The JUpiter ICy moons Explorer Lunar-Earth Gravity Assist (LEGA): Overview

Vallat, Claire; Witasse, Olivier; Altobelli, Nicolas; Dietz, Angela; Aberasturi, Miriam; Aldebert, Clement; Andres, Rafael; Boutonnet, Arnaud; Budnik, Frank; Cornet, Thomas; Costa, Marc

doi:10.5194/egusphere-2026-3074

Preprints

https://doi.org/10.5194/egusphere-2026-3074

Preprints

11 Jun 2026

| 11 Jun 2026

The JUpiter ICy moons Explorer Lunar-Earth Gravity Assist (LEGA): Overview

Claire Vallat, Olivier Witasse, Nicolas Altobelli, Angela Dietz, Miriam Aberasturi, Clement Aldebert, Rafael Andres, Arnaud Boutonnet, Frank Budnik, Thomas Cornet, and Marc Costa

Abstract. The European Space Agency’s JUpiter ICy moons Explorer (JUICE), launched in April 2023, aims to assess the potential habitability of Jupiter’s icy moons–Ganymede, Europa, and Callisto–while investigating the Jovian system as a whole. During its cruise phase, JUICE performed in August 2024 the first-ever combined Moon–Earth gravity assist (LEGA), a critical milestone for optimizing its trajectory toward Jupiter. Beyond its navigation objectives, this unique event provided an exceptional opportunity for in-flight calibration and validation of the scientific payload under known Earth environment. Despite significant operational constraints, all instruments were operated. The Moon flyby enabled precise calibration of remote sensing instruments, radar testing and in situ measurements of the Moon’s environment while within the Earth’s magnetotail, as well as the detection of a spacecraft outgassing event observed by multiple instruments and sensors. The subsequent Earth flyby complemented these efforts through atmospheric, magnetospheric, and radiation belt observations. These measurements confirmed the excellent performances of the JUICE instruments and demonstrated the mission’s capability to deliver high-quality scientific data already during cruise. This campaign represents a key step in preparing for nominal operations at Jupiter while providing valuable scientific results.

Received: 28 May 2026 – Discussion started: 11 Jun 2026

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Claire Vallat, Olivier Witasse, Nicolas Altobelli, Angela Dietz, Miriam Aberasturi, Clement Aldebert, Rafael Andres, Arnaud Boutonnet, Frank Budnik, Thomas Cornet, and Marc Costa

Status: final response (author comments only)

RC1:
'Comment on egusphere-2026-3074', Anonymous Referee #1, 13 Jul 2026

The manuscript entitled "The JUpiter ICy moons Explorer Lunar-Earth Gravity Assist (LEGA): Overview" summarizes the JUICE spacecraft operations and science instrument activities during the Lunar Earth Gravity Assist, intended to be read as an overview to the LEGA special issue manuscripts, without entering too much in detail to prevent overlapping with specific manuscripts.
I recommend minor revisions. I find the manuscript to be informative enough, however the Figures need reworking to improve their readability and some parts of the text need further clarification. Please, find bellow my comments with the intention to help improving the clarity of the manuscript.

Title: LEGA acronym is defined in the title but not the Juice one. For consistency, I would suggest to introduce “Juice” acronym also in the title.
line 34: “After Jupiter Orbit Insertion” -> “After the Jupiter Orbit Insertion”
lines 36-37: the sentence is difficult to read. Here is a suggestion: “… first on an elliptical orbit and circular orbit at 5000km altitude, before going down to 500km and 200km” -> “... first on an elliptical orbit, then on three different circular orbits at altitudes of 5000km, 500km and 200km, respectively.”
lines 38-43: it is not clear that the facility instruments are not considered as scientific instruments. Though 3GM is now responsible for HAA, I believe this facility instrument should also be listed in the text. Also, having a table listing all the instruments with respective descriptions and acronym definition would help those readers that are not familiar with the Juice mission.
lines 47-49: this sentence is not a new paragraph, as the text is a continuation of the previous sentence topic.

line 45: would change the verb to past simple. “This combined Moon-Earth gravity assist has been a major navigation milestone’’ -> “This combined Moon-Earth gravity assist was a major navigation milestone”
section 2.1 could benefit of a summary related to Table 1.
Table 1: to be in harmony with the acronyms, I would suggest to update as follows: “Closest Approach Moon” -> “Moon Closest Approach”, “Closest Approach Earth” -> “Earth Closest Approach”
line 61: would add HAA to the list of facility instruments.
Figure 1 and Figure 2 are too small, making it hard to read the labels and text. I would suggest to merge first panel from Figure 1 and Figure 2, in a separated figure, supporting Table 1 in the previous section.
Figure 2, I would suggest having 2 separated panels one for the incidence angle and another for the spacecraft altitude, as done for the Moon flyby, as the superposition doesn’t allow to read the spacecraft altitude.
Figure 3 legend: typo “22d” -> “22nd”
lines 91-94 no need of making it a new paragraph.
line 91: for precision, I would suggest to define from which body the magnetotail is from: “The Moon Closest Approach is occurring while the Moon is in the magnetotail (trajectory part on the left). “ -> “The Moon Closest Approach is occurring while the Moon is in the Earth’s magnetotail (trajectory part on the left). “
line 93: “… before exiting in the Solar wind” -> “… before reaching the Solar wind”
line 96: can the authors precise the amount of time available for the observations?
line 97-98 “In addition, this period also presented several challenging operational constraints that are briefly described below.” -> “In addition, this period also presented several challenging operational constraints (navigation, eclipse, spacecraft attitude, thermal, electromagnetic interferences) that are briefly described below.”
line 100: please remove “, they” from the sentence. “Gravity assists being key manoeuvres for the mission success, they require robust “ -> “Gravity assists being key manoeuvres for the mission success require robust …”
line 101: I would also capitalize Approach in “Closest approach”, to use CA acronym in the following text.
line 104, please revise the verb tense, as this was an activity schedules in the past.
line 105: “and one 4 hours before “ -> “and another 4 hours before”
lines 104-105: I believe information of the possible eTCM could be included in Table1, even if this is not a certain event, it constrains the payload activities, as the Eclipse does.
line 115: I believe the eclipse needs to be defined in this spacecraft navigation context, this will mainly benefit those readers that have reduced knowledge of spacecraft navigation.
line 123: “resp. Earth” -> “respective to Earth”
section 2.3.3: an introduction to Figure 4 is missing in the text.
Figure 4 legend:

“up” -> “Top panel”

“The green background indicates when the target is within the JANUS science camera field of view, showing that Earth was not in the camera FoV at Earth closest approach.” -> “The green background indicates when the target is within the JANUS science camera field of view. Note that the Earth was not in the camera FoV at Earth closest approach.”
lines 139-144: Figure 5 can be referred to, here.
section 3 title is quite loose term, could the authors be more precise? I am missing a sentence to introduce what this section will entail.
Figure 5 needs improvement, as it is difficult to read. The x axis is too small to be easily read, as well as the text in the different colored bars associated to instruments. The shade “Rime Quiet Time” is not grey as indicated in the legend, effect from overlapping with violet color. “Min Altitude” in red should be called “Moon Closest Approach” to be in agreement with the legend and text. However, I wouldn’t add either “Min Altitude” or MCA or “Terminator” to the figure, but only describe the dashed red and black lines in the Figure’s legend, for simplicity. The shaded colors should be placed behind the instrument activities colored bars (or on top of the Figure as the blue ‘+Zsc intersecting the surface’ bar is) to prevent the instrument bar color change over time.

Also, I would suggest to add the other facility instruments (e.g., RADEM, NAVCAM, JMC, …) to the graphic.
Figure 5 legend: some important features from the figure need to be described in the legend, e.g., red and black vertical dashed lines. What are the vertical blue dotted lines? Is the violet color indicating nightside, as the yellow is indicating the dayside? Why some instruments have a colored single bar, and others have 2 (same color) bars?
lines 152-154section 3.1 would highly benefit of a summary of the Figure 5.
line 156 first time acronym is used “IR”, definition is missing.
lines 163-164: Is the “130 meters” the image resolution? Then I would suggest to slightly rephrase it to “During the Moon flyby, MAJIS was able to provide local coverage of the lunar surface with a resolution down to ~130 meters, ranging from the visible up to the thermal infrared.”
lines 169-170: this sentence reads odd: “could allow provide high quality scientific data “, removing the “allow” word could work.
lines 172-173: please define acronyms when first used.
line 180-186: authors could add the information that these instruments were operating during the lunar approach within the Earth’s magnetotail at distances from the Earth that were previously little sampled. Also, I would move reference to LaMoury et al. when describing the magnetometer measurements in the next.
line 189: too many “radar” in the sentence.
line 195: “response to Moon gravity” -> “response to the Moon gravity”
Figure 6: labels are too small, this will probably be solved by increasing the size of the figure.
Figure 7: the figure appears to be cropped. One of the JMAG sensors it not displayed. What is the “LP” text appearing in the Figure but not pointing to anywhere? Also, were the solar arrays in that position relative to the spacecraft axis during the lunar flyby, or were they rotated? If they were with a certain angle compared to what is shown, then it needs to be stated in the figure legend, to prevent induce a reader in error.
Figure 8: labels and axis marks are too small. Please, fix Figure.
line 219: the authors can specify that De Filippis et al. used accelerometer data in the following sentence “This outgassing event is reported in De Filippis et al., this collection.”
line 226: “ like e.g. radiance and radiance variability” -> “like radiance and radiance variability, for example.”
lines 230-231: I believe the sentence needs rephrasing for clarity. “The UV emissions from Earth’s atmosphere provided an especially good test of the plans for JUICE-UVS to observe Ganymede’s UV aurora and other atmospheric features.” -> “The UV emissions from Earth’s atmosphere provided a particularly relevant test case for JUICE-UVS planning to observe Ganymede’s UV aurora and other atmospheric features.”
lines 244-245: typo “to the ring current” -> “of the ring current”?
line 245: “On the outbound part of the flyby, …” -> On the outbound part of the Earth flyby, ….”
line 248: direction -> directions
line 252: redundant having “like” and “e.g.” next to each other. I would remove “like” from the sentence.
line 261: first time using the SNR acronym, please define.
line 261: situ particles -> in situ particles
line 264-265 The sentence is in the wrong tense, please correct. Also, “radiations belts” -> “radiation belts”
line 179 “both NAVCAM and JMC” not JANUS, right?
Figures 9 and 10 are not introduced in the text but they should. Figure 9 and 10 could be a single Figure with top and bottom panels.
What about outreach activities, couldn’t these also be reported in such summary paper?
line 292: phases -> phase (I assume the authors refer to the flyby)
line 293: “provided excellent calibration targets” -> “proved to be excellent targets”

line 303: no need to mention “data” after JMC.
General comment: Please, prevent having paragraphs with single sentences.

Citation: https://doi.org/10.5194/egusphere-2026-3074-RC1
- AC1: 'Reply on RC1', Claire Vallat, 23 Jul 2026
  
  The manuscript entitled "The JUpiter ICy moons Explorer Lunar-Earth Gravity Assist (LEGA): Overview" summarizes the JUICE spacecraft operations and science instrument activities during the Lunar Earth Gravity Assist, intended to be read as an overview to the LEGA special issue manuscripts, without entering too much in detail to prevent overlapping with specific manuscripts.
  
  I recommend minor revisions. I find the manuscript to be informative enough, however the Figures need reworking to improve their readability and some parts of the text need further clarification. Please, find bellow my comments with the intention to help improving the clarity of the manuscript.
  [Author's feedback]: the author thanks the reviewer for the time and effort spent to provide a detailed review of the text and figures that will help to improve the article. All comments have been considered and a revised version of the manuscript will be provided, taking into account almost all points, as detailed below.
  
  Title: LEGA acronym is defined in the title but not the Juice one. For consistency, I would suggest to introduce “Juice” acronym also in the title.
  [Author's feedback]: Done.
  
  line 34: “After Jupiter Orbit Insertion” -> “After the Jupiter Orbit Insertion”
  [Author's feedback]: Done.
  
  lines 36-37: the sentence is difficult to read. Here is a suggestion: “… first on an elliptical orbit and circular orbit at 5000km altitude, before going down to 500km and 200km” -> “... first on an elliptical orbit, then on three different circular orbits at altitudes of 5000km, 500km and 200km, respectively.”
  [Author's feedback]: rephrased. ’It will then enter in orbit around Ganymede for an additional year towards the end of 2034, first on an elliptical orbit followed by a circular orbit at 500km altitude and finally at 200km altitude.’
  
  lines 38-43: it is not clear that the facility instruments are not considered as scientific instruments. Though 3GM is now responsible for HAA, I believe this facility instrument should also be listed in the text. Also, having a table listing all the instruments with respective descriptions and acronym definition would help those readers that are not familiar with the Juice mission.
  [Author's feedback]: Table 1 added.
  
  lines 47-49: this sentence is not a new paragraph, as the text is a continuation of the previous sentence topic.
  [Author's feedback]: Done.
  
  line 45: would change the verb to past simple. “This combined Moon-Earth gravity assist has been a major navigation milestone’’ -> “This combined Moon-Earth gravity assist was a major navigation milestone”
  [Author's feedback]: Done.
  
  section 2.1 could benefit of a summary related to Table 1.
  [Author's feedback]: Added.
  
  Table 1: to be in harmony with the acronyms, I would suggest to update as follows: “Closest Approach Moon” -> “Moon Closest Approach”, “Closest Approach Earth” -> “Earth Closest Approach”
  [Author's feedback]: Done.
  
  line 61: would add HAA to the list of facility instruments.
  [Author's feedback]: Done.
  
  Figure 1 and Figure 2 are too small, making it hard to read the labels and text. I would suggest to merge first panel from Figure 1 and Figure 2, in a separated figure, supporting Table 1 in the previous section.
  [Author's feedback]: Changes implemented as suggested.
  
  Figure 2, I would suggest having 2 separated panels one for the incidence angle and another for the spacecraft altitude, as done for the Moon flyby, as the superposition doesn’t allow to read the spacecraft altitude.
  [Author's feedback]: Done.
  
  Figure 3 legend: typo “22d” -> “22nd”
  [Author's feedback]: Corrected
  
  lines 91-94 no need of making it a new paragraph.
  [Author's feedback]: Done.
  
  line 91: for precision, I would suggest to define from which body the magnetotail is from: “The Moon Closest Approach is occurring while the Moon is in the magnetotail (trajectory part on the left). “ -> “The Moon Closest Approach is occurring while the Moon is in the Earth’s magnetotail (trajectory part on the left). “
  [Author's feedback]: Modified as suggested.
  
  line 93: “… before exiting in the Solar wind” -> “… before reaching the Solar wind”
  [Author's feedback]: Modified as suggested.
  
  line 96: can the authors precise the amount of time available for the observations?
  [Author's feedback]: Added.
  
  line 97-98 “In addition, this period also presented several challenging operational constraints that are briefly described below.” -> “In addition, this period also presented several challenging operational constraints (navigation, eclipse, spacecraft attitude, thermal, electromagnetic interferences) that are briefly described below.”
  [Author's feedback]: Modified as suggested.
  
  line 100: please remove “, they” from the sentence. “Gravity assists being key manoeuvres for the mission success, they require robust “ -> “Gravity assists being key manoeuvres for the mission success require robust …”
  [Author's feedback]: Modified as suggested.
  
  line 101: I would also capitalize Approach in “Closest approach”, to use CA acronym in the following text.
  [Author's feedback]: Done
  
  line 104, please revise the verb tense, as this was an activity schedules in the past.
  [Author's feedback]: corrected. ‘’In addition, to cope with the unlikely case where the latest orbit determination and trajectory optimisation revealed that Jupiter would not be reached within the given mission constraints (or with a prohibitive Delta-v penalty), an emergency Trajectory Correction Manoeuvre (eTCM) was scheduled a few hours before the actual flyby’.
  
  line 105: “and one 4 hours before “ -> “and another 4 hours before”
  [Author's feedback]: Done.
  
  lines 104-105: I believe information of the possible eTCM could be included in Table1, even if this is not a certain event, it constrains the payload activities, as the Eclipse does.
  [Author's feedback]: Table updated
  
  line 115: I believe the eclipse needs to be defined in this spacecraft navigation context, this will mainly benefit those readers that have reduced knowledge of spacecraft navigation.
  [Author's feedback]:eclipse section merged with Navigation and details added.
  
  line 123: “resp. Earth” -> “respective to Earth” —> ‘respectively’
  [Author's feedback]: added ‘respectively’ instead of ‘resp.’
  
  section 2.3.3: an introduction to Figure 4 is missing in the text.
  [Author's feedback]: Done
  
  Figure 4 legend:
  
  “up” -> “Top panel”
  
  “The green background indicates when the target is within the JANUS science camera field of view, showing that Earth was not in the camera FoV at Earth closest approach.” -> “The green background indicates when the target is within the JANUS science camera field of view. Note that the Earth was not in the camera FoV at Earth closest approach.”
  [Author's feedback]: updated legend.
  
  lines 139-144: Figure 5 can be referred to, here.
  [Author's feedback]: Done
  
  section 3 title is quite loose term, could the authors be more precise? I am missing a sentence to introduce what this section will entail.
  [Author's feedback]”: modified section name. ‘Observations during LEGA ’. Added introductory sentence :’This section provides a description of all activities performed around the Moon and Earth flybys by the various instruments, together with an overview of the main findings that are further described in the different articles of this collection or on other scientific publications.’
  
  Figure 5 needs improvement, as it is difficult to read.
  The x axis is too small to be easily read, as well as the text in the different colored bars associated to instruments.
  The shade “Rime Quiet Time” is not grey as indicated in the legend, effect from overlapping with violet color.
  “Min Altitude” in red should be called “Moon Closest Approach” to be in agreement with the legend and text.
  However, I wouldn’t add either “Min Altitude” or MCA or “Terminator” to the figure, but only describe the dashed red and black lines in the Figure’s legend, for simplicity.
  The shaded colors should be placed behind the instrument activities colored bars (or on top of the Figure as the blue ‘+Zsc intersecting the surface’ bar is) to prevent the instrument bar color change over time.
  
  Also, I would suggest to add the other facility instruments (e.g., RADEM, NAVCAM, JMC, …) to the graphic.
  Figure 5 legend: some important features from the figure need to be described in the legend, e.g., red and black vertical dashed lines. What are the vertical blue dotted lines? Is the violet color indicating nightside, as the yellow is indicating the dayside? Why some instruments have a colored single bar, and others have 2 (same color) bars?
  [Author's feedback]: Figure 5 reworked taking into account all above comments.
  
  lines 152-15 4section 3.1 would highly benefit of a summary of the Figure 5.
  [Author's feedback]: added
  
  line 156 first time acronym is used “IR”, definition is missing.
  [Author's feedback]: corrected
  
  lines 163-164: Is the “130 meters” the image resolution? Then I would suggest to slightly rephrase it to “During the Moon flyby, MAJIS was able to provide local coverage of the lunar surface with a resolution down to ~130 meters, ranging from the visible up to the thermal infrared.”
  [Author's feedback]: implemented as suggested.
  
  lines 169-170: this sentence reads odd: “could allow provide high quality scientific data “, removing the “allow” word could work.
  [Author's feedback]: corrected
  
  lines 172-173: please define acronyms when first used.
  [Author's feedback]: done
  
  line 180-186: authors could add the information that these instruments were operating during the lunar approach within the Earth’s magnetotail at distances from the Earth that were previously little sampled. Also, I would move reference to LaMoury et al. when describing the magnetometer measurements in the next.
  [Author's feedback]: ok. Correction implemented.
  
  line 189: too many “radar” in the sentence.
  [Author's feedback]: Corrected.
  
  line 195: “response to Moon gravity” -> “response to the Moon gravity”
  [Author's feedback]: Corrected.
  
  Figure 6: labels are too small, this will probably be solved by increasing the size of the figure.
  [Author's feedback]: figure revised
  
  Figure 7: the figure appears to be cropped. One of the JMAG sensors is not displayed. What is the “LP” text appearing in the Figure but not pointing to anywhere? Also, were the solar arrays in that position relative to the spacecraft axis during the lunar flyby, or were they rotated? If they were with a certain angle compared to what is shown, then it needs to be stated in the figure legend, to prevent induce a reader in error.
  [Author's feedback]: Figure modified
  
  Figure 8: labels and axis marks are too small. Please, fix Figure.
  [Author's feedback]: Updated figure
  
  line 219: the authors can specify that De Filippis et al. used accelerometer data in the following sentence “This outgassing event is reported in De Filippis et al., this collection.”
  [Author's feedback]: Added.
  
  line 226: “ like e.g. radiance and radiance variability” -> “like radiance and radiance variability, for example.”
  [Author's feedback]: modified as suggested.
  
  lines 230-231: I believe the sentence needs rephrasing for clarity. “The UV emissions from Earth’s atmosphere provided an especially good test of the plans for JUICE-UVS to observe Ganymede’s UV aurora and other atmospheric features.” -> “The UV emissions from Earth’s atmosphere provided a particularly relevant test case for JUICE-UVS planning to observe Ganymede’s UV aurora and other atmospheric features.”
  [Author's feedback]: modified as suggested.
  
  lines 244-245: typo “to the ring current” -> “of the ring current”?
  [Author's feedback]: in this sentence distinction was made between plasmasphere and ring current to distinguish between different energy ranges. Ring current reference was removed and only reference to the plasmasphere and the radiation belts was kept.
  
  line 245: “On the outbound part of the flyby, …” -> On the outbound part of the Earth flyby, ….”
  [Author's feedback]: modified as suggested.
  
  line 248: direction -> directions
  [Author's feedback]: corrected
  
  line 252: redundant having “like” and “e.g.” next to each other. I would remove “like” from the sentence.
  [Author's feedback]: corrected
  
  line 261: first time using the SNR acronym, please define.
  [Author's feedback]: added
  
  line 261: situ particles -> in situ particles
  [Author's feedback]: corrected
  
  line 264-265 The sentence is in the wrong tense, please correct. Also, “radiations belts” -> “radiation belts”
  [Author's feedback]: corrected
  
  line 179 “both NAVCAM and JMC” not JANUS, right?
  [Author's feedback]: No. Reference to NAVCAM and JANUS is correct here.
  
  Figures 9 and 10 are not introduced in the text but they should. Figure 9 and 10 could be a single Figure with top and bottom panels.
  [Author's feedback]: Introduced in the text but not merged in a single figure since they do not correspond to the same period of time (flyby vs farewell, 3 weeks later)
  
  What about outreach activities, couldn’t these also be reported in such summary paper?
  [Author's feedback]: Section added
  
  line 292: phases -> phase (I assume the authors refer to the flyby)
  [Author's feedback]: corrected
  
  line 293: “provided excellent calibration targets” -> “proved to be excellent targets”
  [Author's feedback]: modified as suggested.
  
  line 303: no need to mention “data” after JMC.
  [Author's feedback]: corrected
  
  General comment: Please, prevent having paragraphs with single sentences.
  [Author's feedback]: revised as suggested.
  
  Citation: https://doi.org/10.5194/egusphere-2026-3074-AC1
RC2:
'Comment on egusphere-2026-3074', Anonymous Referee #2, 04 Aug 2026

The paper by Vallat et al. intends to act as a wrapper of all the JUICE/LEGA special issue papers and provide some basic context for all relevant instrument and spacecraft operations. In that sense, the paper fulfills its purpose: the amount of text and the references to the special issue papers are well balanced, there is a good selection of Figures, text is clear and of the appropriate length, and the overlap with Dietz et al. on operational aspects is minimal. The reader can easily discover what each special issue paper focuses on and select which ones to read in more detail. Overall, besides few issues outlined below that need to be address in a minor revision, I believe this contextual paper is suitable for publication in the special issue.
1) Lines 33-35: "Proximity observations at Europa...". Either specify it like this, otherwise add also extend this comment to account for remote Io and smaller moon observations which are also planned.
2) Line 39: Somewhere in this part specify that many of these instruments are instrument packages with several sub-units. This may especially be relevant for PEP, since PEP-Hi & PEP-Lo are mentioned separately later, thus an earlier clarification is needed.
3) Section 2.2: It may be appropriate to define here the incidence angle definition (or at least before the first reference to the relevant Figure is mentioned), rather that in the caption.
4) Figures 1,2 : All Figure labelling is too small and difficult to read unless one zooms in (but then the full figure fills the page and scrolling is needed to read different parts). Figures are too blurry when zoomed in, so maybe better quality versions are available. Arrangement of the Figures may allow for some panels to appear larger to begin with. For instance, the top panel may be considered as separate Figure? These comments for improvement apply for both Figure 1 & 2, but are more critical for Figure 1.
5) Figure 3: Can you iterate on the aspect angle of this illustration? Much of the figure is just black space, and the region of interest in the Figure is too small. If much of the black space is removed left and right of the flyby graphics, then the trajectory geometry with respect to the magnetospheric interfaces will become much better visible.
6) Section 2.3.3 & Figure 4: These plots and this section are very useful, but my understanding is that the blue curves in each panel show the incidence angle variation in Figures 1,2. In that sense, a link to these Figures should be added, possibly with a comment that during CA at either Moon or Earth was inertial (not the typical nadir one in many flybys), explaining the aspect angle variations. If indeed what plotted in the y-axes of Fig. 4 is the incidence angle, maybe the axes titles can be adjusted for consistency with Fig. 1,2? Otherwise, clarify what the difference is between the angles plotted in those figures.
7) Section 2.35 & line 144: Operations were resumed from all instruments, not just remote sensing ones.
8) Figure 5: "JENI-JoEE" should be renamed as "PEP-Hi"
7) Line 182: By plasma environment package, do you mean PEP or the in-situ instruments as a whole?
8) Section 3.1.2: This is a bit too short. Some other details that can be added is that e.g. for all in-situ instruments this flyby offered an excellent calibration opportunity due to the proximity of JUICE to the two ARTEMIS spacecraft, which carry similar types of instruments as JUICE (magnetometer, charged particle detectors overlapping in energy with PEP and a plasma wave instrument). Also, the JENI instrument on PEP-Hi would attempt the first imaging of the moon in very high Energetic Neutral Atoms (ENAs). RADEM and Pinto et al. can also be mentioned her. Understandably, there are not yet many in-situ observations shown from the lunar flyby in the special issue (excluding RADEM - showing that the environment was rather quiet and not ideal to resolve strong lunar magnetosphere interactions), but relevant data for all aforementioned activities has been collected.
9) Section 3.4: PEP-Lo (with JDC and JEI) were also operational during this short campaign and a short comment can be added for completeness. The conditions for JDC were especially favorable for forming a baseline against which lunar flyby observations can be compared against to.

Citation: https://doi.org/10.5194/egusphere-2026-3074-RC2
- AC2: 'Reply on RC2', Claire Vallat, 25 Aug 2026
  
  1) Lines 33-35: "Proximity observations at Europa...". Either specify it like this, otherwise add also extend this comment to account for remote Io and smaller moon observations which are also planned.
  [author]: can be modified as suggested.
  2) Line 39: Somewhere in this part specify that many of these instruments are instrument packages with several sub-units. This may especially be relevant for PEP, since PEP-Hi & PEP-Lo are mentioned separately later, thus an earlier clarification is needed.
  [author]: ok can be clarified in the text:’’JUICE is equipped with ten advanced scientific instrument (or instrument packages with several sub-units) and dedicated to geophysical (RIME, GALA, 3GM), in-situ (JMAG, RPWI, and PEP organised into two groups: PEP-Lo and PEP-Hi), and remote-sensing investigations (JANUS, MAJIS, SWI, UVS), along with the Planetary Radio Interferometer and Doppler Experiment (PRIDE).
  3) Section 2.2: It may be appropriate to define here the incidence angle definition (or at least before the first reference to the relevant Figure is mentioned), rather that in the caption.
  [author]: Ok. Can add in the text before Figure 2 :’The temporal evolution of the incidence angle, defined as the angle is measured between the outward normal (zenith direction) at the surface point and the direction from the surface point to the illuminator, is displayed in Figure 2 and Figure 3 (bottom plots).’ And removed from caption.
  4) Figures 1,2 : All Figure labelling is too small and difficult to read unless one zooms in (but then the full figure fills the page and scrolling is needed to read different parts). Figures are too blurry when zoomed in, so maybe better quality versions are available. Arrangement of the Figures may allow for some panels to appear larger to begin with. For instance, the top panel may be considered as separate Figure? These comments for improvement apply for both Figure 1 & 2, but are more critical for Figure 1.
  [author]: figures can be modified following comments from referee 1 recommendations
  5) Figure 3: Can you iterate on the aspect angle of this illustration? Much of the figure is just black space, and the region of interest in the Figure is too small. If much of the black space is removed left and right of the flyby graphics, then the trajectory geometry with respect to the magnetospheric interfaces will become much better visible.
  [author]: Figure can be regenerated with better viewpoint and larger zoom over the two relevant regions. Reference to the models used for magnetopause and bow shock crossing will be added as well.
  The text will be modified as follows: ‘The plot in Figure 4 represents the JUICE trajectory in the Geocentric Solar Ecliptic (GSE) frame, X_GSE indicating the Sun direction (Z_GSE being perpendicular to the plane of the Earth's orbit around the Sun, positive North)). JUICE is arriving from the far magnetotail and approaching Earth from the night side, almost aligned with the Sun-Earth line and followed by a flyby on the dawn side. The Moon Closest Approach is occurring while the Moon is in the Earth magnetotail (trajectory part on the bottom right). While approaching and departing from Earth, JUICE also flew over the outer and inner radiation belts (not shown). A few hours after Earth Closest Approach, JUICE crossed the magnetopause (light blue) and the bow shock (yellow) before reaching the Solar Wind on the dusk side of the Earth.’
  6) Section 2.3.3 & Figure 4: These plots and this section are very useful, but my understanding is that the blue curves in each panel show the incidence angle variation in Figures 1,2. In that sense, a link to these Figures should be added, possibly with a comment that during CA at either Moon or Earth was inertial (not the typical nadir one in many flybys), explaining the aspect angle variations. If indeed what plotted in the y-axes of Fig. 4 is the incidence angle, maybe the axes titles can be adjusted for consistency with Fig. 1,2? Otherwise, clarify what the difference is between the angles plotted in those figures.
  [author]: The blue curve shows the angular separation between the Moon center and the +Zsc axis as seen from the spacecraft while incidence angle is the angular separation between the surface normal and the Sun direction. Incidence angle evolves from night (>90 degrees) to day (<90 degrees) while Figure 4 blue line shows that the angle is minimum close to CA but increase before and after that.
  The text will be modified to clarify the approach :’[…] The flexibility in setting the spacecraft attitude is therefore limited to a rotation around the spacecraft to Sun direction while keeping the High Gain antenna always pointing to the Sun, therefore preventing to follow an ideal nadir tracking approach.
  To maximize the observations time and conditions despite this pointing constraint, a strategy had to be defined to ensure that the Spacecraft +Zsc axis (nadir deck) could intercept the Moon (respectively Earth) disc for as long as possible while maintaining the HGA pointed to Sun. A rotation angle strategy (around the HGA to Sun orientation) has been selected to ensure that the Spacecraft +Zsc axis (nadir deck) intercepts the Moon (respectively Earth) disc for as long as possible. ’
  7) Section 2.35 & line 144: Operations were resumed from all instruments, not just remote sensing ones.
  [author]: yes, correct, sentence to be modified accordingly :’, […] when all instruments could resume their observations, and as shown in the observations’ timeline overview in Figure 6’
  8) Figure 5: "JENI-JoEE" should be renamed as "PEP-Hi"
  [author]: Ok. Will be corrected.
  7) Line 182: By plasma environment package, do you mean PEP or the in-situ instruments as a whole?
  [author]: I refer to the PEP sensors here (JDC and JEI), but in practice this can be extended to the whole in situ.
  8) Section 3.1.2: This is a bit too short. Some other details that can be added is that e.g. for all in-situ instruments this flyby offered an excellent calibration opportunity due to the proximity of JUICE to the two ARTEMIS spacecraft, which carry similar types of instruments as JUICE (magnetometer, charged particle detectors overlapping in energy with PEP and a plasma wave instrument). Also, the JENI instrument on PEP-Hi would attempt the first imaging of the moon in very high Energetic Neutral Atoms (ENAs). RADEM and Pinto et al. can also be mentioned her. Understandably, there are not yet many in-situ observations shown from the lunar flyby in the special issue (excluding RADEM - showing that the environment was rather quiet and not ideal to resolve strong lunar magnetosphere interactions), but relevant data for all aforementioned activities has been collected.
  [author]: Thanks for the suggestion, the following text can be added in this section: ‘The JENI instrument on PEP-Hi could also attempt the first imaging of the moon in very high Energetic Neutral Atoms (ENAs). This flyby also provided an excellent calibration opportunity, owing to JUICE's close proximity to the two ARTEMIS (Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun) spacecraft, which carry similar types of instruments as JUICE (magnetometer, charged particle detectors and plasma wave instrument). Given JUICE's location within the magnetosphere during the flyby, RADEM—operating continuously—could only detect the interplanetary GCR background (Pinto et al, this issue).
  9) Section 3.4: PEP-Lo (with JDC and JEI) were also operational during this short campaign and a short comment can be added for completeness. The conditions for JDC were especially favorable for forming a baseline against which lunar flyby observations can be compared against to.
  [author]: ok text can be completed with the following sentence :’For the in-situ, both RPWI and PEP-Lo were operating during this time. For PEP-Lo (JDC in particular), this provided a baseline against which lunar flyby observations could be compared.
  
  Citation: https://doi.org/10.5194/egusphere-2026-3074-AC2
EC1: 'Comment on egusphere-2026-3074', Joana S. Oliveira, 04 Aug 2026

Publisher's note: this comment is a copy of RC3, and its content was therefore removed on 5 August 2026.

Citation: https://doi.org/10.5194/egusphere-2026-3074-EC1
RC3:
'Comment on egusphere-2026-3074', Stephanie C. Werner, 05 Aug 2026

Congratulations on achieving this mission milestone!
The opportunity of observing Earth and Moon through the eyes of JUICE during this manoeuvre produced exciting data sets of Earth and Moon that can spark the interest of research communities beyond those who study the Jupiter system.
The manuscript The JUpiter ICy moons Explorer Lunar-Earth Gravity Assist (LEGA): Overview provides the overview of this unique in-flight calibration and validation opportunity for JUICE’s scientific payload and introduces the content of the special issue in ANGEO.

Comments and suggestions:
It may be useful to add a glossary table that spells out all acronyms of the instruments, manoeuvres etc. to enhance the readability of the paper and the entire special issue.
Overall, the manuscript contains some community slang and could be revised to more physical precise wording (e.g.: “the spacecraft … was illuminated by Moon albedo” Line 208), however, the extent of modifications I leave to the authors’ discretion. Additionally, I recommend homogenization of spelling and wording and capitalization, e.g., Moon or moon, this issue or this collection.
In section 3.1.4, you indicate the presence of water vapour around the spacecraft and its relevance in the context of the lunar water cycle, but this relation is no further defined or discussed. Is it a detection or is it considered a potential issue when inferring presence of water vapour at the target? Is this further discussed in any of the special issue contributions? If so please refer to the contribution.

Minor suggestions or additional examples to the above main comment:
Line 167: did you mean “radiations” or rather “radiation variations”?
Line 169: please consider rephrasing “could allow provide high quality scientific data”

Citation: https://doi.org/10.5194/egusphere-2026-3074-RC3
- AC3: 'Reply on RC3', Claire Vallat, 25 Aug 2026
  
  [Author's feedback]: the author thanks the 3 reviewers for the time and effort spent to provide a detailed review of the text and figures that will help to improve the article. All comments from RC3 have been addressed and commented below.
  It may be useful to add a glossary table that spells out all acronyms of the instruments, manoeuvres etc. to enhance the readability of the paper and the entire special issue.
  [author]: a Table will be added, also following suggestion from reviewer 1. It lists the instruments acronyms. For the other acronyms, I will try to remove them in the text as much as possible or/and make sure they were defined at least the first time they were used.
  Overall, the manuscript contains some community slang and could be revised to more physical precise wording (e.g.: “the spacecraft … was illuminated by Moon albedo” Line 208), however, the extent of modifications I leave to the authors’ discretion.
  [author]: for this sentence, the following rephrasing is suggested :’At this time, the cold side of the spacecraft (face oriented along +Xsc) was illuminated by the Moon albedo (see Figure 7 and 8), consistent with these effects. In parallel, Orbit Determination estimates a parasitic Δv of about 0.3 mm/s (constrained along +X) shortly after the lunar closest approach.’
  Additionally, I recommend homogenization of spelling and wording and capitalization, e.g., Moon or moon, this issue or this collection.
  [author]: agreed, thanks for pointing that out. I tried to correct it and homogenize it over the paper. Same for JUICE acronym (expect if this is part of an acronym, in which case lower case is used, like e.g Juice Monitoring Camera for JMC)
  In section 3.1.4, you indicate the presence of water vapour around the spacecraft and its relevance in the context of the lunar water cycle, but this relation is no further defined or discussed. Is it a detection or is it considered a potential issue when inferring presence of water vapour at the target? Is this further discussed in any of the special issue contributions? If so please refer to the contribution.
  [author]: Section 3.1.4 reports the detection of an outgassing event during the lunar closest approach. Given that the interpretation is supported by three independent and consistent observations, namely spacecraft torque measurements, accelerometer data, and SWI instrument observations, we consider the outgassing scenario to be well founded. We chose to include this topic in the paper because (1) the event was unexpected, and (2) it is important to establish that the detected water originated from the spacecraft rather than the lunar environment, a question that remains under discussion in the literature cited in Section 3.1.4. Apart from the contribution by De Filippis et al., this topic is not addressed elsewhere in the topical collection. As such, it represents an interesting and potentially important result that will hopefully be explored in greater detail in future publications.
  Minor suggestions or additional examples to the above main comment:
  Line 167: did you mean “radiations” or rather “radiation variations”?
  [author]: we suggest to replace 'observed' by 'identified' (‘Thermal emission and reflectance radiations due to seas or “maria” and lunar highlands could be identified’) to distinguish from other signatures.
  Line 169: please consider rephrasing “could allow provide high quality scientific data”
  [author]: rephrased (also following reviewer #1 suggestion): ‘The observations acquired by the JANUS camera, although primarily devoted to calibrations, could provide high-quality scientific data to support geological and spectrophotometric analysis of the surface, like e.g. the Langrenus crater (Lucchetti et al, this issue).’
  
  Citation: https://doi.org/10.5194/egusphere-2026-3074-AC3

Claire Vallat, Olivier Witasse, Nicolas Altobelli, Angela Dietz, Miriam Aberasturi, Clement Aldebert, Rafael Andres, Arnaud Boutonnet, Frank Budnik, Thomas Cornet, and Marc Costa

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Short summary

The first ever double gravity assist of Moon and Earth on the 19th and 20th of August 2024 by the ESA JUICE spacecraft provided a unique opportunity for payload calibration in preparation for the nominal science phase. Despite the complex set of constraints applicable to such operationally critical phases, the payload and facility instruments were able to operate. The excellent scientific dataset acquired confirmed the instrument performances.


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