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Fig. 7. First X-ray observation of the lunar farside is shown. The XRF at Al and Si are
detected along with the cosmic X-ray backgrounds and dark noise. The XRF at Mg is
not detected so that Al/Si and the upper limit of Mg/Si are derived.
Just before the Earth swing-by on May 17, 2004, the XRS has observed
X-rays off the farside of the Moon for the very first time. 15 The apparent
diameter of the Moon was 0.5 so that the X-ray spectrum was a mixture
of the X-rays from the Moon with the cosmic X-ray backgrounds. The line
spectra of Al and Si that are considered from the Moon can be separated
from the continuous CXB but not for that of Mg, indicating high Al/Si and
low Mg/Si. This result shows that the global average composition of the
lunar far-side is Al-enriched and Mg-poor, most suitable to anorthosetic
rather than basaltic (shown in Fig. 7).
7. Conclusions
The XRS has been developed for Hayabusa to determine major elemen-
tal composition of Asteroid 25143 Itokawa under such severe conditions as
small mass and low bit-rate telemetry. In order to achieve scientific goals,
new technologies have been adopted such as charge-coupled devices as X-ray
detectors, an ultra-thin beryllium windows for light-shield, a latticed col-
limator to limit the fields of view, onboard standard sample for in situ
calibration, the thermal design to keep CCDs cool enough through radi-
ation cooling, and the compact electronics using SH-OBC and FPGAs.
Onboard analysis including event extraction and grade classification highly
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