The Story of 1I/`Oumuamua, the First Visitor from Another Star System: Still an Enigma
Karen J. Meech
Institute for Astronomy, University of Hawaii



On October 19, 2017 R. Weryk discovered a rapidly moving object in the data from the Pan-STARRS1 telescope. Additional astrometry obtained with pre-discovery observations on October 18 and with data obtained with the CFHT on October 22 showed that the object had the highest hyperbolic eccentricity ever detected, confirming that this object clearly originated from outside the solar system. Beginning on October 22 there was an intense effort to secure observing resources to characterize the object. Because it was receding rapidly from the Earth and Sun, within a week of discovery the brightness had dropped by a factor of 10 and in less than a month it had dropped by a factor of 100. Thus, there was a period about 2 weeks where the target could be relatively easily characterized. Deep images of `Oumuamua showed no hint of cometary activity, with limits on the amount of dust that could be present at less than 7-8 orders of magnitude that of a typical comet at similar distances. Light curve observations showed that the object was rotating with an instantaneous rotation period of 7.34 hours, and a light curve range of 2.5 magnitudes, implying an extremely elongated axis ratio perhaps as large as 10:1. Assuming a low albedo typical of comets (4%) this implies a size of 800x80x80 m. However, as more time series data were obtained, it was evident that `Oumuamua was in an excited spin state with the long axis precessing around the total angular momentum vector with an average period of 8.67±0.34 hr. The timescale for damping an excited spin in a body this size is very long, so the spin state may reflect the violent process of ejection of `Oumuamua from its host planetary system. The color of `Oumuamua was found to be quite red by many teams, consistent with comet surfaces, the dark side of Iapetus, and other minerals, however, some data suggest that the color may change with rotational phase. Infrared measurements were made over a period of 30 hours in November with the Spitzer Telescope, resulting in a strong upper limit on the flux. This suggests that `Oumuamua was smaller and brighter than previously believed, and put strong upper limits on any outgassing from CO or CO 2 . Our final experiment was to combine position measurements obtained from the Hubble Space Telescope with ground based measurements to do a detailed study of the orbit and trace back to it's home. Our analysis of 207 astrometric positions showed that the orbit cannot be fit by a purely gravity-only trajectory, but are well matched (at the 30-sigma level) by the addition of a radial acceleration. We explored several explanations for the non-gravitational motion, and found that cometary outgassing is the most physically plausible, but requires that `Oumuamua has a somewhat different nature from solar system comets. Using the new orbit and the data from the Gaia DR2 release we found 4 possible home systems for `Oumuamua. In this talk I will summarize what we know, how well the story "fits" and will discuss prospects for missions to future "once in a lifetime" targets such as `Oumuamua.