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. It exhibited non‑gravitational acceleration, potentially due to outgassing or a push from solar radiation pressure. It has a rotation rate similar to that of Solar System asteroids, but many valid models permit it to be more elongated than all but a few other natural bodies. 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 an extrasolar origin. Extrapolated and without further deceleration, its path cannot be captured into a solar orbit, so it will eventually leave the Solar System and continue into interstellar space. Its planetary system of origin and age are unknown.
ʻOumuamua would be remarkable for its extrasolar origin, high obliqueness, and observed acceleration without an apparent coma. By July 2019, most astronomers concluded that it was a natural object, but its exact characterization is contentious given the limited observation window. While an unconsolidated object (rubble pile) would require ʻOumuamua to be of a density similar to rocky asteroids, a small amount of internal strength similar to icy comets would allow it to have a relatively low density. Proposed explanations of its origin include the remnant of a disintegrated rogue comet, or a piece of an exoplanet rich in nitrogen ice, similar to Pluto. On 22 March 2023, astronomers proposed the observed acceleration was "due to the release of entrapped molecular hydrogen that formed through energetic processing of an H2O-rich icy body", consistent with 'Oumuamua being an interstellar comet, "originating as a planetesimal relic broadly similar to solar system comets".
Avi Loeb has suggested that it could be a product of extraterrestrial technology, but there is insufficient evidence to support any hypotheses, "despite all [its] strangeness". In January 2022, researchers proposed Project Lyra, where a spacecraft launched from Earth could catch up to 'Oumuamua in 26 years for closer studies.
