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TOP > Report & Column > The Forefront of Space Science > 2007 > Understanding of the Solar System through Research of Minor Bodies - Bridging Groundbased-Observation and Meteorite-Analysis Research -

The Forefront of Space Science

Understanding of the Solar System through Research of Minor Bodies
- Bridging Groundbased-Observation and Meteorite-Analysis Research -
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HAYABUSA explorer and near-infrared spectrometer

Such was the background to the planning of the HAYABUSA mission. Although the primary objective of HAYABUSA is to demonstrate engineering technologies necessary for next-generation solar explorers, it also had the major scientific objective of close observations of the asteroid, collection of surface materials and returning samples to earth.

HAYABUSA was launched in May, 2003, and arrived at the asteroid Itokawa in September, 2005. Itokawa was classified as an S-type asteroid by observations from the ground, but it was also confirmed that the asteroid had different feature from ordinary chondrites (Fig. 1).

Figure 1
Figure 1. Reflectance spectrum of the S-type asteroid Itokawa (dots) and that of the ordinary chondrite (solid line) (Modification of Binzel et al., 2001)

A near infrared spectrometer (NIRS) was installed on HAYABUSA as an instrument for spectroscopic observations (Fig. 2). We observed Itokawa with this instrument.

The wavelength coverage of the NIRS is approx. 800 to 2,100nm. The instrument was designed to identify the features of pyroxene and olivine that are thought to exist mostly on the surface of asteroids (their existence is actually confirmed by meteorite analytical research). Furthermore, in order to obtain different data from the groundbased observations, the field-of-view size was set to 0.1 deg., which allows enough spatial resolution in observation of the asteroid surface. A detailed map of Itokawa’s surface observed by NIRS is being produced. The differences in color and brightness of the surface revealed by the visible light camera (see the photo by Saito in the May, 2006, edition of “ISAS News”), were also observed by NIRS.

Figure 2
Figure 2. Near Infrared Spectrometer (NIRS) onboard HAYABUSA

New findings discovered by HAYABUSA’s spectroscopic observations

NIRS first investigated the standard features of the spectroscopic data, which also served for calibration with the ground observations. In the NIRS data, we saw the features of pyroxene and olivine, indicating that they are present in the asteroid’s surface materials. Information on their composition ratio was also provided. In addition, NIRS discovered that Itokawa’s surface materials mostly resemble the ordinary chondrites among meteorites falling to earth and, in particular, are close to the LL chondrites among the ordinary chondrites.

With the spatially resolved observation data, it became clear that areas with varying color and brightness have almost the same mineral composition as other areas. This means that Itokawa’s surface comprises mostly homogeneous substances and has not undergone a major thermal evolution process. It is thought that ordinary chondrites are primitive meteorites that have not undergone thermal evolution. From this standpoint, S-type asteroids and ordinary chondrites have a closer relationship. This result was also supported by observations from the fluorescent X-ray spectrometer onboard HAYABUSA.

When HAYABUSA approached close to the surface, more precise observation of the bright area became possible. With this observation, we found that the spectroscopic data of the brighter area are more similar to the features of ordinary chondrites than other relatively darker areas.

Once the surface materials of Itokawa were estimated, an interesting fact emerged from Itokawa’s mass obtained by gravity measurement and volume obtained by camera observation. Itokawa’s entire density was calculated from its mass and volume. Its value was 1.9g/cm3, which differs greatly from the average density of LL chondrites at 3.2g/cm3. This means that voids exists inside Itokawa.

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