November 14, 2026
By Burak Oktenli
Key Takeaways:
Revised observations show asteroid 1998 KY26 is only about 11 metres across and rotates every 5.35 minutes—half its previously estimated size and twice as fast—raising mission-design challenges for Hayabusa2’s planned 2031 rendezvous.
New thermal modeling indicates orbit-averaged temperatures are too high for long-lived buried water ice, challenging the dark-comet outgassing explanation and favoring ordinary radiation-pressure and thermal-recoil effects for its nongravitational orbital acceleration.
The encounter will test whether a well-planned mission can update its scientific questions with pre-arrival discoveries, refining shape, spin, surface and thermal data relevant to both solar-system science and planetary-defence operations near small, rapidly rotating bodies.
New research challenges the idea that buried ice explains the unusual motion of asteroid 1998 KY26. Japan’s 2031 rendezvous is becoming a test of how scientists revise a mission’s expectations before it arrives.
In July, Japan’s Hayabusa2 spacecraft flew within roughly 400 metres of the surface of asteroid Torifune at about 5.3 kilometres per second. The encounter succeeded in capturing valuable observations while testing the navigation needed for more demanding small-body exploration. The spacecraft’s next destination is much smaller: 1998 KY26, an asteroid approximately 11 metres across that spins once every 5.35 minutes.
Hayabusa2 is scheduled to meet it in July 2031. But the scientific picture of the target is changing well before the spacecraft arrives. In the space of two years, astronomers have substantially revised its size and rotation, offered a physically detailed explanation for a puzzling orbital acceleration, and challenged the idea that escaping water vapour is pushing it through space. The lesson is not that another cosmic mystery has been solved. It is that a well-planned mission must be capable of learning from the discoveries made on the way.
A target that became smaller and faster
For decades, estimates suggested KY26 was roughly 30 metres in diameter and completed a rotation in about 10.7 minutes. A 2025 observational study revisited those numbers using 2024 photometry and earlier radar measurements. The revised estimate was 11 ± 2 metres, with a rotation period of 5.3516 minutes. What had looked like a fast-spinning small asteroid was now a far smaller body spinning twice as fast as previously thought.
That is not merely a correction to a catalogue entry. At this scale, the balance among weak gravity, rotation and the forces holding material together becomes a mission-design concern. A surface may behave very differently from the loose terrain that a spacecraft encounters around a larger asteroid. The 2025 study considered both a loosely bound structure and a solid body; it did not settle which description is right. JAXA identifies operations close to such a rapidly rotating object as a central technical challenge of the extended mission.
The revision also illustrates how international observational work earns its keep. Ground-based telescopes and radar changed the questions mission engineers must ask before the vehicle had even arrived. More powerful instruments on a future spacecraft do not make that preparatory work expendable.
Two natural explanations for an unusual orbit
KY26’s trajectory contains a small nongravitational component: gravity alone does not fully account for the positions astronomers have measured. That phrase can sound exotic, but it simply means that additional physical forces must be considered. In a small body, forces almost negligible for a large asteroid can matter over years of precise tracking.
One proposed explanation placed KY26 among the objects called “dark comets”: bodies without an obvious dust coma whose motions may nonetheless be affected by escaping gas. If hidden water ice slowly warms and releases vapour, the recoil could alter the orbit. In this case, however, sustained outgassing was an inference to investigate, not a direct detection of gas.
There was already another contender. In October 2025, Davide Farnocchia and colleagues published a model that accounted for KY26’s nonspherical shape and combined the pressure of incoming sunlight with recoil from heat radiated by its surface. The resulting physical model fitted the positional observations without requiring unseen cometary activity. It did not turn every thermal property into an observed fact, but it showed that familiar physics could explain the motion.
Now a new study available online in Icarus, by Georgia Tech researchers Korbin Hansen and Masatoshi Hirabayashi, attacks the outgassing hypothesis from a different direction. Across 13 sets of thermal assumptions, their calculations indicate that orbit-averaged temperatures are too high for buried water ice to persist over long periods. A hypothetical ice-rich interior would lose its water too quickly to serve as a durable source of vapour under those modelled conditions.
That is a substantial challenge to the long-lived buried-water explanation for this particular asteroid. It is not an inventory taken inside KY26; the investigators have modelled a body they have not visited. Nor does it rule out all dark comets or establish that every aspect of the radiation model is beyond question. The correct scientific gain is narrower and more useful: a proposed mechanism now has to satisfy both the measured orbital motion and a test of whether its proposed physical source could survive.
What the spacecraft can actually settle
The distinction between sunlight and escaping gas matters for more than a label. Direct solar radiation pressure depends on how incoming light interacts with a body. Thermal recoil—the family of effects that includes the Yarkovsky effect—depends on how absorbed energy is emitted as heat. Those predictions respond to size, shape, reflectivity, temperature, rotation and bulk density. Outgassing, by contrast, requires material capable of escaping and imparting momentum. These mechanisms leave different questions for an observing programme.
A productive rendezvous would refine the shape, spin state and surface characteristics against which the competing models can be checked. Thermal observations can constrain heat transport and the plausibility of different recoil calculations. Measurements relevant to composition or possible activity can sharpen the volatile question, subject to the instruments’ actual sensitivities. A failure to detect activity could constrain one hypothesis without proving that no volatile material exists anywhere inside the asteroid.
Researchers have also identified possible James Webb Space Telescope observing windows before arrival that could refine size and thermal estimates. Those are opportunities proposed in the literature, not observations already completed or guaranteed observing time. Between now and 2031, the scientific priority should be to make the most discriminating measurements possible, not to defend whichever interpretation first attracted headlines.
Why this small object matters for planetary defence
No evidence presented in these studies establishes that KY26 poses an imminent impact threat to Earth. Its value for planetary defence is more practical: it belongs to a population of small, difficult-to-characterise objects that planetary-defence planners must learn to identify, track and, where necessary, approach. The Hayabusa2 extended mission treats the ability to operate near a rapidly rotating, decametre-scale asteroid as part of that preparation.
The 2026 Torifune flyby supplied a concrete engineering step. The 2031 rendezvous asks a different question: how do you operate around an object whose gravity is feeble, whose spin is rapid and whose physical properties have been repeatedly revised? Those capabilities are valuable even if KY26 proves to be an ordinary, largely dry asteroid responding to sunlight. Planetary defence needs reliable knowledge of the ordinary as much as it needs a response to the extraordinary.
There is a funding lesson here, but it is not a plea to treat every uncertainty as a reason for a larger mission. The public return should be tied to what can be learned: better trajectories, stronger physical models, tested close-approach methods and clearer limits on remote observations. International telescope networks, space agencies and mission teams share that work. Its continuity matters during the years between a result that changes expectations and a spacecraft encounter capable of testing them.
The best result in 2031 will not be a sensational answer to whether KY26 “really is” a dark comet. It will be an explanation of its motion that survives comparison with the object itself—and a more dependable method for exploring the small bodies we cannot afford to misunderstand.
For a mission already in flight, changing the scientific question is not a setback. It is often the beginning of the discovery.
About Burak Oktenli
Burak Oktenli holds an MBA and a Master of Professional Studies in Applied Intelligence from Georgetown University. His research addresses the governance of authority in autonomous and AI-enabled systems, and his writing has appeared at the Modern War Institute at West Point, RUSI, RealClearDefense, RealClearMarkets, and Geopolitical Monitor. He is the author of Authority Architectures for Autonomous Systems, a ten-volume series on how authority in autonomous systems is delegated, monitored and recovered, at authority-architecture.me.
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