'Oumuamua

ʻOumuamua broke the mold in nearly every measurement. Its brightness swung by a factor of ten as it tumbled, which implies an extreme shape — somewhere between five and ten times longer than it was wide, a cosmic cigar (or a pancake, depending on whose model you trust). It was reddish, the color of a surface baked by cosmic radiation over a very long time. And despite passing close to the Sun, it showed no comet tail, no coma, no detectable outgassing of any kind.

Then, on the way out, it sped up. Not by much — but measurably, more than the Sun's gravity alone could account for, confirmed by Hubble tracking. For a comet this is routine: venting gas acts like a tiny thruster. But ʻOumuamua vented nothing anyone could see. The push was real. The usual cause was missing.

NASA and JPL reclassified it from asteroid to comet and attributed the acceleration to outgassing — hydrogen or other volatiles too faint for our instruments to catch. Reasonable. Also unproven, because we never actually detected the gas.

In 2018, Harvard's Shmuel Bialy and Avi Loeb ran the other number: what if the push was sunlight? Solar radiation pressure could account for the acceleration if ʻOumuamua were extraordinarily thin — about a millimeter — and broad, like a sail. A natural object that thin would be absurdly fragile. An engineered one would not. They did not claim it was built. They said the data did not rule it out.

The definitive review arrived in 2019, when fourteen scientists convened at the International Space Science Institute specifically to settle the question. Their verdict: everything observed is consistent with a natural body, and there is no evidential basis for an artificial one.

Here's the uncomfortable part: both explanations require something we have never seen. A hydrogen iceberg, a nitrogen shard, a fractal dust aggregate — the exotic-but-natural candidates each fit some of the data and none of it cleanly. The sail fits the acceleration and nothing else in nature. The honest position is that ʻOumuamua was genuinely strange, the consensus is the safer bet, and the one measurement that would have closed the case — the gas — was never made.

And it never will be. ʻOumuamua is beyond the reach of any instrument we have or plan to build. The few weeks of data from late 2017 are the entire scientific record, permanently. The second interstellar visitor, Borisov, turned up in 2019 looking like a perfectly ordinary comet — which only makes the first one stranger.