Astronomers have discovered that asteroid 1998 KY26, the target of Japan’s Hayabusa2 extended mission, is far smaller and faster-spinning than previously thought.
Astronomers have conducted a new study of the asteroid 1998 KY26 using observatories across the globe, including the European Southern Observatory’s Very Large Telescope (ESO’s VLT). Their findings show that the asteroid is nearly three times smaller than earlier estimates and spins much faster than expected.
This small but fast-moving object has been chosen as the 2031 destination for Japan’s Hayabusa2 extended mission. With only six years remaining until the spacecraft arrives, the updated measurements provide crucial details to help plan operations at the asteroid.
“We found that the reality of the object is completely different from what it was previously described as,” says astronomer Toni Santana-Ros, a researcher from the University of Alicante, Spain, who led a study on 1998 KY26 published today in Nature Communications.
The new observations, combined with previous radar data, have revealed that the asteroid is just 11 meters wide, meaning it could easily fit inside the dome of the VLT unit telescope used to observe it (see image below). It is also spinning about twice as fast as previously thought: “One day on this asteroid lasts only five minutes!” he says. Previous data indicated that the asteroid was around 30 meters in diameter and completed a rotation in 10 minutes or so.
A Challenge for Hayabusa2
“The smaller size and faster rotation now measured will make Hayabusa2’s visit even more interesting, but also even more challenging,” says co-author Olivier Hainaut, an astronomer at ESO in Germany. This is because a touchdown maneuver, where the spacecraft ‘kisses’ the asteroid (see video below), will be more difficult to perform than anticipated.
1998 KY26 is set to be the final target asteroid for the Japanese Aerospace eXploration Agency (JAXA)’s Hayabusa2 spacecraft. In its original mission, Hayabusa2 explored the 900-metre-diameter asteroid 162173 Ryugu in 2018, returning asteroid samples to Earth in 2020.
With fuel remaining, the spacecraft was sent on an extended mission until 2031, when it’s set to encounter 1998 KY26, aiming to learn more about the smallest asteroids. This will be the first time a space mission encounters a tiny asteroid — all previous missions visited asteroids with diameters in the hundreds or even thousands of meters.
This animation shows the touchdown manoeuvre that Japan’s Hayabusa2 spacecraft is likely to perform when it reaches its target in 2031, in a brief encounter with the asteroid 1998 KY26. Now that a new study has shown that this asteroid is roughly three times smaller than previously expected, and spinning twice as fast, this procedure may be more difficult to conduct. Credit: ESO/M. Kornmesser. Asteroid: T. Santana-Ros et al. Hayabusa2 model: SuperTKG (CC-BY-SA)
Observing a Faint Target
Santana-Ros and his team observed 1998 KY26 from the ground to support the preparation of the mission. Because the asteroid is very small and, hence, very faint, studying it required waiting for a close encounter with Earth and using large telescopes, like ESO’s VLT in Chile’s Atacama Desert.
The observations revealed that the asteroid has a bright surface and likely consists of a solid chunk of rock, which may have originated from a piece of a planet or another asteroid. However, the team could not completely rule out the possibility that the asteroid is made up of rubble piles loosely sticking together. “We have never seen a ten-meter-sized asteroid in situ, so we don’t really know what to expect and how it will look,” says Santana-Ros, who is also affiliated with the University of Barcelona.
“The amazing story here is that we found that the size of the asteroid is comparable to the size of the spacecraft that is going to visit it! And we were able to characterise such a small object using our telescopes, which means that we can do it for other objects in the future,” says Santana-Ros. “Our methods could have an impact on the plans for future near-Earth asteroid exploration or even asteroid mining.”
“Moreover, we now know we can characterize even the smallest hazardous asteroids that could impact Earth, such as the one that hit near Chelyabinsk, in Russia in 2013, which was barely larger than KY26,” concludes Hainaut.
Reference: “Hayabusa2 extended mission target asteroid 1998 KY26 is smaller and rotating faster than previously known” by T. Santana-Ros, P. Bartczak, K. Muinonen, A. Rożek, T. Müller, M. Hirabayashi, D. Farnocchia, M. Micheli, R. E. Cannon, M. Brozović, O. Hainaut, D. Oszkiewicz, A. K. Virkki, L. A. M. Benner, A. Campo Bagatin, P. G. Benavidez, A. Cabrera-Lavers, C. E. Martínez-Vázquez and K. Vivas, 18 September 2025, Nature Communications.
DOI: 10.1038/s41467-025-63697-4
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