearly defining this gap would strengthen the paper’s contribution and provide a clearer rationale for its significance.

 

Comment 1:

-Page 1, Lines 1-3- Correction:

Original Version: "The increasing population of space debris, also known as space junk, presents a significant challenge for all space economic activities, including those involving human-onboard spacecraft, due to the rising collision threats..."

Suggestion: "The growing population of space debris poses a critical risk to space operations, requiring urgent removal strategies. This review examines the mechanics of debris detection, capture, and mitigation, analyzing contact-based and contactless removal techniques. Special focus is given to net capturing methods and their mechanical constraints. Finally, we discuss regulatory frameworks and future research directions."

 

Response to Comment 1: Thank you for your valuable suggestion. We have revised and updated the abstract in the manuscript to align with the reviewer’s suggestions.

Abstract

The growing population of space debris poses a critical risk to space operations, requiring urgent removal strategies. Numerous scientific investigations have focused on debris capture mechanisms in Earth orbits, including contact and contact-less capturing methods. However, the known debris population exhibits a multiscale distribution with broad statistics concerning size, shape, etc., making any general-purpose removal approach challenging.

This review examines the mechanics of debris detection, capture, and mitigation, analyzing contact-based and contactless removal techniques. Special focus is given to net capturing methods and their mechanical constraints. We also aim to provide comprehensive discussion, beginning with a statistical overview of current debris followed by detection and removal methods by analysing key mechanical parameters relevant to removal.

Therefore, we delve into the key parameters essential for the engineering of novel debris removal technologies. Finally,  we discuss the preventive measures, regulative frameworks and future research directions.

 

 

 

Comment 2:

-Page 2, Lines 21-30- Correction: Include a paragraph emphasizing the paper’s main contribution compared to previous reviews, for example: "While various studies have addressed space debris removal, a systematic analysis of the mechanics involved in different methods is still lacking. This article seeks to fill this gap by presenting a critical review of the main approaches, emphasizing the efficiency of proposed methods and their technical limitations."

 

Response to Comment 2: Thank you for the suggestion. We have included the text in the introduction of the revised manuscript.

Line 112 – 115 (revised manuscript)

While various studies have addressed space debris removal, a systematic analysis of the mechanics involved in different methods is still lacking. This article seeks to fill this gap by presenting a critical review of the main approaches, emphasizing the efficiency of proposed methods and their technical limitations.

 

Comment 3:

-Page 3, Lines 40-50: Include additional text explaining the direct relation of these figures to the mechanical challenges of debris removal.

Suggestion: " Fig. 1 illustrates the significant damage caused by small debris impacts, highlighting the necessity of robust mitigation strategies. This representation reinforces the importance of developing durable shielding materials and advanced removal techniques. Similarly, Fig. 2 shows the effects of debris collisions on spacecraft surfaces, emphasizing the mechanical challenges associated with impact resistance and the need for effective debris capture and mitigation solutions."

Response to Comment 3: Thank you for your suggestion. We have included the text in the revised manuscript as suggested by the reviewer.

Line 51-54 (revised manuscript)

Thus, the figures illustrate the significant damage caused by small debris impacts, highlighting the necessity of robust mitigation strategies. This representation reinforces the importance of developing durable shielding materials and advanced removal techniques.

Line 80-82 (revised manuscript)

Therefore, the effects of debris collisions are clearly shown in Fig. 2, highlighting the need to emphasize the mechanical challenges associated with impact resistance and the development of effective debris capture and mitigation solutions.

 

 

Comment 4:

-Page 5, Lines 147-149 – Optical Telescopes:

Original Version: "The ground-based telescopes have the capability to detect debris as small as 10 cm in size, despite the substantial distance of over 36000 km between ground observers and GEO."

suggestion "Ground-based telescopes can detect debris as small as 10 cm in GEO, despite the large observational distance of over 36,000 km."

Response to Comment 4: Thank you for your suggestion. We have revised the sentence accordingly in the manuscript to improve clarity and accuracy.

 

Line 200-201 (revised manuscript)

             Ground-based telescopes can detect debris as small as 10 cm in GEO, despite the large observational distance of over 36,000 km \citep{schildknecht2007optical,LUO20222618}.

 

Comment 5:

Page 12, Lines 335-343 – Contactless Capture Methods- Correction: include a critical section at the end of the description of contactless methods, highlighting advantages and disadvantages.

"While contactless debris removal methods, such as laser ablation and electrostatic tractors, provide the advantage of non-invasive maneuvering, their effectiveness is constrained by high energy demands and reduced efficiency over long distances. Furthermore, the large-scale deployment of these technologies remains uncertain due to both technological limitations and regulatory challenges."

 

Response to Comment 5: Thank you for your valuable suggestion. We have incorporated the critical section at the end of the description of contactless methods, as recommended.

Line 347 -351 (revised manuscript)

 

While contactless debris removal methods, such as laser ablation and electrostatic tractors, provide the advantage of non-invasive maneuvering, their effectiveness is constrained by high energy demands and reduced efficiency over long distances. Furthermore, the large-scale deployment of these technologies remains uncertain due to both technological limitations and regulatory challenges.

 

Comment 6:

page 47 Equation 28- Question: is v a vector after the operation?  

page 48 Equation 29- Question: is v a vector after the operation?  

 

Response to Comment 6:  Thank you for your insightful question regarding Equations 28 and 29. In Equation 28, the expression  confirms that  remains a vector after the operation. Since  is a scalar matrix and  is a vector, the resulting preserves its vector properties. Similarly, in Equation 29, the operation maintains  as a vector.

We have carefully reviewed these equations and ensured their correctness in the revised manuscript. Additionally, we have made minor adjustments to enhance clarity and explicitly indicate the vector nature of  where necessary.

 

 

Old equation:

 

 

New updated equation:

 

 

Comment 7:

page 56 line 1642: Conclusion  

Suggestion for inclusion in line 1642: "This review provides a critical analysis of space debris removal methods, highlighting their advantages, limitations, and practical feasibility. Future research should prioritize improving the efficiency of contactless techniques, optimizing energy management in laser-based systems, and developing scalable capture mechanisms to address the growing challenge of orbital congestion."

Response to Comment 7:  Thank you for your valuable suggestion regarding the conclusion section. We have incorporated the suggested text in the last paragraph of the conclusion to provide a more comprehensive summary of space debris removal methods.

 

Line 1693-1697 (revised manuscript)

This review provides a critical analysis of space debris removal methods, highlighting their advantages, limitations, and practical feasibility. Future research should prioritize improving the efficiency of contactless techniques, optimizing energy management in laser-based systems, and developing scalable capture mechanisms to address the growing challenge of orbital congestion.

 

 

Evaluation Conclusion: 

1. Originality / Relevance: The article provides a comprehensive and well-structured overview of the subject but with a medium significance for the field.
2. Clarity / Organization: The section organization enhances comprehension;
3. Methodology/ critical Analysis: The article discusses technological limitations and practical implementation challenges;
4. Academic writing / references: The language is formal and scientific, the English used correct and readable and includes a broad range of relevant references.

 

With these revisions, the article will have the standards required for publication in journal.

 

 

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The research discussion on the detection of space debris and the utilization of various techniques for measurement and their mechanical characterization. Including the key parameters for the engineering of novel debris removal technologies, ongoing mitigation strategies, the net capturing method and its contact mechanics aspects. However, several points need to be improved before final approval. The detailed comments are as follows:

1. Figures 1 and 2 do not align with the theme.
2. The writing in the second section is confusing, as much of it applies to all robotic scenarios in general and does not serve as a key approach for the current observation of debris cleanup. This part requires revision.
3. It is recommended to merge Sections 2 and 3, there is a lot of overlapping content within it.
4. The article mentions the mechanical properties of space debris removal, yet throughout the text, the review of mechanical research is notably scant and inadequately discussed. This part requires expansion.
5. The review of contact-based capture methods is incomplete. Robotic arms represent a typical contact-based approach to debris removal, and there is a need to expand the discussion in this area. However, the discussion from the introduction can be improved by adding the following references. For example, FSTSMC Compliance Control for Dual-Arm Space Robot with SDBD Capture Satellite Operation, Integrated sliding mode control with input restriction, output feedback and repetitive learning for space robot with flexible-base, flexible-link and flexible-joint, Buffer Compliance Control of Space Robots Capturing a Non-Cooperative Spacecraft Based on Reinforcement Learning.
6. Many figures in the article need to be adjusted for clarity, as they are too blurry, and some do not align well with the theme.
7. It is recommended to restructure the article’s framework, as much of the content feels disconnected from the topic of space debris removal

Author Response

Reviewer 2:

Final Evaluation of the Article "Mechanics of Space Debris Removal: A Review"

Mohammad Bigdeli, Rajat Srivastava, and Michele Scaraggi

Comment 1: Figures 1 and 2 do not align with the theme.

Response to Comment 1:

Figure 1 and Figure 2 are important to show to the criticality of the presence of space debris that can cause severe damage, thus highlighting the necessity of robust mitigation strategies. This representation reinforces the importance of developing durable shielding materials and advanced removal techniques. Similarly, Fig. 2 shows the effects of debris collisions on spacecraft surfaces, emphasizing the mechanical challenges associated with impact resistance and the need for effective debris capture and mitigation solutions. 

These revisions have been incorporated into the revised manuscript in alignment with the suggestions provided by Reviewer 1.

 

Comment 2: The writing in the second section is confusing, as much of it applies to all robotic scenarios in general and does not serve as a key approach for the current observation of debris cleanup. This part requires revision.

 

Response to Comment 2:

Thank you for the suggestion, but this section basically to provide a general overview related to current debris detection technology and just for the sake of completeness of this review article it is merged with Section 3 as suggested in the next comment and updated in the revised manuscript.

 

 

Comment 3: It is recommended to merge Sections 2 and 3, there is a lot of overlapping content within it.

 

Response to Comment 3: Thank you for the suggestion. We have merged and restructured sections 2 and 3 to improve the logical flow of the discussion

 

Comment 4: The article mentions the mechanical properties of space debris removal, yet throughout the text, the review of mechanical research is notably scant and inadequately discussed. This part requires expansion.

 

Response to Comment 4: Thank you for your insightful feedback. In response to your suggestion, we have expanded the discussion on the mechanical aspects of space debris removal in the introduction:

 

Line 102 – 112 (revised manuscript)

 

This review provides in-depth information on the mechanics of space debris removal, which involves various phenomena, including efficient debris capture, deorbiting strategies, collision prevention, and reducing future debris generation.

Some key mechanisms for space debris removal include robotic capture systems, electrodynamic tethers, laser ablation, drag enhancement devices, and removal methods using nets, harpoons, magnetic tethers, and gravitational tractors, which are discussed in detail in this article.

Several challenges persist, such as the high costs and technological complexities, as much of the research remains in its initial prototype phase and requires significant investment. This review aims to outline the pathway for effective space debris removal, emphasizing the need for a combination of innovative technologies, international cooperation, and long-term investment to ensure the sustainability of Earth's orbital environment.

 

 

Comment 5: The review of contact-based capture methods is incomplete. Robotic arms represent a typical contact-based approach to debris removal, and there is a need to expand the discussion in this area. However, the discussion from the introduction can be improved by adding the following references. For example, FSTSMC Compliance Control for Dual-Arm Space Robot with SDBD Capture Satellite Operation, Integrated sliding mode control with input restriction, output feedback and repetitive learning for space robot with flexible-base, flexible-link and flexible-joint, Buffer Compliance Control of Space Robots Capturing a Non-Cooperative Spacecraft Based on Reinforcement Learning.

 

Response to Comment 5: We appreciate the reviewer’s suggestion to expand the discussion on contact-based capture methods, particularly robotic arms. In response, we have substantially revised the manuscript by incorporating a more detailed discussion of robotic arm-based debris removal, emphasizing advancements in compliance control, adaptive manipulation, and multi-arm coordination. Following lines are added in the revised manuscript.

 

Line 699 – 739 (revised manuscript)

 

Recent advancements in robotic arm-based debris removal system have focused on enhancing compliance control, adaptive manipulation, and multi-arm coordination. Researchers have explored robust control techniques, such as fast terminal sliding mode control (FSTSMC) for compliance in dual-arm space robots, which enhances precision in capturing debris while mitigating dynamic disturbances. In particular, a Spring Damper Buffer Device (SDBD) has been introduced to protect robotic joints from impact forces during debris capture. The SDBD absorbs impact energy and ensures stable control of the hybrid system by matching compliance strategies with dynamic constraints. The system dynamics, derived using the Lagrange function, integrate velocity constraints and closed-chain geometric relationships to optimize post-capture stability\cite{zhu2023fstsmc}.

 

Additionally, integrated sliding mode control (ISMC) strategies have been developed for space robots with flexible-base, flexible-link, and flexible-joint (FBFLFJ) configurations. Traditional ISMC methods struggle to maintain stability and precise motion control, but FBFLFJ configuration provides inherent structural flexibility to the space robots. This method achieves vibration suppression improvements ranging from 50\% to 80\% and enhances trajectory tracking accuracy by 37\% compared to traditional ISMC methods lacking vibration suppression. Furthermore, the upgraded ISMC approach ensures stable control while considering actuator constraints and leveraging output feedback control, making it a suitable solution for real-time space operations\cite{fu2023integrated}.

 

It is also observed that robotic arm faces issues such as joint damage caused by the impact torque of a non-cooperative spacecraft during on-orbit capture. To address this issue, buffer compliance control mechanism is used based on a reinforcement learning control algorithm. The compliant mechanism not only absorbs impact energy through the deformation of its internal spring but also limits the impact torque within a safe range. In the reinforcement learning control algorithm, the dynamics of space robot and the target spacecraft are formulated using the Lagrange approach and the Newton-Euler method before capture.  Following the capture, the integrated dynamic model of the post-capture hybrid system is derived based on the law of conservation of momentum and velocity constraints. The buffer compliance control strategy is used to enhance stability of an unstable hybrid system. The approach employs an associative search network (ASN) to approximate unknown nonlinear functions and an adaptive critic network (ACN) to generate reinforcement signals, optimizing the learning process in real-time. Numerical simulations demonstrate that the proposed control scheme effectively reduces impact torque acting on the joints by up to 76.6\% at maximum and 58.7\% at minimum during the capture phase. Additionally, in the stabilization phase, the impact torque on the joints remains within the safety threshold, preventing overload and structural damage. These improvements enhance the resilience and adaptability of space robots for future on-orbit servicing and debris removal missions\cite{ai2021buffer}. These advancements in compliance control and trajectory optimization significantly improve the reliability of robotic manipulators in space debris capture. However, challenges remain, particularly in capturing debris with high angular momentum.

 

These discussions have been seamlessly integrated into the manuscript, particularly in the section focusing on robotic arm-based debris removal methods.

 

 

 

Comment 6: Many figures in the article need to be adjusted for clarity, as they are too blurry, and some do not align well with the theme.

 

Response to Comment 6: Thank you for your feedback. In response to your suggestion, we have improved the clarity of Figures 22 and 31 to enhance readability. After a thorough review, we confirm that all figures effectively support the discussion and align with the key themes presented in the manuscript.

 

 

Comment 7: It is recommended to restructure the article’s framework, as much of the content feels disconnected from the topic of space debris removal

 

Response to Comment 7: Thank you for your valuable suggestion. In response to reviewers recommendation, we have restructured the manuscript to enhance coherence and ensure that all sections align more effectively with the central theme of space debris removal. The following revisions have been made in the updated manuscript:

1. The abstract has been rewritten to provide a clearer and more focused summary of the article's key contributions.
2. The introduction has been expanded to emphasize the relevance of this review in the context of the mechanics of space debris removal techniques.
3. As suggested by the reviewer, Sections 2 and 3 have been merged and restructured to improve the logical flow of the discussion.
4. Additional details regarding the dynamics of robotic arms, as suggested by the reviewer, have been incorporated into the revised Section 2.

These modifications enhance the overall structure and clarity of the manuscript, ensuring that all content remains closely aligned with the topic of space debris removal. We appreciate your insightful feedback, which has contributed to strengthening the coherence and impact of the manuscript.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The author has revised the manuscript according to the requirements and it is recommended for accept.