Spectral and photometric characterization of the two targets of the extended mission of Hayabusa2

Following a successful nominal mission resulting in the return of samples from the Near-Earth Object (NEO) Ryugu, the JAXA mission Hayabusa2 has been extended to study two other objects. The first is the NEO (98943) 2001 CC21, at which the spacecraft will perform a flyby in July 2026. This encounter will be followed by the rendezvous with the asteroid 1998 KY26, in 2031. There is limited information about these two objects in the literature. For 2001 CC21, its diameter ranges between 440 and 530 m with an albedo of 0.23 ± 0.04 (Geem et al., 2023). It has been classified as S-complex or L-type (Binzel et al., 2004; Lazzarin et al., 2005). 1998 KY26 has a diameter of ~30 m with a spin period of ~11min (Ostro et al., 1999). Previous photometric observations suggest that 1998 KY26 may belong to the “primitive” types, likely B-, C-, F-, G-, D-, or P-type in the Tholen taxonomy (Tholen, 1984; Ostro et al., 1999).

To prepare for the spacecraft encounter with 2001 CC21, we conducted several observations using the 2.6 m Nordic Optical Telescope (NOT) based in La Palma, Spain. 2001 CC21 was observable between October 2022 and March 2023. We seized the opportunity to observe it spectroscopically and photometrically in visible and near-infrared ranges. The objective of our study is to obtain more information about the mineral composition and the physical characteristics of 2001 CC21. We obtained eleven spectra in the visible range and combined them with the near-infrared part of Geem et al. (2023) to obtain full spectra. From this, we calculated several parameters, such as spectral slope and band parameters, and determined its taxonomy following the Bus-DeMeo taxonomy (DeMeo et al., 2009). In the photometric observations, we observed 2001 CC21 for four nights, covering several rotation phases. We used several filters (B, V, R, Y, Z, J, H, and Ks) to detect surface heterogeneities such as large-scale craters or boulders. We combined our results with those of previous studies to precisely characterize 2001 CC21 before the spacecraft's arrival.

Taxonomic classification and color studies reveal that this asteroid belongs to the Sq or Sr-type (see Figure), contradicting the classification of Binzel et al. (2004), which estimated 2001 CC21 to be an L-type asteroid. Indeed, the spectra clearly present a band at 1 µm, usually absent in L-type asteroid spectra. From this classification and knowing the size of this asteroid, we estimated its mass to be between 1.53 and 2.67 x1011 kg.

Regarding surface mineralogy, we found a good match with ordinary chondrites of L or LL types. This result is confirmed by matching with meteorite analogs using the RELAB database (Pieters, 1983). Considering the albedo value of 2001 CC21 to be 23 ± 4 % (Geem et al, 2023), we found a good match with the spectra of the Chateau Renard L6 meteorite. With this petrographic grade (6), we should not find any traces of hydration within the minerals that compose its surface.

With a rotational period of 5.02124 ± 0.00001 hours (Fornasier et al, submit.), we looked for spectroscopic variations to detect geological features, but we found no regular variations indicating such features.

Finally, we attempted to compare the visible spectral slope of 2001 CC21 with the average slope of inner main belt families dominated by S-complex members. We found a slight correlation with the Lucienne family. While this is insufficient to conclude that either 2001 CC21 or its parent body is a member of this family, it suggests that it may be related to the history of 2001 CC21. Further dynamical studies are needed to confirm this relationship.

Regarding 1998 KY26, we observed this object for two nights between May and June 2024, using the 3.58m Telescopio Nazionale Galileo at La Palma, Spain, and the VLT at the Silla Paranal Observatory, Chile. We observed this object spectroscopically in the visible and near-infrared ranges to precisely determine its taxonomy and surface mineralogy. Additionally, we conducted multi-filter photometric observations at different intervals to cover several rotational phases and potentially detect surface heterogeneities such as large-scale craters or boulders. Data reduction is underway, and we will present preliminary results and interpretations. 

 

Acknowledgements : Based on the observations made with the Nordic Optical Telescope, owned in collaboration by the University of Turku and Aarhus University, and operated jointly by Aarhus University, the University of Turku and the University of Oslo, representing Denmark, Finland, and Norway, the University of Iceland and Stockholm University at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias. This work received financial support from the Italian Space Agency (ASI) under contract No. 2022-12-HH.0.

References : Geem et al. (2023), MNRAS, 525:L17, Lazzarin et al. (2005), MNRAS 359:1575, Ostro et al. (1999) Science 285, 557-559, Tholen, D.J. (1984), PhD thesis, DeMeo et al. (2009), Icarus, 202:160, Binzel et al. (2004), M&PS, 39:351, Pierters, C. M. (1983), J.Geophys. Res., 9534-9544.