Mysterious Asteroid 1998 KY26 Moves Like Something Is Pushing 2026

Asteroid 1998 KY26 is a small near-Earth object that has puzzled astronomers since its discovery in 1998. The 11-meter asteroid moves with an acceleration that cannot be fully explained by the Yarkovsky effect, the thermal force from solar radiation that gently pushes most asteroids. This excess acceleration suggests something else is at play, and a new study has just ruled out the most obvious explanation: that 1998 KY26 is a comet in disguise.

What Is Asteroid 1998 KY26?

Discovered in 1998 by an astronomer at the University of Arizona, 1998 KY26 was initially classified as a typical asteroid. It is about 11 meters in diameter, roughly the size of a small house, and it orbits the Sun on a path that brings it close to Earth. But its motion revealed a peculiarity: it was accelerating more than expected from the Yarkovsky effect alone.

Asteroids are not self-propelled; they move under the influence of external forces like gravity. The Yarkovsky effect is a subtle thermal force caused by the Sun heating one side of the asteroid, which then re-radiates that heat and produces a tiny thrust. For 1998 KY26, the observed acceleration exceeded what the Yarkovsky effect could account for, hinting at an additional, unknown source of propulsion.

The Comet Hypothesis and Why It Failed

Comets, unlike asteroids, contain significant amounts of ice. When sunlight heats a comet, the ice sublimates into gas, creating a jet that propels the comet forward—much like a rocket engine. This outgassing is why comets often have bright tails and why they can accelerate beyond the influence of gravity alone.

For years, astronomers wondered if 1998 KY26 might be a "dark comet"—a comet that has lost its bright tail but still contains enough ice to produce subtle outgassing. Dark comets are known to exist, and they can exhibit comet-like acceleration without a visible coma or tail. However, the new study, which has not yet been published in a peer-reviewed journal but is available as a preprint, throws cold water on this idea.

The research team analyzed the object's motion and found that the acceleration pattern does not match what would be expected from outgassing. The object appears to be too hot and too dry to sustain the sublimation needed for comet-like jets. In other words, 1998 KY26 is not a comet masquerading as an asteroid—it is something else entirely.

What Could Be Causing the Extra Acceleration?

With the comet explanation ruled out, scientists are left searching for alternative mechanisms. Several possibilities exist, but none are fully satisfactory yet.

• Enhanced Yarkovsky effect: The asteroid's shape, spin, and surface properties might amplify the thermal thrust beyond standard estimates.
• Rotational breakup: If the asteroid is spinning rapidly, it could be shedding material, providing a small thrust.
• Solar radiation pressure: Light itself carries momentum, and for a small object, this pressure can cause measurable acceleration.
• Unmodeled forces: There may be subtle gravitational interactions or other forces not yet accounted for in the orbital models.

Each of these hypotheses has its own challenges. For instance, the Yarkovsky effect is highly dependent on the asteroid's thermal inertia and spin axis, which are difficult to measure for such a small object. Rotational breakup would likely produce observable debris, which has not been seen. Solar radiation pressure is typically negligible for objects of this size. And unmodeled forces would require new physics or unknown perturbers.

Why This Matters for Planetary Defense

1998 KY26 is not on a collision course with Earth, but it is a near-Earth asteroid, and understanding its motion is crucial for predicting its long-term trajectory. Small asteroids like this one are notoriously hard to track because their orbits are easily perturbed by non-gravitational forces. If we can't predict their paths accurately, we can't assess the risk they pose. The longer history behind the story is traced in Mysterious Asteroid 1998 KY26 Moves Like Something Is Pushing.

Moreover, 1998 KY26 is the target of a proposed Japanese mission called Hayabusa2 Extended Mission. The spacecraft, which previously visited asteroid Ryugu, is scheduled to rendezvous with 1998 KY26 in 2031. Understanding the asteroid's acceleration is essential for planning the encounter and interpreting the data the spacecraft will collect. The background to this story is set out in Alfa Romeo Design Chief: Big Screens? Buy Something Else. A related thread runs through How to Safely Remove Something Stuck in Your Eye. Earlier coverage in Science: latest articles shows how the argument developed.

What to Watch Next

The new study calls for more observations of 1998 KY26 to pin down its physical properties. Future telescopic observations could measure its spin rate, shape, and thermal emission, which would help refine models of the Yarkovsky effect. The Hayabusa2 Extended Mission will provide close-up data that could finally solve the mystery. A closer look at the same subject appears in China Challenges US Leadership in Asteroid Defense. Anyone following the wider picture can start with Australia's Six Mystery Spheres Identified as Space Debris. The same theme surfaces in China's Tianwen-2 Probe Reaches Kamo'oalewa Asteroid After 400 Days, which takes a different angle.

In the meantime, the case of 1998 KY26 serves as a reminder that even small asteroids can harbor big surprises. As our ability to detect and track near-Earth objects improves, we may find more of these enigmatic movers, each offering clues to the diverse forces shaping our solar system. Context on how this played out before sits in Anthropic's AI Development Accelerates Despite Calls for Caution. A companion piece, Streaming's Long Wait: Why You Forget Your Favorite Show, approaches the same subject from another angle.

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