In the former case, rare earth elements are sometimes mixed with radioactive elements such as uranium and thus are difficult to handle. In the latter case, they are comparatively easy to handle, but are very scarce because they only exist in extremely limited regions such as China.

It is thought that the first life forms arose from organic compounds in the early oceans approximately 4 billion years ago. Photosynthetic organisms such as blue-green algae appeared 3.9 billion years ago, and the oxygen level in the atmosphere of the early Earth started to increase. The oxygen then formed the ozone layer encircling the Earth, and protected the Earth from the sun's ultraviolet radiation. Because the atmospheric composition became similar to the current one and the climate became stable, the foundation of terrestrial ecosystem had been constricted. Subsequently, plants moved onto land and ancient forests grew, and animals moved into the terrestrial ecosystems. These dynamics caused terrestrial ecosystems to be complex.

Fertile soil was created from the organic matter produced by those plants and animals. The organic matter mainly in the ocean got buried deep underground by movements of the plates, and strong pressure and enormous amounts of heat caused the organic matter to turn into the fossil fuels such as oil and coal that support our human activities today.

The various resources that we today obtain from the Earth were created over a long time since the Earth was born, and therefore they are limited. Mineral resources and fossil fuels are non-renewable on the human time scale, and they are biological resources that will be lost forever unless they are used in a sustainable way.

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# The Asteroid Itokawa and the Earth

Our solar system consists of the Sun, eight planets and their satellites, and many small celestial bodies such as asteroids and comets. Counting only the asteroids and other small celestial bodies whose orbits are calculated, there are as many as 100,000, mostly existing between the orbits of Mars and Jupiter.

Except for the moon, humans have never landed directly on celestial bodies and obtained rocks and fine particles from them. Because large celestial bodies such as the Earth and the Moon have changed in composition significantly since their birth until today, it is not possible to know the composition of substances in the early stage of the solar system. If technology were available to bring back samples from asteroids (so-called “sample return”) which have a comparatively fixed record of the composition of substances when the planets were born, it would be possible to obtain clues about the materials that created planets and asteroids.

The Japan Aerospace Exploration Agency (JAXA) probed the asteroid “Itokawa” in a mission of the

Source: The Japan Aerospace Exploration Agency (JAXA)

asteroid explorer “Hayabusa (MUSES-C)” launched in May 2003.

Detailed observation by Hayabusa found that Itokawa is a small celestial body of 540m in longest diameter and an average diameter of 320m, shaped like a sea otter floating on the ocean, with a surface comprising areas where rocks are exposed and areas where sand and gravel are accumulated.

This “Hayabusa” mission achieved a variety of results, such as the operation of an ion engine, a rendezvous with Itokawa using autonomous optical navigation for landing on asteroids, and various scientific observations of Itokawa. One of the greatest results was that it successfully brought particles from Itokawa’s surface back to the Earth.

The fine particles were collected from the capsule that returned to the Earth in June 2010.

Some of the particles were identified as rocky particles by observation using an electron microscope. They have been under a series of physical and chemical analysis of things such as particle surface and inside composition, elemental composition, existence of macromolecular organic complexes and the types and content of mineral substances. It is expected that from the results of such analysis we will be able to obtain extremely valuable scientific knowledge related to the birth of the Earth.

When Hayabusa landed on Itokawa in September 2005, Itokawa was in a position approximately 320 million kilometers away from the Earth. To make an analogy of the space traveling of Hayabusa (1m x 1.6m x 2.0m), it was precision work as if an approximately 0.02mm grain of sand ("Hayabusa") launched from the northernmost point of Etorofu Island in Hokkaido directly hit a grain of rice approximately 5mm long ("Itokawa") situated at the westernmost point of