MAJIS observation strategies during the JUICE science nominal phase: coordinated perijoves and moon flyby

The Jupiter Icy Moons Explorer (JUICE) mission will perform an extensive exploration of the Jovian system, combining regular investigations of Jupiter’s atmosphere with multiple flybys of the Galilean moons during the tour phase (2031-2034) and a Ganymede orbital phase until end of 2035 [1]. Among the JUICE payload, Moons and Jupiter Imaging Spectrometer (MAJIS) is an imaging spectrometer designed to characterize the composition, cloud structure, aerosols, and auroral emissions of Jupiter, while also investigating the surfaces and exospheres of the icy moons through visible and infrared hyperspectral imaging observations [2].

To prepare the nominal science phase of the mission, JUICE planning exercises are regularly conducted in coordination with the ESA Science Operations Centre. These exercises aim to consolidate science observations, test operational procedures and planning tools, and evaluate coordinated multi-instrument observation strategies. The latest exercise, performed in 2026, simulated a representative segment of the nominal tour including two perijoves and one Ganymede flyby, providing a framework to test coordinated Jupiter and Galilean moon observation strategies under realistic operational constraints. 

Geometry of the perijoves and Ganymede flyby:

The latest planning exercise "TR03" focuses on a representative segment of the JUICE nominal tour extending from 23 November to 20 December 2033, encompassing the ORB37 and ORB38 perijoves together with the Ganymede flyby 27G6 on November 27th. The selected configuration provides a favorable context to test coordinated observation strategies because it combines two consecutive perijove passages with a low-altitude (900 km) flyby of Ganymede on the trailing hemisphere, although under night-side inbound conditions. The geometry also includes Jupiter eclipse conditions offering opportunities for high-phase observations of Jupiter’s rings, as well as monitoring of Io, the Io plasma torus, and volcanic hot spots. The succession of two closely spaced perijoves enables the exercise to compare different observation strategies or to evaluate the implementation of a more generic and reusable perijove observation template within a multi-instrument framework.

MAJIS Jupiter planning and investigations:

The MAJIS Jupiter observation strategy aims to address several key Level-1 science objectives through the combination of high-resolution perijove observations and pre-/post-perijove monitoring segments. Perijove measurements provide access to cloud microphysics, atmospheric chemistry, and auroral emissions at high spatial resolution, while longer-term monitoring observations provide the temporal and longitudinal context required to investigate atmospheric variability and large-scale dynamics. The exercise started with the implementation of 3 coordinated Jupiter campaigns: a distant monitoring campaign, an auroral monitoring campaign, and a zonal wind campaign. For MAJIS, the exercise included the implementation of several observation types at Jupiter, such as global monitoring sequences, auroral observations, limb scans, and high-resolution perijove measurements, while simultaneously accommodating observations related to Ganymede and other satellite targets. 

Auroral observations included nightside and dayside measurements of the northern and southern auroral ovals at spatial resolutions of 150–250 km/pixel, together with a long-distance auroral monitoring campaign at intermediate phase angle (130°) at 300–350 km/pixel. The monitoring strategy additionally included 13 disk scan observations (250–350 km/pixel), high-resolution perijove mosaics, half-disk observations, and a rapid-revisit sequence consisting of two disk scans separated by 40 minutes (150 km/pixel). Limb observations included north and south polar scans at 160–180 km/pixel together with a global limb campaign composed of 12 acquisitions distributed around Jupiter’s disk at phase angles close to 90°, sampling both dayside and nightside conditions. A few stellar occultations were also implemented. 

MAJIS satellite planning and investigations:

The operational strategy and science return of Galilean moons flybys depends primarily on the geometry of the flyby: 1) if it is a high vs low altitude flyby (altitudes ranging 200-1000km vs >1000 km), 2) how much of the closest approach is during day vs night side, 3) the S/C velocity to a lesser extent. The latest training exercise featured a low velocity, medium altitude (900 km) and mostly nightside trajectory at low altitude. As a result, the main science focus of MAJIS observations was the exosphere of Ganymede. This is achieved by observing the disk and limb with both forward and back scattering geometries (low and high solar phase angles). A typical exospheric observation scans the first 100 to 200 km vertical profile of the exosphere with spatial resolution ranging 1 - 15 km/pixel. Whenever possible, the MAJIS slit is placed tangent to the limb and the instrument scans outwards. Several latitudinal positions are investigated both on the day and night sides. Other pointings have been designed including using a S/C slew motion or scanning in the north-south direction, in an effort to harmonize pointing requirements with other instruments. Partial coverage of the sunlit surface is achieved on the dayside for spatial resolutions ranging 1-15 km/pix, with limited coverage at high resolution given the geometry of the flyby. The search for Ganymede’s mid-latitude auroral emission is achieved by scanning the surface of the moon on its nightside. The final plan for Ganymede has MAJIS observing Ganymede over 20 observing slots: 6 dedicated to dayside surface mapping, 12 of the exosphere (full range of latitudes and solar phase angles) and 2 nightside surface observations dedicated to the search for aurora. 

Substantial effort was dedicated to finding commonalities in the pointing design between MAJIS and the other remote sensing instruments SWI, UVS and JANUS. The plan shows that each MAJIS observing block of Ganymede also has between one and all of these other instruments observing at the same time, demonstrating the strong pointing and science synergy that is possible with these instruments, despite very different pointing requirements and operational modes. 

In addition to Ganymede observations, long distance observations of Io and Europa are planned at low resolution (150-250 km/pix) on their day/night sides, and high solar phase angle observations of the Jupiter rings are implemented in coordination with JANUS. 

This contribution summarizes the main lessons learned from the MAJIS participation, including both scientific and operational aspects of coordinated observation planning within the JUICE mission.

References: [1] Boutonnet et al., 10.1007/s11214-024-01093-y (2024), [2] Poulet et al., 10.1007/s11214-024-01057-2 (2024)

Acknowledgements: JUICE is a mission under ESA leadership with contributions from its Member States, NASA, JAXA and the Israel Space Agency. This work was supported by CNES. This work has been developed under the ASI-INAF agreement no. 2023-6-HH.0.