JAXA’s Hayabusa2#: Flyby observations and characterizations of Asteroid (98943) Torifune

JAXA’s Hayabusa2#, the extended mission of Hayabusa2, will fly by the target asteroid (98943) Torifune on July 5, 2026. Telescopic observations have suggested that the asteroid is about 400 m in diameter and highly elongated. The spin period is about 5 hr, which is below a disruption limit of a rubble pile asteroid but is relatively shorter than the spin periods of the asteroids visited by other small body exploration missions. No tumbling state has been detected. An earlier taxonomic classification was L-type, while later observations have suggested Sq-type features. 

 

The spacecraft will approach Torifune at a flyby speed of about 5.25 km/s, aiming for a ~1-10 km distance from the asteroid’s center at the closest encounter, though it may vary depending on orbit determination accuracy. The spacecraft’s orientation will be almost fixed without major rotational maneuvers. Until 5 min before the closest encounter, the spacecraft will face the ongoing direction so that the major remote sensing instruments can keep Torifune in their fields of view. After this phase, the spacecraft will reorient by a few degrees so that all the planned instruments can capture the asteroid a few seconds before the closest encounter. This reorientation will enable observations at a higher resolution. The remote sensing instruments planned to be used are the Optical Navigation Cameras (ONCs), Thermal Imager (TIR), Near Infrared Spectrometer (NIRS3), and Light Detection and Ranging (LIDAR) instrument. The ONC-W2 camera will not be used for observation this time due to its poor viewing geometry, however. 

 

Given that the spacecraft was designed not for flyby operations but for rendezvous and sampling, the team will address various challenges not encountered during the nominal mission. If the spacecraft successfully passes by the asteroid at a few kilometers from its center, it will be one of the closest encounter distances that small body explorations have ever achieved. The ideal resolution of ONC-T will be 2 m/pix about four seconds before the closest encounter, while that of TIR will be 18 m/pix under the same conditions. NIRS3 will continue its observations for the last hour. LIDAR will attempt to acquire at least one data sample. 

 

If successful, not only will Hayabusa2#’s flyby of Torifune address scientific questions for material transport in the solar system, but it will also be directly relevant to planetary defense, particularly the fast reconnaissance concept, in which measurements of target asteroids prior to deflection under a limited preparation time are considered to play a key role in constraining the deflection outcomes. The spacecraft is aging and not designed for ideal observational operations. However, using an in-flight spacecraft like Hayabusa2# can demonstrate how the properties of the target asteroid could be determined under such limited conditions. Importantly, Hayabusa2#’s flyby will be complementary to but will not address the fast reconnaissance concept solely. The concept addresses targeting an asteroid between 50 and 100 m in diameter, while Torifune is about 400 m in diameter. This presentation summarizes the initial results from Hayabusa2#.