In 2031, Japan’s Hayabusa2 spacecraft will arrive at the near-Earth asteroid 1998 KY26: a tiny body that is accelerating in a way that astronomers have struggled to explain. Some studies have suggested that the asteroid is pushed along by water escaping from it as gas, making it an as-yet theoretical “dark comet.”
But through new simulations, Korbin Hansen and Masatoshi Hirabayashi at the Georgia Institute of Technology show that the asteroid is far too warm to hold on to ice, pouring cold water on the idea that comet-like accelerations could occur without a visible cloud.
Their findings have been published in Icarus.
Escaping gas or radiation pressure?
At just 11 m (36 feet) across, 1998 KY26 is one of the fastest-spinning asteroids ever discovered, completing a full rotation in just under five and a half minutes. As it orbits the sun, it also experiences a small but unexpected push out of the plane of its orbit.
This unusual motion has led some astronomers to propose that it is a dark comet: an object propelled by escaping gas, without forming the glowing cloud and tail of an ordinary comet. However, it has remained unclear whether the asteroid’s interior could stay cold enough to keep ice frozen for long enough for this to happen.
Other studies have argued that the motion could be explained by pressure from sunlight, combined with heat radiating away from the asteroid’s surface—without any escaping gas. This idea is supported by measurements of light reflected from its surface, which suggest a bright, rocky asteroid of a type thought to contain very little water.
Virtual asteroid
To investigate further, Hansen and Hirabayashi built a 3D computer model of the asteroid, which allowed them to calculate how heat from the sun flows through its surface and deep into its interior. They then compared temperatures across the asteroid’s entire orbit.
Since many of its physical properties are still uncertain, the pair varied factors including its size, reflectivity, how easily it conducts heat and the direction of its spin axis in different runs of their simulation. Finally, they used these modeled temperatures to estimate how quickly any ice buried inside the asteroid would turn to vapor and escape.
Water-depleted rock
Across all 13 runs, the results were consistent. For ice to survive inside the asteroid over long periods, temperatures would need to stay below about –128°C (-198 degrees Fahrenheit). Yet averaged over its orbit, even the coldest parts of 1998 KY26 sat at around –47°C (-53 degrees Fahrenheit) or warmer. No point on its surface or in its interior stayed cold enough to preserve ice in the long term.
Even if water had made up a generous 1% of the asteroid’s mass, the researchers estimate it would all be lost in roughly 2,000 years, suggesting that 1998 KY26 is strongly depleted in ice.
These results make it highly unlikely that 1998 KY26 is a dark comet and strengthen the case that its strange acceleration is driven by sunlight and heat radiation instead. Since it is unclear how long the asteroid has occupied its current orbit, its true nature can only be confirmed once Hayabusa2 arrives. Until then, the study offers important clues about what astronomers can expect to find there.
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Publication details
Korbin Hansen et al., Thermally unfavorable conditions of volatile existence in near-Earth asteroid 1998 KY26, the Hayabusa extended mission’s target, Icarus (2027). DOI: 10.1016/j.icarus.2026.117361
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Simulations cast doubt on unusual asteroid’s ‘dark comet’ status (2026, November 10)
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