Physical characterization of the D-type Near-Earth Asteroid (162998) 2001 SK162: the primary target of NGSR-D

D-type asteroids are among the most primitive, organic-rich objects in the Solar System and are widely thought to have originated in the outer regions of Solar System, being them key witnesses to early Solar System chemistry and volatile delivery. (162998) 2001 SK162 is a Near-Earth Asteroid (NEA) belonging to the Apollo group and designated as a Potentially Hazardous Asteroid (PHA). Discovered by the LINEAR program in 2001, the object follows an eccentric orbit (e ~ 0.47) with a small orbital inclination of 1.7° and a perihelion distance of ~1.01 au. Some previous studies showed that this asteroid is possible D/T types [1,2,3,4]. These orbital and spectral characteristics led to its selection as the D-type primary target of the Japanese Next Generation Small-body Sample Return mission (NGSR-D) [5]. Precise knowledge of the target’s fundamental physical properties—in particular its geometric albedo, diameter, rotation state, and shape—is essential for mission planning, including spacecraft navigation, approach trajectory design, and sample acquisition operations. However, these properties remain poorly constrained: previous photometric and thermal-infrared observations [7,6] have yielded significant uncertainties that could directly impact mission design and operations.

To address this, we conducted a coordinated, multi-technique observing campaign of 2001 SK162 between October 2025 and March 2026, using a worldwide network of facilities: lightcurve photometry with the Two-meter Twin Telescope (TTT) at Teide Observatory; visible multi-band photometry with MuSCAT3/4 on the Las Cumbres Observatory (LCO) 2-m telescopes at Haleakala Observatory and with the Pirka 1.6-m telescope at Nayoro Observatory; visible spectroscopy with OSIRIS on the 10.4-m Gran Telescopio Canarias (GTC) at Roque de los Muchachos Observatory (ORM); NIR multi-band photometry with MOSFIRE on Keck I and polarimetry with the Subaru Telescope at Maunakea Observatories and the 2.5-m Nordic Optical Telescope (NOT) at ORM. In parallel, we reanalyzed archival WISE and Spitzer thermal-infrared data to independently constrain the size and albedo.

Lightcurves were obtained with TTT over 37 nights between 2025-10-21 and 2025-12-22. Periodogram analysis yields a rotational period of ~178 h, much longer than the previous estimate [8] and placing 2001 SK162 firmly in the slow-rotator regime. The lightcurve shows a peak-to-peak amplitude of 0.34 mag, indicating a moderately elongated shape. Combining our TTT photometry with archival ZTF data, we fit the H–G phase function and obtain H_V = 18.04 mag and G = 0.10, consistent with the photometric behavior of dark, primitive asteroids.

Visible multi-band photometry with MuSCAT3/4 (LCO 2-m) and Pirka 1.6-m telescope and visible spectroscopy with OSIRIS/GTC consistently reveal a featureless, red visible reflectance spectrum. Within the recent taxonomic framework of Mahlke et al. [9], the spectrum is best classified as D- or Z-type, although the color variation with different rotational phase is little. This confirms the spectral classification on which the NGSR-D target selection was based and reinforces the scientific case for sampling 2001 SK162. Moreover, we also obtained the near infrared reflectance at Y, J, and K bands by MOSFIRE/Keck I.

Because the geometric albedo of 2001 SK162 has been poorly constrained, we performed polarimetric observations with Subaru and NOT to discriminate between high- and low-albedo solutions, and reanalyzed archival WISE and Spitzer thermal-infrared data to provide an independent estimate of the geometric albedo and effective diameter. Joint analysis of these complementary techniques is ongoing and will be presented at the meeting, with the goal of delivering a self-consistent set of physical parameters for the NGSR-D mission. Together, these results provide the first comprehensive physical characterization of (162998) 2001 SK162. The combination of an unusually long rotation period, elongated shape, and primitive D/Z-type spectral classification makes this object a scientifically compelling target for NGSR-D.

 

[1] Binzel et al. (2004) MAPS 39, 351, [2] Ye (2011) AJ  141, 32, [3] Thomas et al. (2014) Icarus 228, 217, [4] Marsset et al. (2022) AJ 163, 165, [5] Okada, T. et al. (2026) EPSC 2026, [6] Mainzer et al. (2011) AJ 743, 156, [7] Mueller et al. (2011) AJ 141, 109, [8] Pravec, P. et al. (2005) https://space.asu.cas.cz/~ppravec/neo.htm, [9] Mahlke, M., Carry, B., & Mattei, P.A. (2022) A&A 665, A26.