Kohei Kitazato

Near-Infrared Spectral Results of Asteroid Itokawa from the Hayabusa Spacecraft

The near-infrared spectrometer on board the Japanese Hayabusa spacecraft found a variation of more than 10% in albedo and absorption band depth in the surface reflectance of asteroid 25143 Itokawa. Spectral shape over the 1-micrometer absorption band indicates that the surface of this body has an olivine-rich mineral assemblage potentially similar to that of LL5 or LL6 chondrites. Diversity in the physical condition of Itokawas surface appears to be larger than for other S-type asteroids previously explored by spacecraft, such as 433 Eros.

Developing space weathering on the asteroid 25143 Itokawa

Puzzlingly, the parent bodies of ordinary chondrites (the most abundant type of meteorites) do not seem to be abundant among asteroids. One possible explanation is that surfaces of the parent bodies become optically altered, to become the S-type asteroids which are abundant in the main asteroid belt. The process is called ‘space weathering’—it makes the visible and near-infrared reflectance spectrum of a body darker and redder. A recent survey of small, near-Earth asteroids suggests that the surfaces of small S asteroids may have developing stages of space weathering. Here we report that a dark region on a small (550-metre) asteroid—25143 Itokawa—is significantly more space-weathered than a nearby bright region. Spectra of both regions are consistent with those of LL5-6 chondrites after continuum removal. A simple calculation suggests that the dark area has a shorter mean optical path length and about 0.04 per cent by volume more nanophase metallic iron particles than the bright area. This clearly shows that space-weathered materials accumulate on small asteroids, which are likely to be the parent bodies of LL chondrites. We conclude that, because LL meteorites are the least abundant of ordinary (H, L, and LL) chondrites, there must be many asteroids with ordinary-chondrite compositions in near-Earth orbits.

New photometric observations of asteroids (1862) Apollo and (25143) Itokawa - an analysis of YORP effect

Aims. Asteroid (1862) Apollo is one of two asteroids in which the YORP effect was detected. We carried out new photometric observations of Apollo in April 2007 to enlarge the time line and to derive a more precise shape and spin state model. We also observed another YORP-candidate, asteroid (25143) Itokawa, in December 2006 and January 2007 to obtain a longer time line. An estimation of the YORP strength on Itokawa based on its precise shape model from the Hayabusa mission predicted the deceleration to be already observable during the 2007 apparition. Methods. We used the lightcurve inversion method to model the shape and spin state of Apollo. For Itokawa, the shape and pole direction are known to a high degree of accuracy from the Hayabusa mission, so we used a modified version of lightcurve inversion with only two free parameters – the rotation period and its linear change in time. Results. The new model of Apollo confirms earlier results. The observed acceleration of Apollo’s rotation rate is (5.5 ± 1.2) × 10 −8 rad d −2 , which is in agreement with the theoretically predicted value. For Itokawa, the theoretical YORP value is sensitive to the resolution of the shape model and lies in the range from − 2t o−3 × 10 −7 rad d −2 . This is inconsistent with results of lightcurve inversion that place an upper limit to the change of Itokawa’s rotation rate ∼1.5 × 10 −7 rad d −2 .

Rotational characterization of Hayabusa II target Asteroid (162173) 1999 JU3

Abstract The Japanese Space Agency’s Hayabusa II mission is scheduled to rendezvous with and return a sample from the near-Earth Asteroid (162173) 1999 JU3. Previous visible-wavelength spectra of this object show significant variability across multiple epochs which has been attributed to a compositionally heterogeneous surface. We present new visible and near-infrared spectra to demonstrate that thermally altered carbonaceous chondrites are plausible compositional analogs, however this is a tentative association due to a lack of prominent absorption features in our data. We have also conducted a series of high signal-to-noise visible-wavelength observations to investigate the reported surface heterogeneity. Our time series of visible spectra do not show variability at a precision level of a few percent. This result suggests two most likely possibilities. One, that the surface of 1999 JU3 is homogenous and that unaccounted for systematic effects are causing spectral variation across epochs. Or two, that the surface of 1999 JU3 is regionally heterogenous, in which case existing shape models suggest that any heterogeneity must be limited to terrains smaller than approximately 5% of the total surface area. These new observations represent the last opportunity before both the launch and return of the Hayabusa II spacecraft to perform ground-based characterization of this asteroid. Ultimately, these predictions for composition and surface properties will be tested upon completion of the mission.

The Hayabusa Spacecraft Asteroid Multi-band Imaging Camera (AMICA)

Abstract The Hayabusa Spacecraft Asteroid Multi-band Imaging Camera (AMICA) has acquired more than 1400 multispectral and high-resolution images of its target asteroid, 25143 Itokawa, since late August 2005. In this paper, we summarize the design and performance of AMICA. In addition, we describe the calibration methods, assumptions, and models, based on measurements. Major calibration steps include corrections for linearity and modeling and subtraction of bias, dark current, read-out smear, and pixel-to-pixel responsivity variations. AMICA v-band data were calibrated to radiance using in-flight stellar observations. The other band data were calibrated to reflectance by comparing them to ground-based observations to avoid the uncertainty of the solar irradiation in those bands. We found that the AMICA signal was linear with respect to the input signal to an accuracy of ≪1% when the signal level was

25143 Itokawa: direct detection of the current decelerating spin state due to YORP effect

Aims. The Hayabusa mission revealed fundamental physical properties of the small near-Earth asteroid 25143 Itokawa, such as shape and mass, during its rendezvous with the asteroid in 2005. Resulting from this, the YORP-induced change in the asteroid’s spin state has been predicted theoretically. The purpose of this study is to investigate the YORP effect for Itokawa directly from an observational perspective. Methods. A long-term campaign of ground-based photometric observations of Itokawa were performed from March 2001 to December 2006. The observed asteroid lightcurves were compared with numerical modeling using the detailed shape model and global surface photometric properties derived from the Hayabusa mission. Results. As a non-linear time evolution of rotational phase lag is shown, we found that Itokawa has been decreasing its spin rate by u ω = −1.2(±0.2) × 10 −17 rad s −2 . The detected deceleration rate is almost consistent with, but slightly smaller than, the theoretically predicted value due to the YORP effect.

An evaluation method of reflectance spectra to be obtained by Hayabusa2 Near-Infrared Spectrometer (NIRS3) based on laboratory measurements of carbonaceous chondrites

We conducted ground-based performance evaluation tests of the Near-Infrared Spectrometer (NIRS3) onboard Hayabusa2 spacecraft in November 2013 and from April to May 2014 and established a method for evaluating its measured reflectance spectra. Reflectance spectra of nine powdered carbonaceous chondrite samples were measured by both NIRS3 and a Fourier transform infrared (FT-IR) spectrometer. We have established two methods for correcting the NIRS3 data by comparing them with the corresponding FT-IR data because raw data obtained by NIRS3 underwent spectral distortion caused by systematic offsets in sensitivity of individual pixels. The corrected NIRS3 spectra of carbonaceous chondrite samples are comparable with their FT-IR spectra. The depth of each band component Dλ is defined for each wavelength λ (μm) to characterize the absorption bands in NIRS3 spectra. It is suggested that the relationship between the D2.72/D2.79 ratio and the D2.76/D2.90 ratio would be useful for estimating the degree of heating of the asteroid surface, if contributions of terrestrial adsorbed water on D2.79 and D2.90 are properly corrected. The degrees of heating and space weathering are also comprehensively evaluated by the relationship between D2.90 and the D2.76/D2.90 ratio. Reflectance spectra of asteroid Ryugu, the target asteroid of Hayabusa2, to be recorded by the NIRS3 instrument are expected to reveal the characteristics of the surface materials by using the evaluation technique proposed in this paper. Such information will be used for choosing the touchdown points for sampling and also for investigating the distribution of the materials similar to the returned samples on Ryugu.Graphical abstract.