ESO DOI Service

• Hainaut, Olivier R. [European Southern Observatory (ESO)]
• Meech, Karen J. [Institute for Astronomy, University of Hawai‘i, Honolulu, USA]
• Micheli, Marco [ESA SSA-NEO Coordination Centre, Frascati, Italy; INAF–Osservatorio Astronomico di Roma, Monte Porzio Catone, Italy]
• Belton, Michael S. J. [Belton Space Exploration Initiatives, Tucson, USA]

On 19 October 2017 the Panoramic Survey Telescope And Rapid Response System (Pan-STARRS) discovered a rapidly moving object near the Earth. In itself this was nothing unusual but over the course of a few days astronomers realised that this was the first detection of an unbound object travelling through the Solar System. At the time of its discovery, the interstellar visitor, 1I/2017 U1 (`Oumuamua), was quite faint and already speeding away. In the ensuing days, thanks to the efforts of about 10 separate teams, over 100 hours on 2.5- to 10-metre telescopes were devoted to observing the object during the short, exhilarating and frantic period over which it was visible. This is an account of our observations and how they have contributed to the current view that 1I/2017 U1 is an elongated object in an excited rotation state with surface colours similar to those of Solar System comets and asteroids.