Another Success for Hayabusa 2 as it Completes a Flyby of Asteroid Torifune
JAXA's remarkable Hayabusa 2 spacecraft continues to rewrite the boundaries of deep-space exploration, adding yet another chapter to one of the most ambitious extended mission profiles in the history of planetary science. The spacecraft has successfully completed a close flyby of asteroid Torifune (98943 Torifune), capturing unprecedented imagery and scientific data that promises to deepen our understanding of near-Earth asteroids and the early solar system.
A Mission That Keeps on Giving
Hayabusa 2's primary mission is now well in the past. JAXA's asteroid-sampling spacecraft rendezvoused with the carbonaceous asteroid Ryugu back in June 2018, spending approximately 1.5 years studying it in extraordinary detail. During that time, the spacecraft deployed multiple rovers onto the surface, detonated a small explosive charge to expose subsurface material, and collected pristine samples. Those samples were returned to Earth in a heat-shielded capsule in December 2020, delivering some of the most scientifically valuable extraterrestrial material ever retrieved — including complex organic molecules and hydrated minerals that speak to the wet, chemically rich history of the early solar system.
Rather than retiring Hayabusa 2 after that triumphant sample return, JAXA engineers made the bold decision to repurpose the spacecraft for an extended mission. While another full sample-return is not feasible given remaining propellant reserves, the spacecraft is now charting a course toward a tiny near-Earth object called 1998 KY26, a diminutive body only about 11 meters in diameter. En route, the mission team seized the opportunity to conduct a bonus flyby of another near-Earth asteroid: Torifune.
Introducing Torifune: A Stony Relic of the Solar System
Prior to the flyby, ground-based observations had already provided a tantalizing preview of Torifune's character. The asteroid measures approximately 450 meters in diameter and is classified as an S-type (siliceous or stony-type) asteroid — a category of high-density, rocky bodies rich in silicate minerals such as olivine and pyroxene, along with metallic nickel and iron. S-type asteroids represent roughly 17% of the known asteroid population, making them the second most common type after the darker, carbon-rich C-type (carbonaceous) asteroids.
The distinction matters greatly to planetary scientists. S-type asteroids are believed to be the parent bodies of ordinary chondrites — the most common type of meteorite found on Earth — and studying them up close offers a window into the processes of planetary formation and differentiation that occurred in the first tens of millions of years after the Sun ignited. Unlike C-type asteroids, which tend to preserve more pristine, volatile-rich material from the outer solar system, S-type bodies like Torifune have typically experienced more thermal processing, making their mineralogy a record of heating and cooling cycles from the solar system's infancy.
"Contact binary asteroids like Torifune offer a rare opportunity to study two distinct primordial bodies that have merged over time, preserving a record of early solar system dynamics in a single, accessible object."
Ground-based radar and photometric observations had also hinted at something particularly intriguing about Torifune's shape: it appeared notably elongated, suggesting a possible contact binary structure. Only the Hayabusa 2 flyby imagery could confirm this suspicion.
The Flyby: Precision Navigation at 5 km/s
Hayabusa 2 began its observational campaign of Torifune in June, using its Optical Navigation Camera – Telescopic (ONC-T) to first resolve the asteroid against the stellar background. The ONC-T captured direct images of Torifune on June 20th for navigation purposes — a critical step in fine-tuning the spacecraft's trajectory for the close approach.
The main event arrived on July 5th, when Hayabusa 2 swooped to within approximately 800 meters of Torifune's surface. At this remarkable proximity, the ONC-T delivered images of the asteroid's surface in detail impossible to achieve from Earth, decisively confirming its contact binary nature. In a contact binary system, two separate bodies orbit a common center of mass and, over potentially millions or billions of years, gradually spiral inward — slowed by tidal forces, radiation pressure effects like the YORP effect, and gravitational interactions — until they gently merge into a single conjoined body. The resulting "peanut" or "dumbbell" shape is a telltale signature of this process.
Contact binary asteroids are not considered rare in the near-Earth population. In fact, some estimates suggest that up to 15% of near-Earth asteroids may be contact binaries, with well-known examples including 433 Eros and 25143 Itokawa — the latter being the target of the original Hayabusa mission. Torifune's confirmation as a contact binary adds valuable data to the growing census of these fascinating objects.
Navigation and image acquisition during the flyby were technically demanding. Hayabusa 2 was traveling at a relative speed of 5 km/s (approximately 3.1 miles per second or over 18,000 km/h), meaning that the window for close-range imaging was extremely brief. The mission team had to plan camera exposures and instrument activation sequences meticulously in advance, with no time for real-time corrections given the communication delay between Earth and the spacecraft.
These are two different top-down views of Hayabusa 2 and asteroid Torifune. The left panel shows a view in a rotating frame tied to Earth's orbit, while the right panel shows a stationary inertial frame. Image Credit: Original by Hirabayashi et al. 2026, modified by Nrco0e — Modified from Figure 2 of arXiv:2604.08832, CC BY 4.0, Wikimedia Commons.
A Multi-Instrument Scientific Campaign
The Hayabusa 2 team made the most of the close approach by activating multiple scientific instruments in the hour leading up to and during closest approach. Each instrument probed a different aspect of Torifune's physical and chemical character:
- ONC-T (Optical Navigation Camera – Telescopic): Captured high-resolution visible-light images revealing the asteroid's shape, surface texture, albedo variations, and structural features — confirming the contact binary morphology.
- NIRS3 (Near-Infrared Spectrometer): Analyzed the spectral signature of sunlight reflected from Torifune's surface, providing compositional information about surface minerals, hydration states, and organic compounds if present.
- TIR (Thermal InfraRed Imager): Measured thermal emissions from the asteroid's surface to derive its thermal inertia — a critical parameter that reveals whether the surface is covered in fine regolith (low thermal inertia) or exposed bare rock (high thermal inertia), and which influences how solar radiation pressure alters the asteroid's orbit over time via the Yarkovsky effect.
- LIDAR (Light Detection and Ranging): Fired laser pulses toward the asteroid to measure precise distances, helping to reconstruct a three-dimensional shape model of Torifune and refine understanding of its topography.
Hayabusa 2 imaged asteroid Torifune with its Thermal InfraRed Imager (TIR) from approximately 10 km away. Image Credit: JAXA, Maebashi Institute of Technology, Chiba Institute of Technology, The University of Aizu, Hokkaido University of Education, AIST.
This multi-instrument approach mirrors the strategy used so successfully at Ryugu and reflects the maturity of JAXA's asteroid science program. Even without the ability to collect a physical sample, the combination of optical, spectroscopic, thermal, and ranging data can yield a remarkably complete scientific portrait of a small body. NASA's own asteroid research programs have similarly emphasized the importance of combining these observational modalities to fully characterize near-Earth objects.
Propellant Constraints and Careful Mission Planning
Hayabusa 2's longevity is a triumph of careful engineering and meticulous fuel budgeting. The spacecraft relies on highly efficient xenon-ion thrusters, which generate thrust by ionizing xenon gas and accelerating the ions to extreme velocities using electric fields — achieving specific impulses far beyond what chemical rockets can deliver. However, the spacecraft now has less than half of its xenon propellant remaining, enough to complete the planned trajectory to 1998 KY26 but leaving no margin for significant unplanned maneuvers.
The Torifune flyby was incorporated precisely because it fell naturally along the spacecraft's trajectory toward 1998 KY26, requiring only minimal propellant expenditure for navigation adjustments. It is a testament to the ingenuity of JAXA's mission planners that they were able to design an extended mission profile that squeezes extraordinary scientific value out of a spacecraft already far beyond its original design scope. The European Space Agency's complementary asteroid missions reflect a similarly growing international commitment to small-body exploration.
The Road Ahead: Earth Flybys and a Date with 1998 KY26
The journey to Hayabusa 2's next major destination will unfold across the remainder of this decade. The spacecraft's planned milestones include:
- December 2027: A gravity-assist flyby of Earth, using our planet's gravitational field to adjust the spacecraft's trajectory without expending precious propellant.
- June 2028: A second Earth flyby, further refining the trajectory toward the target asteroid.
- July 2031: Rendezvous with 1998 KY26, the mission's ultimate destination.
The target itself is a fascinating and unusual object. At only about 11 meters in diameter, 1998 KY26 is one of the smallest asteroids ever targeted by a spacecraft. Optical and radar observations suggest it may be a water-rich asteroid, potentially harboring hydrated minerals similar to those found in carbonaceous meteorites. Its rapid rotation rate — completing a full rotation in under 11 minutes — almost certainly rules out a "rubble pile" composition held together by self-gravity alone. Such a fast spin would fling loose material off the surface; instead, 1998 KY26 is likely a monolithic chunk of coherent rock.
There is also the intriguing possibility that 1998 KY26 could be an X-type asteroid — a broad taxonomic category that serves as something of a scientific catch-all for objects that appear spectrally similar in telescope observations but may be composed of radically different materials. X-types can include metallic M-type bodies, primitive P-types, and enstatite-rich E-types. Distinguishing between these subtypes requires close-range observation, making Hayabusa 2's eventual rendezvous scientifically invaluable. The NASA Planetary Data System Small Bodies Node maintains extensive databases of asteroid classifications and orbital parameters that inform mission planning for targets like 1998 KY26.
Preliminary Results and What Comes Next
It is important to note that not all of Hayabusa 2's data from the Torifune flyby has yet reached Earth. Deep-space data transmission is a slow and painstaking process, constrained by the vast distances involved and the limited downlink bandwidth of the spacecraft's radio systems. JAXA scientists are currently processing and analyzing the data that has been received, and the agency is expected to release more comprehensive findings in the near future.
When the full dataset is available, scientists anticipate being able to construct detailed shape models of Torifune, derive thermal inertia maps of its surface, and produce spectral maps revealing compositional variations across the two lobes of its contact binary structure. Any differences in surface composition or thermal properties between the two lobes could provide important clues about whether the two progenitor bodies formed in similar or different regions of the early solar system before their eventual merger.
For now, the successful flyby of Torifune stands as a remarkable demonstration of what extended spacecraft missions can achieve with careful planning and scientific creativity. JAXA's official Hayabusa 2 mission page continues to be updated with the latest findings as data flows in from the spacecraft's instruments. From Ryugu's carbon-rich plains to the stony lobes of Torifune, and eventually to the tiny spinning enigma of 1998 KY26, Hayabusa 2's odyssey through the solar system's smallest worlds is far from over — and the science it returns will inform asteroid research, planetary defense, and our understanding of solar system origins for decades to come.
- Mission: Hayabusa 2 Extended Mission (JAXA)
- Flyby Target: 98943 Torifune — S-type contact binary NEA, ~450 m diameter
- Closest Approach Distance: ~800 meters
- Flyby Speed: 5 km/s (~18,000 km/h)
- Instruments Used: ONC-T, NIRS3, TIR, LIDAR
- Ultimate Destination: 1998 KY26 (rendezvous July 2031)
