Hayabusa2#’s (98943) Torifune flyby in July 2026: Rapid asteroid characterization using an already-flying spacecraft for planetary defense

Planetary defense is an international effort to collaboratively assess and mitigate upcoming threats from small bodies potentially hitting the Earth. The recent success in NASA’s DART, the mission that successfully smashed its spacecraft into the target asteroid Dimorphos, the smaller secondary of the S-type asteroid (65803) Didymos, proved the maturity of kinetic impact deflection [1]. This accomplishment, further enhanced by the visit of ESA’s Hera to this asteroid system [2], deepens the knowledge about this deflection technique. At the same time, NASA’s NEO Surveyor will also significantly increase the ability to monitor potentially hazardous objects (PHOs) [3].

Such efforts have been a remarkable catalyst for the community's greater confidence in resolving an unexpected event that causes non-negligible negative impacts on the Earth. However, a new problem emerged as being critical to resolve. When an asteroid is found to hit the Earth soon, what information is needed, and how can we identify such information before an expected terminal event? One way is a fast reconnaissance (fast recon), where a quick campaign (telescopic or spaceflight characterization) identifies the target’s key properties regardless of its limitations [4].

A flyby mission is the quickest way to visit and detail a target object directly. Thus, it is the most realistic as a fast recon in the spaceflight option, though others, such as rendezvous, landing, and sampling, may also be reasonable if time is allowed. Nevertheless, the major challenge in a fast recon mission is to prepare a spacecraft for its flight whenever needed. If mission development starts from scratch, countless processes, including the cruise phase, may delay the planned mission from reaching the target.

One possible solution emphasized in this paper is to apply an already-flying spacecraft to flyby operations. Spacecraft flying in a heliocentric orbit, particularly in the near-Earth region, can generally have high accessibility to HPOs. Such spacecraft can reach a target object in a much shorter time than the typical process of building a probe. When nominal spaceflight missions are completed, their spacecrafts are generally healthy enough for extended missions. Adding operational flexibility to already flying (used) spacecraft or making it intentionally taxi in heliocentric orbits may be another approach for a fast recon. However, using such used (and usually old) spacecraft is not optimal for getting key information because of the limited capabilities. For example, the spacecraft’s platform may not be designed for necessary operations, and onboard instruments may be degraded already.

The Hayabusa2 extended mission attempts to address this issue directly during its critical flyby operation next year [5, 6]. The Hayabusa2 extended mission follows its nominal mission, Hayabusa2, which made a triumphant sample return from the C-type asteroid (162173) Ryugu in December 2020. With its nickname, Hayabusa2# (SHARP: Small Hazardous Asteroid Reconnaissance Probe), the mission is operating its 10-year-old spacecraft en route to the final rendezvous destination, the asteroid 1998 KY26, a 15-30 m object spinning at 5-10 min [7, 8]. Until the rendezvous with 1998 KY26 in 2031, the mission will conduct various scientific and engineering investigations, including monitoring exoplanets and zodiacal light, flying by the S-type asteroid Torifune (2001 CC21), and characterizing long-term spacecraft behaviors (Figure 1). One recent effort is to plan the flyby operation at Torifune.

The spacecraft is planned to fly by Torifune in July 2026. At 0.81 AU from the Sun, the spacecraft approaches the asteroid from the inner Solar System at an encounter speed of about 5.25 km/s and a phase angle of approximately 20 deg. Hayabusa2# will challenge its engineering limit to make the spacecraft as close as possible to the asteroid and conduct comprehensive scientific investigations using the onboard instruments: the Optical Navigation Camera Telescope (ONC-T/W1), Thermal Infrared Imager (TIR), Near Infrared Spectrometer (NIRS3), and Laser Altimeter (LIDAR). The expected observational conditions will be extremely limited because the spacecraft was not designed for flyby operations. Still, the mission will attempt to maximize the determination of the asteroid’s physical properties regardless of many challenges.

Hayabusa2# has made a two-year-long effort to find an optimal approach for maximizing science return during the Torifune flyby. Torifune is a likely S-type [9-12] near-Earth asteroid rotating at a spin period of 5.02 h with no indication of a tumbling mode [9, 13]. While the equivalent diameter ranges between 0.3 and 0.53 km [9-15], the asteroid is likely to be elongated [9-11, 13-14]. The flyby sequence will consist of two phases. The first phase will be until 5 minutes before the closest approach and will focus on the spacecraft’s guidance, navigation, and control (GNC), so science observations will be limited. Within five minutes before the closest approach, the spacecraft’s GNC will be turned off (or at least less prioritized) to focus on science investigation. The mission developed a scheme to identify the best view geometry and flyby timing, as well as observational sequences, given all the existing constraints on spacecraft operations and instruments.

While recommended fast recon targets are between 50 and 100 m in diameter [4], the developed approach aligns directly with the fast recon concept, particularly using flying spacecraft. This paper discusses Hayabusa2#’s flyby planning effort, which closely ties up with planetary defense.

Figure 1. Hayabusa2#’s mission summary

 

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