In October 2017, a telescope in Hawaii caught something that had never been confirmed before: an object passing through our solar system that did not come from it. It was named 'Oumuamua, Hawaiian for 'scout.' It was tumbling, oddly elongated, reddish, and moving far too fast to be bound to the Sun. Then, on its way out, it did the thing that turned a curiosity into a controversy that still hasn't ended. It accelerated, and nobody could see why.

Start with what is solid, because the hard data here is genuinely strange. 'Oumuamua's hyperbolic orbit was nailed down well enough to be certain of its interstellar origin. Its brightness swung by a factor of about ten as it tumbled, implying an extreme shape, possibly several times longer than it was wide, a geometry essentially unseen among ordinary asteroids and comets. And in a 2018 paper in Nature, Marco Micheli and colleagues reported a non-gravitational acceleration in its trajectory, detected at high statistical significance, pushing it outward as if something were gently shoving it away from the Sun. That measurement is not in serious dispute. The argument is entirely about its cause.

Normally an outward push like that is easy to explain: comets outgas, jets of vapor act like little rockets, and you see a glowing tail. 'Oumuamua had no detectable tail. Deep imaging found no coma, no visible dust, and spectroscopy found no gas emission, even though it passed inside the orbit of Mercury, close enough that any ordinary comet would have been screaming with activity. So you are left with a body that accelerated like an outgassing comet while showing none of the evidence of outgassing. That is the anomaly, stated without embellishment.

This is where the Harvard astronomer Avi Loeb stepped in. With Shmuel Bialy, he published a 2018 paper in The Astrophysical Journal Letters proposing that the acceleration could be explained by solar radiation pressure, if 'Oumuamua were extremely thin and sail-like, and noting that such a structure 'may be a lightsail' of artificial origin. Loeb's argument is not that aliens are proven; it is that the conventional explanations each require something unprecedented anyway, so a manufactured object should not be ruled out by reflex. He has made the fuller case in the scientific literature and in his book Extraterrestrial.

The mainstream pushed back hard, and fairly. A 2019 collective study in Nature Astronomy concluded that 'Oumuamua's behavior was consistent with a natural object, and various teams proposed natural fixes for the missing tail: a comet venting molecular hydrogen, which is invisible and produces little dust; a fluffy fractal aggregate with a huge surface-to-mass ratio; or a shard of nitrogen ice chipped off a Pluto-like world. Each is plausible. Each is also a never-before-seen kind of body invented specifically to fit this one object. That is the uncomfortable symmetry skeptics sometimes gloss over: the natural explanations are exotic too.

The deciding evidence, the kind that would settle it, does not exist, and that is the heart of the problem. 'Oumuamua was discovered on its way out. Nobody got a close look, a radar return, or a resolved image. Everything we will ever know about it comes from a few weeks of a fading dot of light. It is gone, accelerating into the dark, and it is not coming back.

So the question stays open in the most honest way a question can. We have a rigorously measured push with no visible engine, and a menu of explanations every one of which requires inventing a new category of object, whether icy, fluffy, or built. The follow-up interstellar visitors since, like 2I/Borisov, behaved like ordinary comets and told us nothing about this one. 'Oumuamua remains a single data point that fits no template we had, and the only certainty is that we let the first known piece of another star system pass through our reach without ever getting close enough to know what it was.