（Ⅲ）m=0类二次系统的极限环问题，（Ⅰ）

彗星的空间探测是当代前沿课题．本文首先概述彗星的主要研究结果．彗星的本体是不大的‘脏雪球’彗核；随着接近太阳，其表面冰升华并带出尘埃而形成彗发，发生复杂的物理-化学过程；太阳辐射压和太阳风推斥彗发的尘埃和离子而形成彗尾；彗星尘散失在轨道附近而成为流星群．虽然地面观测到彗发和彗尾的丰富资料，但对彗核却了解甚少，需要发射飞船进行空间探测；接着，从探测彗核的真实性质、采集彗星样品分析成分、探索彗星活动和机制、彗星的有机物与生命起源、彗星-地球关系、太阳系起源和演化线索几方面阐述彗星的空间探测的科学意义；然后，综述彗星空间探测的一些计划进展情况，总结彗星空间探测的重要成果；最后，讨论未来的彗星空间探测的科学目标，包括高轨卫星采集彗星尘、搜寻近地小彗星、搜寻未知彗星、发射更优秀飞船探测不同彗星．

On the Scientific Objective of Space Exploration to Comets

This paper reviews the history of space exploration to comets and summarizes the important achievements from these missions.The missions to comets have gone through three phases: flyby,impact and sample collection.These activities have greatly enhanced our understanding of the solar system formation and evolution history.Here we illustrate the scientific perspectives of the deep space exploration to comets.The nature of cometary nuclei is poorly understood.What are the nuclei made of and what physical properties do they have? These questions can be resolved with the space missions to comets.The astronomical observation of comets on ground is often hampered by the earth's atmosphere.The space telescope can overcome such problem and get better information of ultra-V,X-ray and infrared emissions from comets.Spacecraft exploration of comets can also better determine the chemical and mineralogical compositions of cometary nuclei,analyze the isotopic compositions of comic dusts,and study the organic compounds and water contents of comets.Stardust is bringing back our first dust samples from the comet 81P/Wild 2.Deep Impact stroke the comet to test the internal structure of a cometary nucleus(9P/Tempel 1).Rosetta will make extended in situ analysis of both the nucleus and coma environment of a comet(67P/Churyumov-Gerasimenko).Due to their small bodies,comets do not experience large degree of geological processes since their formation.They record abundant information about the conditions and processes during the earliest stages of our solar system.Exploration of comets thus provides a new way to study the formation of the solar system and the planetary evolution.The origin of life started very early in the earth history.One theory is proposed that life-supporting organic molecules were derived from comets.Deep space exploration of comets will provide much vital information about the chemical evolution of organism in the solar system.To explore comets,we will face new challenges in various technical areas such as deep space communication,dynamical resources,navigation system and material science.In turn,the achievements we obtain in developing these new techniques will change our life style and improve our living standards.And they will help promoting economic development and national defense capability.In summary,space exploration of comets has played an important role in planetary sciences and space exploration missions.As a large nation in the world,China should also carry out deep space exploration of comets,which could improve our space exploration capability and exploit the new space frontier.