1998 KY26 asteroid: new study rejects ice-driven acceleration, Loeb proposes lost Soviet probe

A study challenges the ice explanation

A study by two Georgia Institute of Technology researchers, published in the journal Icarus, tests the leading explanation for 1998 KY26, an object about 11 metres across. That explanation, the 'dark comet' hypothesis, holds that buried ice turns to gas and pushes the body like a small engine. The researchers built a three-dimensional thermal model to calculate how warm the object gets along its orbit around the Sun. Even its coldest regions average about minus 47 to minus 61 degrees Celsius or warmer, while water ice would need to stay colder than about minus 128 degrees to survive for long. The authors, Korbin Hansen and Masatoshi Hirabayashi, conclude that the object is probably a dehydrated asteroid and therefore not a dark comet at present.

The models also put a timescale on the ice idea. A hypothetical ice reserve equal to 1% of the object's mass would be lost over just over 2000 years. If that same ice were producing the observed acceleration, the reserve would run out after about 62 years. The study does not offer an explanation of its own. It points instead to an earlier study which argued that solar radiation pressure and heat radiated by the body could together account for the deviation in its orbit.

Ice reserve at 1% of mass, no acceleration2,000

Ice reserve at 1% of mass, if driving acceleration62

Ice reserve at 1% of mass, no acceleration

2000 years

Ice reserve at 1% of mass, if driving acceleration

62 years

Data from the article text

Discovery and the acceleration puzzle

A University of Arizona astronomer discovered the object in 1998, according to El Confidencial, and El Mundo reports it was first seen that May. In 2023, researchers analysing about 25 years of telescope observations found that it moves differently from what the gravity of the Sun and planets alone would predict. Since then it has been treated as a possible dark comet, although astronomers have observed neither a tail nor a dust cloud around it. The object completes one rotation every 5.35 minutes, a figure El Confidencial attributes to observations and radar measurements in a 2025 study in Nature Communications. ESA's asteroid database gives an orbital period of 498 Earth days. ESA projects the object at about 51.5 million km from Earth in 2028, and at its May 2039 approach Il Fatto Quotidiano cites about 16.3 million km, while Adevarul reports under 16.1 million km. The object is currently roughly 300 million km away, according to TheSkyLive.

The Phobos 1 hypothesis

Harvard astrophysicist Avi Loeb has proposed a far stranger explanation: that the object could be Phobos 1, a Soviet probe launched in 1988 on a mission to Mars and lost a few months after departure. Loeb, together with Adam Hibberd and Adam Crowl, built a simplified model of the probe and compared how it would reflect sunlight with the object's behaviour. They argue that the object's size, shape and unusual acceleration could point to an artificial origin. In their account, solar radiation pressure acting on the probe's shiny metal surfaces and solar panels could produce the acceleration. NASA sees no sign that the object is anything other than an asteroid, and El Mundo stresses that the probe link remains an unconfirmed theory.

Hayabusa2 and what comes next

Japan's Hayabusa2 spacecraft, launched in 2014, has already made several asteroid flybys and collected samples. JAXA expects it to reach 1998 KY26, the mission's last target, in July 2031. El Mundo reports that the probe would be able to study the object's composition, its surface and the forces that change its trajectory. Until then the question stays open: the Georgia Tech study rejects the ice explanation, Loeb's probe theory is unconfirmed, and NASA treats the object as an asteroid.