Near-perihelion Observations of 3I/ATLAS with the MAJIS Instrument onboard the Juice Spacecraft

We present the results of observations of interstellar object (ISO) 3I/ATLAS close to its perihelion (October 29, 2025) by MAJIS, the VISNIR imaging spectrometer (Poulet et al.,2024) on-board the JUICE ESA mission to Jupiter (Grasset et al., 2013). ISO’s provide a unique opportunity for investigating the properties of planetesimals from distant planetary systems. 3I/ATLAS became exceptionally active close to perihelion, which made possible detailed spectroscopic investigations in the spectral range of MAJIS, 0.5 – 5.63 µm (two channels, VISNIR from 0.5 to 2.35 µm and IR from 2.28 µm to 5.54 µm)

Observing 3I/ATLAS with MAJIS was a challenge as while JUICE was relatively close to its trajectory, the minimum distance during observations was ~ 66 million km, so that the expected MAJIS signal levels were very small (a few e^- to a few 10 e^- for the dust continuum, up to a few 100 e^- for H₂O and CO₂ emissions from the coma). H1RG detectors had been selected by MAJIS so as to provide adequate performances for weak signals of interest in the Jupiter system, in particular those for rings and exospheres. All observations of 3I/ATLAS were performed using a dedicated “low signal” readout mode with an integration time of 2 s and on-board co-adding of 4 acquisitions. Each observation generated image cubes with 400 x 23 spatial samples and 508 spectral samples for each channel. Given the time available, two such observations were performed for the first 3 slots, eight observations for the last slot (Table 1).

Table 1: 3I/ATLAS MAJIS observations; R_h: heliocentric distance of 3I/ATLAS; D: distance from Juice to 3I/ATLAS; sampling: projected MAJIS IFOV (0.15 mrad) at the comet distance D (spatial sampling of the coma)  

As shown by Fig. 1, H₂O and CO₂ molecular emissions were unambiguously detected during the first slot (2025/11/02). The dispersion for MAJIS data elements away from the comet is ~ 10 e^- in both spectral ranges, in line with the noise model, so that the SNR is in the range of 15 to 20 for the brighest spectral samples, making it possible to determine with high accuracy the column content of H₂O and CO₂.

Figure 1: H₂O (left) and CO₂ (right) molecular emissions for bright IFOV’s in the coma (2025/11/02 slot). The red profiles show the observed spectra. The black profiles are H₂O (CO₂) synthetic spectra modeled with the MAJIS spectral sampling. The blue stars are signal levels for 8 frames away from the comet. Gaps corrrespond to unoperable spectels (~ 10% for the IR channel with a 2 s integration time due to a high dark current).

 

The SNR decreases from one slot to the next due to the increasing distances of the comet to the sun and to the JUICE S/C (see Table 1). As shown by Fig. 2,  MAJIS could still obtain information on the spatial distribution of H₂O and CO₂ emissions, which made it possible to determine H₂O and CO₂ production rates for all 4 observation slots. Emissions were also observed in the C-H spectral range (~ 3.4 µm).

Figure 2: spatial distribution of the H₂O emission for the 2025/11/02 slot (left), the CO₂ emission for the 2025/11/12 slot (center) and the the CO₂ emission for the 2025/11/19 slot (right)

The dust scattering continuum of 3I/ATLAS was observed during the first observation slot (2025/11/02) mainly in the VISNIR range (0.5 – 2.35 µm). As shown in Fig. 3, the signal for the brightest IFOV’s peaks at ~ 25 e^- to be compared with a noise level of 4 e^-, smaller than that in the IR as the dark current is very small for the VISNIR H1RG detector. The dust continuum is expected to be spectrally smooth. An increase of the reflectance with wavelength at a steeper slope from 0.5 to 1 µm could be identified by co-adding the 4 brightest IFOV’s and stacking in the spectral direction.

Fig. 3: Signal  from dust scattering in the coma;  dashed red lines: ± 1 sigma (4 e^-)

The MAJIS results on the H₂O and CO₂ production rates and rotational temperatures, and on the dust continuum will be presented in a forthcoming article and at the conference. The comparison of these results with measurements by other instruments of the H₂O and CO₂ production rates farther from the Sun revealed strong variations of the CO₂ / H₂O ratio with heliocentric distance. The radial distributions of H₂O and CO₂ do not reveal significant extended production of these species from icy grains near perihelion.

The primary objective of the 3I/ATLAS observations by MAJIS was the science return from an interstellar object. They were also of particular interest to the MAJIS team as a very successful litmus test of the MAJIS readout mode dedicated to weak signals in the Jupiter system (rings and exospheres).

Acknowledgements: JUICE is a mission under ESA leadership with contributions from its Member States, NASA, JAXA and the Israel Space Agency. It is the first Large-class mission in ESA’s Cosmic Vision programme. This work was  supported by CNES, focused on MAJIS. This work has  been developed under the ASI-INAF agreement no. 2023-6-HH.0. 

References 

• Poulet F., Piccioni G., Langevin Y., Dumesnil C. et al., Space Sci. Rev., 2024, 220, 27
• Grasset O., Dougherty M.K., Coustenis A. et al., Planet. Space Sci., 2013, 78, 1