The Object That Won't Close.

On 19 October 2017 a telescope in Hawaii caught something falling through the solar system on a path that could only have come from another star. It was tumbling, wildly elongated, and as it left it sped up — pushed by a force that gravity alone can’t supply, while showing no comet’s tail to explain the push. Every other case in this series ended in a clean verdict. This one doesn’t — and that’s the point. The method’s hardest job isn’t dispelling a mystery. It’s holding one honestly.

Where we land: still an open case — the good kind. We reproduced all three measured facts in our own code, but two very different explanations fit the same push, and the tracking data can’t tell them apart.

Three things about ‘Oumuamua are measured facts, not interpretations. Its orbit was hyperbolic — an open curve; it was never bound to the Sun. Its brightness swung by a huge factor as it tumbled, which only an extreme shape can do. And its outbound path bent away from the pure-gravity prediction, betraying a gentle extra push. We reproduce the orbit and the shape from the numbers, and illustrate the push, in our own code. What we cannot do — what nobody yet can — is say what the push was. Here’s the dark, drawn exactly as wide as it really is.

1 · The shape: a light curve that needs a splinter

‘Oumuamua was a point of light — far too small for any telescope to resolve. But a tumbling body betrays its shape by how its brightness rises and falls: a long object shows you a big face, then an edge, then a big face again. Below is the light curve of a rotating 6:1:1 ellipsoid — a body six times longer than it is wide — computed in our own code from its projected area. Two peaks per rotation, and a swing of 1.95 magnitudes on the astronomers’ brightness scale.

2 · The push: a bend gravity can’t make

From the point on the outbound arc where the object sat at 1.4 AU — the geometry Micheli and colleagues actually tracked — run the path twice from the same starting state: once under the Sun’s gravity alone, once with the measured extra push — 5×10⁻⁶ m/s² at 1 AU, falling off as 1/r² (so about 2.6×10⁻⁶ m/s² out at the 1.4 AU tracking point). Integrated step by step in our own code (RK4, the workhorse of orbit calculations), the two paths peel apart to a 65,633 km gap in 120 days — the same order as the real reported deviation, a 30-sigma detection: the bend stands thirty error bars tall, which is astronomy for “not a fluke.” The push is real, and it follows a comet’s 1/r² law.

3 · The knot: two answers, one number

So something pushed it. A comet outgasses — escaping vapour shoves it along like a slow, gentle rocket — but outgassing carries dust, and dust makes a tail, and ‘Oumuamua had none anyone could see. Take away outgassing and the next candidate is sunlight itself, pushing on a big, light sail of an object. Both explain the exact same 5×10⁻⁶ m/s². Neither can be ruled out by the trajectory, because they leave the same 1/r² fingerprint. That is the knot — and here is why the number alone can’t cut it.

Door one — radiation pressure. Sunlight pushes on area. To supply the measured push, the object needs an area-to-mass ratio of about A/m ≈ 1.10 m²/kg. A rock’s is ~0.0001 — ten thousand times less. A splinter of solid rock can’t be that light. You need something gossamer: a fluffy fractal aggregate… or, as one camp argued, something made. That is the fringe reading.

Door two — cometary outgassing. Escaping ice supplies the thrust at normal density — no exotic thinness required. This is the leading natural explanation. But comets that outgas this hard show a bright dust coma, and the observed tail was: none. The rescue is an invisible propellant — pure H₂ or N₂ ice, which boils straight from ice to gas without dust. Plausible, but never seen before, and its own kind of exotic.

Both doors fit the key. The tracking that proves the push exists is the very thing that can’t tell you which door it came through. The honest result isn’t an answer — it’s the exact shape of what we don’t know.

Why our math sees more

Surface-first thinking wants a headline — “comet” or “aliens,” pick one. Structure-first analysis separates the invariant (orbit, elongation, acceleration — the dark that every observer agrees on) from the interpretation (the mechanism — the light that the data can’t yet decide) and refuses to spend certainty it hasn’t earned. The discipline that debunked five mysteries in a row is the same discipline that, here, says not yet. Holding a doubt at its real size is not failing to answer. It is the answer.