Science planning activities in preparation of JUICE's science phase. The JANUS perspective

The JUICE (Jupiter Icy Moons Explorer) mission is the first Large (L-class) mission selected for the European Space Agency (ESA) Cosmic Vision 2015-2025 program. Its main goal is the exploration of the Jupiter system and the investigation of its icy Galilean satellites Europa, Ganymede and Callisto [Grasset et al. (2013)]. JUICE has been successfully launched on 14 April 2023 from Europe’s Spaceport in Kourou, French Guiana, on an Ariane 5 launcher and, after its 8 years journey throughout the inner Solar System, it will reach the Jupiter system in July 2031. During its nominal science phase, JUICE will spend many months orbiting around Jupiter, performing fly-bys of Europa, Ganymede and Callisto, and finally conducting an orbital tour of Ganymede. JUICE carries 10 state-of-the-art instruments, comprising the most powerful remote sensing, geophysical and in situ payload suite ever flown to the outer Solar System. Among those, JANUS (Jovis Amorum ac Natorum Undique Scrutator) is the scientific optical camera system [Palumbo et al. (2025)]. Its design has been optimised, according to JANUS’ scientific requirements, for observations of a wide range of targets, from Jupiter’s atmosphere, to solid satellite surfaces and their exospheres, rings, and transient phenomena like lightning.

JANUS is a modified Ritchey-Chrétien telescope with a nominal focal length of 467 mm, an effective entrance pupil diameter of 103.6 mm, a FoV of 1.72° x 1.29° and a 2000x1504 pixel CMOS sensor with a pixel dimension of 7 µm. In addition, a filter wheel with 13 filters allows JANUS to acquire multi-spectral images in the 340-1080 nm wavelength range. This camera provides images of the targets with a scale of 7.5 m/pixel at a distance of 500 km. Such characteristics will allow to observe the surfaces of the icy satellites with a spatial resolution ranging from 400 m to 3 m for Europa, Ganymede and Callisto. In addition, Jupiter and other targets, e.g.  Io, small moons and rings, will be observed with a resolution from few km to tens of km.

The achievement of mission and instrument science goals during the science phase is strictly related to the resources available to each instrument. A series of science planning exercises, led by ESA and involving all JUICE instruments, are taking place during the cruise phase. Starting from individual instrument timelines, sets of observations that are fulfilling specific scientific objectives, under ESA's coordination, a harmonization process integrates proposed observations from various payloads into a unified mission timeline.

 ORB17 was the chosen mission segment for the fourth planning exercise and a very challenging one: it is a multi-target scenario over a Medium-Term-Planning (MTP) time scale. MTP time scales of several weeks will be typical during the JUICE science phase at Jupiter, and in that sense this exercise was a huge step forward in developing and testing realistic planning sequences, compared to the more focused planning intervals of past exercises. ORB17 is a highly representative orbit from the 3^rd Phase of the JUICE mission, during which the inclination of the spacecraft orbit gradually increases to >30° with the help of multiple Callisto flybys. Phase-3 orbits thus offer repeated opportunities to observe the polar regions of the planet during perijoves, offer a top-down perspective on the rings and the Europa and Io torus, and allow for repeated close observations of Callisto. Specifically, ORB17 planning covered a three-week period (2032-12-18 to 2033-01-08), and included the Callisto flyby 16C9 at a low altitude on the leading hemisphere of the moon and a perijove at high south latitudes.

 ORB17 planning was successful: Callisto’s surface at the flyby and Jupiter’s atmosphere during perijove could get sufficient observation time. Even more important was that the observation time was accumulated while also finding common or shared pointing strategies with several instruments during perijove and during most of the Callisto flyby period (JANUS, MAJIS, SWI, UVS, PEP-Lo, PEP-Hi, RIME). Common strategies and resolutions will likely propagate as planning templates in follow up exercises, maximizing the efficiency of science planning for JANUS and JUICE overall. An effort will have to be put on optimizing conflicting periods, especially during closest approach of moon flybys, which present most of the challenges for incorporating all instrument pointing requirements and resource allocation.

Here we present the JANUS perspective of the ORB17 planning, with a view to the actions that shall be taken in the future to ensure that all science goals will be reached in the science phase.

 

Acknowledgements: JANUS has been funded by the respective Space Agencies: ASI (lead funding agency), DLR, Spanish Research Ministry and the UK Space Agency. Main hardware-provider Companies and Institutes are Leonardo SpA (Prime Industry), DLR-Berlin, CSIC-IAA and Sener. PI and Italian team members acknowledge ASI support in the frame of ASI-INAF agreement n. 2023-6-HH.0.

 

References: Grasset, O. et al. JUpiter ICy moons Explorer (JUICE): An ESA mission to orbit Ganymede and to characterise the Jupiter system. Planetary and Space Science, 78, 1-21 (2013). Palumbo, P. et al., The JANUS (Jovis Amorum ac Natorum Undique Scrutator) VISNIR multi-band imager for the JUICE mission. Space Science Reviews, 221, 32 (2025).