A small object racing through the solar system is accelerating in a way that gravity alone may not explain. That has prompted speculation that it could be a Soviet spacecraft lost almost 40 years ago.
Called 1998 KY26, the object is about 36 feet wide and is generally thought to be a near-Earth asteroid. Scientists have identified an unexplained change in its motion, but new research challenges one proposed explanation.
NASA Administrator Jared Isaacman addressed the speculation Thursday, saying some people who follow reports of unexplained aerial phenomena have pointed to KY26’s unusual movement. He said he has seen no evidence that it is anything but an asteroid.
“First of all, nothing is about to hit us, to be very clear on this,” Isaacman said. “I’ve not seen any evidence to suggest this is a threat to our planet or anything other than an asteroid.”
Scientists call the unexplained change in KY26’s path “nongravitational acceleration,” meaning an influence other than the gravity of the sun and planets may be at work.
One explanation is that KY26 is a “dark comet.” In this scenario, water trapped beneath its surface turns to gas and pushes the object along, despite astronomers not detecting the cloud or tail typically associated with a comet.
A peer-reviewed study by Georgia Institute of Technology researchers Korbin Hansen and Masatoshi Hirabayashi, published in Icarus, casts doubt on that idea. The researchers found that KY26 is probably too warm for buried water ice to last long enough to explain its motion. Their calculations, they wrote, “preclude 1998 KY26 being a dark comet at present.”
Other explanations remain possible. Earlier research suggested that sunlight warming the object and heat radiating from its surface could create an extra push, without any buried water ice.
A separate team has investigated a more unusual possibility: that KY26 is actually Phobos 1, a Soviet spacecraft that lost contact while on its way to Mars in 1988. The team includes Harvard astrophysicist Avi Loeb and researchers Adam Hibberd and Adam Crowl.
KY26 reflects about 52% of the sunlight that reaches it and spins once every 5.35 minutes. The researchers created a simplified model of Phobos 1 and compared how it would reflect light while rotating with observations of KY26. They found a close match across eight sets of observations.
They also tested whether sunlight pushing on a cylinder or solar panels could account for the object’s movement. That model, they reported, fit hundreds of telescope and radar observations more closely.
The researchers cautioned that their work cannot establish KY26’s identity. Still, they said the results “add strong weight to the Phobos 1 hypothesis,” which they described as “strongly plausible but presently unproven.”
Other findings point toward a more familiar explanation. A study published this year in Astronomy & Astrophysics analyzed light reflected by KY26 and found that its composition appears consistent with a rocky asteroid. The researchers concluded it likely originated in the inner asteroid belt.
Isaacman said NASA is continuing to track KY26 but sees no reason for concern. “I have no reason to believe this is in any way remotely a threat to our planet or anything other than just an asteroid,” he said.
More direct evidence may arrive in the future. Japan’s Hayabusa2 spacecraft is scheduled to reach KY26 in July 2031, according to a 2025 study in Nature Communications. Loeb has said the mission could provide a decisive test of the spacecraft theory.
“Our prediction is that instead of an iceberg, Hayabusa 2’s cameras will reveal the label ‘Made in the U.S.S.R.,’” Loeb wrote.
