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The Forefront of Space Science

Sand of Itokawa
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In June 2010, the spacecraft HAYABUSA returned from asteroid Itokawa to the earth. In November 2010, it became evident that the fine rocky particles it brought back originated from the surface of Itokawa, which attracted great public attention and encouraged us. The fact was confirmed by our compositional analysis of about 1,500 extremely fine rocky particles (almost 10µm or less in size), whose rough mineral assemble is shown in Fig. 1. The 1,500 particles were selected after removing artificial material (mainly aluminum flakes) from about 3,300 particles collected by a special Teflon spatula from inside the sample container called "sample catcher." Since the particles collected by the spatula were very small, i.e., 1ng (nano gram = 1/1,000,000,000 gram) per particle, they were suitable for statistical analysis of their characteristics, but were too small to examine individually in detail. For this reason, we collected slightly larger particles (several tens µm) from the sample catcher using a special tool called a micro-manipulator and started the initial analysis.

Figure 1
Figure 1. Mineral compositions of extremely fine rocky particles recovered from the sample catcher room A

The objective of the initial analysis is to describe the basic characteristics of the samples for the further detailed analyses. The content of the analysis was discussed before the launch of HAYABUSA. Researchers and organizations responsible for the analysis were selected from inside Japan through several international prior evaluations. Foreign researchers are participating in the initial analysis conducted in Japan. The distribution of the samples for analysis started at the end of January 2011. Partial analysis results were reported in the Lunar and Planetary Science Conference (Houston, U.S.A.) in March 2011, the Japan Geoscience Union Meeting (Makuhari) in May 2011, and in special issue of "Science", vol.333, 2011. The analysis is still proceeding.

As of today, we have acquired additional solid evidence showing that the samples are extraterrestrial material: information on the detailed structure of the surface and interior of the particles, information from elemental composition and mineral composition, information from isotopic composition of noble gases, oxygen, etc., and observed textural characteristics of extraterrestrial material such as space weathering. Based on these findings, we gradually learned about phenomena occurring on the surface of the S-type asteroid Itokawa, its relation to ordinary chondrites (most abundant stony-meteorites). We are also planning further analysis, e.g., distributing samples to NASA and announcing a public offering research, though these schedules have yet to be decided.

Below is the summary about the recovery of the solid fine particles for analysis and other information related to the recovery and analysis.

Recovery of solid fine particle samples

The samples were fewer than our initial expectation and extremely minute in size. First, we removed the lid of the sample catcher and replaced it with a transparent synthetic quartz glass lid. Figure 2 shows the interior of the catcher seen through the transparent lid. The catcher has two rooms and the picture shows room A. Room B is below room A. When viewed with the naked eye, there seemed to be no samples in both room A and B. Recovery work was performed with a micro-manipulator operated by gloves of a clean chamber while observing the inside of the catcher with an optical microscope.

Figure 2
Figure 2. Sample catcher room A
Almost nothing is visible to the naked eye. Room B is the same. We conducted direct collection with an electrostatic-controlled micro-manipulator used for fine particle handling and indirect collection with a Teflon spatula. Aperture diameter is 48mm.

Since there were no particles visible to the naked eye, we were unable to adopt simple methods such as picking up particles with tweezers. In addition, since the samples were tiny fine particles, they stuck to the catcher with static electricity and were hard to remove. We also needed to avoid polluting the samples by touching them, to protect them from mechanical damage, and to handle them carefully so as not to lose any. The samples were recovered in a clean chamber filled with ultra pure nitrogen to prevent pollution by the earth's atmosphere, etc.

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