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The Forefront of Space Science

Exploring the Asteroid Itokawa:Subsequent Analysis and Research
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Analysis of X-ray data

Regarding Itokawa’s surface materials, we have also made progress analyzing the data collected by the X-ray fluorescent spectrometer (XRS). When X-rays from the Sun hit Itokawa’s surface, the surface materials emit X-rays called fluorescent X-rays. By examining this X-ray’s spectrum, we can estimate the elemental composition of the surface materials.

Further analysis of the XRS data was conducted by members led by Takehiko Arai and Tatsuaki Okada. Although the XRSEview is much wider than that of NIRS by about 3.5 deg., when HAYABUSA approaches very close to the meteorite it actually looks at a part of Itokawa’s surface. From a detailed analysis of the XRS data, it was revealed that, although the elemental composition of the surface materials of Itokawa is mostly uniform, it is possible that the abundance of sulfur, a volatile element, varies according to the location. From the XRS data, we also confirmed the high probability that the surface materials of Itokawa are ordinary chondrites.

Figure 3
Figure 2. Reflectance ratio in 1.57µm infrared
Dark blue shows reflectance ratio of 0.126 while red shows 0.145.

Figure 3
Figure 3. Most precise shape model of Itokawa
3-D model can be replicated on PC display by using digital shape data.

Analysis of distance and orbital data

Another scientific instrument onboard HAYABUSA is a laser altimeter called LIDAR. This instrument precisely measures the distance between HAYABUSA and Itokawa using laser beams. In conjunction with the camera images, the relative position of Haybusa to Itokawa is obtained accurately. LIDAR gathered about 1,670,000 data during the mission period. The data analysis was carried out by Shinsuke Abe and Olivier S. Barnouin-Jha. As a result, Itokawa’s mass and the undulations of its surface were precisely obtained.

Hitoshi Ikeda is now studying Itokawa’s mass from the standpoint of orbital engineering. In addition to the value of mass itself, we are also examining to confirm if there is a difference in density within Itokawa. As of today, however, there has been no evidence found showing non-uniformity in density.

To obtain the density, we need to know Itokawa’s volume. Robert W. Gaskell has continually refined the geometric model of Itokawa. The density value calculated from the precise model is almost the same as the initially calculated value of 1.9g/cm3. The density of ordinary chondrite is approximately 3.2g/cm3. So, if we assume that Itokawa is composed of ordinary chondrite to its center, the void ratio of its interior is estimated to be about 40%.

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