  Comets hold the key to the understanding of our solar system, its formation
and its evolution, and to the fundamental plasma processes at work both in it
and beyond it. A comet nucleus emits gas as it is heated by the sunlight. The
gas forms the coma, where it is ionised, becomes a plasma and eventually
interacts with the solar wind. Besides these neutral and ionised gases, the
coma also contains dust grains, released from the comet nucleus. As a cometary
atmosphere 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 fully
grown, quasi-steady state. In situ measurements at comets enable us to learn
both how such large-scale structures are formed or reformed and how small-scale
processes in the plasma affect the formation and properties of these large
scale structures. Furthermore, a comet goes through a wide range of parameter
regimes during its life cycle, where either collisional processes, involving
neutrals and charged particles, or collisionless processes are at play, and
might even compete in complicated transitional regimes. Thus a comet presents a
unique opportunity to study this parameter space, from an asteroid-like to a
Mars- and Venus-like interaction. Fast flybys of comets have made many new
discoveries, setting the stage for a multi-spacecraft mission to accompany a
comet on its journey through the solar system. This white paper reviews the
present-day knowledge of cometary plasmas, discusses the many questions that
remain unanswered, and outlines a multi-spacecraft ESA mission to accompany a
comet that will answer these questions by combining both multi-spacecraft
observations and a rendezvous mission, and at the same time advance our
understanding of fundamental plasma physics and its role in planetary systems.
