The nature of asteroid Itokawa revealed by Hayabusa

In September 2005, Hayabusa, the spacecraft of the Japanese asteroid sample return mission, finally arrived at Asteroid Itokawa, which is one of the small Near Earth Objects. We were surprised to see the images of Itokawa taken by the camera on board, because the asteroid was strange looking, covered with a lot of boulders, and there are only a few craters on it. After the detailed scientific observations by using the imaging camera, near-infrared spectrometers, X-ray spectrometers, and Laser Altimeter, we found following things: The surface material of Itokawa is uniform and it corresponds to LL-chondrite of ordinary chondrite. The bulk density of Itokawa is about 1.9 g/cm³, which indicates that the macroporocity is about 40%. The surface of Itokawa is clearly divided into two parts, smooth terrain and rough terrain. The smooth terrain is covered with small pebbles and there are no such pebbles on the rough terrain. We also get some evidences for the space weathering. From these results, we concluded that Itokawa i s a rubble-pile asteroid. And the scenario of the evolution of Itokawa may be as follows. In the inner part of the asteroid belt, a large parent body was once destroyed by an impact. Then the fragments gravitationally attracted each other to form two bodies, and these two bodies attached to form Itokawa. After that, the orbit or Itokawa was changed to the current one by some dynamical mechanisms. In the future, most probably Itokawa will collide to the sun or inner planets. Maybe this is the typical evolution of small Near Earth Asteroids of S-type. Itokawa is a common small S-type asteroid, and now we know many features of Itokawa. The knowledge about Itokawa will be quite useful from the point of S paceguard.