Orbital Evolution of (25143) ITOKAWA, the target asteroid of HAYABUSA (MUSES-C) mission

The HAYABUSA (MUSES-C) spacecraft was launched on May 9, 2003, and now it is on its way to the asteroid (25143) ITOKAWA, which was known as 1998 SF36. HAYABUSA will arrive at this asteroid in the summer of 2005, get the surface material, and come back to the Earth in 2007. This is the first sample return mission from asteroid. In this paper, the orbital evolution of this target asteroid is studied. The target asteroid ITOKAWA is one of the near-earth asteroids, the orbital element of which is a=1.32, e=0.28, i=1.73 (a : semimajor axis [AU], e : eccentricity, i : inclination [deg]). Since this asteroid is almost in the ecliptic plane and the orbit crosses the orbits of the Earth and Mars, many close encounters with the Earth and Mars occur. Therefore, the motion of this asteroid is chaotic. We studied the orbital evolution of this asteroid by calculating the motions for many clone asteroids, which have similar initial orbital elements. The results show that it is most probable that this asteroid exist around the present orbit for about several thousand years to the past. Moreover further calculations suggest that this asteroid has been existing in the near-earth region for one million years or longer. However, the chaotic behavior continues almost all the time, so we cannot deny the possibility that this asteroid came outside of the near-earth region not so long ago. The information of orbit evolution will be important when the sample is analyzed, so we need much more analyses to know the orbital evolution of this asteroid in much longer periods.