1I/ʻOumuamua (Hawaiian: “scout” or “messenger from afar arriving first”) was the first confirmed interstellar object detected passing through the Solar System, discovered in October 2017. It displayed several anomalous properties: a cigar-like or disc-like shape, extreme brightness variation, and — most controversially — non-gravitational acceleration not consistent with cometary outgassing (no coma or tail was detected). avi-loeb and collaborators proposed it could be of artificial origin (a light sail), though most astronomers favor exotic natural explanations (e.g., hydrogen iceberg, nitrogen chunk, fractal dust aggregate). It serves as the benchmark on Loeb’s informal “Loeb Classification scale” of interstellar-object anomalousness, against which 3i-atlas was rated slightly less anomalous in src-3i-atlas-22-anomalies-loeb-2026-06.
Comparison to 3I/ATLAS
| Property | 1I/ʻOumuamua | 3i-atlas |
|---|---|---|
| Non-grav. acceleration | Unexplained (no coma) | Detected; includes sideways component |
| Shape | Elongated | Comet-like nucleus |
| Coma/tail | None | Present (active comet) |
| Loeb Classification | Higher anomalousness | Slightly lower |