image: An artist's impression of Japan’s Hayabusa2 space mission touching down on the surface of the asteroid 1998 KY26. New observations with ESO’s Very Large Telescope (VLT) have revealed that 1998 KY26 is just 11 m wide, almost three times smaller than previously thought, and is spinning once every 5 minutes, which is much faster than expected. The image above shows an updated size comparison between the asteroid and spacecraft.
Credit: ESO/M. Kornmesser. Asteroid: T. Santana-Ros et al. Hayabusa2 model: SuperTKG (CC-BY-SA).
Astronomers have used observatories around the world, including the European Southern Observatory's Very Large Telescope (ESO’s VLT), to study the asteroid 1998 KY26, revealing it to be almost three times smaller and spinning much faster than previously thought. The asteroid is the 2031 target for Japan’s Hayabusa2 extended mission. The new observations offer key information for the mission’s operations at the asteroid, just six years out from the spacecraft’s encounter with 1998 KY26.
“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 metres wide, meaning it could easily fit inside the dome of the VLT unit telescope used to observe it. 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 metres in diameter and completed a rotation in 10 minutes or so.
"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 manoeuvre, where the spacecraft ‘kisses’ the asteroid, 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 metres.
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-metre-size 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 characterise 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.
More information
This research was presented in a paper titled “Hayabusa2♯ mission target 1998 KY26 preview: decametre size, high albedo and rotating twice as fast” to appear in Nature Communications (doi: 10.1038/s41467-025-63697-4).
The team is composed of T. Santana-Ros (Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Universidad de Alicante, and Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Spain), P. Bartczak (Instituto Universitario de Física Aplicada a las Ciencias y a las Tecnologías, Universidad de Alicante, Spain and Astronomical Observatory Institute, Faculty of Physics and Astronomy, A. Mickiewicz University, Poland [AOI AMU]), K. Muinonen (Department of Physics, University of Helsinki, Finland [Physics UH]), A. Rożek (Institute for Astronomy, University of Edinburgh, Royal Observatory Edinburgh, UK [IfA UoE]), T. Müller (Max-Planck-Institut für extraterrestrische Physik, Germany), M. Hirabayashi (Georgia Institute of Technology, United States), D. Farnocchia (Jet Propulsion Laboratory, California Institute of Technology, USA [JPL]), D. Oszkiewicz (AOI AMU), M. Micheli (ESA ESRIN / PDO / NEO Coordination Centre, Italy), R. E. Cannon (IfA UoE), M. Brozovic (JPL), O. Hainaut (European Southern Observatory, Germany), A. K. Virkki [Physics UH], L. A. M. Benner (JPL), A. Cabrera-Lavers (GRANTECAN and Instituto de Astrofísica de Canarias, Spain), C. E. Martínez-Vázquez (International Gemini Observatory/NSF NOIRLab, USA), K. Vivas (Cerro Tololo Inter-American Observatory/NSF NOIRLab, Chile).
The European Southern Observatory (ESO) enables scientists worldwide to discover the secrets of the Universe for the benefit of all. We design, build and operate world-class observatories on the ground — which astronomers use to tackle exciting questions and spread the fascination of astronomy — and promote international collaboration for astronomy. Established as an intergovernmental organisation in 1962, today ESO is supported by 16 Member States (Austria, Belgium, Czechia, Denmark, France, Finland, Germany, Ireland, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom), along with the host state of Chile and with Australia as a Strategic Partner. ESO’s headquarters and its visitor centre and planetarium, the ESO Supernova, are located close to Munich in Germany, while the Chilean Atacama Desert, a marvellous place with unique conditions to observe the sky, hosts our telescopes. ESO operates three observing sites: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope and its Very Large Telescope Interferometer, as well as survey telescopes such as VISTA. Also at Paranal ESO will host and operate the Cherenkov Telescope Array South, the world’s largest and most sensitive gamma-ray observatory. Together with international partners, ESO operates ALMA on Chajnantor, a facility that observes the skies in the millimetre and submillimetre range. At Cerro Armazones, near Paranal, we are building “the world’s biggest eye on the sky” — ESO’s Extremely Large Telescope. From our offices in Santiago, Chile we support our operations in the country and engage with Chilean partners and society.
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Contacts
Toni Santana-Ros
Planetary Scientist, University of Alicante and University of Barcelona
Alicante and Barcelona (Catalonia), Spain
Tel: +34 965903400 Ext: 2645 / 600948703
Email: tsantanaros@icc.ub.edu
Olivier Hainaut
ESO Astronomer
Garching bei München, Germany
Tel: +49 89 3200 6754
Cell: +49 151 2262 0554
Email: ohainaut@eso.org
Bárbara Ferreira
ESO Media Manager
Garching bei München, Germany
Tel: +49 89 3200 6670
Cell: +49 151 241 664 00
Email: press@eso.org
Journal
Nature Communications