Yukio YAMAMOTO

X-ray fluorescence spectrometry of asteroid 25143 Itokawa was performed by the x-ray spectrometer onboard Hayabusa during the first touchdown on 19 November 2005. We selected those data observed during relatively enhanced solar activity and determined average elemental mass ratios of Mg/Si =0.78 +/- 0.09 and Al/Si = 0.07 +/- 0.03. Our preliminary results suggest that Itokawa has a composition consistent with that of ordinary chondrites, but primitive achondrites cannot be ruled out. Among ordinary chondrites, LL- or L-chondrites appear to be more likely than H-chondrites. No substantial regional difference was found on the asteroid surface, indicating its homogeneity in composition.

We present the results of remote X-ray fluorescence spectrometry of a near-Earth asteroid 25143 Itokawa with the X-ray fluorescence spectrometer (XRS) onboard Hayabusa. As has been reported, major elemental ratios of Mg/Si and Al/Si indicate that the surface of asteroid is more like an ordinary chondrite meteorite, especially LL- or L-chondrite, although some kinds of primitive achondrite are not ruled out. In this study, compositions of heavier elements such as Ca/Si and Fe/Si are quantitatively analyzed as well as the upper limit of S/Si. These results also support that the surface of Itokawa is like an ordinary chondrite, but also suggest that the surface must have experienced a little bit of thermal alteration or micro-impact processes.

In HAYABUSA/XRS, X-ray CCD needs to be kept below -40 degree Celsius to achieve the energy resolution to analyze elemental composition of the asteroid surface. XRS has three different techniques cooling CCDs; 1.thermal filament between XRS and HAYABUSA, 2. Pertie cooler on CCDs, 3. passive radiation by the hood to remove the stray light. The thermometer attached on the hood is available to measure thermal emissivity from the asteroid. To construct a thermal model of Itokawa, we considered the thermal balance between XRS and other environments such as the solar flux, heat radiation of HAYABUSA and Itokawa, heat generation of inside instruments that affected the temperature observed by this thermometer. To estimate the heat generation of inside instruments we divided XRS into 26 elements and solved equations of thermal conduction between nodes. To reveal the radiation from Itokawa we constructed the thermal model considering the shape of Itokawa and tried to investigate the most suitable characteristic for the observation and compared our analysis with infrared and near infrared ground observations.

The Hayabusa will rendezvous 25143 Itokawa, an S (IV)-class near-earth asteroid, in September to October, 2005. The XRS is among the remote sensing instruments aboard. We present here the main objectives of the XRS observation. The XRS will determine major elemental composition of surface of the asteroid. Elemental variation will be surveyed by asteroid rotation in longitudinal direction, maybe with spatial resolution of 60 degrees. The XRS always detect such major elements as Mg, Al, Si, and S, but it also can do for higher atomic number elements such as Ca and Fe, especially during the occurrence of solar flare. Furthermore, microscopic roughness of the surface is informed by observation at various solar phase angles due to particle size effect in X-ray fluorescence. Since thermal design of the XRS has been well calibrated, thermal radiation intensity off the asteroid surface will be estimated by monitoring the temperature profile during descent of the spacecraft for asteroid touchdown.

SELENE XRS will determine major elemental composition of lunar surface. In this study, we will introduce the intelligent observation system of XRS with onboard computer-control.

The SELENE (SELenological and Engineering Explorer) is a Japanese lunar polar orbiter with 14 scientific instruments. The XRS is to map major elemental composition in 20km spatial resolution, to provide the clue to understanding the lunar evolution. Energy resolution and efficient detection area of the XRS has been much improved by using arrays of Si CCD with 100cm² detection area and ultra-thin beryllium window for better transparency at low energy. We present the specification, performance and current status of the XRS and method of remote X-ray spectrometry.

The SELENE (SELenological and Engineering Explorer) is a Japanese lunar polar orbiter and performs lunar global mapping with more than 10 scientific instruments. The XRS will map major elemental composition in 20km spatial resolution and provide the clue to understanding the lunar evolution. For that purpose, the XRS has been designed to improve energy resolution and efficient detection area. Then the X-ray detector has arrays of CCD with 100cm² detection area, and is kept below -50C by radiation cooling. Ultra-thin beryllium window are developed for better transparency at low energy. We present the specification and the current status of the XRS as prepared for the interface test, as well as the plan of pre-flight tests and observation plan.

We present the results of initial operation, the current status, and the future observation plan of the X-ray fluorescence spectrometer (XRS) onboard Hayabusa, which was launched with the fifth M-V launch vehicle on May 9th, 2003. The XRS observes X-rays characteristic of elements excited by solar irradiation, to determine major elemental composition of asteroid Itokawa (1998SF36). Then we classify the S-class asteroid and prospect the degree of evolution processes. During the two-year long cruising phase, the XRS are checked out its function and the detectors against radiation damage, and observes X-ray bodies such as super nova remnants or active galactic cores as well as cosmic backgrounds of X-rays for in-flight calibration. The XRS also monitors solar X-rays using the standard sample. Flight demonstration of the newly developed onboard computer proves usability for more than 20 days.

X-ray fluorescence spectrometer (XRS) onboard HAYABUSA (MUSES-C) spacecraft that was launched by the fifth M-V launch vehicle on 9 May 2003. The XRS has plans to observe supernova remnants (SNR), active galactic nuclei (AGN) and cosmic X-ray backgrounds for in-flight calibration and scientific observation during the transfer and return phase. When HAYABUSA spacecraft performed tests of the ion electric propulsion system (IES) and its heater control system, the XRS observed spectra were noisy. This study shows method of reducing heat noise.

X-ray spectrometer onboard HAYABUSA spacecraft has a standard sample to perform comparative analysis. Comparing with the X-rays from the standard sample and asteroid surface, the dependency of induced X-ray, solar X-rays, is reduced [1].<BR>It takes more than 2 years to reach the Asteroid, and the temporal variance of the XRS performance is investigated periodically by the observation of the X-rays from Cosmic X-ray Backgrounds, astronomical X-ray bodies, and the standard sample.<BR>From 28 May 2003 to 30 May 2003, just after the launch of HAYABUSA, XRS observed an X-ray source: SCO-X1 [2]. The observation continued two days due to the initial checkout. Fortunately, an X.1 class solar flare happened during this term. The spectra from the standard sample were composed of Ca and Si line spectra together with Mg, Al, and Si line spectra. These data is available for the XRS in-flight calibration: energy gain and intensities. In addition, model parameter is determined by this data.<BR>Yamamoto et al reported that the energy calibration, comparison of the energy flux between GOED 10 and XRS, and the comparison of the observation and calculation using thess data [3]. The energy calibration of the line spectra: Mg, Al, Si, Ca, and Fe, was determined within 1% accuracy comparing with ⁵⁵Fe calibration data before launch. Temporal variation of energy flux obtained by XRS showed the consistency with that obtained by GOES 10 satellite. Moreover, the model calculation was performed and compared with the observation. The comparison showed the background noise in the energy range 0.5-3keV clearly existed. This noise was serious problem to determine the intensities of Mg, Al, and Si X-ray fluorescence spectra.Therefore, we report here the method to remove this background noise, and the effect of the method.<BR>[1] Okada, T., Fujiwara, A., Tsunemi, H., and Kitamoto, S. 2000. X-ray fluorescence spectrometer onboard MUSES-C, Adv. Space Res. 25, 345-348.<BR>[2] Arai et al, 2003. Proc. ISAS Lunar Planet. Symp.<BR>[3] Yamamoto et al, 2003. Proc. ISAS Lunar Planet. Symp.<BR>