Japan’s Hayabusa2 probe is hurtling through deep space on an extended mission following its historic drop of asteroid Ryugu samples into the Australian outback on December 6, 2020. Its next destination is a tiny near-Earth object designated 1998 KY26, which continues to baffle astronomers. The target is accelerating in ways that cannot be accounted for by gravity alone, and a newly published scientific paper has just dismantled the leading hypothesis used to explain its erratic trajectory. As the spacecraft races across the solar system toward an eventual 2031 encounter, the true nature of this strange space rock remains an open question.
Key takeaways
- Hayabusa2 is scheduled to rendezvous with the near-Earth object 1998 KY26 in 2031.
- The target is only 36 feet wide and completes a full rotation every 5 minutes.
- A new study published in Icarus proves the space rock is too warm to retain buried ice.
- The object's strange non-gravitational acceleration currently lacks any scientific explanation.
A Shrinking Target With Extreme Physics
Astronomers have realized that 1998 KY26 is far more extreme than previously thought. A paper published in 2025 revealed that the target is about a third of its previously estimated size, measuring just 36 feet (11 meters) wide—an expanse barely twice the width of Hayabusa2's solar panels. For comparison, the spacecraft's first target, asteroid Ryugu, measures nearly 3,000 feet (900 meters) wide. Hayabusa2 famously touched down on Ryugu on February 22, 2019, shot a copper impactor into its surface in April to form an artificial crater, and returned in July to collect subterranean samples. Landing on 1998 KY26 will be exponentially harder, not only because of its minuscule dimensions, but also because it rotates rapidly enough to complete a full spin roughly every five minutes.
Adding to the mission's complexity, trajectory analyses published in The Planetary Science Journal in 2023 showed that the object experiences unusual non-gravitational accelerations. These measured forces are greater than typical alterations caused by the Yarkovsky effect, a tiny pressure imparted by sunlight that gradually shifts asteroid orbits. The authors of that study noted that no known radiation-based, non-radial effect could account for such a large acceleration.
The Collapse of the Dark Comet Hypothesis
To explain the unexpected acceleration, researchers previously proposed that 1998 KY26 might actually be a dark comet. Dark comets are hybrid celestial bodies that resemble asteroids visually but move through space like comets. Under this theory, as the object heats up, subsurface ice turns to gas and vents into space, acting like a faint thruster to nudge the rock forward. If true, this mechanism would neatly account for both the hybrid behavior and the unusual trajectory shifts.
That explanation has now hit a major roadblock. A newly published paper in the journal Icarus demonstrates that 1998 KY26 is far too warm to hold onto any buried ice reserves. Using pre-existing data, the research team modeled the object's thermal profile and discovered that even its coldest regions maintain an average temperature of at least 212 Kelvin (-61 degrees Celsius). In contrast, preserving buried ice would require orbit-averaged temperatures to stay below approximately 145 Kelvin (-128 degrees Celsius). The study concludes that any water outgassing rate would be excessively high, pointing instead to a classification of asteroids largely devoid of water and leaving scientists without an explanation for the rock's acceleration.
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