Most asteroids are fairly predictable. They simply do what they are told, pushed and pulled by external forces such as gravity or the pressure of radiation from the Sun. No 1998 KY26. KY26 is a good-for-nothing, that is, it accelerates in a way that defies expectations about how a well-behaved asteroid should behave. One proposed explanation for these cosmic monotonies is that KY26 is just masquerading as an asteroid. that, deep in its heart, it is actually a “dark” comet, whose undetectable outgassing is leading it down a different path. But now there is a problem with that explanation. According to aerospace engineers Korbin Hansen and Masatoshi Hirabayashi of the Georgia Institute of Technology, the 36-foot (11-meter) space rock is too hot to retain the ice needed to power propulsion. “Results from our temperature model indicate that the water outgassing rate at 1998 KY26 would be too high to keep the ice buried, which would refine the understanding of the asteroid’s composition,” they write in a paper published in Icarus. A model of what KY26 could look like. (Original model by Toni Santana-Ros et al. (2025); edited by Nrco0e/Wikimedia Commons, CC BY 4.0) The rocks that populate interplanetary space generally fall into two broad categories: asteroids and comets. Asteroids are mostly chunks of rock, while comets contain ice that, as it sublimates into gas and escapes into space, can give the comet a boost beyond the forces applied by gravity. as it orbits the Sun. The image that probably comes to mind when you think of a comet is that of a hazy, glowing orb followed by luminous streamers. But not all comets are built the same. What we call dark comets are so named because their comet activity is effectively invisible. They are believed to shed gases like an ordinary comet, without producing the spectacular visible tails normally associated with them. They look like ordinary asteroids, with a little extra acceleration. This is what KY26 could look like. The small asteroid rotates once every 5.35 minutes and its trajectory shows an unusual acceleration perpendicular to the plane of its orbit. Naturally, scientists have considered whether it could be one of these dark comets. One of the defining characteristics of comets is their icy composition, giving scientists an easy way to verify their comet identification. All they have to do is take their temperature. This is what Hansen and Hirabayashi did. Or rather, they combined what is known about KY26—its size, its orbit, its reflectivity, and its likely composition—to model the temperatures on its surface and in its interior. Actual photo of KY26 released in 2025 using Gemini South. The asteroid is the small white dot in the center. (Gemini International Observatory/NOIRLab/NSF/AURA/T. Santana-Ros) For buried water ice to survive for long periods, temperatures must remain below about 145 Kelvin (-128 degrees Celsius). But even the coldest regions of KY26, the researchers found, have an average temperature of at least 212 Kelvin (-61 degrees Celsius) over the course of its orbit around the Sun. That’s deadly cold for a human, sure, but too warm for cometary ice to remain. At these temperatures, the researchers calculated that even if water ice made up 1 percent of KY26’s mass, it would all disappear in just over 2,000 years. And if that ice were responsible for the asteroid’s unusual acceleration, it would disappear in just 62 years. KY26 may be billions of years old, but even so, there is an important caveat. It is unclear how long it has occupied its current orbit. So it’s still possible that it has ice, but between this result and a previous paper tracing KY26 to a possible origin in an asteroid belt, that possibility is narrowing. So what could be pushing KY26 off the straight and narrow path? Well, we can still look at the old peer pressure, if peer pressure means radiation pressure from the Sun. Sunlight exerts a small force but potentially powerful enough to be responsible for the apparent delinquency of the asteroid. The good news is that we may know for sure very soon. The Japanese space agency’s Hayabusa2 spacecraft is heading toward KY26 even as you read these words, with a rendezvous scheduled for July 2031. Speed up all you want, KY26. You can’t hide forever. The research has been published in Icarus. This article was fact-checked by Peter Dockrill and edited by Peter Dockrill. While we take pride in our process, we are human. If you spot an error, please let us know.
