[23] 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.

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.

By July 2019, most astronomers concluded that it was a natural object, but its precise characterization is contentious given the limited time window for observation.

Proposed explanations of its origin include the remnant of a disintegrated rogue comet,[28][29] or a piece of an exoplanet rich in nitrogen ice, similar to Pluto.

[34] In January 2022, researchers proposed that a spacecraft launched from Earth as part of Project Lyra, could catch up to 'Oumuamua in 26 years for closer studies.

[4] The name comes from Hawaiian ʻoumuamua 'scout'[38] (from ʻou 'reach out for' and mua, reduplicated for emphasis 'first, in advance of'[4]), and reflects the way the object is like a scout or messenger sent from the distant past to reach out to humanity.

[20] By the end of October, it had already faded to about apparent magnitude 23,[48] and in mid-December 2017, it was too faint and fast-moving to be studied by even the largest ground-based telescopes.

[53] More detailed observations, using the Breakthrough Listen hardware and the Green Bank Telescope, were performed;[49][53][54] the data were searched for narrowband signals and none were found.

Given the close proximity to this interstellar object, limits were placed to putative transmitters with the extremely low effective isotropically radiated power of 0.08 watts.

[10] On 27 June 2018, astronomers reported a non-gravitational acceleration to ʻOumuamua's trajectory, potentially consistent with a push from solar radiation pressure.

Initial speculation as to the cause of this acceleration pointed to the comet-like outgassing,[25] whereby volatile substances inside the object evaporate as the Sun heats its surface.

[81] In fact, the closeness of ʻOumuamua's velocity to the local standard of rest might mean that it has circulated the Milky Way several times and thus may have originated from an entirely different part of the galaxy.

[82][45] It has been speculated that the object may have been ejected from a stellar system in one of the local kinematic associations of young stars (specifically, Carina or Columba) within a range of about 100 parsecs,[83] 45 million years ago.

[11] In May 2020, it was proposed that the object was the first observed member of a class of small H2-ice-rich bodies that form at temperatures near 3 K in the cores of giant molecular clouds.

[3] In an attempt to confirm any cometary activity, very deep stacked images were taken at the Very Large Telescope later the same day, but the object showed no presence of a coma.

[4] The lack of a coma limits the amount of surface ice to a few square meters, and any volatiles (if they exist) must lie below a crust at least 0.5 m (1.6 ft) thick.

[46] One speculation regarding its shape is that it is a result of a violent event (such as a collision or stellar explosion) that caused its ejection from its system of origin.

[107] A 2021 paper proposed that, if 'Oumuamua is made of nitrogen ice, the extreme shape could be a result of recent evaporation, and that when the object entered the Solar System it likely had an unremarkable 2:1 aspect ratio.

[30] Light curve observations suggest however that the object may be composed of dense metal-rich rock that has been reddened by millions of years of exposure to cosmic rays.

[99] Despite the lack of any cometary coma when it approached the Sun, it may still contain internal ice, hidden by "an insulating mantle produced by long-term cosmic ray exposure".

[112][113][114] In August 2020, astronomers reported that ʻOumuamua is not likely to have been composed of frozen hydrogen, which had been proposed earlier; the compositional nature of the object continues to be unknown.

Nitrogen ice the size of 'Oumuamua could survive for 500 million years in the interstellar medium and would reflect two-thirds of the Sun's light.

[119] This explanation has been further supported in March 2021 when scientists presented a theory based on nitrogen ice, and further concluded that ʻOumuamua may be a piece of an exoplanet similar to the dwarf planet Pluto, an exo-Pluto as noted, from beyond our solar system.

Significant mass loss caused by the sublimation would also explain the unusual cigar-like shape, comparable to how a bar of soap becomes more elongated as it is used up.

[128][129] In 2023, it was proposed the observed non-gravitational acceleration and spectrum of ʻOumuamua can be best explained by hydrogen outgassing from the water ice matrix.

[132][133] Different mission durations and their velocity requirements were explored with respect to the launch date, assuming direct impulsive transfer to the intercept trajectory.

[58][131] Seligman and Laughlin adopt a complementary approach to the Lyra study but also conclude that such missions, though challenging to mount, are both feasible and scientifically attractive.

[145] In response, Loeb wrote an article detailing six anomalous properties of ʻOumuamua that make it unusual, unlike any comets or asteroids seen before.

[146][147] A subsequent report on observations by the Spitzer Space Telescope set a tight limit on cometary outgassing of any carbon-based molecules and indicated that ʻOumuamua is at least ten times shinier than a typical comet.

[79] The solar sail technosignature hypothesis is considered unlikely by many experts owing to available simpler explanations that align with the expected characteristics of interstellar asteroids and comets.