Pluto and asteroids
In the general assembly of the International Astronomical Union (IAU) in 2006, it was decided to exclude Pluto from the definition of so-called planets. The news was reported to millions around the world by the mass media. Of course, this does not mean that Pluto did something wrong. The decision was made because many scientists judged that, “We should reacknowledge Pluto as a typical body representing minor bodies existing in the solar system.”
“Dwarf planet” is now considered as the name representative of the solar system’s minor bodies. Until now, only two names “comets” and “asteroids” had been assigned to the small objects that go around the Sun except for planets.
The first asteroid was discovered in 1801 and since then about 400,000 asteroids have been identified. It is now known that they are distributed across the whole solar system from inside Mercury’s orbit to outside Pluto’s.
Opening of asteroid research from the aspect of material science
Solar system research from the aspect of material science has been advanced mainly by meteorite research. We are able to conduct detailed material analyses of meteorites with facilities on the ground and identify the age of their formation and environment at formation. Most of our information on the origin and evolution of the solar system is derived from meteorite-analysis research.
Since we study meteorites that fell onto the ground, however, we cannot know from where in the solar system they came. First of all, it is doubtful that meteorites falling to earth present information of the whole solar system. Groundbased observation of asteroids is complementary research to clarify the uncertainties.
In the 1970s, astronomers began to believe that meteorites were pieces of asteroids falling to earth. This was because, as spectroscopic observations (i.e., observations to examine fine color differences) of asteroids commenced, similar spectroscopic data were obtained for both asteroids and meteorites.
As spectroscopic observation of asteroids progressed, it became known that there were many kinds of asteroids with various features and that such features varied according to their distance from the Sun. Different trends of asteroids’ materials were identified according to their distance from the Sun (see Fig. 4 in the article “The Role of Solar-System Exploration in the Space Science Vision” (21 July, 2004) by Yano on this site). Meanwhile, with advances in meteorite research, we came to think that, as the age information of most meteorites shows early in the solar system’s formation history, the material distribution of asteroids must be indicative of the early solar system.
Mystery of the relation between asteroids and meteorites
Many researchers advanced their research by combining spectroscopic features of asteroids and material features of meteorites. There remained, however, one large mystery: the features of S-type asteroids that are most found in asteroids are not consistent from a spectroscopic viewpoint with those of ordinary chondrites most found in meteorites.
This issue has been discussed for a long time and several hypotheses remain. They are:
To answer the question, we needed to send an explorer to an S-type asteroid and examine it.