Somewhere between the ordinary rules of celestial motion and the unexplained, a small rock called 1998 KY26 has begun to drift in a way no one can fully account for. Its path bends with a push that gravity and familiar forces do not supply, and for a time the most tempting explanation was that it hides ice beneath a dark crust, a comet wearing the disguise of an asteroid. Researchers at the Georgia Institute of Technology now argue that the object is simply too warm to have kept such ice, which turns attention toward quieter forces such as the pressure of sunlight. The mystery remains open, bu
Mysterious Asteroid 1998 KY26 Is Too Warm to Be a Hidden Comet, Scientists Find
Deathly cold for a human, but far too warm for cometary ice
Mark
So what's the basic finding here?
Mimi
That 1998 KY26 is too warm to hold buried water ice. Its coldest regions still average at least 212 Kelvin over an orbit, and ice needs to stay under about 145 Kelvin.
Mark
Why does ice matter for the acceleration?
Mimi
A dark comet would be steered by gas escaping as ice sublimates. No ice, no thruster.
Luke
But this is a model, not a measurement. Hansen and Hirabayashi built temperatures from size, orbit, reflectivity and likely composition. Nobody has touched the surface.
Mimi
True, and the inputs include a likely composition, which is an assumption. Still, the gap between 212 and 145 Kelvin is wide.
Mark
And the lifetime numbers?
Mimi
One percent ice would last just over 2,000 years; ice powering the acceleration would last 62 years.
Luke
Those are model outputs too. And the paper leaves a gap: we don't know how long KY26 has been in this orbit, so some ice isn't ruled out.
Mark
So what replaces the dark comet idea?
Mimi
Solar radiation pressure is a candidate, but it's a possibility, not a confirmed cause.
Luke
Right. Nothing is settled until Hayabusa2 arrives in July 2031.
Il Polso
- A tiny 11-meter asteroid is accelerating in a direction that ordinary physics cannot explain, and astronomers cannot shake the puzzle.
- The leading theory, that it is a hidden dark comet venting ice, now faces a sharp challenge from thermal models.
- Georgia Tech engineers show its surface and interior run far too warm to keep buried water ice, closing one escape hatch.
- Sunlight pressure has emerged as the new front-runner, and Japan's Hayabusa2 is on course to arrive in July 2031 to settle the matter.
Somewhere between the ordinary rules of celestial motion and the unexplained, a small rock called 1998 KY26 has begun to drift in a way no one can fully account for. Its path bends with a push that gravity and familiar forces do not supply, and for a time the most tempting explanation was that it hides ice beneath a dark crust, a comet wearing the disguise of an asteroid. Researchers at the Georgia Institute of Technology now argue that the object is simply too warm to have kept such ice, which turns attention toward quieter forces such as the pressure of sunlight. The mystery remains open, but it is the kind of question that humanity answers slowly, one careful measurement at a time.
Most asteroids keep to their predicted paths, shaped by gravity and the gentle push of sunlight. The small body 1998 KY26 does not. Its orbit bends perpendicular to its plane, a nudge that ordinary forces cannot account for, and scientists have been searching for the cause.
One idea was that KY26 is a dark comet: an object that looks like a rock but carries ice that vents gas and quietly pushes it along, the way visible comets do with their glowing tails. The asteroid is about 11 meters across and spins every 5.35 minutes, and it fits the profile well enough to deserve a test. The most direct test is whether it is cold enough to keep ice at all.
Aerospace engineers Korbin Hansen and Masatoshi Hirabayashi at Georgia Tech modeled its temperatures from its size, orbit, reflectivity, and likely composition. Buried water ice survives only below about 145 Kelvin, yet even the coldest regions of KY26 average at least 212 Kelvin over an orbit. Their numbers are stark: a 1 percent ice fraction would vanish in roughly two thousand years, or in just 62 years if that ice drove the acceleration.
Some uncertainty remains, since the asteroid's time in its current orbit is unknown and ancient ice could linger. Still, the dark-comet explanation is weakening, and attention is turning to solar radiation pressure as a plausible cause. Hayabusa2 is already on its way, with a rendezvous planned for July 2031, and it may finally reveal what is pushing this small stone through the dark.
Most asteroids behave. They follow orbits shaped by gravity and by the gentle push of sunlight, and astronomers can predict where they will be. The small asteroid 1998 KY26 does not fit that pattern. Its trajectory shows an unexpected acceleration perpendicular to the plane of its orbit, a nudge that gravity and ordinary forces do not account for.
One proposed explanation was that KY26 is not a plain asteroid at all, but a dark comet. Ordinary comets are known for glowing halos and luminous tails, produced as ice turns to gas and escapes into space. That outflow also acts as a small thruster, pushing the body beyond what gravity alone would do. Dark comets are thought to do the same thing without the visible show. They look like asteroids with a bit of extra acceleration, which is what KY26 appears to be.
The object is a good candidate on other counts. It is about 11 meters (36 feet) across and spins once every 5.35 minutes. Because ice is the defining ingredient of a comet, the test for a cometary identity is fairly direct: check whether it is cold enough to keep any.
That is what aerospace engineers Korbin Hansen and Masatoshi Hirabayashi of the Georgia Institute of Technology did, in a paper published in Icarus. They did not take a literal temperature reading. They combined what is known about KY26, including its size, orbit, reflectivity and likely composition, to model temperatures across its surface and deep within it.
Buried water ice needs to stay below roughly 145 Kelvin (-128 degrees Celsius) to survive over long periods. The model found that even the coldest regions of KY26 average at least 212 Kelvin (-61 degrees Celsius) over a full orbit around the Sun. That is brutally cold by human standards but far too warm for cometary ice. In the researchers' words, the water outgassing rate would be too high to maintain buried ice.
They put numbers on it. If water ice made up 1 percent of KY26's mass, it would vanish in just over 2,000 years. If that ice were also responsible for the observed acceleration, it would be gone in only 62 years.
There is a caveat. KY26 may be billions of years old, but it is unclear how long it has been in its current orbit, so it could still hold some ice. Still, an earlier paper traced the asteroid to a possible origin in the asteroid belt, and together with the new result, that possibility is shrinking.
If not outgassing, what is pushing it? Radiation pressure from the Sun is a candidate. Sunlight exerts a small force that may be strong enough to explain the behavior. Answers may come soon: Japan's Hayabusa2 spacecraft is already heading toward KY26, with a rendezvous scheduled for July 2031.
Citazioni salienti
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.— Korbin Hansen and Masatoshi Hirabayashi, Georgia Institute of Technology, in Icarus
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