Physical Characterization of Near-Earth Asteroid (52768) 1998 OR2: Evidence of Shock 1 Darkening/Impact Melt

We conducted photometric and spectroscopic characterization of near-Earth asteroid (52768) 1998 OR2 during a close approach to the Earth in April of 2020. Our photometric measurements confirm the rotation period of the asteroid to be 4.126 +/- 0.179 hours, consistent with the previously published value of 4.112 +/- 0.001 hours. By combining our visible spectroscopic measurements (0.45 - 0.93 microns) with archival MITHNEOS near infrared spectra (0.78 - 2.49 microns), 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 microns and Band II at 2.07 +/- 0.02 microns 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 the 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-NIR spectrum fall 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 aerial 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 ~63% dark lithology and ~37% light lithology.