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 .