 Full paper
 Open Access
 Published:
Geometric correction for thermographic images of asteroid 162173 Ryugu by TIR (thermal infrared imager) onboard Hayabusa2
Earth, Planets and Space volume 73, Article number: 115 (2021)
Abstract
The thermal infrared imager (TIR) onboard the Hayabusa2 spacecraft performed thermographic observations of the asteroid 162173 Ryugu (1999 JU\(_3\)) from June 2018 to November 2019. Our previous reports revealed that the surface of Ryugu was globally filled with porous materials and had high surface roughness. These results were derived from making the observed temperature maps of TIR using a projection method onto the shape model of Ryugu as geometric corrections. The pointing directions of TIR were calculated using an interpolation of data from the SPICE kernels (NASA/NAIF) during the periods when the optical navigation camera (ONC) and the light detection and ranging (LIDAR) observations were performed. However, the mapping accuracy of the observed TIR images was degraded when the ONC and LIDAR were not performed with TIR. Also, the orbital and attitudinal fluctuations of Hayabusa2 increased the error of the temperature maps. In this paper, to solve the temperature image mapping problems, we improved the correction method by fitting all of the observed TIR images with the surface coordinate addressed on the highdefinition shape model of Ryugu (SFM 800k v20180804). This correction adjusted the pointing direction of TIR by rotating the TIR frame relative to the Hayabusa2 frame using a least squares fit. As a result, the temperature maps spatially spreading areas were converged within highresolved \(0.5^\circ\) by \(0.5^\circ\) maps. The estimated thermal inertia, for instance, was approximately 300\(\sim\)350 Jm\(^{2}\)s\(^{0.5}\)K\(^{1}\) at the hot area of the Ejima Saxum. This estimation was succeeded in case that the surface topographic features were larger than the pixel scale of TIR. However, the thermal inertia estimation of smooth terrains, such as the Urashima crater, was difficult because of surface roughness effects, where roughness was probably much smaller than the pixel scale of TIR.
Introduction
Hayabusa2 is a Japanese asteroid sample return mission that rendezvoused with the asteroid 162173 Ryugu (previously known as 1999 JU\(_3\)) from June 2018 to November 2019 (Tsuda et al. 2013). Ryugu is classified as a Ctype asteroid (Binzel et al. 2004), which is considered a parent body of carbonaceous chondrites. Based on the ground and space telescopic observations of Ryugu before the arrival of the Hayabusa2, its rotation period was found to be 7.63 h, and the thermal inertia was estimated to be 150\(\sim\)300 Jm\(^{2}\)s\(^{0.5}\)K\(^{1}\) (Müller et al. 2017).
After the arrival of Hayabus2, the observation using the nearinfrared spectrometer, NIRS3, suggested that the surface materials of Ryugu are similar to thermally or shockmetamorphosed carbonaceous chondrites (Kitazato et al. 2019). The optical navigation camera, ONC, found several characteristic features of the surface on the scale of centimeters to meters; they are large ridges, regolith deposits, and cracked rocks (Sugita et al. 2019). The gravitational observation indicated that the bulk density of Ryugu is 1.19 ± 0.02 gcm\(^{3}\) (Watanabe et al. 2019). The thermal infrared imager, TIR, suggested that the surface of Ryugu is filled with porous materials derived by the thermal inertia estimation (Okada et al. 2020). These results implied that Ryugu is composed of coalesced rubble piles.
TIR onboard Hayabusa2 is a thermographic camera with \(328 \times 248\) pixels resolution (Okada et al. 2017). Its sensor is an array of microbolometers, and one pixel has a size of 37 \(\mu\)m. The observation wavelength is integration energy ranges from 8 to 12 \(\mu\)m. The field of view is an angle of \(16.74^\circ \times 12.66^\circ\), and the spatial resolution is 0.051\(^\circ\)/pixel. The goal of TIR is to reveal the thermophysical states of the Ryugu surface, such as grain size and/or its porosity (e.g. Grott et al. 2020), which affects the orbit of the asteroid due to radiation force (Yarkovsky effect, e.g., Bottke et al. 2006). The thermophysical state is usually characterized by thermal inertia, which is written as \(\Gamma =\sqrt{\rho c_p k}\), where \(\rho\) is the bulk density, k is the thermal conductivity, and \(c_p\) is the specific heat.
Okada et al. (2020) estimated the global thermal inertia of Ryugu by comparing the calculation results of a thermophysical model, TPM1 (Takita et al. 2017), with the observed images on 1 August 2018 (the midaltitude observation; hereafter MidAlt observation). TPM1 utilized a shape model of Ryugu with 196,608 polygons (SFM 200k v20180804, Watanabe et al. 2019) whose polygon size was comparable to the pixel scale of TIR in the MidAlt observation (\(\sim\)5 m / pixel). TPM1 anticipated the surface temperature by calculating incident solar flux for all polygons, including secondary radiations and solar ray scatterings from surrounding polygons. The resulting global thermal inertia of Ryugu was roughly estimated to be 300 ± 100 Jm\(^{2}\)s\(^{0.5}\)K\(^{1}\). Nevertheless, TPM1 could not reproduce the observed temperature distribution of Ryugu; it generated higher temperature than the observed TIR images around the subsolar region of Ryugu and lower temperature at the dawn and dusk regions (\(\sim\)asteroid limb). The observed TIR images were broadly homogeneous over the hemisphere (e.g., Fig. 2 in Okada et al. 2020). This phenomenon is considered to be caused by surface roughness within a spatial resolution of the sensor (e.g. Groussin et al. 2007).
Shimaki et al. (2020) evaluated temperature profiles by using a thermophysical model that utilized a local rough surface model, TPM2, which successfully reproduced the observed temperature profiles of Ryugu. TPM2 simulates solar incident flux and thermal radiation from a local rough surface, which corresponds to be smaller than the spatial resolution of TIR. The temperature profiles of the local areas were obtained by projecting observed brightness temperature images for one asteroid rotation onto each polygon face of the shape model of Ryugu. By comparing the calculated and observed temperature profiles, the thermal inertia and surface roughness were derived; the mean value was estimated to be 225 ± 45 Jm\(^{2}\)s\(^{0.5}\)K\(^{1}\) with the mean RMS slope of 47 ± 5\(^{\circ }\). TPM2 produces only local temperature variation using the surface roughness model smaller than the pixel scale of TIR. Therefore, temperature profiles do not depend on the topographic shape of Ryugu, such as shadowing by large boulders (e.g. Ejima Saxum, Fig. 12 in Shimaki et al. 2020).
The quality of the observed temperature profiles, which depends on the accuracy of the image mapping onto a shape model, is important for comparing the observed and calculation results. The mapping was carried out using the numerical geometry tool kit, SPICE kernels (NASA/NAIF). It includes spacecraft attitudes, spacecraft altitudes, and the TIR alignment. The attitudes and altitudes were derived from data of the orbit control system (AOCS), ONC images, and measured altitudes by LIDAR. The pointing directions of TIR were calculated using interpolation of the data from SPICE kernels during the periods when the ONC or LIDAR observation was performed. However, the mapping accuracy of the observed TIR images was degraded when the ONC and LIDAR observations were not performed together with TIR. Also, the orbital and attitudinal fluctuations of Hayabusa2 increased the error of temperature maps.
The TIR alignment is defined by the TIR frame relative to the Hayabusa2 frame as Euler angles. The alignment was firstly determined in the Moon observation (Moon distance >763,000 km) during the Earth swingby (Okada et al. 2018); the resulting Euler angle was \((\theta _z, \theta _y, \theta _x) = (1.077^\circ , 180.080^\circ , 0.141^\circ )\) (hyb2_v09.tf, SPICE kernel). This Euler angle was a specific value obtained from the point source observation. We updated the alignment of TIR just after the arrival of Hayabusa2 using the observation data of MidAlt observation (Okada et al. 2020; Shimaki et al. 2020); the resulting value was \((\theta _z, \theta _y, \theta _x) = (1.052^\circ , 180.007^\circ , 0.115^\circ )\) (hyb2_v14.tf, SPICE kernel). This Euler angle was the mean value determined by using the same method of geometric correction in this study. However, we found that the value of \(\theta _z\), the rotation angle of the TIR boresight, has a bimodal path in the clockwise and the counterclockwise rotations. This value was specific and inadequate for other observation data. Therefore, determining the precise alignment and pointing directions are required for improving the image mapping of TIR.
In this paper, to improve the mapping quality of observed images of TIR, we developed a geometric correction method by performing a onetoone correspondence between the observed points in the TIR images and the surface coordinate addressed on the highdefinition shape model of Ryugu. We show the corrected projection maps and discuss the temperature and thermal inertia estimations, which depend on the topographic variation of the Ryugu surface.
Data
We adapted a geometric correction for all TIR images observed in the early major observations period (BoxA on 20180710, BoxC on 20180720, MidAlt on 20180801, and BoxB on 20180831). The nominal observations of Hayabusa2 were called BoxA, B, and C (Watanabe et al. 2019). The BoxA (Home Position) was performed at an altitude of around 20 km at the subEarth point. The BoxB was performed at an altitude of 20 km, moving the horizontal direction within 9 km. The BoxC was performed at a low altitude below 20 km at the subEarth point. Besides the Box observations, several observations were carried out at low altitudes, such as the MidAlt operation at an altitude of 5 km and the MASCOT deployment operation at an altitude of about 50 m (Jaumann et al. 2019).
In this study, we used a highdefinition shape model of 786,432 polygons (SFM 800k v20180804) derived from ONC observations, which was smaller than the TIR pixel scale; our previous studies used the model of 196,608 polygons (SFM 200k v20180804), which is the comparable size of the TIR pixel scale (Okada et al. 2020; Shimaki et al. 2020).
TIR images
TIR took thermal images of Ryugu as brightness temperature, which is represented as thermal emission from a black body in radiative equilibrium with sunlight (e.g., Hapke 2012). The observed data of TIR were converted from digital data to brightness temperature using a calibration database “HEAT” (Endo et al. 2017). The calibration table was constructed in prelaunch experiments (Okada et al. 2020; Arai et al. 2017). The observed data are released in JAXA/DARTS (Yamamoto et al. 2016) and NASA/PDS, which are raw digital images (Level 1 product) and brightness temperature images (Level 2 product).
Shape model of Ryugu
The surface terrain of Ryugu has been modeled by numerical shape models constructed from ONC images (Watanabe et al. 2019). The shape models were derived by a structure from motion (SfM) method (e.g., Szeliski 2010) and a stereo photoclinometry (SPC) method (Gaskell et al. 2010). The 3M, 800k, and 200k shape models of Ryugu are composed of 3,145,728, 786,432, and 196,608 triangles; the sizes of the polygon triangles are tens centimeters, about 1 meter, and a few meters, respectively. We used the 800k model (SfM) in this study because the polygon size was small enough for TIR analysis and the SPICE kernel of this SFM model was compatible with TIR analysis.
SPICE kernels
NASA’s numerical toolkit for the geometry system of the space mission called SPICE kernels (NASA/NAIF) for the Hayabusa2 mission has been released on the website of JAXA/DARTS. The SPICE kernels include the spacecraft information on trajectories derived from data of AOCS and LIDAR. The alignment information of TIR is included in a frame kernel (FK) of the SPICE kernel. The observation coordinate of TIR could be calculated by complementing data of ONC and LIDAR observation; TIR and ONC nominally took images alternately. However, the pointing coordinate of TIR was uncertain when observations with ONC and LIDAR were not performed.
We analyzed data of early major observations (BoxA, BoxC, MidAlt, and BoxB) using version 03 of the SPICE kernel, released in March 2020. Version 03 kernels contain the improved trajectory of Hayabusa2 derived from LIDAR (Matsumoto et al. 2020) but do not include the relative position and attitude between Ryugu and Hayabusa2 derived from the shape model. The new version of the SPICE kernels for Hayabusa2 will be released in 2021 and includes a precise attitude of Hayabusa2 derived from the shape model, along with the results of this study.
Method: geometric correction
To fit positions observed by TIR to the coordinates of a shape model, rotating the TIR image frame using the SPICE toolkit (N0066) is performed. The temperatures of the observation images are represented by T(i, j) in the image pixel coordinate, where i and j are the index of pixels (\(i=1\sim 328\), \(j=1\sim 248\)), respectively. On the other hand, the observed brightness temperatures projected on polygons of a shape model are represented by T(X, Y, Z) in the Cartesian coordinate of the Ryugu system (Fig. 1), where the positional vector of the polygon faces are denoted by \(\mathbf{V }(X, Y, Z)\).
Projection from image to shape
The brightness temperature projection from a TIR image to a shape model of Ryugu is expressed as a coordinate transformation from the pixel coordinate to the Ryugu coordinate. We assume that the observed brightness temperature of a pixel value is the mean temperature of the observed region because the TIR pixel detects total radiation fluxes from a region of the Ryugu surface (e.g. \(\sim\)5 m squares in MidAlt observation). The brightness temperature value of an image is directly converted to a shape model as \(T(i, j) \rightarrow T(X, Y, Z)\). This transformation is represented by using the vector sum of positional vectors. A ray vector \(\mathbf{V }_{\mathrm{R}}\) from a polygon of the shape model of Ryugu to TIR is written as \(\mathbf{V }_{\mathrm{R}} = \mathbf{V }_{\mathrm{T}}  \mathbf{V }_{\mathrm{P}}\), where \(\mathbf{V }_{\mathrm{P}}\) and \(\mathbf{V }_{\mathrm{T}}\) are position vectors of the polygon and TIR, respectively (Fig. 1). The ray vector in the coordinated system of Ryugu \(\mathbf{V }_{\mathrm{R}}\) is transformed into that of the coordinate system of Hayabusa2 \(\mathbf{v }_{\mathrm{R}}\) using a coordinate conversion tool of the SPICE kernels, as \(\mathbf{V }_{\mathrm{R}} \rightarrow \mathbf{v }_{\mathrm{R}}\).
A normalized boresight vector of TIR in the coordinated system of TIR is \((x, y, z)=(0, 0, 1)\). The frame of TIR is the coordinate system of Hayabusa2 that rotated the angle of \(180^\circ\) in the yaxis (Fig. 1). The focal point length of the TIR is 42.2 mm, and thus the positional vector of the focal point is written as \(\mathbf{v }_{\mathrm{F}}(x, y, z)=(0, 0, 0.0422)\). The incident rays from polygons of the shape model to TIR intersect the image plane of TIR at the positional vector \(\mathbf{v }_{\mathrm{I}}\), where \(\mathbf{v }_{\mathrm{I}} = \mathbf{v }_{\mathrm{R}} + \mathbf{v }_{\mathrm{F}}\). The pixel size of the TIR sensor is 37 square \(\mu\)m, and thus the scalar value \(\mathbf{v }_{\mathrm{I}}(x, y, z)\) normalized by the pixel size is equivalent to the pixel coordinate (i, j).
Projection from shape to image
The projection temperature from a shape to an image is written as an inverse process of the image projection to shape coordinate. However, this reprojection process is irreversible because it is not onetoone correspondence as \(T(X, Y, Z) \nrightarrow T(i, j)\). Here, the radiant flux F from the polygons to a pixel of the image as the sum of their radiations is calculated, using the StefanBoltzmann law and the scattering component of the solar ray, as follows:
where \(\epsilon _k\) is the emissivity, \(\sigma\) is the Boltzmann constant, \(s_k\) is the area of the polygons, \(\phi _k\) is the emission angle to the TIR direction, and k denotes the index of the polygon. The observation area for one pixel is written as the sum of the polygon areas, as follows:
We neglect the scattering component because the intensity of the radiation flux in the 8 to 12 \(\mu\)m wavelength range is more than two orders of magnitude smaller than that of the infrared radiation of a body around 300 K. We assume a constant emissivity for each polygon face. Thus, the reimaged temperature \(\tau\) for a pixel is rewritten using Eqs. (1) and (2), as follows:
Fitting alignment
The TIR frame \(\mathbf{v }_{\mathrm{T}}(x, y, z)\) is defined based on the Hayabusa2 frame \(\mathbf{v }_{\mathrm{H}}(x_o, y_o, z_o)\). The frame transformation is described using the Euler rotation, as follows:
where \(\mathrm{R}_{z}\), \(\mathrm{R}_{y}\), and \(\mathrm{R}_{x}\) form the Euler rotation matrix in the righthanded coordinate system. The matrix \(\mathrm{R}_{z}\) is the rotation of the TIR boresight vector, which has a bimodal path in the clockwise and the counterclockwise rotations. In this study, \(\mathrm{R}_{z}\) is fixed as a unit matrix because this parameter is not effective in minimizing the converge value of the least squares fittings. Hence, Eq. (4) is written, as follows:
where \(\theta _x\) and \(\theta _y\) are the Euler angles.
TIR observation points are determined by fitting the observation image to the projection image and onto the shape model of Ryugu. This adjustment is performed by minimizing the difference between the observed temperature image T(i, j) and the reprojected temperature image from the Ryugu shape \(T'(i, j)\) using the least squares fit, as follows:
where RSS is a residual sum of squares in temperature. The free parameters of this fitting are the Euler angles of \(\theta _y\) and \(\theta _x\) in Eq. (5). The fitting is performed by using the algorithms of the LevenbergMarquardt and Simplex fitting method (e.g. Press et al. 2007). The fitting converged value of RSS is ideally zero.
Figure 2 shows an example of the geometric correction procedure for an image (MidAlt observation, 23:07), which is the observed image projected onto the shape model and reprojected onto images. The bestfit Euler angles as free parameters of \(\theta _y\) and \(\theta _x\) are determined by reducing the residuals of the observed image and the reproduced image.
Results
The geometric correction was performed for all observed images of TIR in the early major observations. Table 1 shows the bestfit alignment values (Euler angle), along with the observation configuration for each observation, such as the solar distance, the solar phase angle, the subspacecraft latitude, the spacecraft altitude, and the number of observed images. The error of the Euler angles is the standard deviation of each bestfit value.
Figure 3a and b show the bestfit Euler angles of \(\theta _y\) and \(\theta _x\) in time series for all images of each observation. The resulting Euler angles were increased (decreased) along with Hayabusa2 attitude while the Ryugupointing was not performed, such as BoxA (20180710) and MidAlt (20180801) observations. On the other hand, the Euler angles were stable in the observations of BoxC (20180720) and BoxB (20180831) because the Ryugupointing was continuously performed. Ryugu scanning observations were carried out in BoxC to measure the sensitivity of the TIR’s angle of view, and thus the Euler angles had the bimodal feature in BoxC. Figure 3c shows the difference of the TIR observed point (footprint) between before and after the correction results in the Ryugu’s geographic coordinate; the alignment value before the correction was the inverse frame of the Hayabusa2 yframe \((\theta _z, \theta _y, \theta _x) = (0^\circ , 180^\circ , 0^\circ )\). Thus, this figure implies the relative difference between the TIR frame and the Hayabusa2 frame. Although large differences were seen in the resulting values of BoxA and MidAlt observations, caused by the attitude changes of Hayabusa2, that of BoxB was stable. Therefore, we believe that the resulting value of BoxB represented the accurate alignment value of TIR. We concluded that the alignment value of TIR is \((0.00^\circ , 180.00^\circ , 0.08^\circ )\) and release this value as a new FK of the Hayabusa2 SPICE kernels in 2021.
Figure 4 shows the maximum temperature maps projected onto Ryugu maps with \(0.5^\circ\) by \(0.5^\circ\) gridded before and after the geometric correction (BoxA, BoxC, MidAlt, and BoxB). These maximum temperatures were merged and extracted, combined with all observed images of the Ryugu’s one rotation period obtained with the observed time interval of 76 s. We can see that the temperature maps spatially spreading areas are converged in the map of \(0.5^\circ\) gridded after the correction; for instance, the temperature of the splitting area at the Ejima Saxum before the correction (left figure of 20180801) is about 20 K larger than the surrounding temperature, but the area is successfully converged after the correction (middle figure of 20180801). This correction is important for thermal inertia estimation because the maximum temperature of the temperature profile affects the derived value of thermal inertia. The geometric correction succeeded in detailed map construction with the low altitude observations (BoxC and MidAlt), and the correction was also effective for observed data at a high altitude observation of 20 km (BoxA and BoxB) because the spreading temperature areas were more converged.
Discussion
The geometric correction carried out the temperature map construction within the accuracy of the pixel scale of TIR; the spatially spreading temperature areas were converged in \(0.5^\circ\) by \(0.5^\circ\) gridded after the correction. Here, we discuss the reproducibility of the observed temperature projected onto the shape model and map, compared with numerical simulation results, such as the specific region of Ejima Saxum and Urashima crater.
Figure 5a shows an example of one image projected on a map using the shape model of Ryugu (SFM 800k v20180804). Figure 5b and c show examples of the temperature profiles of the hot areas of the Ejima Saxum and Urashima crater in time series, derived from the temperature map projected onto a Ryugu map using 31 observed images with \(0.5^\circ\) by \(0.5^\circ\) gridded. The profile before and after the correction varied in the Ejima Saxum as a characteristic area, but the profile was not significantly changed in the Urashima crater as a smooth area (top Figures of 5b and c). Therefore, the geometric correction is important for the detailed estimation of the temperature profile in topographically characteristic areas, such as the Ejima Saxum; the difference of the profile will cause the error of thermal inertia estimation at least \(\sim\) 50 Jm\(^{2}\)s\(^{0.5}\)K\(^{1}\).
The observed temperature profiles of the Ejima Saxum and the Urashima crater and the simulated temperature profiles derived from TPM1 (Takita et al. 2017) in the thermal inertia steps of 100 Jm\(^{2}\)s\(^{0.5}\)K\(^{1}\) are shown in bottom Figures of 5b and c. The thermal inertia of the Ejima Saxum was estimated to be 300 Jm\(^{2}\)s\(^{0.5}\)K\(^{1}\) except the temperature profile at dawn time (\(\sim\)14:30) and dusk time (\(\sim\)17:00). On the other hand, the thermal inertia of the Urashima crater was approximately 200 to 400 Jm\(^{2}\)s\(^{0.5}\)K\(^{1}\). This difference between the observed profile and the simulated profiles of the Urashima crater was also probably caused by the surface roughness. These results indicate that the brightness temperatures of observed areas larger than a TIR pixel scale, such as characteristic areas, are dependent on largescale surface topography that can be reproduced by TPM1. On the other hand, the brightness temperatures of observed areas smaller than a TIR pixel scale, such as smooth areas, are dependent on smallscale roughness is required to be reproduced by TPM2 (Shimaki et al. 2020).
Figure 6 shows the observed image and the anticipated image reproduced by TPM1 using the shape model (SFM 800k v20180804). The hot regions are found at the bottom of Ejima Saxum and Otohime Saxum in both of the observed and the anticipated images (see the closedup images of ONC, Figs. 1(B) and 2(A) Sugita et al. 2019). The simulation parameters of TPM1 were assumed to be an emissivity of 1.0, an albedo of 0.045 (Sugita et al. 2019), and a thermal inertia of 350 Jm\(^{2}\)s\(^{0.5}\)K\(^{1}\). The observed image (Fig. 6a) is similar to the anticipated image (Fig. 6b) at the hot areas. This model involves the effect of secondary radiation, and thus this result implies that the secondary radiations between surrounding terrains generate the hot areas. Although the thermal inertia at the bottom of Ejima Saxum resulted in the thermal inertia profile of approximately 300 Jm\(^{2}\)s\(^{0.5}\)K\(^{1}\) (Fig. 5b), this anticipated image was reproduced well by using the value of 350 Jm\(^{2}\)s\(^{0.5}\)K\(^{1}\). Also, the globally homogeneous temperature of the observed image is not reproduced by TPM1. These differences are probably caused by surface roughness because the surface roughness can flatten the whole temperature of the surface.
We conclude that the development of a thermophysical model, including a hybrid model that implements topography (TPM1) and smallscale surface roughness (TPM2), is required.
Summaries
The accurate alignment value of TIR was determined, \((\theta _z, \theta _y, \theta _x)=(0.00^\circ , 180.00^\circ , 0.08^\circ )\). This positioning accuracy of TIR was improved equivalent to a pixel scale (\(\sim\) 5 m at an altitude of 5 km observation). The resulting alignment value is released as the new version FK of the Hayabusa2 SPICE kernels. The higher degree products of TIR constructed by using the geometric correction are also released on the website of JAXA/DARTS and NASA/PDS in 2021.
The observed temperature maps—which were projected onto the Ryugu map spatially spreading areas are improved by the geometric correction and constructed with highresolved 0.5° by 0.5° gridded.
The thermal inertia estimation of a characteristic area was succeeded in case that the surface topographic features larger than the TIR pixel scale, which the temperature profile can be reproduced by TPM1. In contrast, the thermal inertia estimation of the smooth area was inaccurate. This phenomenon was probably caused by surface roughness much smaller than the TIR pixel scale; such temperature profiles can be reproduced by TPM2. We conclude that the development of a thermophysical model, including a hybrid model that implements topography and smallscale surface roughness, is required to reveal the actual surface state of Ryugu.
Availability of data and materials
The original data of this study are available and downloadable on the following website:
TIR data: https://www.darts.isas.jaxa.jp/pub/hayabusa2/tir_bundle/browse/
SPICE kernels: https://www.darts.isas.jaxa.jp/pub/hayabusa2/spice_bundle/document/spiceds_v001.html.
Abbreviations
 AOCS::

Attitude and Orbit Control System
 DARTS::

Data Archive and Transmission System
 FK::

Frame Kernel
 JAXA::

Japan Aerospace Exploration Agency
 LIDAR::

Light detection and ranging
 MASCOT::

Mobile Asteroid Surface Scout
 MidAlt::

Middle Altitude
 NAIF::

Navigation and Ancillary Information Facility
 NASA::

National Aeronautics and Space Administration
 NIRS3::

Near infrared spectrometer
 ONC::

Optical navigation camera
 RSS::

Residual sum of squares
 SC::

Spacecraft
 SfM::

Structure from motion
 SPC::

Stereo photoclinometry
 SPICE::

Spacecraft Planet Instrument Cmatrix Events
 TIR::

Thermal infrared imager
 TPM::

Thermophysical model
 UTC::

Coordinated Universal Time
References
Arai T et al (2017) Thermal imaging performance of TIR onboard the Hayabusa2 Spacecraft. Space Sci Rev 208:239–254. https://doi.org/10.1007/s1121401703539
Binzel RP et al (2004) Observed spectral properties of nearearth objects: results for population distribution, source regions, and space weathering processes. Icarus 170(2):259–294. https://doi.org/10.1016/j.icarus.2004.04.004
Bottke WF Jr et al (2006) The Yarkovsky and YORP effects: implications for asteroid dynamics. Annu Rev Earth Planet Sci. 34:157–91
Endo K et al (2017) HEAT: Image and database browser for the thermal imager on Hayabusa2. IEEE Aerospace Conference 1–10. https://doi.org/10.1109/AERO.2017.7943827
Gaskell RW et al (2010) Characterizing and navigating small bodies with imaging data. Meteor Planet Sci. 43(6):1049–1061. https://doi.org/10.1111/j.19455100.2008.tb00692.x
Grott et al (2020) Macroporosity and grain density of rubble pile asteroid (162173) Ryugu. J Geophys Res. https://doi.org/10.1029/2020JE006519
Groussin O et al (2007) Surface temperature of the nucleus of Comet 9P/Tempel 1. Icarus 187(1):16–25. https://doi.org/10.1016/j.icarus.2006.08.030
Hapke (2012) Thermal emission and emittance spectroscopy, Theory of Reflectance and Emittance Spectroscopy. Cambridge University Press 412–439. https://doi.org/10.1017/CBO9781139025683
Jaumann R et al (2019) Images from the surface of asteroid Ryugu show rocks similar to carbonaceous chondrite meteorites. Science 365:817–820. https://doi.org/10.1126/science.aaw8627
Kitazato K et al (2019) The surface composition of asteroid 162173 Ryugu from Hayabusa2 nearinfrared spectroscopy. Science 364(6437):272–275. https://doi.org/10.1126/science.aav7432
Matsumoto K et al (2020) Improving Hayabusa2 trajectory by combining LIDAR data and a shape model. Icarus. https://doi.org/10.1016/j.icarus.2019.113574
Müller TG et al (2017) Hayabusa2 mission target asteroid 162173 Ryugu (1999 JU3): searching for the object’s spinaxis orientation. Astron Astrophys 599:A103. https://doi.org/10.1051/00046361/201629134
Okada T et al (2017) Thermal infrared imaging experiments of Ctype asteroid 162173 Ryugu on Hayabusa2. Space Sci Rev 208:255–286. https://doi.org/10.1007/s1121401602868
Okada T et al (2018) Earth and moon observations by thermal infrared imager on Hayabusa2 and the application to detectability of asteroid 162173 Ryugu. Planet Space Sci 158:46–52. https://doi.org/10.1016/j.pss.2018.05.007
Okada T et al (2020) Highly porous nature of a primitive asteroid revealed by thermal imaging. Nature 579:518–522. https://doi.org/10.1038/s4158602021026
Press WH et al (2007) Modeling of data, numerical recipes, 3rd edn. Cambridge University Press, Cambridge, pp 773–839
Shimaki Y et al (2020) Thermophysical properties of the surface of asteroid 162173 Ryugu: infrared observations and thermal inertia mapping. Icarus. https://doi.org/10.1016/j.icarus.2020.113835
Sugita S et al (2019) The geomorphology, color, and thermal properties of Ryugu: Implications for parentbody processes. Science. https://doi.org/10.1126/science.aaw0422
Szeliski R (2010) Computer vision: algorithms and applications. Berlin, Springer
Takita J et al (2017) Feasibility and accuracy of thermophysical estimation of asteroid 162173 Ryugu (1999 JU\(_3\)) from the Hayabusa2 thermal infrared imager. Space Sci Rev 208:287–315. https://doi.org/10.1007/s112140170336x
Tsuda Y et al (2013) System design of the Hayabusa 2  asteroid sample return mission to 1999 JU\(_3\). Acta Astronaut 91:356–362
Watanabe S et al (2019) Hayabusa2 arrives at the carbonaceous asteroid 162173 Ryugu  a spinningtopshaped rubble pile. Science 364(6437):268–272. https://doi.org/10.1126/science.aav8032
Yamamoto Y et al (2016) Scientific data archives in Hayabusa2 mission. Trans JSASS Aerospace Tech Japan 14:151–154. https://doi.org/10.2322/tastj.14.Pk_151
Acknowledgements
We are grateful to all members of the Hayabusa2 team and thank NEC Inc. Co., who made the flight model of TIR. We thank Dr. Tsuneo Matsunaga of National Institute for Environmental Studies, who provides us computer resources.
Funding
The JSPS Kakenhi JP17H06459 partly supports TO.
Author information
Affiliations
Contributions
TA: Preparation of the manuscript, Methodology, and Data analysis; TO: Methodology, and Administration of TIR science team; ST, TF: Administration of TIR science team; HD: Software development; TK, YS, HS, TS: Methodology; NS, JT: Data analysis; NH: Software development; YY: Science operation of the spacecraft. All authors read and approved the final manuscript.
Corresponding author
Ethics declarations
Competing interests
The authors declare that they have no competing interests.
Additional information
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Rights and permissions
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
About this article
Cite this article
Arai, T., Okada, T., Tanaka, S. et al. Geometric correction for thermographic images of asteroid 162173 Ryugu by TIR (thermal infrared imager) onboard Hayabusa2. Earth Planets Space 73, 115 (2021). https://doi.org/10.1186/s4062302101437w
Received:
Accepted:
Published:
Keywords
 Hayabysa2
 Asteroid
 Ryugu
 Thermal infrared imager
 TIR
 Shape model
 Thermal model
 Surface roughness