On 19 October 2017 Robert Weryk, working with the Pan-STARRS telescope on Haleakalā in Hawaii, spotted a faint object moving fast. Its path was not an ellipse around the Sun but an open curve: it had come from interstellar space and was leaving again. It had already passed its closest point to the Sun about 40 days earlier.
It became the first object given an “I” designation, for interstellar: 1I/2017 U1. The Pan-STARRS team named it ʻOumuamua, a Hawaiian word meaning roughly “scout” or “first distant messenger”.
A strange shape
ʻOumuamua was too small and too far away to be seen as more than a point of light. But its brightness changed dramatically as it tumbled, which means it must have been very elongated, perhaps five or more times longer than it was wide, or else flat like a disc. Assuming a typical dark surface, one estimate made it about 230 metres long and 35 metres wide.
The acceleration
In June 2018 Marco Micheli and colleagues reported in Nature that ʻOumuamua had been pushed slightly faster than gravity alone could explain. Comets do this when gas jets off their ices, but ʻOumuamua showed no visible tail or coma. Explanations since then include hydrogen escaping from water ice altered by cosmic rays, a fragment of nitrogen ice from a Pluto-like world, and others. A proposal that it was an alien light sail drew headlines, but most experts consider it unlikely.
It is gone now, heading out of the Solar System, and we will never get a closer look.