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. The Rosetta mission and previous fast flybys of comets have together made many new discoveries, but the most important breakthroughs in the understanding of cometary plasmas are yet to come. The Comet Interceptor mission will provide a sample of multi-point measurements at a comet, setting the stage for a multi-spacecraft mission to accompany a comet on its journey through the Solar System. This White Paper, submitted in response to the European Space Agency’s Voyage 2050 call, reviews the present-day knowledge of cometary plasmas, discusses the many questions that remain unanswered, and outlines a multi-spacecraft European Space Agency 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.

彗星是了解我们太阳系、它的形成和演化，以及在它内部和外部工作的基本等离子体过程的关键。彗核在被阳光加热时会释放出气体。气体形成彗发，在那里被电离，变成等离子体，并最终与太阳风相互作用。除了这些中性和电离气体外，彗发还含有彗核释放的尘埃颗粒。当彗星穿过太阳系时，随着彗星大气层的形成，等离子体边界等大型结构会发展和消失，而在行星上，此类大型结构只能在完全成长的准稳态状态下才能进入。彗星的原位测量使我们能够了解这种大尺度结构是如何形成或改造的，以及等离子体中的小尺度过程如何影响这些大尺度结构的形成和性质。此外，彗星在其生命周期中经历了广泛的参数机制，其中涉及中性粒子和带电粒子的碰撞过程或无碰撞过程都在起作用，甚至可能在复杂的过渡机制中竞争。因此，彗星提供了一个独特的机会来研究这个参数空间，从类小行星到类火星和金星的相互作用。罗塞塔任务和之前的彗星快速飞越共同取得了许多新发现，但在理解彗星等离子体方面最重要的突破尚未到来。彗星拦截器任务将提供彗星多点测量的样本，为陪伴彗星穿越太阳系的多航天器任务奠定基础。这份白皮书是为响应欧洲航天局的 2050 年航程号召而提交的，它回顾了目前对彗星等离子体的了解，讨论了许多仍未得到解答的问题，并概述了欧洲航天局的一项多航天器任务，以陪伴一颗即将到来的彗星。通过结合多航天器观测和交会任务来回答这些问题，同时增进我们对基本等离子体物理学及其在行星系统中的作用的理解。为陪伴彗星穿越太阳系的多航天器任务奠定基础。这份白皮书是为响应欧洲航天局的 2050 年航程号召而提交的，它回顾了目前对彗星等离子体的了解，讨论了许多仍未得到解答的问题，并概述了欧洲航天局的一项多航天器任务，以陪伴一颗即将到来的彗星。通过结合多航天器观测和交会任务来回答这些问题，同时增进我们对基本等离子体物理学及其在行星系统中的作用的理解。为陪伴彗星穿越太阳系的多航天器任务奠定基础。这份白皮书是为响应欧洲航天局的 2050 年航程号召而提交的，它回顾了目前对彗星等离子体的了解，讨论了许多仍未得到解答的问题，并概述了欧洲航天局的一项多航天器任务，以陪伴一颗即将到来的彗星。通过结合多航天器观测和交会任务来回答这些问题，同时增进我们对基本等离子体物理学及其在行星系统中的作用的理解。