小行星深空探测的科学意义和展望

DEEP SPACE EXPLORATION OF ASTEROIDS: THE SCIENCE PERSPECTIVES

太阳系深空探测活动方兴未艾,小行星探测已成为主要发展方向。通过回顾近年来几个主要的国际小行星空间探测计划以及取得的研究成果,总结了小行星深空探测从早期的近距离飞越到小行星低空绕轨勘探,到目前的表面软着陆和采集样品返回的发展进程。同时深空探测也给行星科学研究者提出了新的挑战,鉴于目前行星科学的研究热点,详细叙述了小行星深空探测急需解决的重大科学问题及其科学意义,随后简单介绍了未来小行星深空探测计划的科学目标,为小行星深空探测的具体任务提供了科学目标选择方向。最后呼吁我国及时介入小行星深空探测,提升航天能力,开拓深空领域。

This paper reviews the history of deep space exploration of asteroids and summarizes the important achievements from these missions. The missions to asteroids have gone through three phases： ①flyby; ②orbiting; and ③sample collection. These activities have greatly enhanced our understanding of the solar system formation and evolution history. Here we illustrate the science perspectives for the deep space exploration of asteroids. （1） The major fraction of asteroids orbits the sun between Mars and Jupiter. But a small number of asteroids has orbital radius very close to that of the Earth. Some of them collided with the Earth in the past, and some will hit the Earth in the near future. These objects possess great threaten to the Earth and our human being. How did the asteroids change their orbits over the time and what affected their orbit evolution？ These questions become top priorities in planetary research areas. To better under stand the orbital evolution of asteroids and evaluate their impact probability to the Earth, it is necessary to carry out deep space exploration of asteroids. （2） How do asteroids collide with the Earth？ In what way can we change their trajectories before they hit the Earth？ This is an important practice to avoid impact or minimize impact damage. To do this,we need to know the internal structures of asteroids. We can make plan to deflect potentially hazardous objects before they impact the Earth. （3） Due to their small bodies, asteroids did not experience large degree of geological processes since their formation. They recorded abundant information about the conditions and proeesses during the earliest stages of our solar system. Exploration of asteroids thus provides a new way to study the formation of the solar system and the planetary evolution. （4） Exploration of asteroids can provide us a deeper understanding of the origin and evolution of the solar system. It is not clear why some asteroids preserved their primitive nature and others went through large degree of differentiation during the earliest stage of the solar system. This is the main goal of the Dawn mission scheduled to launch next year by NASA. This mission will explore two large asteroids, 1 Ceres and 4 Vesta. The former has a very primitive surface, water-bearing minerals, and possibly a very weak atmosphere and frost. The latter is a dry, differentiated body whose surface has been resurfaced by basaltic lava flows. Dawn will investigate the internal structure, density and homogeneity of these two complementary asteroids. （5） Most meteorites were derived from asteroids. But we do not know where they come from and what the genetic relationships between meteorites and their parent bodies are. Among all the meteorites found so far, about 80% are ordinary chondrites. These meteorites have reflected spectrum similar to those of S type asteroids. But S type asteroids only account for about 20% of asteroid population. Exploration of asteroids can help us to establish a genetic relationship between asteroids and meteorites. （6） Looking for new primitive materials in the solar system. It is generally believed that carbonaceous chondrites come from C type asteroids, ordinary chondrites from S type asteroids, and iron meteorites from M type asteroids. But we do not know which group of meteorites comes from T, D, O, Ld type asteroids. Do these asteroids have unique chemical compositions or mineral assemblages？ They are very important in studying the solar system formation and evolution. （7） The origin of life started very early in the Earth history, One theory proposed that life supporting organic molecules were derived from asteroids via meteorite delivery. Currently, several important organic molecules （e. g. , amino acids） were found in carbonaceous chondrites. This type of meteorites is believed to have come from C type asteroids. Among the asteroids observed so far, about 75% is C type asteroids. It is interesting to know what organics these asteroids have. In this way, we can understand the origin of life on the Earth. Deep space exploration of asteroids, especially the C type asteroids, will provide much vital information about the chemical evolution of organics in the solar system. （8） In recent years, short-lived nuclides and pre-solar grains were observed in primitive meteorites. These new discoveries have greatly enhanced our understanding of the stellar nucleosynthesis, galactic chemical evolution, the origin and evolution of stars as well as our solar system. The stellar materials are very rare and only found in primitive chondrites. Deep space exploration of asteroids will provide us a new frontier for the study of stellar material. （9） Utilization of the resources is another important goal for the deep space exploration of asteroids. For example, M type asteroids contain numerous precious metals （e. g. , Au, Ru, Rh, Pd, Os, Ir, and Pt）. It was estimated that a one-km sized M type asteroid contains 4×10^8 kg noble metals, with a current market value of 5 trillion U.S. dollars. C Type asteroids have organics and water/ice which are very critical to the space missions as they can be important energy sources. It will be much easy to deliver water to the spacecrafts from asteroids than from the earth. （10） To explore asteroids, 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, deep space exploration of asteroids has played an important role in planetary sciences and space exploration missions. In 2006, NASA will launch a new mission （Dawn） to explore two large asteroids （Ceres and Vesta） with a goal to understand the internal differentiation of asteroids at the beginning of the solar system. As a large nation in the world, China should also carry out deep space exploration of asteroids, which could promote our space exploration capability and exploit the new space frontier.