Japan’s Hayabusa2 spacecraft is currently racing toward a target that defies standard orbital physics.
As the probe travels toward the near-Earth object 1998 KY26, researchers are struggling to identify the source of the asteroid’s anomalous acceleration.
"Our temperature model results indicate that the water outgassing rate on 1998 KY26 would be too high to maintain buried ice, refining the understanding of the asteroid’s composition," aerospace engineers Korbin Hansen and Masatoshi Hirabayashi of the Georgia Institute of Technology wrote in the journal Icarus.
Their findings challenge earlier theories that the object might be a "dark comet," a hybrid body that releases hidden ice to create a faint, thruster-like effect.
Disproving the Dark Comet Theory
The dark comet hypothesis suggested that 1998 KY26 was venting gas as ice beneath its surface turned to vapor. However, the new research indicates the rock is far too warm for this to occur. Modeling shows that even the coldest regions of the object maintain an average temperature of at least 212 Kelvin (-61 degrees Celsius). For buried ice to survive, temperatures would need to stay below 145 Kelvin (-128 degrees Celsius).
Because the object is too warm to retain ice, it likely lost any water reserves long ago. This leaves the cause of the asteroid’s non-gravitational acceleration, first identified in a 2023 Planetary Science Journal paper, once again a mystery.
Surprising Physical Characteristics
The mission to reach 1998 KY26 has become increasingly complex as new data emerges. Research published in 2025 revealed the object is about three times smaller than initial estimates, measuring just 36 feet (11 meters) wide.
The asteroid also rotates at a high velocity, completing a full spin every five minutes. Because the target is barely twice the width of the spacecraft’s solar panels, the planned 2031 encounter will require extreme precision compared to the 2019 touchdown on the much larger asteroid Ryugu.
Future Rendezvous in 2031
Hayabusa2 continues its mission after successfully returning samples from Ryugu to the Australian outback in December 2020. The spacecraft is scheduled to reach 1998 KY26 in July 2031. Scientists expect that this close-range inspection will finally provide concrete data on why the rock moves in ways that gravity alone cannot explain.
Why 1998 KY26 Accelerates and How It Compares to Ryugu
Why does 1998 KY26 accelerate in a way that ignores gravity?
The cause remains unknown. While scientists previously suspected outgassing from ice—a "dark comet" effect—new temperature models show the rock is too hot to hold onto ice, meaning another force, such as thermal emission from sunlight, may be responsible.
How does the size of 1998 KY26 compare to previous targets?
The target is significantly smaller than the Japanese space agency’s previous destination, asteroid Ryugu. While Ryugu measures nearly 3,000 feet (900 meters) wide, 1998 KY26 is only 36 feet (11 meters) across, making it a much more difficult target for a spacecraft to approach.
What is the status of the Hayabusa2 mission?
The spacecraft is currently in transit following its successful 2020 sample return mission. It is on track for a rendezvous with 1998 KY26 in July 2031 to collect further geological data.