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The Forefront of Space Science

Exploring the Asteroid Itokawa:Subsequent Analysis and Research
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I feel the passage of time when considering that three years have already passed since HAYABUSA arrived at the asteroid Itokawa. Many events and accidents have occurred with HAYABUSA through this period, including the observation of Itokawa, attempted touch-downs, communication breaks and recoveries. HAYABUSA has gone through dramatic ups and downs. At present (September 2008), HAYABUSA is operating smoothly and we are now making various arrangements in preparation for its return to the earth in June 2010.

At the same time, analysis of data retrieved by HAYABUSA is steadily advancing. Recent findings on Itokawa are not as surprising as the early ones, but we can now review the entire collected data and gain a deeper understanding of Itokawa. In this article, I will summarize the progress following the initial data analysis.

Analysis of visible images

The most outstanding result of HAYABUSA’s observation of Itokawa is around about 1,500 photos. These were shot by Asteroid Multiband Imaging Camera called AMICA, which can take images in seven wavelength regions in visible light. To compare data in different wavelengths, we need to know accurately the properties of filters used and to remove extra light (scattering light) arising within the instrument. Such work is called calibration and requires very careful operation. The task was done by the members led by Masaaki Ishiguro and Jun Saito. Eventually, the number of calibrated photos reached around 1,400.

From the calibrated images, we were able to identify the distribution of albedo (i.e., reflectance ratio) across the entire surface of Itokawa. Its mean value is 0.25±0.03, almost the same as that obtained by observation from the ground. In addition, more accurate analysis was performed for the differences in color depending on location on Itokawa’s surface, which was discovered from the beginning. As a result, it is clear that the difference is closely related to a phenomenon called “space weathering.ESpace weathering describes changes in the apparent color and brightness of the surface materials, caused by the collision of solar wind and/or micro-meteorites with the surface of minor bodies.

Analysis of near-infrared data

HAYABUSA carries the near-infrared spectrometer called NIRS. With this instrument, we can observe the difference in intensity (spectrum) of near-infrared extending from 0.8 to 2.1µm per wavelength. While HAYABUSA stayed near Itokawa, NIRS collected data on more than 80,000 spectra. The analysis of this data was conducted mainly by researchers led by Kouhei Kitazato and Masanao Abe.

Calibration per wavelength is also important for NIRS. Another critical point is where NIRS measures the spectrum on the surface of Itokawa. The instrument’s view angle is 0.1 deg. square i.e., it corresponds to an area of 6m2 to 90m2 of Itokawa’s surface depending on the distance of HAYABUSA from the surface of Itokawa (except for touch-down phase). After much time-consuming work, data covering almost the entire Itokawa were analyzed and an albedo map represented in near-infrared was created. From near-infrared data, we can estimate the mineral composition on the surface. It was verified that the mineral composition of Itokawa was similar to that of meteorites called ordinary chondrite which have fallen on the earth much. As previously suggested, Itokawa is one of homes of such meteorites.

Figure 1
Figure 1. Reflectance ratios of p band (around 960nm) and w band (around 700nm)
The figure shows that reflectance ratio of p band is high in the reddish part while that of w band is high in the bluish part. By comparing the intensity of light in various wavelengths, we can ascertain if the surface materials have denatured.

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