Hayabusa2's next asteroid, 1998 KY26, is too warm to hide ice, a new study finds

A rock about as wide as a house travels near Earth’s orbit, and a Japanese spacecraft is on its way to meet it. Some scientists have wondered whether this asteroid, 1998 KY26, is secretly a “dark comet”. That would mean it hides ice that slowly turns to gas and gives it a tiny push.

A new study says that is very unlikely. In a paper published online in the journal Icarus on 28 October, Korbin Hansen and Masatoshi Hirabayashi of the Georgia Institute of Technology report that it is too warm to keep ice, on the surface or deep inside.

That matters because JAXA’s Hayabusa2 spacecraft is due to reach 1998 KY26 in July 2031. Knowing what the rock is likely made of helps scientists plan. (The picture at the top is an artist’s illustration from ESO; nobody has yet seen the asteroid up close.)

1998 KY26: a house-sized rock with a small, odd drift

Hayabusa2 has done this before: it explored the 900-metre asteroid Ryugu in 2018 and brought samples back to Earth in 2020, ESO notes. With fuel to spare, JAXA sent it on an extended Hayabusa2 mission, with 1998 KY26 as the final stop.

In September 2025, a team led by Toni Santana-Ros of the University of Alicante reported in Nature Communications that the asteroid is only about 11 metres wide. That is almost three times smaller than earlier estimates of around 30 metres. It spins once every 5.35 minutes. No spacecraft has ever visited an asteroid this small.

Tracking also shows something odd. The asteroid is pushed slightly out of the plane of its orbit, in a way gravity alone does not explain. A 2023 study in the Planetary Science Journal by Seligman and colleagues listed it among small asteroids with pushes like this. It raised the dark-comet idea: invisible jets of gas, with no visible tail, steering the rock.

Thirteen heat models, one answer

Hansen and Hirabayashi tested that idea with physics. They modelled how sunlight warms the asteroid as it travels around the Sun. Many details are still unknown, so they ran 13 versions with different settings. These covered its size, how well it carries and stores heat, how much light it reflects and which way its spin axis points.

Every version gave the same result. Averaged over a full orbit, every point on the surface and inside stays well above the temperature at which ice turns straight into gas in space. The researchers calculated, as reported by ScienceAlert, that ice must stay below about 145 kelvin (−128 °C) to last. Even the asteroid’s coldest spots average at least 212 K (−61 °C).

So water would escape too fast for buried ice to survive, the authors conclude.

What the heat model cannot tell us yet

The study removes one explanation, not every one. Sunlight itself can nudge a small rock, because light carries a very small pressure. ScienceAlert notes this remains a possible cause. The paper adds that 1998 KY26 may belong to a largely dry type of asteroid, which fits the bright colours seen in the 2025 observations.

There are honest limits. Nobody knows how long the asteroid has been in its current orbit. And this is a careful model, but a calculation, not a sample. The paper’s title calls conditions for ice “thermally unfavorable”: very unlikely, not impossible in every case.

Why a tiny rock is worth your attention

For students and space fans, this is a clear lesson in testing an idea. The dark-comet theory predicted ice. Physics checked, and the rock is too warm.

For planetary defence, small details matter. Predicting where small near-Earth asteroids go depends on understanding even tiny pushes like this one. A close look helps, as Europe’s Hera mission showed when it saw the asteroid pair NASA’s DART hit. JAXA is busy elsewhere too, with its MMX mission to Mars’ moon Phobos.

Ruling one idea out is quiet progress

On balance, this is a modest but useful result. It does not solve the drift, and it does not claim to. It removes a tempting explanation with clear physics, helping the mission team prepare for what is most likely a dry, warm rock.

The caution is that models depend on their inputs, and some, like the asteroid’s real shape and spin axis, are still uncertain. Thirteen runs reduce that risk; they cannot remove it.

Headlines about a mysterious asteroid accelerating in space oversell it a little. The odd push was noticed years ago. What is new is a firmer reason to doubt the ice. In practice, puzzles about small bodies are often settled by a close look rather than by more models.

That close look is coming. In July 2031, Hayabusa2 should arrive, and a rock the size of a house can finally answer for itself.