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تسجيل مستخدم جديدChange-2 spacecraft observations of asteroid 4179 Toutatis
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On 13 December 2012, Change-2 completed a successful flyby of the near-Earth asteroid 4179 Toutatis at a closest distance of 770 meters from the asteroids surface. The observations show that Toutatis has an irregular surface and its shape resembles a ginger-root of a smaller lobe (head) and a larger lobe (body). Such bilobate shape is indicative of a contact binary origin for Toutatis. In addition, the high-resolution images better than 3 meters provide a number of new discoveries about this asteroid, such as an 800-meter depression at the end of the large lobe, a sharply perpendicular silhouette near the neck region, boulders, indicating that Toutatis is probably a rubble-pile asteroid. Change-2 observations have significantly revealed new insights into the geological features and the formation and evolution of this asteroid. In final, we brief the future Chinese asteroid mission concept.
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On 13 December 2012, Change-2 conducted a successful flyby of the near-Earth asteroid 4179 Toutatis at a closest distance of 770 $pm$ 120 meters from the asteroids surface. The highest-resolution image, with a resolution of better than 3 meters, reve
als new discoveries on the asteroid, e.g., a giant basin at the big end, a sharply perpendicular silhouette near the neck region, and direct evidence of boulders and regolith, which suggests that Toutatis may bear a rubble-pile structure. Toutatis maximum physical length and width are (4.75 $times$ 1.95 km) $pm$10$%$, respectively, and the direction of the +$z$ axis is estimated to be (250$pm$5$^circ$, 63$pm$5$^circ$) with respect to the J2000 ecliptic coordinate system. The bifurcated configuration is indicative of a contact binary origin for Toutatis, which is composed of two lobes (head and body). Change-2 observations have significantly improved our understanding of the characteristics, formation, and evolution of asteroids in general.
This paper presents analysis of the rotational parameters of Toutatis based on the observational results from Change-2s close flyby. The 3-D shape model derived from ground-based radar observation is used to calculate the 3-1-3 Euler angles at the fl
yby epoch, which are evaluated to be $-20.1^circpm1^circ$, $27.6^circpm1^circ$ and $42.2^circpm1^circ$. The large amplitude of Toutatis tumbling attitude is demonstrated to be the result of the large deviation of the angular momentum axis and the rotational axis. Two rotational periods are evaluated to be $5.38pm0.03$ days for rotation about the long axis and $7.40pm0.03$ days for precession of the long axis about the angular momentum vector based on Fourier analysis. These results provide a further understanding of rotational state of Toutatis.
In this work, we investigate the rotational dynamics of the ginger-shaped near-Earth asteroid 4179 Toutatis, which was closely observed by Change-2 at a distance of $770pm120~$ meters from the asteroids surface during the outbound flyby citep{Huang20
13} on 13 December 2012. A sequence of high-resolution images was acquired during the flyby mission. In combination with ground-based radar observations collected over the last two decades, we analyze these flyby images and determine the orientation of the asteroid at the flyby epoch. The 3-1-3 Euler angles of the conversion matrix from the J2000 ecliptic coordinate system to the body-fixed frame are evaluated to be $-20.1^circpm1^circ$, $27.6^circpm1^circ$ and $42.2^circpm1^circ$, respectively. The least-squares method is utilized to determine the rotational parameters and spin state of Toutatis. The characteristics of the spin-state parameters and angular momentum variations are extensively studied using numerical simulations, which confirm those reported by citet{Takahashi2013}. The large amplitude of Toutatis precession is assumed to be responsible for its tumbling attitude as observed from Earth. Toutatis angular momentum orientation is determined to be described by $lambda_{H}=180.2^{+0.2^circ}_{-0.3^circ}$ and $beta_{H}=-54.75^{+0.15^circ}_{-0.10^circ}$, implying that it has remained nearly unchanged for two decades. Furthermore, using Fourier analysis to explore the change in the orientation of Toutatis axes, we reveal that the two rotational periods are 5.38 and 7.40 days, respectively, consistent with the results of the former investigation. Hence, our investigation provides a clear understanding of the state of the rotational dynamics of Toutatis.
We derive the geometric albedo of a near-Earth asteroid, (4179) Toutatis, to investigate its surface physical conditions. The asteroid has been studied rigorously not only via ground-based photometric, spectrometric, polarimetric, and radar observati
ons but also via textit{in situ} observation by the Chinese Change-2 space probe; however, its geometric albedo is not well understood. We conducted V-band photometric observations when the asteroid was at opposition in April 2018 using the three telescopes in the southern hemisphere that compose the Korea Microlensing Telescope Network (KMTNet). The observed time-variable cross section was corrected using the radar shape model. We find that Toutatis has a geometric albedo $p_mathrm{V} = 0.185^{+0.045}_{-0.039} $, which is typical of S-type asteroids. We compare the geometric albedo with archival polarimetric data and further find that the polarimetric slope--albedo law provides a reliable estimate for the albedo of this S-type asteroid. The thermal infrared observation also produced similar results if the size of the asteroid is updated to match the results from Change-2. We conjecture that the surface of Toutatis is covered with grains smaller than that of the near-Sun asteroids including (1566) Icarus and (3200) Phaethon.
The Hayabusa Spacecraft Asteroid Multiband 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 performan
ce 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 < 3800 DN. We verified that the absolute radiance calibration of the AMICA v-band (0.55 micron) was accurate to 4% or less, the accuracy of the disk-integrated spectra with respect to the AMICA v-band was about 1%, and the pixel-to-pixel responsivity (flatfield) variation was 3% or less. The uncertainty in background zero-level was 5 DN. From wide-band observations of star clusters, we found that the AMICA optics have an effective focal length of 120.80 pm 0.03 mm, yielding a field-of-view (FOV) of 5.83 deg x 5.69 deg. The resulting geometric distortion model was accurate to within a third of a pixel. We demonstrated an image-restoration technique using the point-spread functions of stars, and confirmed that the technique functions well in all loss-less images. An artifact not corrected by this calibration is scattered light associated with bright disks in the FOV.
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