the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 May 2026
Geometric in-flight calibration of MAJIS (JUICE) during early cruise phase and LEGA flyby
Abstract. In August 2024, MAJIS, the visible and infrared mapping spectrometer onboard the ESA-Juice spacecraft, collected its first observations of resolved targets during Lunar and Earth Gravity Assist maneuver (LEGA). One month before, during payload checkout 2 (PC2), MAJIS performed two starfield observations to evaluate its post-launch geometric performances that can be compared with the LEGA data. This work presents an overview of the spatial distribution of the MAJIS signal collected at different scan angles and their position in MAJIS field of view. In PC2 starfields, 14 stars were identified, allowing us to derive a new in-flight alignment with an accuracy of 0.7 ± 0.4 MAJIS instantaneous field of view (IFOV). The offset between the nominal boresight of Juice remote sensing platform (+Z) and MAJIS boresight was evaluated to be 27 samples (along-slit) and 31 lines (cross-slit). With this new alignment, MAJIS observations on the Moon are co-aligned with JANUS camera with an accuracy of 2.2 ± 0.9 IFOV which could be improved to 1.1 ± 0.6 IFOV with bundle adjustment. A comparison of MAJIS Earth observations with simultaneous weather satellites observations confirmed that the MAJIS alignment is accurate for the full close Earth flyby. Residual misalignments with JANUS and NavCam remain visible in the Earth dataset but they can be easily corrected using small timing and scan angle adjustments (<10 IFOV). The instrument and frame kernels for MAJIS will be updated on the basis of these results. The improved co-alignment between MAJIS, JANUS and NavCam ensures strong synergy between the remote sensing instruments during the nominal scientific phase when they operate at the same time.
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Status: open (until 26 Oct 2026)
- RC1: 'Comment on egusphere-2026-2655', Anonymous Referee #1, 15 Jun 2026 reply
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RC2: 'Comment on egusphere-2026-2655', Anonymous Referee #2, 29 Sep 2026
reply
This paper highlights the importance of in-flight calibration and its impact of the science outcome, as well as the use of gravity assists as unique opportunities to verify the instrument performances with resolved targets.
Comparisons between LEGA data acquired by MAJIS, JANUS and NavCam demonstrate what could be performed in the science phase using synergistic approaches.
The paper is well written and clear. I have a number of minor comments and suggestions that the authors may choose to implement (I give them by line number).
A significant improvement might come from adding some discussion about why the residual misalignments of the Earth dataset are significantly larger wrt the ones of the Moon datasets (instead of simply stating that the issue is still under investigation). The S/C attitude was the same during both flybys and the (most likely incorrect) expectation was that both datasets behave in similar ways.
Abstract: specify that during LEGA MAJIS obtain Moon and Earth data.
Line 20 and 22: remove the (Before/After launch)
Line 33: explain better what you mean with “planning development”
Line 37: in section 4, the observations (instead of these observations)
Line 41: remove flyby.
Throughout the paper: be consistent with figure / Figure, table / Table and dates (use always the same format)
Line 58: specify/list which are the SPICE reference frames because it is not obvious for a non-experienced reader.
Line 71: what does it mean “the length of the slit was measured at 14.4 mm”? is 14.4 mm the length of the slit? Please clarify the sentence
Line 71: i.e. instead of ie.
Page 5, note: remove comma after term.
Line 100: mention that the acquisition of a subpart of the slit is referred to as windowing. This is then used in the caption of Fig. 5, 6, 7 and it would help the reader to have it introduced before.
Fig. 5, 6, 7: the labels, which are very useful are difficult to read.
Line 118 and 126: it is not obvious how those equations are derived, some more explanation could help the reader.
Line 148: correct +/- 4 deg (the unit is currently before the number)
Line 167: I suggest to add (PC2) after Payload Checkout 2.
Line 170: you could add the ref to Sarri et al 2026 (SSR), which explains why the -X axis shall be kept aligned with the Sun direction.
Fig. 9: the green circles are very poorly visible inside the red ones. For sake or clarity, maybe you could replace the green circle with a green x?
Line 173: two inertial stares of (instead of during) 42 minutes each …
Are those 42 minutes after the S/C reached stability? How sensitive is MAJIS to pointing stability? A short mention to this could be added.
Line 177: we first queried the Gaia DR3 catalogue () FOR all the expected… (for is missing)
Table 2: add in the caption the details about RA, Dec which are written in lines 178-179
Line 196: you state “Then we relaxed the constraint on the fixed rotation matrix between the SPACECRAFT and MAJIS BASE frames”, however it was never explicitly stated before about the fixed rotation matrix. This information should be added (explicitly, for non-experienced readers) in the relevant section.
Line 204: are there additional calibration observations foreseen to improve the determination of this parameter?
Line 208: Figure 12 represents the position of the … (I would add position for clarity)
Line 209: Please add the origin of the boresight position and instrument FOVs. I expect to be the instrument IKs, but it should be added explicitly for completeness.
Line 215: please clarify the sentence “Even though the spacecraft was not allowed to rotate, …”. There is a good description of the S/C attitude during LEGA in Dietz et al 2026, which could be referenced.
Line 222: … we planned: (replace , with :)
Line 223: remove “then”
Fig 14: top panel: the label of the average offset is not readable, it would be better to place it on the plot (in both panels)
Line 245: what did you use as acquisition time for the JANUS image? An important aspect, which might cause a time discrepancy, is that the S/C was not tracking the target, but using a quasi-inertial pointing.
Line 254: remove “it” from “This new TKFRAME value, it will…”
Line 261: replace “,” with “:” after surface
Line 261: remove And from “And only …”
Line 264: when the scan angle is larger (is missing in the original sentence)
Line 266: remove “been” at the end of the line
Line 270: replace “,” with “:” after planned
Fig 16: it would have to have the EGA1-16 added to each MAJIS observation
Line 291, 296: uniformity of date format would be good throughout the paper.
Figure 17: EGA5 should not be in brackets, which (right) is missing after EGA5
Line 329: remove “the” in of the three of…
Line 369: remove are at the end of the line
Line 375: remove the at the end of the line
Line 380: I would replace “was just collected” with which occurred
Line 383: I would rephrase as: they will be performed only a limited number of times during the science phase.
Line 389: synergistic rather than synergetic observations
Citation: https://doi.org/10.5194/egusphere-2026-2655-RC2
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Cited
3 citations as recorded by crossref.
- The JUICE 2024 close flyby of the Moon: thermal assessment from MAJIS F. Tosi et al. https://doi.org/10.5194/angeo-44-547-2026
- Post launch spectral and radiometric performances of MAJIS, the VIS-NIR imaging spectrometer of JUICE Y. Langevin et al. https://doi.org/10.5194/angeo-44-825-2026
- JUICE-MAJIS Earth observations during the 2024 gravity assist: first analysis and comparison with PRISMA data F. Oliva et al. https://doi.org/10.5194/angeo-44-511-2026
Thank for letting me reviews this work, it is very good and represents an important contribution to the community. The presentation is clear, the language is precise, and the conclusions regarding the updated instrument boresight and SPICE reference frames are well supported. This work is essential to ensure the co-alignment and synergy between MAJIS, JANUS, and NavCam during the nominal mission at Jupiter.
I would like to see this published, after addressing some minor comments I have.
Comments :
Suggestion: The authors state this is "still under investigation", I would suggest they add a brief discussion or hypothesis regarding the physical cause. Is it a result of the high-speed slew motion during the Earth flyby compared to the inertial pointing of starfields? Providing even a preliminary error budget for the -1.1s temporal shift identified in the Earth-JANUS comparison would be highly valuable.
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Suggestion: The authors should clarify if they have investigated the source of this potential sub-second latency. Is this a known internal clock drift within the MAJIS instrument, or a trigger delay in the scan mirror mechanism?.
Quantifying this would help other JUICE instrument teams (like JANUS or NavCam) who are also performing cross-calibrations.
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The authors admit that the dominant source of uncertainty in the new TKFRAME alignment is the rotation around the spacecraft +Z axis, which might reflect a misalignment between the mirror rotation axis and the slit.
Suggestion: Clarify if the author plan to use the more recent PC4 data (February 2026) which to specifically refine this mirror axis parameter before the final publication or in future works. Additionally, there is a mention that the linearity between mirror steps and viewing scan angles still needs further in-flight validation. A brief comment on whether ground-calibration linearity holds true in-flight would strengthen the "Conclusions and perspectives" section.
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A major lesson learned in the paper is the need for synchronized observations between MAJIS, JANUS, and NavCam,.
Suggestion: The authors could be more specific in their recommendation to the JUICE SOC (Science Operations Center). For example, they could suggest a maximum allowable time-offset between instrument triggers to ensure the common coverage needed for geometric context during windowing modes.
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While the raw data is under embargo until 2029, the manuscript mentions that the source code to reproduce these results will be available on the Nantes University Gitlab.
Suggestion (probably not needed, authors are aware of this) : I strongly suggest the authors provide a persistent DOI for this code repository upon publication. This ensures that the geometric reconstruction methods , such as the sub-pixel centroiding described in Section 3.2, are transparent and usable by the wider community.
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