A mysterious object speeding through the Solar System and showing unusual acceleration has sparked speculation that it could be a Soviet spacecraft that went missing nearly 40 years ago. The object, known as 1998 KY26, is an 11-meter-wide body passing near Earth, Euronews reports.
Scientists have noticed that its trajectory is not fully explained by the gravitational pull of the Sun or Earth. This behavior is called “non-gravitational acceleration,” meaning that forces other than gravity are influencing the object’s path and speed. NASA chief Jared Isaacman said on Thursday that the agency is monitoring KY26, but stressed that there is no reason for concern.
According to him, the object poses no known threat to Earth and there is no evidence that it could collide with the planet. A group of researchers has put forward an unusual hypothesis. Harvard astrophysicist Avi Loeb and his two colleagues, Adam Hibberd and Adam Crowl, believe that KY26 could be Phobos 1, an uncrewed Soviet spacecraft launched on July 7, 1988.
The probe lost contact with Earth while en route to Mars. The researchers built a simplified model of Phobos 1 and compared how it would reflect sunlight as it rotated with observations of KY26. The model closely matched eight separate sets of observations of the object.
KY26 reflects almost 52% of the sunlight that hits it and completes a full rotation every 5.35 minutes. Loeb and his colleagues argue that these characteristics match their hypothesis that KY26 could be an artificial object. One explanation, first proposed in a 2023 study, is that KY26 is a so-called “dark comet” containing trapped water, which turns into gas and creates thrust as the object moves through space.
However, astronomers have not detected gas clouds or a tail, as is usually the case with comets. Another scientific study has offered an opposing explanation. The researchers concluded that 1998 KY26 is most likely a dehydrated asteroid.
This would mean that water ice is unlikely to be responsible for its unusual acceleration, since temperatures are too high for ice to survive for a long time. Another possible explanation is solar radiation, which could exert pressure on the object’s surface and slightly alter its trajectory.
Other observations also point toward a natural rocky asteroid, not an old spacecraft. A study published in the journal Nature Communications in 2025 estimated its diameter at just 11 meters. A Japanese space mission may eventually provide important answers.
The Hayabusa2 spacecraft is expected to reach 1998 KY26 in July 2031 and study it up close. This would be the first mission to directly investigate an asteroid of such a small size. Images and measurements taken during the encounter could reveal whether the object is truly a natural rocky body or something potentially artificial.
Until then, Avi Loeb’s suggestion remains a fascinating but unproven theory. The Harvard astrophysicist himself has even joked that Hayabusa2’s cameras could reveal a label reading “Made in U.S.S.R.” instead of a block of ice.