The JUICE Lunar-Earth gravity assist from trajectory design, navigation and spacecraft operations perspective

Dietz, Angela; Boutonnet, Arnaud; Budnik, Frank

doi:10.5194/egusphere-2026-1015

Preprints

https://doi.org/10.5194/egusphere-2026-1015

Preprints

05 Mar 2026

| 05 Mar 2026

The JUICE Lunar-Earth gravity assist from trajectory design, navigation and spacecraft operations perspective

Angela Dietz, Arnaud Boutonnet, and Frank Budnik

Abstract. This paper describes the world’s first-ever Lunar–Earth Gravity Assist (LEGA) performed by ESA’s JUICE spacecraft on 19–20 August 2024 from trajectory design, navigation and spacecraft operations point of view.

This double flyby – Moon first, Earth second – enabled a large Delta-V gain while minimizing propellant use, redirecting JUICE toward its next destination: Venus (August 2025) and ultimately Jupiter (2031). The manoeuvre was unprecedented in complexity, requiring extremely accurate navigation, rigorous preparation, and coordinated operations across engineering, flight dynamics, and science teams.

Overall, JUICE demonstrated outstanding platform stability, navigation accuracy, and subsystem robustness during this critical milestone, validating the operational feasibility of LEGA as an enabling technique for complex interplanetary trajectories.

This paper is an executive summary of papers published on LEGA trajectory design [Schoenmaekers et al. (2014); Boutonnet et al. (2023)], navigation [Syndercombe et al. (2025)] and spacecraft operations [Heck et al. (2025)].

Received: 22 Feb 2026 – Discussion started: 05 Mar 2026

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Angela Dietz, Arnaud Boutonnet, and Frank Budnik

Status: final response (author comments only)

RC1: 'Comment on egusphere-2026-1015', Anonymous Referee #1, 06 Apr 2026

The paper provides a clear and useful overview of the moon-earth gravity assist from a trajectory design, navigation and spacecraft operations perspective.
A few suggestions are provided below, mainly to provide a few explanations which will help a broader audience to understand the paper.
- use always the same format for the dates (e.g. 14 April 2023 (line 22)) throughout the paper. Different formats are use: e.g. caption Fig. 2, line 190, …
- Line 15-16 can be removed, they look out of scope in the abstract and the papers are references elsewhere in the article.
- Line 41: explain what cold- and hot-redundancy means
- Line 50 to 55: add references to the listed instruments. Here are a few: JANUS: Palumbo et al 2025, MAJIS: Poulet et al. 2024, RIME: Bruzzone et al. 2015
- Line 66: explain what delta-V is
- Line 104: Ariane 5 ECA: explain acronym (ECA)
- Fig 2: Change 19^th and 20^th August to 19 and 20 August (to be consistent with the other dates throughout the paper)
- Fig 2: S/C vel change into S/C velocity
- Fig 3: the label “outgoing” would be better place on the left-hand side of the plot, close to the line
- Line 127: Different “cases”, scenarios might be better than cases
- Line 129: explain what periselenium is
- Line 155: replace “see next section” with “see Sec. XXX” with ref to the section number.
- Line 176: replace “the previous section” with “Sec. XXX” with ref to the section number.
- Line 180: replace 1/1/2025 with 1 January 2025
- Fig 5: in the caption: remove schedule after TCM. Write “data-cutoff” in a consistent way throughout the paper: either data cutoff or data-cutoff
- Line 190: replace 16/07/2024 with 16 July 2024
- Line 204: replace 31/07 and 06/08/2024 with 31 July and 6 August 2024
- Line 205: correct reference to Fig. 7 (and not Fig. 6)
- Line 211: caption Fig. 7: replace 31/07/2014 with 31 July 2024 and 06 August/2024 with 6 August 2024
- Line 228, 231, 236, 237, 247, 254: correct date format
- Line 247: change Km into km
- Line 263: change closest approach + 5 minutes into CA+5min
- Line 289: replace dry-run with rehearsal (it’s a more broadly understood term)
- Line 291: after “-X Sun pointing throughout the entire phase” add what that means, e.g. though using the high-gain antenna as a sun shield.
- Line 300 and 301: replace figure with Figure
- Line 302 and 306: caption Fig. 9 and Fig. 10: correct date format
- Line 311: use flyby instead of swingby to be consistent throughout the paper.

Citation: https://doi.org/10.5194/egusphere-2026-1015-RC1
RC2:
'Comment on egusphere-2026-1015', Anonymous Referee #2, 10 Apr 2026
Dear authors,
thank you for this nice paper describing the JUICE LEGA very clearly. The figures support the paper very well and help explaining the content of the paper. Overall, I do have some minor comments.
The title and abstract of the paper describe and summarize the paper well and allow for a good overview of the paper.
The text is mostly written in a clear and structured manner and mostly easy to understand. It clearly shows the novelty of the LEGA and the gain for the mission, but it is hard to see where the authors directly contributed to the work.
As a general remark please consider to add a plural “s” to abbreviations when needed.
In detail, please consider the following comments:
Line 9: “… redirecting JUICE toward its…” should be “towards”.

Line 28: I suggest to replace the “-“ between “500-km” and “200-km” with a “ “.

Lines 19-29: Could you add some references to papers describing the JUICE mission (e.g. the special issue of JUICE describing all instruments which is hopefully published this year).

Line 38: You mentioned that the main engine is a bi-propellant engine consider adding for the attitude and orbit control thrusters that these are bi-propellant as well.

Lines 50-58: Consider adding references to instrument descriptions (if not published yet maybe also reference the JUICE special issue which is hopefully published this year).

Lines 59-61: Maybe consider adding RADEM here?

Line 72: B-plane is not introduced here as Body-plane. Maybe introduce it here already and not in line 168-170.

Lines 70-75: The paragraph in general is written a bit unclear and interrupting the flow of the text. Would it be possible to connect it better to the previous paragraph for example by explaining the “patched conic model” in more detail and with some reformulations?

Line 78: “measurements” should be singular here (“measurement).

Lines 86-98: The three paragraphs do not link together very well. Maybe move the paragraph lines 90-93 to another place or reformulate it to get a better connection as you did not write about the launch window so far.

Lines 91-93: Reformulate this sentence and potentially split in two as the term “… close these days…” for example is not clearly stating that you mean: “remove these days from the launch window”.

Line 104: “Ariane 5 ECA” Why do you use this special version? Maybe clearly state that it was used to bring JUICE to orbit and add a reference to it? You could also already introduce it in the paragraph spanning lines 90-93.

Line 110: Consider replacing “specificity” by “characteristic”.

Lines 110-113: This paragraph feels out of place here and disturbs the text flow maybe consider linking it differently to the previous paragraph.

Line 114: This was already mentioned in line 94. Is it needed here?

Lines 126-130: Why does this concern safety? From your first sentence in this paragraph the impression is that you will talk about safety but this is not the case here.

Line 134: “This difference partially compensates the deterministic Delta-V saving.” Does it really compensate for the Delta-V saving and not reduce the Delta-V saving?

Line 135: “The preliminary estimate…” The estimate of the compensation or the estimate of Delta-V? Please clarify.

Line 140: Suggested to remove the part in parenthesis.

Lines 140-141: “… than without it (). This is…” Consider adding a paragraph before “This is…”.

Line 141: “(which is slightly different for both the LEGA and the EGA cases)” you did so far not state that an EGA case was considered at certain parts of the launch windows. This should be added earlier maybe mention this already in the paragraph lines 90-93?

Lines 144-145 (Figure4): if you adjust the x-axis in figure for according to my comment bellow remove the date conversion in parenthesis.

Lines 151-155: You mentioned safety before. Consider merging both paragraphs into one and expand more on the safety criteria. It is not clear why the safety aspect is important and has to be considered with the current formulation.

Line 159: Do you have a reference for the LEGA inception from 2012?

Line 164: Consider removing “by the interested readers”.

Line 166: “… a-priori zero and…” to what is the zero referring to? Please clarify.

Lines 168-170: Could be removed if the B-plane was introduced before.

Lines 170- 171: “The two directions in the plane are defined as T and R whereby the former…” it is not clear if the former refers to the “T” or the plane and the R axis is not defined. Please clarify.

Line 205: “… Figure 6…” This reference should be Figure 7.

Line 241: “Jupiter system” suggest to change to “Jovian system”.

Line 247: “Km” should be “km”.

Line 263: Consider removing “shortly”.

Line 267: “…optical head blindings and straylight happened…” consider adding “detections” (or something suitable) after “straylight”.

Lines 269-270: “…using only internal measurement unit (IMU)” insert “the” → “…using only the internal measurement unit (IMU)”.

Lines 285-286: Consider changing the layout here, as the “-“ sign does not clearly belong to the number following. The same counts for the “+” sign.

Lines 293-298: Layout change from lines 285-286 should be propagated to here as well.

Line 299: “-“ can be mistaken as a minus.

Line 317: remove “team” in “Flight Control team”.

Add section “Author Contributions” before Acknowledgements.

Figures:
Figure 2: The dotted lines which intersect between the lunar orbit and the JUICE trajectory are not mentioned anywhere. Either remove them or explain their meaning by e.g. indicating that each intersection of one of these dotted lines with one of the trajectories corresponds to some specific date. Change either red or green color due to red green blindness.

Figure 3: Consider to add arrows to indicate the direction. Move “Moon Orbit” to the left of dashed line and use the blue color from the dashed line.

Figure 4: Consider to convert the x-axis into a format of either DOJ or actual calendar dates to improve readability. Additionally, consider in the image caption to change “vertical black lines” to “vertical black lines with star marks”. Consider changing red or green color to something else.

Figure 5: Replace Figure so that selection of middle box is removed.

Figure 6: “… overlaid contour plot…” Why is the contour plot so much smaller than the actual figure? Maybe increase the size so in the contour plot it is visible that the estimated impact point and the target are within the contour. Explain legend of the right plot in the image caption more as the dates are not present in Figure 5 and STP# references are not explained at all. Move “Decision to execute TCM-4w!” to bottom left of figure to make it consistent with Figure 7.

Figure 7: The estimated impact point seems to not have moved in the left panel of the figure. Compared to Figure 6 is this correct? If so why is a 10m/s correction maneuver enough?

Figures 2-8: Improve resolution.
Citation: https://doi.org/10.5194/egusphere-2026-1015-RC2

Angela Dietz, Arnaud Boutonnet, and Frank Budnik

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Short summary

ESA's JUICE spacecraft performed the first-ever Lunar–Earth Gravity Assist (LEGA) on 19–20 August 2024, executing a double flyby – first the Moon, then Earth – to gain a significant ΔV boost while conserving propellant. This paper describes this manoeuvre from trajectory design, navigation and spacecraft operations point of view.


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