the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Apr 2026
Planning of the JUICE/JANUS camera observations during the first ever Lunar-Earth Gravity Assist
Abstract. The Jupiter Icy Moons Explorer (JUICE) spacecraft, launched in April 2023, will reach the Jovian system in July 2031 and will conduct an extensive science campaign of the Jupiter system, with a strong focus on Jupiter itself and of its icy Galilean satellites. During its cruise phase the spacecraft performed a double gravity assist maneuver (Moon–Earth) in August 2024, followed by a "farewell" observational sequence on 9 September 2024. These events constitute the Lunar Earth Gravity assist (LEGA) campaign. JANUS, the high resolution optical camera of JUICE, used this opportunity to perform an imaging campaign under realistic illumination, viewing geometry and thermal conditions that closely mimic the forthcoming Jupiter operations. JANUS acquired a total of 461 images in full frame, lossless mode, covering the complete set of 13 spectral filters (340–1080 nm). This paper summarizes the end‑to‑end workflow from the definition of scientific and engineering constraints, through the design and ground‑segment validation of the observation sequences, to the acquisition, processing and preliminary analysis of the LEGA data. The campaign demonstrated that a rapid thermal‑pre‑warm strategy using the spacecraft survival heaters can bring the JANUS optics to the required −20 °C thermal stability significantly limiting the power draw but preserving image quality. Exposure times were limited by a 1/4‑pixel smearing criterion; nevertheless, exposure times ranging from 0.22 ms to 163 ms yielded adequate signal‑to‑noise ratios across all filters, and full 13‑filter colour sequences were successfully obtained at high ground‑track speeds. A series of images acquired with increasing compression ratios showed no perceptible degradation, establishing a realistic data‑volume budget for the science phase. Simultaneous measurements by MAJIS, NavCam and the external Earth‑observation satellites enabled a three‑way cross‑calibration that will improve the absolute radiometric accuracy of JANUS. The LEGA experience shows that extending payload operations during cruise‑phase flybys dramatically enhances calibration quality, cross‑instrument synergy and scientific return. These lessons will directly inform the planning of the remaining JUICE Earth flyby windows (September 2026 and January 2029) and provide a best‑practice template for the Jupiter phase of the mission.
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Status: open (until 26 May 2026)
- RC1: 'Comment on egusphere-2026-2008', Anonymous Referee #1, 04 May 2026 reply
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This paper presents a useful description of how JANUS observations were planned for the unique double gravity assist flyby JUICE performed in 2024. Such details are often lost in papers focussed on the results of observations, and it is good to see the effort that goes into making the observations carefully described. The paper is well written and clear, and I have only minor comments and suggestions that the authors may choose to implement. I give them by line number:
L35: the CA distances are apparently from the surface, not the body centre, but it would be good to state this for clarity.
L50/51 (and elsewhere): various papers are referenced without a year (Agostini et al., Lucchetti et al.) while Hueso et al. (2026) is cited properly. I understand that those without a year are others in this issue - perhaps cite them as such in the text? Or at least remember to update these references in the final version when all these papers are available (perhaps more a comment for the editors than the authors).
L72: It is stated that thermal stability is reached 12 hours after the instrument is switched on, but the operations windows in the flybys are shorter than this. Later it is explained that this was solved using survival heaters, but it appears contradictory here - maybe point to the later section already so the reader is not initially confused?
L165-170: I realise that the authors say that the results of the compression test are preliminary, but I wonder if there is any way that they can show how well photometric accuracy is preserved? I agree that there is no visible degradation of the image quality with compression, but are pixel-to-pixel brightness variations affected? Perhaps showing some image histograms for the lower panels of fig 4 would be a way to do this? Alternatively, if a more detailed analysis is planned, perhaps the authors can simply cite the (in prep?) paper that will look at it.
L236: For the farewell image the instrument was brought to operating temperature 'normally' rather than using the survival heaters. Was there any difference in performance (other than this taking more power)? Some statement on this would help reinforce the conclusion that the survival heaters can be used in future to save power.
Fig 7: Could the MAJIS footprints also be shown in this figure?
Fig 8: Please list which filters were used to make this (very nice!) colour composite.
L287: I would go further in the final conclusion - these observations show that instrument operations are useful in general during spacecraft gravity assists, and making the effort to do them should be considered for all missions, not just JUICE.