Thermal Infrared Imaging Experiments of C-Type Asteroid 162173 Ryugu on Hayabusa2

The thermal infrared imager TIR onboard Hayabusa2 has been developed to investigate thermo-physical properties of C-type, near-Earth asteroid 162173 Ryugu. TIR is one of the remote science instruments on Hayabusa2 designed to understand the nature of a volatile-rich solar system small body, but it also has significant mission objectives to provide information on surface physical properties and conditions for sampling site selection as well as the assessment of safe landing operations. TIR is based on a two-dimensional uncooled micro-bolometer array inherited from the Longwave Infrared Camera LIR on Akatsuki (Fukuhara et al., 2011). TIR takes images of thermal infrared emission in 8 to 12 μm with a field of view of 16×12∘ and a spatial resolution of 0.05∘ per pixel. TIR covers the temperature range from 150 to 460 K, including the well calibrated range from 230 to 420 K. Temperature accuracy is within 2 K or better for summed images, and the relative accuracy or noise equivalent temperature difference (NETD) at each of pixels is 0.4 K or lower for the well-calibrated temperature range. TIR takes a couple of images with shutter open and closed, the corresponding dark frame, and provides a true thermal image by dark frame subtraction. Data processing involves summation of multiple images, image processing including the StarPixel compression (Hihara et al., 2014), and transfer to the data recorder in the spacecraft digital electronics (DE). We report the scientific and mission objectives of TIR, the requirements and constraints for the instrument specifications, the designed instrumentation and the pre-flight and in-flight performances of TIR, as well as its observation plan during the Hayabusa2 mission

Highly porous nature of a primitive asteroid revealed by thermal imaging

Carbonaceous (C-type) asteroids are relics of the early Solar System that have preserved primitive materials since their formation approximately 4.6 billion years ago. They are probably analogues of carbonaceous chondrites and are essential for understanding planetary formation processes. However, their physical properties remain poorly known because carbonaceous chondrite meteoroids tend not to survive entry to Earth’s atmosphere. Here we report on global one-rotation thermographic images of the C-type asteroid 162173 Ryugu, taken by the thermal infrared imager (TIR) onboard the spacecraft Hayabusa2, indicating that the asteroid’s boulders and their surroundings have similar temperatures, with a derived thermal inertia of about 300 J m−2 s−0.5 K−1 (300 tiu). Contrary to predictions that the surface consists of regolith and dense boulders, this low thermal inertia suggests that the boulders are more porous than typical carbonaceous chondrites and that their surroundings are covered with porous fragments more than 10 centimetres in diameter. Close-up thermal images confirm the presence of such porous fragments and the flat diurnal temperature profiles suggest a strong surface roughness effect. We also observed in the close-up thermal images boulders that are colder during the day, with thermal inertia exceeding 600 tiu, corresponding to dense boulders similar to typical carbonaceous chondrites. These results constrain the formation history of Ryugu: the asteroid must be a rubble pile formed from impact fragments of a parent body with microporosity of approximately 30 to 50 per cent that experienced a low degree of consolidation. The dense boulders might have originated from the consolidated innermost region or they may have an exogenic origin. This high-porosity asteroid may link cosmic fluffy dust to dense celestial bodies.Additional co-authors: Tsuneo Matsunaga, Takeshi Imamura, Takehiko Wada, Sunao Hasegawa, Jörn Helbert, Thomas G. Müller, Jens Biele, Matthias Grott, Maximilian Hamm, Marco Delbo, Naru Hirata, Naoyuki Hirata, Yukio Yamamoto, Seiji Sugita, Noriyuki Namiki, Kohei Kitazato, Masahiko Arakawa, Shogo Tachibana, Hitoshi Ikeda, Masateru Ishiguro, Koji Wada, Chikatoshi Honda, Rie Honda, Yoshiaki Ishihara, Koji Matsumoto, Moe Matsuoka, Tatsuhiro Michikami, Akira Miura, Tomokatsu Morota, Hirotomo Noda, Rina Noguchi, Kazunori Ogawa, Kei Shirai, Eri Tatsumi, Hikaru Yabuta, Yasuhiro Yokota, Manabu Yamada, Masanao Abe, Masahiko Hayakawa, Takahiro Iwata, Masanobu Ozaki, Hajime Yano, Satoshi Hosoda, Osamu Mori, Hirotaka Sawada, Takanobu Shimada, Hiroshi Takeuchi, Ryudo Tsukizaki, Atsushi Fujii, Chikako Hirose, Shota Kikuchi, Yuya Mimasu, Naoko Ogawa, Go Ono, Tadateru Takahashi, Yuto Takei, Tomohiro Yamaguchi, Kent Yoshikawa, Fuyuto Terui, Takanao Saiki, Satoru Nakazawa, Makoto Yoshikawa, Seiichiro Watanabe & Yuichi Tsud

Thermal inertia of asteroid Ryugu using dawn-side thermal images by TIR on Hayabusa2

A thermal inertia map of the C-type Near-Earth asteroid 162173 Ryugu has been derived using the one-rotation global thermal image sets observed from the Home Position at 20 km altitude [1]. This time the thermal images of the night side areas of the surface just before sunrise were taken during observations from the dawn side. The coldest brightness temperature of the surface indicates another information on the thermal inertia of the surface. The thermal inertia is preliminary estimated at 250 [J m-2K-1s-0.5] or lower, which is consistent with other than those derived from the daytime observations [2]

Analysis of Solar Radiation Shading Effects by Trees in the Open Space around Buildings

Work and leisure in outdoor spaces influences people’s physiological and psychological, while it can also lead to a reduction of energy consumed via air conditioning and lighting. In this study, the effects of solar radiation shading by trees in the open space around buildings on microclimatic development and pedestrian radiation environment are evaluated through a case study on the redevelopment buildings in front of Central Osaka Station, based on both observation and calculation. In both open space and green garden, direct solar radiation is shielded mainly by the buildings located behind the trees rather than by the trees themselves. Solar radiation shielding by trees is necessary in the range of more than 10 m from the south side of the buildings and more than 6 m from the west or east sides of the buildings

In-orbit recalibration of Longwave Infrared Camera onboard Akatsuki

Abstract Akatsuki has been in operation since Venus orbit insertion-revenge 1 (VOI-R1) in December 2015 and has been making observations of Venus’ cloud-top temperature with Longwave Infrared Camera (LIR) since the start of nominal observations in April 2016. LIR was originally designed to maintain its performance for at least 4 years after the VOI originally planned in December 2010. Although the operation time of LIR has exceeded its designed lifetime as of August 2022, it is still functioning normally. The mechanical shutter plate has been kept at a normal temperature and used as a hot reference in determining the brightness temperature of objects when in the closed position. Since the observed temperature of the background deep space is merely a value representing the output for no radiation input, it should be the same in any observation. This was around 180 K just after the launch of Akatsuki in May 2010; however, it has gradually increased to approximately 200 K by February 2022. Average Venus disk temperatures also show a slight increasing trend. The increases of the background and Venus’ disk temperatures are most likely due to degradation of the sensitivity of the bolometer array used in LIR as an image sensor. These temperatures have apparently been increasing since LIR was activated in October 2016. While LIR is activated, the bolometer temperature is kept at 40 °C and a moderate baking effect may have accelerated degassing in the bolometer package, and the resulting increase of thermal conductivity or decrease of transmittance of the window contaminated by evaporated components may have degraded the sensitivity of the bolometer. A sensitivity degradation of 5% from October 2016 to February 2022 is estimated from the increasing trend of the background temperature. A correction has been made to the LIR data to keep the background temperature constant. The corrected data show no increasing trend in either the background or Venus’ disk temperature. The corrected data are open to the public as a more reliable dataset for investigating the long-term variability of thermal condition at cloud-top altitudes. Graphical Abstrac

How waves and turbulence maintain the super-rotation of Venus' atmosphere

Venus has a thick atmosphere that rotates 60 times as fast as the surface, a phenomenon known as superrotation. We use data obtained from the orbiting Akatsuki spacecraft to investigate how the super-rotation is maintained in the cloud layer, where the rotation speed is highest. A thermally induced latitudinal-vertical circulation acts to homogenize the distribution of the angular momentum around the rotational axis. Maintaining the super-rotation requires this to be counteracted by atmospheric waves and turbulence. Among those effects, thermal tides transport the angular momentum, which maintains the rotation peak, near the cloud top at low latitudes. Other planetary-scale waves and large-scale turbulence act in the opposite direction. We suggest that hydrodynamic instabilities adjust the angular-momentum distribution at mid-latitudes

Earth and moon observations by thermal infrared imager on Hayabusa2 and the application to detectability of asteroid 162173 Ryugu

Thermal Infrared Imager TIR on Hayabusa2 has proven its in-flight performance to detect celestial bodies during the cruise phase, especially by the observations of the Earth and the Moon before and after the Earth swing-by on 3rd of December 2015. The result indicates that the target C-type asteroid 162173 Ryugu will be detected from the distance of 3000 km at the beginning of the approach phase, and that a C-type small moon larger than 1 m will be detected from the home position, 20 km from the asteroid, if it orbits around the asteroid.Additional co-authors: Hayabusa2 TIR Tea