The orbit and size of (87) Sylvia’s Romulus from the 2015 apparition

•Romulus, one of two moons around asteroid (87) Sylvia, has been imaged.•The 3.5 m telescope used adaptive optics and a laser to image the moon.•This telescope is the smallest to ever image a moon of an asteroid from the ground.•From the moon’s orbit, the mass and density of asteroid (87) Sylvia was found.•The relative lightcurve between the two suggests an elongated figure for the moon.

Using the US Air Force’s Starfire Optical Range 3.5 meter telescope with adaptive optics and a laser guidestar, we obtained 68 images of asteroid (87) Sylvia and its satellite Romulus over 6 nights in March and May of 2015. Adding an additional 3 images from earlier observations on one night in November 2012, we are able to derive a circular (but not an eccentric) orbit for Romulus, leading to a density for Sylvia of 1.37 ± 0.04 gm/cm3. Extending the time base to 14 years by combining our data with previous observations from Keck, HST, and the VLT reported in the literature, we can fit for a new circular orbit and change the density estimate slightly to 1.35 ± 0.04 gm/cm3. By fitting a ratio of two Fourier series to the measured magnitude difference between Sylvia (V=12.5) and Romulus, which ranged from 4.1 to 5.0 in the J-band (λ=1.2μm), and modeling both as triaxial ellipsoids, we are able to derive prolate spheroid equatorial diameters for Romulus of 41(±27) × 30(±16) km. This assumes that Romulus is rotating synchronously with its 3.64 d orbital period. However, decomposing the differential lightcurve between Sylvia and Romulus reveals a much shorter 7.96 hr rotational period, leading to more elongated prolate spheroid diameters of 82(±7) × 21(±2) km. As far as we know, our 3.5 m telescope is the smallest ground-based telescope to ever image any asteroid’s moon.