JANUS: the Scientific Camera Onboard the ESA Juice Mission. Science Planning Activities in Preparation of the Science Phase

The Jupiter Icy Moons Explorer (JUICE) mission is the first Large (L-class) mission selected for the European Space Agency (ESA) Cosmic Vision 2015-2025 program. It is devoted to exploring the Jupiter system and investigating its icy Galilean satellites Europa, Ganymede and Callisto [Grasset et al., (2013)]. JUICE has been succesfully 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 is planned to 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., (2014)]. 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. It has a nominal focal length of 467 mm, an effective entrance pupil diameter of 103.6 mm, a FoV of 1.72°x1.29° and a 2000x1504 pixel CMOS sensor with a pixel dimension of 7 µm. In addition, a filter wheel with 13 filters allows JANUS to obtain 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, lead by ESA and involving all 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 wise timeline. The outcome of the planning exercises is the identification of the available resources (and in particular of pointing, data volume and power) during each scenario and the definition of the best observation approach which ensures the best scientific outcome at instrument and mission level for each scientific target.Here we present the JANUS planning strategy that we have developed for Jupiter’s atmosphere observations in the framework of the Perijove 12 (PJ12) planning exercise. A detailed science planning is pivotal to assess the capabilities of the instrument and estimate the resources required to achieve the JANUS and JUICE science goals, identify the best observation approach which is in line with the available resources. Acknowledgement: authors acknowledge support from National Space Agencies (ASI*, DLR, Spanish Research Ministry and UKSA) in the frame of JANUS-JUICE project.* ASI-INAF agreement n. 2023-6-HH.