Doubt Creeps Into Hayabusa 2’s Planned Visit To Its Next Asteroid

In November 2019, Japan’s Hayabusa 2 mission departed the asteroid Ryugu after 1.5 years of observations. It had successfully collected a sample from the near-Earth object and in December 2020, the spacecraft returned the valuable sample to Earth. If its mission had ended there, it would’ve been deemed one of the most successful and challenging missions ever conducted.

But Hayabusa 2 still had 30 kg of xenon propellant remaining for its ion drives. JAXA, the Japan Aerospace Exploration Agency, implemented a mission extension. They directed the spacecraft to visit two more asteroids. It would perform a fly-by of the 500-meter 98943 Torifune in July 2026, coming to within 100 meters of the asteroid. In July 2031, the spacecraft would rendezvous with 1998 KY26. At the time, astronomers thought that 1998 KY26 was about 30 meters in diameter, and rotating every 10.7 minutes.

Visiting KY26 was an opportunity to visit and understand a small, decameter-sized asteroid. While our knowledge of larger asteroids has grown because we’ve sampled them and they’re easier to observe, these smaller asteroids are more challenging to study. Asteroids hold clues to the evolution of the Solar System and how planets formed, so they’re important scientific targets. However, there’s another compelling reason to study smaller ones like KY26. Both the Tunguska and Chelyabinsk impactors were in this size range, and so are most other potential impactors, so understanding KY26 is important for planetary defense. Additionally, KY26 is about the same size of the boulders observed on the surfaces of the asteroids Itokawa, Ryugu, and Bennu, all of which have have been successfully sampled.

These computer 3D models of 1998 KY26 were generated from data available in 1998. It took decades before better observations were available. Image Credit: By NASA’s JPL Digital Image Animation Laboratory. – [1]: Steven Ostro’s Homepage: 1998KY26Transferred from en.wikipedia to Commons by User:Bulwersator using CommonsHelper., Public Domain, https://commons.wikimedia.org/w/index.php?curid=17725693

New research suggests that Hayabusa 2 may be limited to observing KY26 and that a touchdown is impossible. The asteroid appears to be much smaller, and also rotating more quickly, than thought. That could prohibit any contact.

The research is titled “Hayabusa2 extended mission target asteroid 1998 KY26 is smaller and rotating faster than previously known,” and it appears in Nature Communications. The lead author is astronomer Toni Santana-Ros, a researcher from the University of Alicante in Spain.

“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!” – Toni Santana-Ros, University of Alicante.

1998 KY26 was discovered, obviously, in 1998. Even though it’s very small, it reached magnitude 16 during its close approach to Earth. It was only 2 lunar distances away. Observations determined that its spin period was only 10.7 min, which was the fastest asteroid rotation rate ever measured at that time. Observations also showed that its diameter was about 30m, plus or minus 10 m. With those measurements being the only published characteristics of the asteroid, JAXA chose it as a target for Hayabusa 2’s extended mission.

The problem is that in the intervening decades, the asteroid never exceeded magnitude 24. Only in 2024 did the asteroid reach magnitude 20.2 during another close approach to Earth at 12 lunar distances. This was an opportunity to observe KY26 again and to check its characteristics. The new research is based on those observations.

“We found that the reality of the object is completely different from what it was previously described as,” said lead author Santana-Ros in a press release. The research team’s new observations, combined with previous radar data, show that the asteroid is only 11 meters wide. Also, it’s rotation rate is almost double the original measurement. “One day on this asteroid lasts only five minutes!” Santana-Ros said. This is a big change from previous measurements.

The left and right columns each show a different method of determining 1998 KY26’s shape used in the research. The model is shown from the perspective of each principal axis (X, Y, Z), which are three mutually perpendicular directions that form a right-handed coordinate system. The rotation axis is aligned with the Z axis. Both models show that 1998 KY26 is an asymmetric, moderately elongated object. Image Credit: Santana-Ros et al. 2025. NatComm

“The smaller size and faster rotation now measured will make Hayabusa2’s visit even more interesting, but also even more challenging,” said co-author Olivier Hainaut, an astronomer at ESO in Germany.

“These results suggest the need to reassess the scientific objectives and mission operations of Hayabusa2, while also highlighting the value of combining multiple remote sensing techniques for the study of decametre-scale objects,” the authors write in their research.

There’s no doubt that Hayabusa 2 can still visit KY26. The visit is a chance to validate the way astronomers observe these small objects and how the results are best understood. Since objects in this size range pose the most threat to Earth, understanding how their orbits evolve is important for planetary defense. “Additionally, gaining insights into the non-gravitational forces acting on these bodies will enable more accurate orbit predictions. This knowledge is critical for future planetary defense operations, where it will help identify and mitigate potential impact threats,” the authors write.

“However, 1998 KY26’s smaller size, faster rotation, and higher albedo present challenges for mission operations,” the researchers explain. For example, some of its instruments require long integration times that might not be possible with such a rapid rotation. But there’s six years until the spacecraft reaches the asteroid, so some planning may mitigate some of the challenges.

JAXA will only have a limited opportunity to get this visit right. Hayabusa 2 will be at the end of its fuel when it reaches KY26, so efficiency will be key. Even if the spacecraft can’t touch down on the asteroid, the mission will still be a success because it will be the only visit to a decameter-sized object. “We have never seen a ten-meter-size asteroid in situ, so we don’t really know what to expect and how it will look,” said Santana-Ros.

This artist’s illustration shows Hayabusa 2 touching down and gathering a sample from an asteroid. Will that be possible at 1998 KY26? Image Credit: By JGarry at English Wikipedia – Transferred from en.wikipedia to Commons by Drilnoth using CommonsHelper., Public Domain, https://commons.wikimedia.org/w/index.php?curid=6385449

Beyond the mission itself, this research shows how astronomers can characterize even small asteroids.

“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 characterize 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,” said Hainaut.

“If successful, Hayabusa2 will provide invaluable in situ data on a rare bright decametre object, offering unique insights into this class of asteroids,” the researchers conclude.