Scientific objectives of JANUS Instrument onboard JUICE mission and key technical solutions for its Optical Head

JUICE (JUpiter ICy moons Explorer) is the first Large Mission in the frame of ESA Cosmic Vision 2015-2025 program. JANUS (Jovis, Amorum ac Natorum Undique Scrutator) is the camera system selected to fulfill the mission optical imaging scientific requirements. It is being developed by a team involving Institutes and Industries in Italy, Germany, Spain and UK, supported by respective Space Agencies. The science team includes Co-Investigators also from USA, France, Japan and Israel.Resources limitations, S/C characteristics, mission design, environment and the great variability of observing conditions for several objects put stringent constraints on optical head characteristics. High radiation environment in terms of irradiation dose and flux in specific mission phases has to be considered.The wide range of targets, that includes Jupiter atmosphere, to solid satellite surfaces, exosphere, rings, and lightning, all to be observed in several color and narrow-band filters, have all to be considered in instrument development.The many science objectives of JANUS are described together with the technical solution adopted for the optical head.