We conducted observations of asteroid Ryugu using the Thermal Infrared Imager (TIR) onboard Hayabusa2. Figure 1 shows the thermographic image taken using the TIR at an altitude of 20 km (the home position) from Ryugu.
The image shows the temperature differences on Ryugu's surface during one rotation, with red indicating regions with a high temperature. Distinct regions at different temperatures are captured by the TIR. Features in a thermal image can be seen even if they are in a shaded location in the visible photograph. This lets us confirm that the overall shape of the asteroid is well understood, and also the characteristic topography such as craters and large boulders that show up as a difference in temperature.
A temperature difference can also be seen between the north and south hemispheres of the asteroid. At present, it is summer in the southern hemisphere (the upper part of the figure) and the temperature is higher in this region. In the northern hemisphere in the lower part of the figure, it is currently winter and colder. This difference is due to the inclination of the rotation axis, which results in different levels of radiation reaching the north and south. The TIR has therefore spotted that asteroids also undergo a “seasonal change”.
High temperatures on the asteroid reach 100°C, while the coldest regions sit at about room temperature. Temperatures also change depending on the solar distance of the asteroid, lowering as Ryugu moves further away from the Sun.
During the Hayabusa2 mission, we will investigate the formation process of asteroids by examining the characteristics of the surface material revealed by differences in surface temperature. From the TIR data, we can also look for scientifically important landing sites with millimeter-sized grains, and avoid landing Hayabusa2 in severe temperature environments or locations with obstacles such as boulders.
Reference: An article by Takehiko Arai, member of the TIR team, was posted on the Ashikaga University website (Japanese only).
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