## 6 Concluding Remarks  
During the asteroid proximity phase of Hayabusa2, thermography was applied for the first time in planetary missions. Thermal infrared imaging by TIR has been conducted through the mission to investigate C-type near-Earth asteroid 162173 Ryugu, for achieving both scientific and mission objectives [4; 19; 23] (see Supplementary Materials). Much progress has been made in asteroid science, offering a new insight for understanding planetary formation scenario and hypothesizing a highly porous nature of small bodies in the current and probably ancient Solar System [4]. Global, local, and close-up thermal images by the TIR have unveiled the thermophysical properties of the surface of Ryugu and provided us with new technical information and experiences. The observations of TIR on Hayabusa2 mainly from the stational home position for asteroid one-rotation are very helpful for data interpretation even for the boulder-rich rough surface, and are achievable for the mission success.  
The limitation of this method using the TIR on Hayabusa2 or the instruments based on the same type of uncooled micro-bolometer array is the minimum detection temperature of 150 K or higher. This is mainly caused by the filtered wavelength range of 8-12 \(\upmu\)m and the background thermal radiation from optics. The wavelength of typical commercial-based bolometer arrays ranges 8-14 \(\upmu\)m, which is not substantially different from the current one. A two-dimensional detector to cover 20 \(\upmu\)m or longer wavelengths would detect the thermal radiation from the colder planetary surface at a lower temperature such as the dark side of the Moon, any asteroid within the main belt, or the satellites of Jupiter. The thermal design to cool the optics might be feasible by passive radiative cooling or using a state-of-the-art active cooling system.  
Thermography has been proven during the Hayabusa2 mission as a potentially useful and applicable tool for future space and planetary missions, for applications such as (1) optical navigation for approaching the distant target body, (2) detection of meteoroids or small satellites (ejected rocks) for hazardous protection of spacecraft, (3) target marker tracking for precise navigation and guidance for smart landing onto the surface of the target bodies, (4) tracking or navigating deployable small robots or landers against the backgrounds of planetary surfaces, (5) operations in the night or in the dark area, such as the dark side of the Moon, (6) landing site selection for determining the best site for sampling and for safety operations in hazardous protect or in thermal conditions, and (7) guidance and navigation of spacecraft around the asteroid or planetary satellites, such as orbiting the spacecraft. Some of these demonstrated technology should be applied in the future missions to the Moon, Mars, asteroids, comets, and other possible targets.  
TIR raw data and temperature converted data with ancillary data are available or will be available online at the Hayabusa2 data site: darts.isas.jaxa.jp/pub/hayabusa2/tir_bundle/browse/. The timing of archival for each data is basically one year after acquisition of the data.  
This work is partially supported by the JSPS Kakenhi No. JP17H06459 (Aqua Planetology).  
The author appreciate all the members of Hayabusa2 project and the people who contributed to its related development, operations, and scientific studies for fruitful discussions and technical supports. The author would give special thanks to the members of TIR teams.