Nuclear planetary science has come to play an important role in our understanding of the origin and evolution of the planetary bodies in our solar system. A newly established branch of planetary science, its study aids in humankind's exploration of the present states of the structures of various planetary bodies (including the Earth), their atmospheres and their satellites, as well as small celestial bodies (e.g. asteroids), through direct observation. Knowing the elemental composition of the planetary bodies is essential in order to understand the formation and evolution of planetary bodies — just as important as it is to know the mass, radius, density and orbit of the celestial body.