A Japanese spacecraft is on a remarkable journey toward one of the most puzzling celestial objects in our solar system — a dark comet. These enigmatic bodies, neither fully asteroids nor classic comets, have baffled scientists with unexplained bursts of acceleration, all without the visible tails that define their icy counterparts.
When the Hayabusa2 probe reaches its target, 1998 KY26, in 2031, it could shed light on a mystery that touches on nothing less than the origins of Earth’s water — and potential threats to our planet’s future.
The Strange Case Of The “Dark Comet”
According to the study, published in The Planetary Science Journal, the concept of dark comets first emerged in 2016, when astronomers spotted an object behaving like a comet but without its signature tail. These bodies look like asteroids — rocky and inert — yet move in ways that suggest hidden jets of gas are pushing them. Such accelerations are common in comets, caused when sunlight vaporizes surface ice, but these newly identified objects show no visible signs of outgassing.
One of the first, 2003 RM, exhibited non-gravitational acceleration that could not be explained even after accounting for solar radiation pressure and the Yarkovsky effect — the tiny thrust caused by heat re-emission. A year later, the interstellar visitor ‘Oumuamua showed a similar unexplained boost as it exited the solar system, reinforcing the idea that a whole category of comet-like bodies might be hiding in plain sight.
By 2024, scientists had catalogued 14 such objects. Research suggests they may be more than curiosities: if some contain hidden water ice, they could be key to understanding how Earth’s oceans formed. They might also represent an overlooked impact hazard, capable of altering their orbits suddenly enough to surprise planetary defense teams.
Two Families, One Mystery
Studies led by planetary scientist Darryl Seligman revealed that dark comets seem to fall into two distinct groups. Outer dark comets, or “outies,” are larger, more reflective bodies — some hundreds of meters across — with comet-like orbits originating near Jupiter.
The smaller inner dark comets, or “innies,” are less than 50 meters wide, with circular orbits confined to the inner solar system. These are harder to explain: despite constant exposure to the Sun’s heat, they still show signs of acceleration that could indicate trapped subsurface ice.
Their tiny size makes them vulnerable to destruction, either by spinning apart or vaporizing completely — yet they persist. The possibility remains that innies are the late-stage remnants of outies, or perhaps main-belt comets that have nearly exhausted their water supply.
Hayabusa2’s Unexpected Mission
The Japanese space agency JAXA launched Hayabusa2 in 2014 to retrieve samples from the asteroid Ryugu, a mission it completed with stunning success in 2020. With fuel to spare, the spacecraft was reassigned to explore smaller objects for planetary defense research. Its final destination — chosen before dark comets were recognized — turned out to be 1998 KY26, an inner dark comet spinning once every ten minutes and measuring only about 30 meters across.
“It’s really exciting,” says mission manager Yuichi Tsuda, who notes the coincidence was pure luck. When Hayabusa2 arrives, it will first observe the object from several kilometers away, searching for any signs of sublimation. If comet-like outgassing is the cause of its odd acceleration, the spacecraft should detect it. Later, the probe could descend closer, possibly even firing a projectile to create a crater and reveal the object’s interior composition.
A Race To Uncover Answers
The Hayabusa2 encounter will be part of a broader scientific push to understand these elusive bodies. Ground-based facilities like the Lowell Discovery Telescope will continue monitoring known dark comets, while the upcoming Vera C. Rubin Observatory promises to find many more once it begins operations.
The stakes are high. Dark comets could rewrite our understanding of the solar system’s formation. They might also reshape planetary defense strategies, ensuring that Earth can anticipate objects whose movements defy conventional prediction. As Aster Taylor of the University of Michigan puts it, “If we can’t detect these objects correctly, we will not be able to know if they’re going to hit us.”
If Hayabusa2’s 2031 rendezvous confirms the icy nature of 1998 KY26, the term “dark comet” may no longer refer to a mystery — but to a well-defined class of objects that has been hiding, quite literally, in the dark.
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