Mysterious object in space: Could it be a Soviet probe missing for 40 years?

A mysterious object speeding through the Solar System has shown unusual acceleration, raising speculation that it could be a spacecraft from the former Soviet Union that went missing nearly 40 years ago.

Known as 1998 KY26, the object is an 11-metre-wide body passing near Earth. Scientists have noticed that its trajectory cannot be fully explained by the gravitational pull of the Sun or Earth.

This unusual behaviour is known as "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 there is no reason for concern. He reassured the public that the object poses no known threat to Earth and that there is no evidence it could collide with our planet.

Has Phobos 1 been found?

A group of researchers has put forward an unusual explanation. Harvard astrophysicist Avi Loeb and two colleagues, Adam Hibberd and Adam Crowl, believe KY26 could be Phobos 1, an unmanned Soviet spacecraft launched on 7 July 1988. The probe lost contact with Earth while on its way 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. They found that the model closely matched eight separate sets of observations of the object.

KY26 reflects almost 52% of the sunlight that hits it and completes one full rotation every 5.35 minutes. Loeb and his colleagues argue that these characteristics are consistent with their hypothesis that KY26 could be an artificial object.

New study challenges the dark comet theory

One explanation, first proposed in a 2023 study, is that KY26 is a so-called 'dark comet' containing trapped water that turns into gas, creating thrust as the object moves through space. However, astronomers have not detected the cloud of gas or tail normally associated with comets.

A separate scientific study has offered a contrasting explanation. 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, as temperatures are too high for sufficient ice to survive over an extended period.

Another possible explanation is solar radiation, which can exert pressure on the object's surface and slightly alter its trajectory.

Other observations also point towards a natural, rocky asteroid rather than an old spacecraft. A study published in the journal Nature Communications in 2025 estimated its diameter at just 11 metres.

Japanese spacecraft could solve the mystery

A Japanese space mission could eventually provide important answers. The Hayabusa2 spacecraft is due to reach 1998 KY26 in July 2031 and study it at close range. It would be the first mission to directly investigate an asteroid of this tiny size.

Images and measurements taken during the encounter could reveal whether the object is indeed a natural rocky body or potentially something artificial.

Until then, Avi Loeb's suggestion remains a fascinating but unproven theory. The Harvard astrophysicist has even joked that Hayabusa2's cameras might discover a label reading "Made in the U.S.S.R." rather than a block of ice.