Ground-based Characterization of (98943) Torifune 2001 CC₂₁, the Target of the Hayabusa2# Space Mission*

Abstract The near-Earth asteroid (98943) Torifune, previously designated 2001 CC 21 , is the flyby target of the Hayabusa2 extended mission, nicknamed Hayabusa2 # (Small Hazardous Asteroid Reconnaissance Probe (SHARP)). The ground-based telescope observations offer a key science input for the mission's scientific investigation. During 2022–2024, this asteroid was at visible apparent magnitudes brighter than 18.5, allowing for a detailed characterization using ground-based telescope observations. We determined its rotation period P = 5.021516 ± 0.000106 hr and its absolute magnitude H = 18.78 ± 0.14. The large number of light curves allows us to estimate its axis ratio, its convex shape, and its pole orientation, λ = 301 ∘ ± 35 ∘ , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>β</mml:mi> <mml:mo>=</mml:mo> <mml:mn>8</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>9</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>6</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>1</mml:mn> <mml:mo>°</mml:mo> </mml:mrow> </mml:msubsup> </mml:math> , and ϵ = 5 ∘ ± 3 ∘ , respectively, which indicate a prograde rotation. We report the semiaxis of the equivalent ellipsoid, a = 0.42 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mn>0.06</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.08</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> km, b = 0.16 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mn>0.04</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.05</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> km, and c = 0.17 ± 0.03 km. Consequently, the volume-equivalent diameter is D eq = 0.44 ± 0.06 km. Using observations conducted simultaneously with four broadband filters, we determined ( g − r ) = 0.663 ± 0.022 mag, ( r − i ) = 0.177 ± 0.012 mag, and ( i − z s ) = −0.061 ± 0.032 mag. Additionally, we found that Torifune exhibits no detectable large-scale heterogeneity. We classified the object using a high signal-to-noise ratio spectrum (over the visible and near-infrared region) as Sq-type in the Bus–DeMeo taxonomy. We estimate a mineralogy similar to LL/L ordinary chondrites, with an ol/(ol+px) = 0.60, a Fa content of 28.5 mol%, and a Fs content of 23.4 mol%. The spectral data indicate a surface affected by moderate space weathering effects.