Free Astronomy Magazine January-February 2018

8 JANUARY-FEBRUARY 2018 At the end of the day on October 19, Rob Weryk, a postdoctoral researcher at the Uni- versity of Hawaii Institute for Astronomy (IfA), was looking at images taken hours earlier by the Panoramic Survey Telescope and Rapid Response System 1 (PanSTARRS 1), a 1.8-meter diameter telescope dedi- cated to the search for potentially danger- ous asteroids. Weryk noticed a luminous track in the Pisces constellation made by an object of about magnitude 20. Excited by what he saw, he examined images from the previous night and found the track on those too, but in an area of the sky so different that it had not been signalled by the auto- matic system detecting moving objects. At that point, Weryk contacted a specialist in the field, Marco Micheli, of the Instituto de Astrofísica de Canarias (a former IfA re- searcher), who confirmed the presence of the object through observations conducted with the 1-meter telescope at the ESA's Op- tical Ground Station in Tenerife. The preliminary calculation of the mysteri- ous object’s trajectory provided a surpris- ing datum: the eccentricity of its orbit is close to 1.2. When eccen- tricity is greater than 1, we are dealing with a hyperbolic tra- jectory, that of a body that does not orbit around the Sun. Weryk and Micheli soon realized that that object came from outside our solar system. This unique opportu- nity to study a body formed in another planetary system mobilised nu- merous researchers, and within a few hours many telescopes, including larger ones, were aimed at the ‘visitor’. Since it initially appeared to be a cometary nucleus, the object was recorded by the Minor Planet Center (MPC, Cambridge, Massachusetts) under the name C/2017 U1. However, the most in-depth images taken with the ESO's Very Large Telescope (8.2- meter) soon showed that there was no trace of cometary activity and therefore the interloper could only be an asteroid, so the designation changed to A/2017 U1. A bove and follow- ing page, this dia- gram shows the orbit of the interstellar as- teroid Oumuamua as it passes through the Solar System. Unlike all other asteroids and comets observed before, this body is not bound by gravity to the Sun. It has come from interstellar space and will return there after its brief encounter with our star system. Its hyper- bolic orbit is highly inclined and it does not appear to have come close to any other Solar System body on its way in. [ESO/K. Meech et al.] O umuamua in a 300-second false-colour image taken on Oct. 29 from Gemini Observatory on Mauna Kea, Hawaii. [Gemini Observatory, NSF, AURA /M. T. Bannister, R. E. Pike, M. E. Schwamb] Below, multiple exposures centered on Oumua- mua combined into one image. [R. Kotulla (Uni- versity of Wisconsin) & WIYN/NOAO/AURA/NSF]