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Here we describe the novel, multi-point Comet Interceptor mission. It is dedicated to theexploration of a little-processed long-period comet, possibly entering the inner Solar System for the first time, or to encounter an interstellar object originating at another star. Theobjectives of the mission are to address the following questions: What are the surface composition, shape, morphology, and structure of the target object? What is the composition ofthe gas and dust in the coma, its connection to the nucleus, and the nature of its interactionwith the solar wind? The mission was proposed to the European Space Agency in 2018, andformally adopted by the agency in June 2022, for launch in 2029 together with the Arielmission. Comet Interceptor will take advantage of the opportunity presented by ESA’s FClass call for fast, flexible, low-cost missions to which it was proposed. The call requireda launch to a halo orbit around the Sun-Earth L2 point. The mission can take advantage ofthis placement to wait for the discovery of a suitable comet reachable with its minimum ΔV capability of 600 ms⁻¹. Comet Interceptor will be unique in encountering and studying, at anominal closest approach distance of 1000 km, a comet that represents a near-pristine sample of material from the formation of the Solar System. It will also add a capability that noprevious cometary mission has had, which is to deploy two sub-probes – B1, provided by theJapanese space agency, JAXA, and B2 – that will follow different trajectories through thecoma. While the main probe passes at a nominal 1000 km distance, probes B1 and B2 willfollow different chords through the coma at distances of 850 km and 400 km, respectively.The result will be unique, simultaneous, spatially resolved information of the 3-dimensionalproperties of the target comet and its interaction with the space environment. We presentthe mission’s science background leading to these objectives, as well as an overview of thescientific instruments, mission design, and schedule.