A 36-foot object named 1998 KY26 is accelerating in space through forces other than gravity. Scientists debate whether the fast-spinning rock is a natural asteroid, a dark comet, or a lost Soviet spacecraft.
Astronomers studying the trajectory of 1998 KY26 have said that unusual accelerations cannot be accounted for by the gravitational pull of the Sun or Earth alone. The object is about 36 feet wide. It completes a full rotation about once every five minutes.
Investigating The Soviet Spacecraft Hypothesis
Harvard astrophysicist Avi Loeb, Adam Hibberd, and Adam Crowl examined whether the object could be Phobos 1. That unmanned Soviet spacecraft launched on July 7, 1988, and stopped communicating while traveling to Mars. The researchers built a simplified model of the probe. They found that sunlight reflecting off its shape closely matched eight separate sets of observations of 1998 KY26, which reflects nearly 52 percent of the sunlight hitting it.
“Well, first of all, it’s not an asteroid because 1998 KY26 appears to have non-gravitational acceleration, meaning it’s being pushed by some force other than gravity, and asteroids are just rocks,” Loeb said.
NASA Administrator Jared Isaacman disputed that interpretation during an appearance on “Fox & Friends.” Isaacman said that he had not seen evidence to suggest the object is anything other than an asteroid. NASA later said that an orbital analysis published in 2025 by a team at the Center for Near-Earth Object Studies concluded the asteroid’s path is driven by solar radiation forces acting on its non-spherical shape.
Challenging The Dark Comet Theory
Another hypothesis proposed that 1998 KY26 is a dark comet. That is a rocky object that moves through space like a comet because trapped ice turns to gas and vents into space, creating thrust. Astronomers have not detected any visible cometary tail or escaping dust particles.
“We should have seen the dust because for comets we see a tail and the dust particles are being pushed away from the sun by the solar radiation,” Loeb said.
Furthermore, research published in the journal Icarus by Georgia Institute of Technology aerospace engineers Korbin Hansen and Masatoshi Hirabayashi cast doubt on the dark comet explanation by modeling the object’s temperatures. The authors found that even the coldest regions of 1998 KY26 have an average temperature of at least 212 Kelvin (-61 degrees Celsius). Buried water ice requires temperatures below about 145 Kelvin (-128 degrees Celsius) to survive.
“Our temperature model results indicate that the water outgassing rate on 1998 KY26 would be too high to maintain buried ice,” Hansen and Hirabayashi wrote.
Planning The Hayabusa2 Rendezvous
The Japan Aerospace Exploration Agency’s Hayabusa2 spacecraft is traveling toward 1998 KY26 following its extended mission after dropping samples from asteroid Ryugu in the Australian outback on December 6, 2020. The spacecraft is scheduled to rendezvous with the object in July 2031 to conduct close-range investigations.
“We predict that when they land on it, instead of seeing an iceberg, they will find the label ‘Made in the USSR,’” Loeb said.