the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Jun 2026
The JUpiter ICy moons Explorer Lunar-Earth Gravity Assist (LEGA): Overview
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.
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- RC1: 'Comment on egusphere-2026-3074', Anonymous Referee #1, 13 Jul 2026 reply
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- 1
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.