Electric Sail Test Cube–Lunar Nanospacecraft, ESTCube-LuNa: Solar Wind Propulsion Demonstration Mission Concept

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1.     The abstract part is too long. It should be much shorter and concise.

2.     The format of Table 3 and 4 is not fit for this journal. Please redraw with the tablet like Table 1 and 2.

3.     The deployment of a 2 km length of E-sail tether would be challenging, and how to avoid potential malfunction? Please provide more citations or other information to prove the availability of such a proposal.

4.     How were the data given in Figures 7 to 9 obtained? And how do you assess the simulation errors? Please give further details.

5.     How do you comment on the attitude control efficiency, as well as orbital transfer efficiency, of such a E-sail proposed in your work?

Author Response

Thank you for the extensive work of reviewing the ESTCube-LuNa manuscript and providing a fresh look on the article. The ESTCube-LuNa consortium appreciates your feedback and, as detailed below, we have taken good care to address your concerns. The submission’s DIFF version includes tracked changes: the deleted text is red and the new text is blue. The changes in the abstract are not tracked, although the abstract has changed. 
The authors noticed minor discrepancies in the original submission: 
We made a slight modification of the arguments about the centre-of-mass (CoM) shift and the related spin-rate change after Equation 1: The CoM shift is insignificant because it is inside the Debye length and inside the sheath. The, also insignificant, CoM to full tether length ratio (as was described in the original submission) is a secondary effect.
We added one sentence at the end of Section 3.3 “E-sail environment characterisation”: explaining that the Langmuir probes will compare the plasma environment inside the E-sail’s sheath between different modes of the E-sail operations.
Figure 11 “Concept of operations” included an outdated element: the 3-m parabolic dish in Tõravere, Estonia which, as it turned out, was not able to deliver the necessary performance in the UHF band. As the text of original submission describes, the low-speed communications is designed with a Yagi–Uda antenna and the concept of operations figure has been updated accordingly. 
The authors noticed two double-used symbols:
I for the moment of inertia is replaced with M_I (Equation 1 and the following paragraph). I for electric current in Equation 2 is left unchanged.
r for the solar distance is replaced with d (Section 1.1). r for the arm length in Equation 1 is left unchanged.

The following revisions, described by RESPONSES, were made according to the valuable comments provided by you.

Reviewer 1: 
1. The abstract part is too long. It should be much shorter and concise.
RESPONSE: Thanks for your review! The abstract is shortened. 
2. The format of Table 3 and 4 is not fit for this journal. Please redraw with the tablet like Table 1 and 2.
RESPONSE: The formatting of Tables 3 and 4 is now aligned with Tables 1 and 2. 
3. The deployment of a 2 km length of E-sail tether would be challenging, and how to avoid potential malfunction? Please provide more citations or other information to prove the availability of such a proposal. 
RESPONSE: The authors agree that deployment of an E-sail tether is a major challenge of technology demonstration (hence we are working on LEO missions). The article’s Section 3.2. “E-sail deployment” includes our first results on 2 km tether deployment. The subsection also discusses the challenges and suggests future research topics. In the revised version, we have added details about simulation and its limitations. The next step is a dedicated tether deployment study in the Electric Sail Mission Expeditor (ESME) which will be an article on its own. In the revised version, we have added the second point to Conclusions and Future work regarding tether dynamics and attitude control.
4. How were the data given in Figures 7 to 9 obtained? And how do you assess the simulation errors? Please give further details.
RESPONSE: Section 3.2. “E-sail deployment” includes new details about the simulation. The simulation is idealistic and preliminary to assess the applicability of thrusters for large angular momentum provision (novel application as far as we know and more studies are certainly needed). 
5. How do you comment on the attitude control efficiency, as well as orbital transfer efficiency, of such a E-sail proposed in your work?
RESPONSE: We have revised Section 1.1. “Electric solar wind sail and plasma brake in the context of propellantless propulsion” with a new paragraph on the E-sail spin and attitude control and efficiency. Thanks for your feedback! 

Reviewer 2 Report

Comments and Suggestions for Authors

Overall comments:

This paper presents important work on a an E-sail concept as a Cubesat payload. A demonstration concept ESTCube-LuNA has also been described in great detail and represents important work in the field of deep space and interplanetary cubesat propulsion and system design in general. 

While some individual sections are well written, it seems like a lot of sections have been written separately and put together without a cohesive structure to the paper. Several sections refer to future sections of the paper, making it hard to follow. At many places technical information is introduced in a way that does not fit the context of the previous paragraph's discussion.

For example, starting with section 1 which is meant to be an introduction, the clear objectives of the paper have not been sufficiently laid out. What does this work hope to achieve? What are the fundamental questions this work attempts to answer? How is the selected approach going to help potentially with answering those questions?

A section by section review is as follows:

1. Introduction

Section 1.1 provides a good introduction to the E-sail concept and a comprehensive study of past missions. It will be good to include a brief description of failures by other missions and why the system architecture presented in this paper will help overcome those failures. Section 1.2 can be made a little more concise keeping information relevant to the ESTCube-LuNA concept. Section 1.3 does not provide the readers with a definitive paper structure but focuses on attributions referring to a lot of subsequent sections in the paper leaving the readers with little context. It is suggested that this section focus on the paper structure and flow of sections and a separate section for attributions and credits created later in the draft. 

The section delves into technical information that is more relevant to future sections. Eg. Section on visual navigation and the end of section 1.2 seems like a deviation from an introductory section

2. E-sail experiment design for solar wind in lunar orbit

The E-sail characterization experiments have been proposed in two different scenarios. The reasoning behind the down-selection of these scenarios is, however, unclear. More description and possibly a graphical illustration is required for the reader to understand how the spin plane is defined.

The concept of and end-mass pulling mechanism described in section 3 is referred to in this section, making it difficult to follow

3. ESTCube-LuNa E-sail payload and experiment operations

This section is well written and provides good descriptions and explanations relevant to this section of the paper.

 

4. ESTCube-LuNa sketch goal : lunar escape maneuver.

This section has is well written but parts of this section do not flow well into one another. The motivation behind objective of the mission concept is not adequately described

5. ESTCube-LuNa preliminary mission requirements and platform design solutions.

This section is written very well and provides a detailed description of the system's preliminary design. Sub sections flow well into each other and the level of description which shows a comprehensive systems engineering approach applied to this development

It is suggested that the authors revise the paper's structure to provide a more useful reference to this very important work.

Comments on the Quality of English Language

Quality of English is adequate. Some spelling corrections are needed.

Author Response

Thank you for the extensive work of reviewing the ESTCube-LuNa manuscript and providing a fresh look on the article. The ESTCube-LuNa consortium appreciates your feedback and, as detailed below, we have taken good care to address your concerns. The submission’s DIFF version includes tracked changes: the deleted text is red and the new text is blue. The changes in the abstract are not tracked, although the abstract has changed. 
The authors noticed minor discrepancies in the original submission: 
We made a slight modification of the arguments about the centre-of-mass (CoM) shift and the related spin-rate change after Equation 1: The CoM shift is insignificant because it is inside the Debye length and inside the sheath. The, also insignificant, CoM to full tether length ratio (as was described in the original submission) is a secondary effect.
We added one sentence at the end of Section 3.3 “E-sail environment characterisation”: explaining that the Langmuir probes will compare the plasma environment inside the E-sail’s sheath between different modes of the E-sail operations.
Figure 11 “Concept of operations” included an outdated element: the 3-m parabolic dish in Tõravere, Estonia which, as it turned out, was not able to deliver the necessary performance in the UHF band. As the text of original submission describes, the low-speed communications is designed with a Yagi–Uda antenna and the concept of operations figure has been updated accordingly. 
The authors noticed two double-used symbols:
I for the moment of inertia is replaced with M_I (Equation 1 and the following paragraph). I for electric current in Equation 2 is left unchanged.
r for the solar distance is replaced with d (Section 1.1). r for the arm length in Equation 1 is left unchanged.

The following revisions, described by RESPONSES, were made according to the valuable comments provided by you.

Reviewer 2:
Overall comments:
This paper presents important work on a an E-sail concept as a Cubesat payload. A demonstration concept ESTCube-LuNA has also been described in great detail and represents important work in the field of deep space and interplanetary cubesat propulsion and system design in general. 
RESPONSE: Thanks for your review!
While some individual sections are well written, it seems like a lot of sections have been written separately and put together without a cohesive structure to the paper. Several sections refer to future sections of the paper, making it hard to follow. At many places technical information is introduced in a way that does not fit the context of the previous paragraph's discussion.
RESPONSE: We agree that the original submission was hard to follow. We have provided the context and additional information across the article and especially in the Introduction which sets the stage for the experiment design and technical solutions. 
For example, starting with section 1 which is meant to be an introduction, the clear objectives of the paper have not been sufficiently laid out. What does this work hope to achieve? What are the fundamental questions this work attempts to answer? How is the selected approach going to help potentially with answering those questions?
RESPONSE: The Introduction has been updated and we have provided additional details on related work on our own LEO demo missions (new Section 1.2. “E-sail and plasma brake demonstration missions”) which is the heritage we are continuing with ESTCube-LuNa. There is also additional context for the “Advances in CubeSat Sails and Tethers” special issue (Section 1.1. “Electric solar wind sail and plasma brake in the context of propellantless propulsion”) as well as for the nanospacecraft topic which relates to the cubesat theme of the special issue (Section 1.3. “Interplanetary and lunar nanospacecraft”). 
A section by section review is as follows:
1. Introduction
Section 1.1 provides a good introduction to the E-sail concept and a comprehensive study of past missions. It will be good to include a brief description of failures by other missions and why the system architecture presented in this paper will help overcome those failures. 
RESPONSE: The revised version includes a new Section 1.2 “E-sail and plasma brake demonstration missions” on previous missions.
Section 1.2 can be made a little more concise keeping information relevant to the ESTCube-LuNA concept. 
RESPONSE: We think now-called Section 1.3 “Interplanetary and lunar nanospacecraft” is central to the ESTCube-LuNa concept and the theme of the special issue: cubesats and propellantless propulsion. More context is given at the beginning of Section 1.3.
Section 1.3 does not provide the readers with a definitive paper structure but focuses on attributions referring to a lot of subsequent sections in the paper leaving the readers with little context. It is suggested that this section focus on the paper structure and flow of sections and a separate section for attributions and credits created later in the draft. The section delves into technical information that is more relevant to future sections. Eg. Section on visual navigation and the end of section 1.2 seems like a deviation from an introductory section
RESPONSE: We agree that the section was difficult to follow. We have revised the now-called Section 1.4 “ESTCube-LuNa consortium and paper structure”.
2. E-sail experiment design for solar wind in lunar orbit
The E-sail characterization experiments have been proposed in two different scenarios. The reasoning behind the down-selection of these scenarios is, however, unclear. More description and possibly a graphical illustration is required for the reader to understand how the spin plane is defined.
RESPONSE: We have revised Section 2 “E-sail experiment design for solar wind in lunar orbit” to provide clear reasoning for why the E-sail experiments are planned with an inertially-fixed spin plane. The main reason is that we do not have the remote unit technology for spin plane control. 
The concept of and end-mass pulling mechanism described in section 3 is referred to in this section, making it difficult to follow
RESPONSE: We have rewritten the beginning of Section 2 “E-sail experiment design for solar wind in lunar orbit”. The reference to Section 3.2 is made for a specific purpose now. 
3. ESTCube-LuNa E-sail payload and experiment operations
This section is well written and provides good descriptions and explanations relevant to this section of the paper.
RESPONSE: Thanks, appreciated! 
4. ESTCube-LuNa sketch goal : lunar escape maneuver.
This section has is well written but parts of this section do not flow well into one another. The motivation behind objective of the mission concept is not adequately described
RESPONSE: We have provided additional context in Sections 1-3. Section 4 is revised. 
5. ESTCube-LuNa preliminary mission requirements and platform design solutions.
This section is written very well and provides a detailed description of the system's preliminary design. Sub sections flow well into each other and the level of description which shows a comprehensive systems engineering approach applied to this development
It is suggested that the authors revise the paper's structure to provide a more useful reference to this very important work.
RESPONSE: Thanks for your feedback! 

Reviewer 3 Report

Comments and Suggestions for Authors

The authors present a study on a mission demonstrating electric sail operation in a lunar orbit. The paper discusses an interesting mission, appropriate for the journal, and is easy to follow.  Following are my comments/questions to authors: 

Major: 

1) It is not clear what the contribution of the paper is. Ref. [22] is on a similar mission concept. Page 3 talks about the mission being designed without DSNs, but the paper development is not aligned with establishing the contribution. Is this a systems engineering study? The challenge in establishing that, as I felt while reading the paper, is that the paper describes the mission requirements and the design choices, rather than rationally deducing the design from the requirements. The lunar escape maneuver (Sec 4) follows Ref. [22] and the novelty therein is not clear. I believe the paper can greatly benefit by having clear statements of the contribution with the novelty explicitly mentioned. 

2) In some places, the paper jumps to future sections or subsections, for instance, "As discussed in the next section, ..." and "see subsection 5.4" in Page 6. I would recommend avoiding this. Perhaps, discussing the organization of the paper in Sec 1 bypasses this issue. 

3) The power management during operations of electric sail is not clearly established in the paper, especially given the high voltage requirement. 

Minor: 

a) How is the solar wind direction determined onboard the spacecraft? 

b) Reference for equation 1? 

c) What is the thrust-to-power ratio used in the GMAT simulations for the modeled electric thruster? 

d) While the orbit is described in terms of semi-major axis and inclination, the effect of orbital transfer is described in terms of change in period. Perhaps also quote the change in semi-major axis. 

e) Why does the tether exhibit a 2-deg coning motion (Page 7)? Is the 2-deg an assumption? 

f) Is the satellite moment of inertia really 70380 kg m2? (Page 11) 

g) Primarily editorial, but Table 2 seems to violate the margin. 

h) The conclusion states that the paper proposed novel navigation and communication solutions without DSN access. The novelty is not clearly established in the paper. Perhaps the focus of the paper can be more focused on these novel aspects of the paper. 

 

Author Response

Thank you for the extensive work of reviewing the ESTCube-LuNa manuscript and providing a fresh look on the article. The ESTCube-LuNa consortium appreciates your feedback and, as detailed below, we have taken good care to address your concerns. The submission’s DIFF version includes tracked changes: the deleted text is red and the new text is blue. The changes in the abstract are not tracked, although the abstract has changed. 
The authors noticed minor discrepancies in the original submission: 
We made a slight modification of the arguments about the centre-of-mass (CoM) shift and the related spin-rate change after Equation 1: The CoM shift is insignificant because it is inside the Debye length and inside the sheath. The, also insignificant, CoM to full tether length ratio (as was described in the original submission) is a secondary effect.
We added one sentence at the end of Section 3.3 “E-sail environment characterisation”: explaining that the Langmuir probes will compare the plasma environment inside the E-sail’s sheath between different modes of the E-sail operations.
Figure 11 “Concept of operations” included an outdated element: the 3-m parabolic dish in Tõravere, Estonia which, as it turned out, was not able to deliver the necessary performance in the UHF band. As the text of original submission describes, the low-speed communications is designed with a Yagi–Uda antenna and the concept of operations figure has been updated accordingly. 
The authors noticed two double-used symbols:
I for the moment of inertia is replaced with M_I (Equation 1 and the following paragraph). I for electric current in Equation 2 is left unchanged.
r for the solar distance is replaced with d (Section 1.1). r for the arm length in Equation 1 is left unchanged.

The following revisions, described by RESPONSES, were made according to the valuable comments provided by you.

Reviewer 3: 
The authors present a study on a mission demonstrating electric sail operation in a lunar orbit. The paper discusses an interesting mission, appropriate for the journal, and is easy to follow.  
RESPONSE: Thanks for your review!
Following are my comments/questions to authors: 
Major: 
1) It is not clear what the contribution of the paper is. Ref. [22] is on a similar mission concept. 
RESPONSE: The contribution of Palos et al. 2023 is now better explained in Section 1.4. “ESTCube-LuNa consortium and paper structure” and in Section 4 “ESTCube-LuNa stretch goal: lunar escape manoeuvre”. The contribution is smaller than initially planned because Palos et al. 2023 implemented an outdated (but still valid) E-sail model for E-sail experiment simulations. In the ESTCube-LuNa concept, we have based our estimates on the original work by Janhunen 2009 ([49] in the revision). Nevertheless, the software toolage of Palos et al. 2023 is relevant for high-fidelity modelling, as now discussed in Section 6 “Conclusions and future work”. 
Page 3 talks about the mission being designed without DSNs, but the paper development is not aligned with establishing the contribution. 
RESPONSE: Section 5 “ESTCube-LuNa preliminary mission requirements and platform design solutions” is dedicated to explaining several now-dubbed “DSN-free” technologies in terms of communications, ranging and navigation: repurposed radio telescopes and orbit determination with on-board cameras. In the revised version, we have provided additional context in the Introduction. Lunar nanospacecraft which do not use space agency DSNs is the major challenge this article addresses and opens up a range of future research topics. We consider this in the context of the “Advances in CubeSat Sails and Tethers” special issue. 
Is this a systems engineering study? The challenge in establishing that, as I felt while reading the paper, is that the paper describes the mission requirements and the design choices, rather than rationally deducing the design from the requirements. 
RESPONSE: Section 5 “ESTCube-LuNa preliminary mission requirements and platform design solutions” clearly discusses system engineering aspects but, as the article’s title suggests, the current paper is a concept and further analyses are necessary to determine the technical feasibility of the concept. In the revised version, we have introduced a clear Section 2.1. “ESTCube-LuNa E-sail experiment requirements”: the design flows from those requirements and considerations at the beginning of Section 2 “E-sail experiment design for solar wind in lunar orbit”. We have constrained ourselves to the cubesat / nanospacecraft form factor in the context of the special issue and it is also our strongest heritage. Larger and more complex systems are possible (e.g. with a remote unit) but would certainly require more resources and extend the schedule. ESTCube-LuNa is the minimum viable E-sail demo concept. 
The lunar escape maneuver (Sec 4) follows Ref. [22] and the novelty therein is not clear. I believe the paper can greatly benefit by having clear statements of the contribution with the novelty explicitly mentioned. 
RESPONSE: Section 4 “ESTCube-LuNa stretch goal: lunar escape manoeuvre” is revised and the Introduction is now updated regarding the Palos et al. 2023 contribution. 
2) In some places, the paper jumps to future sections or subsections, for instance, "As discussed in the next section, ..." and "see subsection 5.4" in Page 6. I would recommend avoiding this. Perhaps, discussing the organization of the paper in Sec 1 bypasses this issue. 
RESPONSE: The article includes many cross-references due to the experimental and spacecraft design aspects being tightly linked (e.g. tether deployment and attitude thrusters and moment of inertia). In the revised version, we have restructured the flow of text throughout Sections 1-4: in some cases extra details are provided along with a cross-reference for the context. 
3) The power management during operations of electric sail is not clearly established in the paper, especially given the high voltage requirement. 
RESPONSE: We have added a paragraph on power management in Section 3.1. “E-sail hardware”. 
Minor: 
a) How is the solar wind direction determined onboard the spacecraft? 
RESPONSE: We have added details about the solar-wind direction in Section 2.1. “ESTCube-LuNa E-sail experiment requirements”.
b) Reference for equation 1? 
RESPONSE: The reference was given in the original submission and now a clear statement is made. 
c) What is the thrust-to-power ratio used in the GMAT simulations for the modeled electric thruster? 
RESPONSE: The power consumption was not considered in the GMAT simulation. Details added to 2.3. “Orbit modification with E-sail thrust”.
d) While the orbit is described in terms of semi-major axis and inclination, the effect of orbital transfer is described in terms of change in period. Perhaps also quote the change in semi-major axis. 
RESPONSE: Details added to 2.3. “Orbit modification with E-sail thrust”.
e) Why does the tether exhibit a 2-deg coning motion (Page 7)? Is the 2-deg an assumption? 
RESPONSE: It is an estimation. Details added to 2.4. “E-sail tether deflection”. 
f) Is the satellite moment of inertia really 70380 kg m2? (Page 11) 
RESPONSE: Yes and we have revised Section 3.2. “E-sail deployment” with extra details on this unusual number. 
g) Primarily editorial, but Table 2 seems to violate the margin. 
RESPONSE: The table size is revised. 
h) The conclusion states that the paper proposed novel navigation and communication solutions without DSN access. The novelty is not clearly established in the paper. Perhaps the focus of the paper can be more focused on these novel aspects of the paper. 
RESPONSE: We have provided a revised focus and context in the Introduction as well as minor edits in Sections 5 and 6. Section 5 “ESTCube-LuNa preliminary mission requirements and platform design solutions” provides most details on the DSN-free approach. Thanks for your feedback! 

Reviewer 4 Report

Comments and Suggestions for Authors

Review of manuscript  MDPI Aerospace-2775546

 “Electric Sail Test Cube – Lunar Nanospacecraft, ESTCube-LuNa: Solar-Wind Propulsion Demonstration Mission Concept”

by

Andris Slavinskis, Mario F. Palos, Janis Dalbins, Pekka Janhunen, Martin Tajmar, Nickolay Ivchenko, Agnes Rohtsalu, Katarina Aas, Aldo Micciani, Nicola Orsini, Karl Mattias Moor, Sergei Kuzmin, Gustavs Plonis, Marcis Bleiders, Marcis Donerblics, Ikechukwu Ofodile, Johan Kütt, Tõnis Eenmäe, Viljo Allik, Jaan Viru, Pätris Halapuu, Katriin Kristmann, Janis Sate, Endija Briede, Marius Anger, Hans Teras, Kristo Allaje, Andris Vaivads, Lorenzo Niccolai, Marco Bassetto, Giovanni Mengali, Petri Toivanen, Iaroslav Iakubivskyi, Mihkel Pajusalu, and Antti Tamm

  

The subject of this paper is very actual and interesting for the space community regarding small missions.

 

General comments:

·         In the literature, E-sail concepts are using multiple tethers. I guess that the concept in this paper aims at reducing complexity by going to a single tether. But wouldn’t the use of two tethers be much better concerning dynamic behavior, e.g. CoM shift etc.?

·         FEEP thrusters with a high divergence angle may affect solar cells or tethers

·         I suggest to use solar wind instead of solar-wind throughout the text

  

Detailed comments:

Line 109: … on ground…

Line 114:  … of Moon, Earth, and Sun

Figure 2: larger text ?

Line 223 no-sail, no-sail-clone? Difference?

Figure 3 yellow line is hard to detect

Line 24: … due to deployment oscillations and thrust coning.

Line 252 There are three….

Figure 4. how realistic is this image? Where comes the brightness information from?

Line 295: ..the spin plane can extend 7 × 104 kg m2?

Line 311: jump to Figure 11…

Line 395: ….solar wind plasma

Check line 449..

Figure 5:  … textures from the Solar…

Figure 5: what is the Earth?

Line 744: .. of Sun, Moon, and Earth.

Reference formats should have a last check.

Comments on the Quality of English Language

Only some minor editing is necessary.

Author Response

Thank you for the extensive work of reviewing the ESTCube-LuNa manuscript and providing a fresh look on the article. The ESTCube-LuNa consortium appreciates your feedback and, as detailed below, we have taken good care to address your concerns. The submission’s DIFF version includes tracked changes: the deleted text is red and the new text is blue. The changes in the abstract are not tracked, although the abstract has changed. 
The authors noticed minor discrepancies in the original submission: 
We made a slight modification of the arguments about the centre-of-mass (CoM) shift and the related spin-rate change after Equation 1: The CoM shift is insignificant because it is inside the Debye length and inside the sheath. The, also insignificant, CoM to full tether length ratio (as was described in the original submission) is a secondary effect.
We added one sentence at the end of Section 3.3 “E-sail environment characterisation”: explaining that the Langmuir probes will compare the plasma environment inside the E-sail’s sheath between different modes of the E-sail operations.
Figure 11 “Concept of operations” included an outdated element: the 3-m parabolic dish in Tõravere, Estonia which, as it turned out, was not able to deliver the necessary performance in the UHF band. As the text of original submission describes, the low-speed communications is designed with a Yagi–Uda antenna and the concept of operations figure has been updated accordingly. 
The authors noticed two double-used symbols:
I for the moment of inertia is replaced with M_I (Equation 1 and the following paragraph). I for electric current in Equation 2 is left unchanged.
r for the solar distance is replaced with d (Section 1.1). r for the arm length in Equation 1 is left unchanged.

The following revisions, described by RESPONSES, were made according to the valuable comments provided by you.

The subject of this paper is very actual and interesting for the space community regarding small missions.
RESPONSE: Thanks for your review!
General comments:
·         In the literature, E-sail concepts are using multiple tethers. I guess that the concept in this paper aims at reducing complexity by going to a single tether. But wouldn’t the use of two tethers be much better concerning dynamic behavior, e.g. CoM shift etc.?
RESPONSE: We have revised Section 2. “E-sail experiment design for solar wind in lunar orbit” with details about the two-tether solution. It is not stable. 
·         FEEP thrusters with a high divergence angle may affect solar cells or tethers
RESPONSE: We have revised the end of Section 3.2. “E-sail deployment” with details on the thruster plume. Full details are in 5.5.1. “Attitude Thrusters, ATs” already since the original submission. 
·         I suggest to use solar wind instead of solar-wind throughout the text
RESPONSE: We use “solar-wind” as an adjective. 
Detailed comments:
Line 109: … on ground…
RESPONSE: We will check with the proofreader upon the final submission.
Line 114:  … of Moon, Earth, and Sun
RESPONSE: We will check with the proofreader upon the final submission.
Figure 2: larger text ?
RESPONSE: We consulted with the artist and decided against the increase of the font size, as it would change the overall readability of the complex graph. 
Line 223 no-sail, no-sail-clone? Difference?
RESPONSE: We solved the discrepancy. 
Figure 3 yellow line is hard to detect
RESPONSE: The figure is revised. 
Line 247: … due to deployment oscillations and thrust coning.
RESPONSE: We will check with the proofreader upon the final submission.
Line 252 There are three….
RESPONSE: Well spotted, thanks! 
Figure 4. how realistic is this image? Where comes the brightness information from?
RESPONSE: We removed Figure 4 “Artistic representation of the E-sail tether as viewed by the camera” which was deemed too far from the expected situation and hence had no scientific value. 
Line 295: ..the spin plane can extend 7 × 104 kg m2?
RESPONSE: Yes and we have revised Section 3.2. E-sail deployment with extra details on this unusual number. 
Line 311: jump to Figure 11…
RESPONSE: Yes, we are referencing the later platform section, as now detailed. 
Line 395: ….solar wind plasma
RESPONSE: We use “solar-wind” as an adjective.
Check line 449..
RESPONSE: We did not find any problems. 
Figure 15:  … textures from the Solar…
RESPONSE: Double “from” removed. 
Figure 5: what is the Earth?
RESPONSE: Figure 14 “Realtime 3D” is updated with two simulated camera images with all bodies visible. 
Line 744: .. of Sun, Moon, and Earth.
RESPONSE: We will check with the proofreader upon the final submission.
Reference formats should have a last check.
RESPONSE: References are updated. Thanks for your feedback! 

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

accept

Reviewer 3 Report

Comments and Suggestions for Authors

Thanks for addressing my comments.