A mysterious object speeding through the solar system is accelerating in a way gravity alone cannot explain, fueling a theory that it could be a Soviet spacecraft lost nearly four decades ago.

Known as 1998 KY26, the roughly 36-foot-wide object is believed to be a near-Earth asteroid, but something other than gravity appears to be affecting its path. New research has now cast doubt on one explanation for what is pushing it through space.

NASA Administrator Jared Isaacman weighed in on the growing speculation Thursday morning on “Fox & Friends,” saying some in the UAP community believe the object is making unusual movements, but he has seen no evidence KY26 is anything other than 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 refer to KY26’s unusual movement as “nongravitational acceleration,” meaning something besides the pull of the sun and planets is affecting its path.

One theory has been that KY26 is a “dark comet” containing buried water that turns to gas and gives the object a push through space, even though astronomers have not detected the cloud or tail normally associated with a comet.

But a new peer-reviewed study by Georgia Institute of Technology researchers Korbin Hansen and Masatoshi Hirabayashi casts doubt on that explanation.

Their study, published in the journal Icarus, found KY26 is likely too warm for buried water ice to survive long enough to account for the unusual movement.

The researchers concluded that calculated water-loss rates “preclude 1998 KY26 being a dark comet at present.”

There are other possible explanations. Previous research found that sunlight hitting KY26 and heat escaping from its surface could produce the extra push without buried water.

Another group of researchers, however, has explored a far more unusual possibility.

Harvard astrophysicist Avi Loeb and researchers Adam Hibberd and Adam Crowl have examined whether KY26 could actually be Phobos 1, an unmanned Soviet spacecraft that stopped communicating while traveling to Mars in 1988.

KY26 is about 36 feet across, reflects roughly 52% of the sunlight that hits it and completes a full rotation every 5.35 minutes.

The researchers built a simplified model of Phobos 1 and compared the way it would reflect sunlight as it spun with observations of KY26. They found the spacecraft model closely matched eight sets of observations of the object.

They also tested whether sunlight pushing against a cylinder or solar panels could help explain KY26’s movement and found the model better matched hundreds of telescope and radar observations.

“Although an investigation of this kind cannot be conclusive,” the researchers wrote, the findings “add strong weight to the Phobos 1 hypothesis.”

The researchers stressed, though, that their model does not prove KY26 is the missing spacecraft, describing the theory as “strongly plausible but presently unproven.”

Other research points to a more conventional answer.

A separate study published this year in Astronomy & Astrophysics examined the light reflected by KY26 and found its makeup appears consistent with a type of rocky asteroid. Researchers concluded it likely came from the inner part of the asteroid belt.

Isaacman said NASA continues to monitor KY26 but sees no reason for alarm.

“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.

Scientists may eventually get a definitive answer.

The Japan Aerospace Exploration Agency’s Hayabusa2 spacecraft is scheduled to rendezvous with KY26 in July 2031, according to a 2025 study published in Nature Communications.

For Loeb, that encounter could provide the ultimate test of the Phobos 1 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.