## 1 Introduction  
The Thermal Infrared Imager TIR [1] (see Figure 1, Table 1) is a remote sensing instrument on Hayabusa2, the Japanese second sample return mission from the asteroid [2], which is based on a two-dimensional uncooled micro-bolometer array with \(328\times 248\) effective pixels, with \(16.7^{\circ}\times 12.7^{\circ}\) field of view, and covering 8-12 \(\upmu\)m wavelength range using a germanium lens and a band pass filter [1]. The absence of a cooling system in the TIR contributes to its small, lightweight, and long-living design. The basic design of the sensor unit (TIR-S), the digital electronics (DE), and the onboard data processing algorithm is inherited from the Longwave Infrared Camera from the Akatsuki Venus mission [3], but the wide detection range of temperature, observation sequences and plans, programming of data processing, and calibration methods are optimized for the TIR [4]. As with a deep-space mission, the maximum downlink rate is limited to within 32 Kbps, and onboard processing is essential for its design [1,2]. Asteroids are primitive bodies of the Solar System and probably the parent bodies of meteorites. Planetary formation started with fluffy dust in the solar nebula, followed by accumulation of dust to planetesimals, or re-accretion to rubble-piles from fragments by meteoroid impacts. The formation and evolution processes will be indicated by the surface physical state of asteroids or planets, such as grain size, porosity, boulder abundance, and roughness, which are informed by thermal infrared observations. Thermophysical models of asteroids [5] have been investiinterpreting those ground observations in thermal infrared wavelengths, but more detailed models are needed for quantitative analysis using data from a spacecraft [1,4]. Remote thermal radiometry is an often-used technique in planetary missions [6; 7; 8; 9; 10; 11] to investigate the physical state of a planetary surface layer, especially in its particle size distribution of regolith and boulder abundance, which are derived from thermal inertia, \(\sqrt{pkc_{p}}\), with density \(\rho\), thermal conductivity \(k\), and heat capacity \(c_{p}\). Thermal inertia is an index of thermophysical properties that \begin{table}
\begin{tabular}{c c} \hline \hline
**Items** & **Performance** \\ \hline Total Mass & 3.28 kg (DE is not included) \\ Total Power & 18 W (nominal) (DE is not included) \\ Uncooled bolometer array NEC 320A (VO) \\ Detector & (anti-reflection coating) \\ Pixels & \(344\times 260\) (effective \(328\times 248\)) \\ Field of View (FOV) & \(16.7\lx@math@degree\times 12.7\lx@math@degree\) \\ IFOV & 0.89 mrad (0.051\({}^{\circ}\)) \\ Aperture & 42 mm (effective), lens diameter is 47 mm \\ MTF (@Nyquist Freq.) & 0.5 \\ F-number & 1.4 \\ Temperature range & 233–423 K (well calibrated), \\  & 150–460 K (detectable range) \\ NETD & \(<\)0.3 \\ Absolute Temperature Range & \(<\)3 K \\ A/D Converter & 12 bit (15 bit after summed) \\ Reference Temperature & Shutter temperature (monitored, nominally at 301 K) \\ Frame Rate & 60 Hz \\ Image Summation & 2’m, m = 0 to 7 (1,2, \(\ldots\),128) \\ Consecutive shot numbers & Programable (max 128 images), current programed: 30 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristic performance of TIR on Hayabusa2 [1]. Figure 1: The flight model of the sensor unit of the thermal infrared imager TIR onboard Hayabusa2 (TIR-S). White squares on the sensor case are the Velcro tapes to attach the multi-layer insulators (MLI, the thermal blankets, not shown) which cover the sensor unit to stabilize the temperature during the operation. A baffled sunshade (not shown) is attached in front of the germanium lens to avoid insolation. The aperture of the lens is 42 mm in diameter (diameter of lens is 47 mm), and the stepping motor for the mechanical shutter is seen at the right hand of the lens. The sheet heater around the lens case is to adjust the temperature of the optics. The scale on the name plate is 10 cm long (1 cm each). (©)Japan Aerospace Exploration Agency (JAXA)).