A Dark Comet Mystery Just Got Stranger After Astronomers Found Its Acceleration May Have Nothing to Do With Ice

Japan is heading toward a mysterious asteroid that may not be what scientists thought.

11 Nov 2026, 19:21 by Tibi Puiu · ZME Science

Credit: ZME Science.

A tiny asteroid is moving through space as though it has a rocket engine. It has a strange acceleration that the Sun’s gravity or that of planets cannot explain. Scientists suspected it was shooting invisible jets of gas from ice buried inside the rock. But there’s a problem: new research suggests the asteroid is far too warm to contain that ice.

The object, known as 1998 KY26, is just 11 meters (36 feet) wide and completes a rotation once every 5.35 minutes.

KY26 may belong to a mysterious class of objects called dark comets. These look like ordinary asteroids but behave like comets, seemingly propelled by gas escaping from their interiors. Unlike familiar comets, however, they have no visible tails.

In a 2023 study, astronomers identified an unusual acceleration pushing KY26 out of its orbital plane. They proposed that escaping water vapor could explain the movement.

The theory made sense. When ice inside a comet warms up, it turns directly into gas and escapes into space. That escaping gas produces a small push, much like a rocket engine.

But what if KY26 has no ice left?

Now, researchers at the Georgia Institute of Technology have found evidence that challenges one of the leading explanations for its unusual motion. The timing couldn’t be more intriguing. Japan’s Hayabusa2 spacecraft is already heading toward the asteroid, with a rendezvous planned for July 2031.

This is the same spacecraft that landed twice on the much larger asteroid Ryugu, blasted a crater into its surface to collect material from beneath, and delivered precious samples back to Earth in 2020. Rather than ending its mission there, Hayabusa2 continued its journey through space, heading toward an even more challenging target.

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An Asteroid Too Warm for Its Own Ice

To investigate, aerospace engineers Korbin Hansen and Masatoshi Hirabayashi built a detailed computer model of KY26.

They used estimates of its shape, size, reflectivity, orbit and physical properties to calculate temperatures across its surface and deep inside the rock.

Overall, the researchers ran 260 simulations covering 13 sets of assumptions, including different possible compositions and orientations.

According to the results, the asteroid’s average temperature was approximately minus 47 degrees Celsius. Even its coldest locations averaged at least minus 61 degrees Celsius over an orbit.

That sounds incredibly cold, but in the vacuum of space, water ice can gradually evaporate away even at temperatures well below freezing. For buried ice to survive over long periods, temperatures generally need to remain below roughly minus 128 degrees Celsius.

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KY26 simply isn’t cold enough.

Nor could ice easily hide deep underground. In the researchers’ main model, the asteroid’s center was only about one degree cooler than its surface. Even modeling the asteroid as a loose pile of insulating rubble didn’t solve the problem.

“Our temperature model results indicate that the water outgassing rate on 1998 KY26 would be too high to maintain buried ice,” the researchers wrote in their study published in Icarus.

Their calculations suggest that even if water initially made up 1% of the asteroid’s mass, that ice would disappear in approximately 2,065 years.

More importantly, if escaping water vapor were responsible for KY26’s unusual acceleration, its ice reserves would be exhausted in just 62 years.

The time window is much too short for an asteroid that may have spent millions of years traveling from the main asteroid belt into its present orbit near Earth.

The researchers therefore conclude that KY26 is probably a largely dry asteroid rather than an active dark comet.

If It’s Not Ice, What’s Pushing It?

There’s another explanation, and it involves something seemingly harmless: sunlight.

Sunlight exerts a tiny pressure on objects it strikes. Asteroids also absorb sunlight and release energy as heat, producing additional forces that can gradually alter their orbits.

Because KY26 is so small, these forces could have a surprisingly large influence on its motion.

In fact, a separate study published in 2025 found that solar radiation pressure and thermal emissions could explain KY26’s unusual acceleration without requiring escaping gas.

The new research strengthens this interpretation, although scientists still don’t know how long KY26 has occupied its current orbit.

The asteroid has already delivered other surprises.

In September 2025, astronomers revealed that KY26 is three times smaller and spinning twice as fast as previously estimated.

“We found that the reality of the object is completely different from what it was previously described as,” said astronomer Toni Santana-Ros of the University of Alicante.

That makes Hayabusa2’s upcoming encounter especially challenging. The Japanese spacecraft previously collected samples from asteroid Ryugu, which is approximately 900 meters wide. Its new target is barely larger than a small house and spins extraordinarily fast. So many things could go wrong.

After successfully flying past asteroid Torifune in July 2026, Hayabusa2 is continuing toward KY26. Its observations could help scientists determine whether the rock is indeed dry and clarify the forces acting on it.