NASA's fact page: discovered 19 October 2017 by Pan-STARRS1, up to about 400 metres long, first classed as a comet, then an asteroid, then seen to accelerate slightly like a comet.
“‘Oumuamua is the first confirmed object from another star to visit our solar system.” source
Press release on ESO telescope observations: a dark, reddish, highly elongated object, about ten times longer than wide, at least 400 metres long, varying in brightness by a factor of ten as it spins every 7.3 hours.
“It appears to be a dark, reddish, highly-elongated rocky or high-metal-content object.” source
JPL's account of the Micheli et al. result: using Hubble and ground telescopes, the team found a small extra push, most likely gas jets from the surface, too little dust to be seen.
CLASSIFIEDDeclassifiedFILE NO. 026SHEET 2 OF 2
Analysis
Our own reading of the file, written from the sources on the other sheets. Opinion is marked as such.
What is beyond dispute: ‘Oumuamua came from outside the Solar System, it was strongly elongated and reddish, and it accelerated slightly in a way gravity alone does not explain. Those facts come from NASA, ESO and two Nature papers by the observing teams.
The puzzle was the combination: comet-like acceleration with no visible comet activity. Micheli and colleagues ruled out several alternatives and concluded outgassing was physically viable. Bialy and Loeb asked whether sunlight could push it instead, which requires a very thin sheet; that is the paper behind the light-sail headlines. The ISSI team's review and the 2023 hydrogen-outgassing study both went the other way: a natural, comet-like icy body.
Why Explained and not Unexplained: the core question (natural or artificial?) has a published, peer-reviewed answer from the people who collected the data, and the one specific gap, the missing gas, now has a mechanism backed by laboratory work. What remains open is detail. The object is long gone, so no explanation can be checked against new observations of ‘Oumuamua itself.
What it shows
That ‘Oumuamua was the first confirmed object from another star system to pass through ours (NASA, ESO).
That it accelerated slightly beyond what gravity explains, detected at high significance in a peer-reviewed study.
That the teams who studied it, reviewed by an international group, found its properties consistent with a natural, comet-like body.
What it doesn't prove
That it was a light sail or probe. Bialy and Loeb explored the radiation-pressure idea; the review team found no basis for an artificial origin.
That the hydrogen model is final. It explains the data without fine-tuning, but the gas itself was never observed.
That its exact shape is known. Shape and length are inferred from how its brightness changed as it rotated.
The other side
The case against Explained: no gas or dust was ever seen coming off ‘Oumuamua, yet it accelerated. Bialy and Loeb showed that sunlight alone could produce the push if the object were a sheet only about 0.3 to 0.9 mm thick, that such a sheet could survive interstellar travel, and that their results apply to any light probe designed for that purpose. Every outgassing model, including the 2023 one, still relies on gas nobody detected, and the object can no longer be observed.
The observing team's characterisation: extrasolar trajectory, spectrally red surface, no hint of cometary activity despite passing within 0.25 au of the Sun, and a shape about ten times longer than wide.
Peer-reviewed. Online 20 November 2017; issue of December 2017.
OFFICIALMarco Micheli, Davide Farnocchia, Karen J. Meech et al., Nature 559, 223–226 · 2018-06-27
Detects non-gravitational acceleration at 30-sigma significance, rules out solar radiation pressure and several other causes, and finds comet-like outgassing physically viable: probably a comet with unusual properties.
Review by an international team convened at the International Space Science Institute in Bern. Finds every observation consistent with a natural origin, explained by what is known of minor bodies and planetary systems.
“Here we review our knowledge and find that in all cases, the observations are consistent with a purely natural origin for ‘Oumuamua.” source
Peer-reviewed Perspective.
OFFICIALJennifer B. Bergner and Darryl Z. Seligman, Nature 615, 610–613 · 2023-03-22
Proposes that cosmic rays turned some of the object's water ice into trapped hydrogen during its interstellar journey, and that the hydrogen escaped as it warmed near the Sun, producing the acceleration without fine-tuning.
Peer-reviewed.
OFFICIALShmuel Bialy and Abraham Loeb · 2018-10-26
Tests whether sunlight could cause the acceleration: it would need a sheet roughly 0.3 to 0.9 mm thick, which could survive interstellar travel. The authors discuss possible origins and note the results apply to light probes.
“We explore the possibility that the excess acceleration results from Solar radiation pressure.” source
Peer-reviewed: accepted for The Astrophysical Journal Letters (November 2018); arXiv copy linked.
TO VERIFY Candidates, not yet checked
Abraham Loeb's later public statements (book and interviews) arguing more directly for an artificial origin were not opened in this session; only the 2018 paper is cited.
Other proposed volatiles (pure hydrogen, nitrogen and carbon monoxide ice models) are mentioned in Bergner and Seligman's abstract; the individual papers were not opened.
Found, but not linked to a source we trust. Not on the sheets until they are.