On 19 October 2017, Robert Weryk, a postdoctoral researcher at the University of Hawaii Institute for Astronomy, identified a moving object in images from the Pan-STARRS1 telescope at Haleakala Observatory on Maui. The object was travelling on a hyperbolic trajectory that could only be interstellar in origin. It was the first such object ever confirmed.
Initially designated C/2017 U1 as a suspected comet, then reclassified as an asteroid when no cometary activity was detected, the object received the permanent designation 1I/2017 U1 on 6 November 2017, when the International Astronomical Union’s Minor Planet Center created a new designation series for interstellar objects. The Pan-STARRS team named it 'Oumuamua, a Hawaiian word in which 'ou means “reach out for” and the repeated mua places emphasis on “first, in advance of”: a messenger from afar arriving first. The object had already passed its closest approach to the Sun on 9 September at 0.25 astronomical units, and its closest approach to Earth on 14 October at a distance of approximately 24 million kilometres. It was discovered on its way out.
The Observational Record
The discovery paper, published in Nature in December 2017 by Karen Meech, Weryk, Marco Micheli, and colleagues at the University of Hawaii and the European Space Agency, characterised 'Oumuamua as “a red and extremely elongated interstellar asteroid.” Its brightness varied by a factor of ten over its rotation cycle, implying an extreme aspect ratio of approximately 10:1, a shape unlike any known solar-system body. Meech et al. estimated a mean radius of approximately 102 metres assuming a low, comet-like albedo of 0.04. The surface was spectrally red, consistent with organic-rich asteroids or comets. No cometary activity was detected despite the object’s close solar approach.
In June 2018, Micheli, Davide Farnocchia of NASA’s Center for Near-Earth Object Studies, Meech, and colleagues published a second paper in Nature reporting the detection, at 30-sigma significance, of non-gravitational acceleration in 'Oumuamua’s trajectory. The acceleration was directed radially outward from the Sun. In solar-system comets, such acceleration is well understood as a product of volatile outgassing, but 'Oumuamua showed no detectable gas or dust. Observations by NASA’s Spitzer Space Telescope, reported by David Trilling and colleagues in the Astronomical Journal in 2018, placed upper limits on carbon dioxide and carbon monoxide emissions, confirming the absence of standard cometary activity.
The object was last observed on 2 January 2018 by the Hubble Space Telescope, near its detection limit. Over its observational arc, 214 observations were recorded by facilities including the Very Large Telescope, Gemini South, Keck Observatory, the Canada-France-Hawaii Telescope, and Hubble.
The Competing Hypotheses
The combination of non-gravitational acceleration and absent outgassing produced a body of published responses, each proposing a different mechanism.
In November 2018, Shmuel Bialy and Avi Loeb of Harvard published a paper in the Astrophysical Journal Letters proposing that 'Oumuamua’s acceleration could be explained by solar radiation pressure acting on a thin, light-sail-like structure, with a required mass-to-area ratio of approximately 0.1 grams per square centimetre. In January 2021, Loeb published “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” (Houghton Mifflin Harcourt), presenting his case that the object’s observed properties were consistent with an artificial artefact.
In 2020, Darryl Seligman of Yale and Gregory Laughlin published a paper in the Astrophysical Journal Letters proposing that 'Oumuamua was composed of molecular hydrogen ice, formed in a giant molecular cloud. Sublimating hydrogen would produce the observed acceleration without a visible coma.
In 2021, Steven Desch and Alan Jackson of Arizona State University published a two-part study in the Journal of Geophysical Research: Planets proposing that 'Oumuamua was a fragment of nitrogen ice from the surface of a Pluto-like exoplanet. Nitrogen sublimation would explain the acceleration and would not have been detectable by Spitzer’s instruments.
In 2023, Jennifer Bergner of UC Berkeley and Darryl Seligman published a paper in Nature proposing that cosmic-ray irradiation of ordinary water ice during interstellar transit had produced trapped molecular hydrogen within the ice. Solar warming released this hydrogen, providing the observed acceleration without requiring exotic compositions and without producing a detectable coma.
Each hypothesis addresses the central observational constraint differently. The observational record is closed: 'Oumuamua left the range of all current instruments in early 2018.
The Documentary Legacy
'Oumuamua’s anomalous properties became one of two named catalysts for the founding of the Galileo Project at Harvard in July 2021. At the project’s founding press conference, Loeb cited the object alongside the Office of the Director of National Intelligence’s preliminary assessment on unidentified aerial phenomena as motivating a systematic scientific search for evidence of extraterrestrial technological artefacts. The project’s interstellar objects branch was designed in part to detect the next 'Oumuamua-like visitor using the Vera C. Rubin Observatory’s Legacy Survey of Space and Time, and to develop concepts for an intercept mission.
The search for interstellar objects also extended to meteors. In 2019, Amir Siraj and Loeb identified a January 2014 bolide in the CNEOS fireball catalogue as a candidate interstellar meteor. Designated IM1, its interstellar origin was confirmed by the United States Space Command in 2022. The Galileo Project’s June 2023 Pacific Ocean expedition to recover material from the IM1 impact site recovered metallic spherules with compositions that the research team described as unprecedented in the published scientific literature.
'Oumuamua itself is beyond retrieval. It departed the solar system at approximately 44 kilometres per second, heading toward the constellation Pegasus.
From the Archive
The archive holds a biography of Avi Loeb, who proposed the lightsail hypothesis and co-founded the Galileo Project in response to 'Oumuamua:
The Galileo Project’s full institutional record, including the interstellar objects research branch founded in response to 'Oumuamua:
Related coverage:
- “Avi Loeb Asks Washington for Raw UAP Data, Not Processed Videos” (23 April 2026)
- “Luna meets Loeb at Harvard” (29 April 2026)
- “After PURSUE: scientists, witnesses and Congress weigh in on the first UAP tranche” (11 May 2026)
