Japanese spacecraft faces a massive challenge from a house-size asteroid

"We found that the reality of the object is completely different from what it was previously described as," says astronomer Toni Santana-Ros of the University of Alicante, Spain, who led the Nature Communications study. By combining the new results with earlier radar data, the team determined that the asteroid is only 11 meters across, small enough to fit inside the dome of the VLT unit telescope used during the observations. They also discovered that the asteroid completes a rotation in roughly five minutes. Previous work suggested a diameter of about 30 meters and a rotation period closer to ten minutes.

A Smaller and Faster Asteroid Raises Mission Challenges

"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. The rapid spin and tiny size mean that performing a touchdown maneuver, in which the spacecraft briefly makes contact with the surface, will be more difficult than mission teams originally expected.

1998 KY26 is planned as the final target of the Japanese Aerospace eXploration Agency (JAXA)'s Hayabusa2 spacecraft. During its primary mission, Hayabusa2 visited the 900-meter-diameter asteroid 162173 Ryugu in 2018 and returned samples to Earth in 2020. With sufficient fuel remaining, the spacecraft was assigned an extended mission ending in 2031, when it will reach 1998 KY26 to investigate very small asteroids. This encounter will mark the first time a spacecraft visits an asteroid of such tiny size, as all previous missions have explored bodies hundreds or thousands of meters wide.

Ground Telescopes Capture Rare Details of a Tiny Target

To support mission planning, Santana-Ros and colleagues observed 1998 KY26 from Earth. Because the asteroid is both extremely small and faint, the team needed to wait until the object made a relatively close pass by Earth and then rely on some of the largest available telescopes, including ESO's VLT in the Atacama Desert of northern Chile.

The observations indicate that the asteroid has a bright surface and is probably a solid piece of rock, possibly originating from a fractured planet or another asteroid. Even so, the researchers cannot completely rule out that it might instead be a cluster of loosely bound debris. "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," says Santana-Ros, who is also affiliated with the University of Barcelona.

Insights for Future Exploration and Planetary Defense

"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," concludes Hainaut.

The findings appear in the paper titled "Hayabusa2♯ mission target 1998 KY26 preview: decametre size, high albedo and rotating twice as fast" published in Nature Communications.

The research team includes 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).