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[PPS03-P08] Investigation of a Method to Reconstruct Asteroid Shape from Hayabusa2# Flyby Observation Data
Keywords:Asteroid shape model, Flyby, Torifune
In July 2026, the asteroid explorer Hayabusa2 will conduct a flyby observation of an asteroid (98943) Torifune as its extended mission. Flyby observation is a method of making observations when a spacecraft passes by a target body; the observation period is short, and it can only observe one side of the asteroid shape. In contrast, Hayabusa2's previous observation of the asteroid (162173) Ryugu involved rendezvous observations, in which the spacecraft stayed around the asteroid and made observations. The observation period was long, and by utilizing the rotation of the asteroid, the direction of observation changed gradually, and eventually observations were made from the entire circumference of the asteroid. Reconstructing asteroid shape models from the explorer’s observed data is crucial. Because the shape model is an important basic data set for asteroids. In fact, Ryugu has also undergone shape reconstruction from observation data. However, this is the first time that Hayabusa2 will perform flyby observations, and it is necessary to consider how to reconstruct the asteroid shape from the restricted data obtained.
In this study, the effects of flyby conditions on shape reconstruction were investigated using data from Hayabusa2 observations of the asteroid Ryugu. Specifically, to reproduce the two constraints characteristic of flyby observations—small amounts of observation images due to a short observation period and observation images of the shape of only one side of the asteroid—shape reconstruction was carried out by varying the image interval and image coverage to confirm the changes. Here, the image interval refers to the angular change in the rotation phase when Hayabusa2 took images of the spinning Ryugu. For flyby observations, this corresponds to an angular change in the direction of the spacecraft as seen from the target asteroid. Similarly, image coverage refers to the extent of the rotation phase and is defined as the angular extent to which the camera shot covers the surface of the asteroid.
The commercial photogrammetry software Metashape was used for shape reconstruction. As a result, if images can be taken of Torifune with a diameter of approximately 500 pixels or more, Metashape is likely to be able to reconstruct the shape if the image interval is less than 3 deg and the image coverage is greater than 45 deg, or if the image interval is less than 12 deg and the image coverage is greater than 90 deg.
In this study, the effects of flyby conditions on shape reconstruction were investigated using data from Hayabusa2 observations of the asteroid Ryugu. Specifically, to reproduce the two constraints characteristic of flyby observations—small amounts of observation images due to a short observation period and observation images of the shape of only one side of the asteroid—shape reconstruction was carried out by varying the image interval and image coverage to confirm the changes. Here, the image interval refers to the angular change in the rotation phase when Hayabusa2 took images of the spinning Ryugu. For flyby observations, this corresponds to an angular change in the direction of the spacecraft as seen from the target asteroid. Similarly, image coverage refers to the extent of the rotation phase and is defined as the angular extent to which the camera shot covers the surface of the asteroid.
The commercial photogrammetry software Metashape was used for shape reconstruction. As a result, if images can be taken of Torifune with a diameter of approximately 500 pixels or more, Metashape is likely to be able to reconstruct the shape if the image interval is less than 3 deg and the image coverage is greater than 45 deg, or if the image interval is less than 12 deg and the image coverage is greater than 90 deg.