Physical Characterization of Near-Earth Asteroid (52768) 1998 OR2: Evidence of Shock Darkening/Impact Melt

We conducted photometric and spectroscopic characterization of near-Earth asteroid (52768) 1998 OR2 during a close approach to Earth in 2020 April. Our photometric measurements confirm the rotation period of the asteroid to be 4.126 +/- 0.179 hr, consistent with the previously published value of 4.112 +/- 0.001 hr. By combining our visible spectroscopic measurements (0.45-0.93 mu m) with archival MITHNEOS near-infrared spectra (0.78-2.49 mu m), we classify the asteroid as an Xn-type in the Bus-DeMeo taxonomy. The combined spectrum shows two weak absorption bands: Band I at 0.926 +/- 0.003 mu m and Band II at 2.07 +/- 0.02 mu m, with band depths of 4.5% +/- 0.15% and 4.0% +/- 0.21%, respectively. The band area ratio is 1.13 +/- 0.05. These spectral band parameters plot at the tip of the S(IV) region of the Gaffey S-asteroid subtypes plot, suggesting an affinity to ordinary chondrite meteorites. We calculated the chemistry of olivine and pyroxene using the Band I center to be 20.1 +/- 2.3 mol% fayalite and 18.2 +/- 1.5 mol% ferrosilite, consistent with H-chondrites. Principal component analysis of 1998 OR2's combined visible-near-IR spectrum falls on the C/X-complex side of the alpha-line, near the end of the shock-darkening trend, consistent with its weak absorption bands (band depth <5%). We use an areal mixing model with lab measurements of the shock-darkened H5 chondrite, Chergach, to constrain the amount of shock-darkened material on the asteroid's surface at similar to 63% dark lithology and similar to 37% light lithology.