Interstellar Interloper 1I/2017 U1: Observations from the NOT and WIYN Telescopes

Abstract

We present observations of the interstellar interloper 1I/2017 U1 ('Oumuamua) taken during its 2017 October flyby of Earth. The optical colors B-V = 0.70$\pm$0.06, V-R = 0.45$\pm$0.05, overlap those of the D-type Jovian Trojan asteroids and are incompatible with the ultrared objects which are abundant in the Kuiper belt. With a mean absolute magnitude $H_V$ = 22.95 and assuming a geometric albedo $p_V$ = 0.1, we find an average radius of 55 m. No coma is apparent; we deduce a limit to the dust mass production rate of only $\sim$ 2$\times$10$^{-4}$ kg s$^{-1}$, ruling out the existence of exposed ice covering more than a few m$^2$ of the surface. Volatiles in this body, if they exist, must lie beneath an involatile surface mantle $\gtrsim$0.5 m thick, perhaps a product of prolonged cosmic ray processing in the interstellar medium. The lightcurve range is unusually large at $\sim$2.0$\pm$0.2 magnitudes. Interpreted as a rotational lightcurve the body has semi-axes $\sim$230 m $\times$ 35 m. A $\sim$6:1 axis ratio is extreme relative to most small solar system asteroids and suggests that albedo variations may additionally contribute to the variability. The lightcurve is consistent with a two-peaked period $\sim$8.26 hr but the period is non-unique as a result of aliasing in the data. Except for its unusually elongated shape, 1I/2017 U1 is a physically unremarkable, sub-kilometer, slightly red, rotating object from another planetary system. The steady-state population of similar, $\sim$100 m scale interstellar objects inside the orbit of Neptune is $\sim$10$^4$, each with a residence time $\sim$10 yr.