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Comets hold the key to the understanding of our solar system, its formationand its evolution, and to the fundamental plasma processes at work both in itand beyond it. A comet nucleus emits gas as it is heated by the sunlight. Thegas forms the coma, where it is ionised, becomes a plasma and eventuallyinteracts with the solar wind. Besides these neutral and ionised gases, thecoma also contains dust grains, released from the comet nucleus. As a cometaryatmosphere develops when the comet travels through the solar system,large-scale structures, such as the plasma boundaries, develop and disappear,while at planets such large-scale structures are only accessible in their fullygrown, quasi-steady state. In situ measurements at comets enable us to learnboth how such large-scale structures are formed or reformed and how small-scaleprocesses in the plasma affect the formation and properties of these largescale structures. Furthermore, a comet goes through a wide range of parameterregimes during its life cycle, where either collisional processes, involvingneutrals and charged particles, or collisionless processes are at play, andmight even compete in complicated transitional regimes. Thus a comet presents aunique opportunity to study this parameter space, from an asteroid-like to aMars- and Venus-like interaction. Fast flybys of comets have made many newdiscoveries, setting the stage for a multi-spacecraft mission to accompany acomet on its journey through the solar system. This white paper reviews thepresent-day knowledge of cometary plasmas, discusses the many questions thatremain unanswered, and outlines a multi-spacecraft ESA mission to accompany acomet that will answer these questions by combining both multi-spacecraftobservations and a rendezvous mission, and at the same time advance ourunderstanding of fundamental plasma physics and its role in planetary systems.

Cometary Plasma Science -- A White Paper in response to the Voyage 2050 Call by the European Space Agency