Evolution of the Binary Asteroid 66391 Moshup-Squannit (1999 KW4)

Near-Earth asteroid (NEA) 1999 KW4 was the first binary asteroid to be imaged with radar at high signal-to-noise (SNR) and decameter resolution. The 2001 observations yielded estimates of the spin states, shapes, masses, and densities of both components (Ostro et al., 2006) and enabled a wide range of studies that advanced our understanding of binary NEAs. In particular, the binary orbit and shape models determined with radar data prompted studies of spin-orbit interactions in close-in binary systems (Scheeres et al., 2006, Naidu and Margot, 2015). Motivated by the prospect of quantifying the orbital evolution of the binary, we obtained additional observations at Arecibo in 2017, 2018, and 2019 with SNRs that reached at most 0.1%, 0.5%, and 2% of the 2001 observations, respectively. These observations enable measurements of the range and Doppler separations between Moshup and Squannit. We are using these measurements in conjunction with measurements obtained in 2001 and 2002 to examine the evolution of the binary orbit and to verify whether the system exhibits binary YORP (BYORP) evolution. It has been postulated that in some binary systems, tidal evolution might be counteracted by BYORP evolution (Jacobson and Scheeres, 2011). In addition to measuring orbital evolution, we are re-analyzing the shapes of the components with accurate modeling of the longitudinal librations of Squannit as part of the shape modeling procedure. Because the previous libration estimate was not part of the fitting procedure, we anticipate that both the libration estimate and the shape of Squannit can be improved, with important consequences for spin-orbit evolution and orbital evolution studies (e.g., Scheirich et al., 2015, Scheirich et al., in prep.).