## 4 TIR Observations in Asteroid Proximity Phase
### Global Thermal Images of Ryugu  
The first global disc-resolved thermal image set of an asteroid in history was taken by TIR for one asteroid rotation on 30 June 2018, just after arrival at the asteroid. The surface temperature in the daytime was around 300 to 370 K at a solar distance of 0.986 au, at the Sun-Probe-Earth (SPE) angle of 18.6\({}^{\circ}\). Comparison with calculated thermal images [25; 26] shows that the surface of Ryugu is colder than in the case of fine regolith surface where the thermal inertia is about 50 tiu and hotter than in the case of base rocks whose typical thermal inertia is higher than 1000 tiu [4; 20]. It was interesting that the temperatures of large boulders with several tens of meters or larger size on Ryugu are almost equal to those of the surrounding surfaces [4]. This was inconsistent with the original idea before arrival, where the large boulders should have high thermal inertia larger than 1000 tiu and looks like \"cold spots\", i.e., much colder than the surrounding [4].  
It was also found that the diurnal temperature profiles of the surface of Ryugu are rather constant compared with the anticipated profiles for a simple thermal calculation where a one-layer homogeneous material and a flat surface without roughness being assumed [4]. The thermal inertia of most geologic sites on Ryugu ranged around \(300\pm 100\) tiu [4; 26], which was derived from the maximum temperature and its delay from the local noon. This implies that the surface physical state of Ryugu is consistent with very porous and rough surface, where boulders have high porosity (30% to 50 %) and the rest of the surrounding surface is dominated by fragments of porous rocks several centimeters larger than the thermal skin depth [4]. It is important that the surface physical state predicted from the global thermal images was confirmed later by the close-up thermal imaging by TIR and also by the optical surface imaging and thermal radiation measurements from the surface lander Mobile Asteroid Surface Scout (MASCOT) [27; 28].  
Most thermal images are taken from the low SPE angles but there are large solar phase angle observation campaigns to view the asteroid from the oblique direction. Figure 3 shows an example of the oblique thermal image of Ryugu taken by TIR at SPE angle of 42\({}^{\circ}\), at rotation phase of 105\({}^{\circ}\)E, where the largest crater Urashima was found at equator of the asteroid (center-left). The flat diurnal temperature profiles and the temperature gradient at the subsolar to the right side should be well investigated since they are influenced by the roughness effect [26].  
\begin{table}
\begin{tabular}{c c c c} \hline
**Date of Observation** & **Distance from Sun [au]** & **Sun–Probe–Earth Angle [deg]** & **Distance from Ryugu [km]** \\ \hline
10 June 2018 & 0.9650 & 17.02 & 1550 \\
11 June 2018 & 0.9655 & 17.09 & 1380 \\
12 June 2018 & 0.9661 & 17.20 & 1150 \\
13 June 2018 & 0.9667 & 17.28 & 1000 \\
15 June 2018 & 0.9681 & 17.45 & 600 \\
16 June 2018 & 0.9689 & 17.53 & 500 \\
17 June 2018 & 0.9698 & 17.63 & 350 \\
18 June 2018 & 0.9707 & 17.71 & 200 \\
19 June 2018 & 0.9716 & 17.78 & 170 \\
20 June 2018 & 0.9727 & 17.87 & 100 \\
22 June 2018 & 0.9749 & 18.01 & 49 \\
29 June 2018 & 0.9843 & 18.48 & 20 \\ \hline \end{tabular}
\end{table}
Table 2: Daily observations of asteroid Ryugu by TIR during the Approach Phase.