Radar Observations and Characterization of Binary Near-Earth Asteroid (35107) 1991 VH

Binary near-Earth Asteroid (35107) 1991 VH approached Earth in August 2008 at a distance of 0.045 au. We used this opportunity to obtain an extensive set of radar observations between Jul. 29 and Aug.12 with Arecibo (2380 MHz, 13 cm) and Goldstone (8560 MHz, 3.5 cm). The range-Doppler images have resolutions as fine as 15 m that spatially resolve both components in the system. Arecibo images show that the primary is roughly spheroidal with a visible range extent of 650 m. The images contain clear signatures of an equatorial ridge with longitudinal variations in its appearance. A concavity 100 meters in extent is present along the ridge. A radar-bright linear feature that casts a radar shadow down-range is visible in some of the images, and occurs at mid- to high-latitudes. A preliminary shape model fit to images of the primary reveals a top-shaped object with a volume equivalent diameter of roughly 1.2 km similar in shape to the (66391) 1999 KW4 primary (Ostro et al. 2006, Science 314, 1296). The mutual orbit determined from the radar data has an orbital period of 32 hours, a semimajor axis of 3.26 km, an eccentricity of 0.05, and a system mass of 1.5e12 kg. These values are consistent with those estimated by Pravec et al. (2006, Icarus 181,63). The orbital fits yield a primary to secondary mass ratio of 12 and a density of the primary of about 1500 kg/m³. The range extents of the secondary echoes vary from less than 100 m to more than 200 m indicating that it is highly elongated. Echo bandwidths of the secondary also vary by a factor of two and are consistent with an elongated shape. Attempts to model the shape of the secondary using a single spin vector have so far yielded poor fits, hinting that the secondary might be spinning in a chaotic manner due to its elongation and eccentric mutual orbit, as suggested by numerical simulations in Naidu & Margot (2015, AJ 149, 80).