Spectral and spin measurement of two small and fast-rotating near-Earth asteroids

In May 2012 two asteroids made near-miss “grazing” passes at distances of a few Earth-radii: 2012 KP24 passed at 9 Earth-radii and 2012 KT42 at only 3 Earth-radii. The latter passed inside the orbital distance of geosynchronous satellites. From spectral and imaging measurements using NASA’s 3-m Infrared Telescope Facility (IRTF), we deduce taxonomic, rotational, and physical properties. Their spectral characteristics are somewhat atypical among near-Earth asteroids: C-complex for 2012 KP24 and B-type for 2012 KT42, from which we interpret the albedos of both asteroids to be between 0.10 and 0.15 and maximum effective diameters of 20 ± 6 and 6 ± 1 m, respectively. Among B-type asteroids, the spectrum of 2012 KT42 is most similar to 3200 Phaethon and 4015 Wilson–Harrington. Not only are these among the smallest asteroids spectrally measured, we also find they are among the fastest-spinning: 2012 KP24 completes a rotation in 2.5008 ± 0.0006 min and 2012 KT42 rotates in 3.634 ± 0.001 min.