Main themes of solar-system explorations in the 21st century
Let's return to the first question. Based on the results of HAYABUSA, what is the goal for next- generation exploration in our solar system?
If there is ever a common goal for the whole space sciences in the 21st century, it must be a revolution in our view of this world, by writing a consecutive "story" from the birth of the universe to our present life. The role of planetary science in writing this story must be to discover a law that will enable the "linkage" between an evolutionary history of this universe from the Big Bang to the current structure (now being revealed by astronomy and cosmology) and the entire history of the Earth (now being interpreted by earth science). In these efforts, the following scenarios must be explained without contradictions adapted by JAXA:
(1) Birth and evolution of all the celestial bodies in the solar system
(2) Formation of the diverse planetary systems other than the solar system
(3) Chemical evolution of precursors to life
During the 1990's, advocating the single keyword "Origins," NASA merged all of its observation and exploration objectives into the quest for the origins of the universe, planets, Earth and life. Cause and effect are linked by a chain of natural laws; therefore we have to find the laws that govern evolutionary tracks of these from their origins in terms of both time and spatial scales. I think "evolution" must become the common keyword for all of the forthcoming space science missions of JAXA.
There are four mid- and long-term themes to planetary exploration:
(A) Origin and evolution of the solar system
(B) Diversity of planets
(C) Origin of life
(D) Unified views of the magnetosphere
The key for (A) through (C) is to understand the evolutionary process of minor bodies that retain information on the primitive solar system. In order to achieve these objectives, we must select relevant bodies to visit, depending upon what major event to be investigated in the evolutionary processes of the primitive solar system: contraction of gas clouds, aggregation from dust to planetisimals, impact growth of planetisimals, and internal thermal differentiation of celestial bodies (Fig. 2).
Considering the above, one candidate method for solar system exploration in the post-HAYABUSA era is to visit appropriate minor bodies (i.e., asteroids, comets, Kuiper Belt objects, etc.) retaining evidences of different evolutionary processes in the primitive solar system and investigate information of their time scale, materials, structure and environment. For the specific theme, we should first conduct multiple sample-return missions like HAYABUSA to visit at least one for each of major spectral-type asteroids, and complete correlation with the meteorite/cosmic dust database as quickly as possible. Secondly, we should study how much degree of chemical evolution in life-precursor can advance in the time scale of the solar system evolution. Thirdly, we should attempt to find the internal structure evolution in the process of growing from undifferentiated planetesimals to the Moon or other planet-size objects (Fig. 3). In Japan, Europe, and the U.S.A, we are entering the second golden era of comet and asteroid explorations following the international Halley armada of the 1980's. I believe that these three themes must remain as the most important targets to be solved in the next decade or two.