The asteroid was studied originally, but briefly, in 1998-1999 using the Goldstone radar and photometric observations and, extensively, in 2024-2025 by an international research team of researchers that used new astronomical observations made at the largest ground-based telescopes such as the Very Large Telescope at the European Southern Observatory (ESO). At the University of Helsinki, Academy Professor Karri Muinonen and Academy Research Fellow Anne Virkki participated in the interpretation of the observations.
The brightness variations (lightcurves) observed for the rotating, nonspherical asteroid allowed to estimate its rotation period, pole orientation and shape, as well as its surface properties. The team retrieved a period of about 5 min, that is, half of the value reported in 1998, and the pole orientation and shape were also thoroughly revised. Reanalysis of the radar measurements with the new rotational characteristics showed that the object is about 11 meters in diameter, one third of the original estimate.
Based on the original size estimate, KY26 was assumed to have a dark surface. The new size determination allowed researchers to conclude that KY26 has a bright surface, similar to rare aubrite meteorite (see the image above of a piece of the Bishopville aubrite meteorite from 1843). Aubrite meteorites are a rare class of meteorites.
“The phase curve indicates a small and bright rocklet instead of a hypothesized dark cometary nucleus”, says Muinonen, who took part in the lightcurve and phase curve analyses. The phase curve describes the brightness variation between the “full-Moon” (zero-degree phase angle) and “new-Moon” geometries (180-degree phase angle), where the surface composition plays a key role.
“The radar measurements showed polarization properties that supported the hypothesis of an Xe-type asteroid, which is a relatively rare spectral type”, concludes Virkki. Finally, the spectral type and the colours observed on the asteroid match what is expected for objects found in the Hungaria region, located in the inner part of the asteroid main belt between Mars and Jupiter.
Although fast spinning and small size make a monolithic structure a more probable scenario, a possibility of a rubble-pile structure held together by gravitation and cohesion was not fully ruled out.
In 2031, the Japanese Hayabusa2 spacecraft will have a rendezvous with the asteroid KY26. The rendezvous will follow a successful sample return from the asteroid (162173) Ryugu using a capsule returned to Earth in 2020 after the sample extraction in 2019.
The physical properties of the asteroid KY26 were discussed in Helsinki at the joint Europlanet Science Congress and Annual Meeting of the Division for Planetary Sciences of the American Astronomical Society (held 8th to 12th of September).
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The research results were published in Nature Communications (Hayabusa2 target asteroid 1998 KY26 is smaller and rotating faster than previously known).