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±0.06, V-R = 0.45±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 HV = 22.95 and assuming a
geometric albedo pV = 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 ∼
2×10−4 kg s−1, ruling out the existence of exposed ice
covering more than a few m2 of the surface. Volatiles in this body, if they
exist, must lie beneath an involatile surface mantle ≳0.5 m thick,
perhaps a product of prolonged cosmic ray processing in the interstellar
medium. The lightcurve range is unusually large at ∼2.0±0.2
magnitudes. Interpreted as a rotational lightcurve the body has semi-axes
∼230 m × 35 m. A ∼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 ∼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, ∼100 m
scale interstellar objects inside the orbit of Neptune is ∼104, each
with a residence time ∼10 yr.Comment: 25 Pages, 2 Tables, 7 Figures; submitted to ApJ