ʻOumuamua is the first interstellar object detected passing through the Solar System. Formally designated 1I/2017 U1, it was discovered by Robert Weryk using the Pan-STARRS telescope at Haleakalā Observatory, Hawaii, on 19 October 2017, approximately 40 days after it passed its closest point to the Sun on 9 September. When it was first observed, it was about 33 million km (21 million mi; 0.22 AU) from Earth (about 85 times as far away as the Moon) and already heading away from the Sun.

ʻOumuamua is a small object estimated to be between 100 and 1,000 metres (300 and 3,000 ft) long, with its width and thickness both estimated between 35 and 167 metres (115 and 548 ft). It has a red color, like objects in the outer Solar System. Despite its close approach to the Sun, it showed no signs of having a coma, the usual nebula around comets formed when they pass near the Sun. Further, it exhibited non‑gravitational acceleration, potentially due to outgassing or a push from solar radiation pressure. It has a rotation rate similar to the Solar System's asteroids, but many valid models permit it to be unusually more elongated than all but a few other natural bodies observed in the solar system. This feature raised speculation about its origin. Its light curve, assuming little systematic error, presents its motion as "tumbling" rather than "spinning", and moving sufficiently fast relative to the Sun that it is likely of extrasolar origin.