February 15, 2018
1I/ʻOumuamua (1I), the clearly hyperbolic object detected, was announced the week of Oct .23 by PANSTARRS as A/2017 U1, and confirmed after the fact in Catalina Survey data. With eccentricity e = 1.19, it was clearly on a hyperbolic interstellar trajectory. Coming from the sky direction of Vega, it was as unbound as an e = 0.8 short period comet is bound to the Sun. The excess velocity of 1I after it leaves the solar system, about 10 kilometers/second, is similar to the differential velocities of stars in the local solar neighborhood. It has now been observed by more than 10 observatories, including the Hubble Space Telescope, Spitzer, APO, NOT, DCT and Gemini. None of these observatories showed any evidence for extended structure, i.e ,it was point-like. Thus 1I is not actively outgassing, despite the object passing within 0.25 AU of the Sun in early September 2017 and 0.20 AU of the Earth in October 2017. It is rocky, not icy, like an asteroid. Measurements show it to be small, < 200 m in effective radius. Its light curve is quite sinusoidal, supporting a simple triaxial ellipsoid shape. The lightcurve varied by a huge two magnitudes peak-to-peak over 8.1 hrs, implying a highly elongated body, more than ever seen for a solar system body. Its color is slightly reddish vs the Sun. Alternative explanations of the lightcurve variations being due to albedo variegations doesn’t match the lack of color variation over a rotation or its sinusoidal shaped lightcurve. On the other hand, it would only take a very small bulk cohesive strength (> 10 Pa) to hold 1I together versus centrifugal force, and models of an object consisting of two objects in tight binary orbit are consistent with what we have observed, no radio signal were detected when the SETI and GBT radio telescopes turned to listen to the object. So what was 1I?
Dr. Carey M. Lisse has over 35 years of experience in experimental and observational research that includes astrophysics, detector physics, optics, electronics, and remote sensing data analysis. Currently at JHU-APL, he is involved with studying the physical properties of primitive solar system objects; the rock-forming dust contained in comets, the IPD cloud, the Proto-Solar Nebula, and YSOs; x-ray emission from solar system bodies and the heliosphere; and designing, building, and operating Solar System spacecraft missions.