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The Forefront of Space Science

Exploring the Asteroid Itokawa:Subsequent Analysis and Research
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Observation from the ground

Itokawa was observed by telescopes on the ground from the end of 2006, about one year after HAYABUSA’s completion of exploration of Itokawa, until the middle of 2007. In the observation, Itokawa’s rotational movement attracted the most interest. In the rotational movement of minor bodies like asteroids, there is a famous effect called YORP effect: as minor bodies are heated by solar light, heat is anisotropically emitted into space in infrared causing a rotational change. The force of the effect is very small but, accumulating over a long time, it changes the rotational speed of minor bodies.

Since we know Itokawa’s shape and surface state in detail from HAYABUSA’s observations, we can calculate the level of its YORP effect. While it was theoretically predicted that Itokawa’s rotation would be slowed, no such sign of change was found by our observation. The reason for this discrepancy with the theoretical conclusion is still unclear. According to one assumption, it can be explained if it is supposed that there is nonuniformity of density within Itokawa and, as a result, its center of gravity is in different point from that already estimated. Researchers including Kouhei Kitazato, Daniel J. Scheeres, and J. Ďurech are now examining the YORP effect.

Observation by infrared astronomy satellite AKARI

The Japanese infrared astronomy satellite AKARI launched in 2006 is bringing remarkable results by its survey of the whole sky and observations of many infrared bodies. In July 2007, the satellite also succeeded in observing the asteroid Itokawa. An asteroid’s size can be estimated by infrared observation. By observing an asteroid of an accurately known size, such as Itokawa, we can improve the accuracy of size estimations of asteroids by infrared observation. The AKARI team cooperated with us on the observation. In addition, Sunao Hasegawa and Thomas Müller are analyzing the data.

Figure 3
Figure 4. Surface inclination map by 3-D GIS (left) and brightness map of AMICA v band (right)
Color bars under the figures show degree of inclination and brightness respectively. Blue indicates the smallest (low in inclination or brightness) while red the largest (high in inclination or brightness)

Other progress

Based on the distribution, location and condition of rocks and stones on Itokawa’s surface, Hideaki Miyamoto et al. conducted research on material flow on the surface. The results seemed to confirm the theory that particles on the surface moved along a gravity field created by Itokawa itself. It is thought that, for example, when meteorites collide with Itokawa, the asteroid’s entire body is shaken and materials on the surface move.

As stated above, a wide range of analysis continues on Itokawa. Software capable of displaying results of analysis in 3-D on a computer was also developed. The software, derived from 3-D GIS (Geographic Information System), was produced by Naru Hirata, Hirohide Demura et al. Using this software, we can display the shape of Itokawa on a computer screen and, on that, depict data of albedo, gravity, surface inclination, and also lines of latitude and longitude.


The amount of data collected by HAYABUSA around Itokawa is far less than that collected by earth-observation satellites such as DAICHI or lunar-orbiting satellite KAGUYA. This is because the distance of data transmission is very different. The mission of the HAYABUSA team and the scientists involved in handling the Itokawa data is to process appropriately the data sent back from 300 million km away and to contribute to the advancement of future planetary science. Our mission has made considerable achievements with its analysis until now, however, there remains much to be done. In addition, if HAYABUSA brings Itokawa materials back to earth, new scientific investigations will begin. As to the exploration of Itokawa, there remains a long way to go.


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