It is already leaving.
By the time anyone on Earth sees it, it has rounded the Sun and is running for the exit. It is not orbiting. It is passing through, the way a bullet passes through a room, on a path that will never bring it back.
Eleven weeks. That is how long the human race gets to look at the first object ever caught arriving from another star.
And in those eleven weeks, it does something a rock is not supposed to do. It speeds up.
The orbit that would not close
On 19 October 2017, Robert Weryk was working through images from the Pan-STARRS survey telescope on Haleakala, on Maui. One faint smear did not behave.
Every object in the solar system runs on an ellipse. Feed enough positions into the software and the ellipse closes, and you know where the thing will be next century. This one refused.
The orbit came back hyperbolic. Eccentricity around 1.2, well past the line where a path stops being a loop and becomes a one-way trip. Run it backward and it does not come from the Kuiper belt or the Oort cloud. Run it forward and it does not come back at all.
The catalogues had no slot for it. It was first logged as a comet, then reclassified as an asteroid when no tail appeared, and finally given a designation invented for the occasion: 1I/2017 U1. The I stands for interstellar. It was the first.
The discovery team named it Oumuamua, from Hawaiian, along the sense of a scout or a first messenger reaching out from far away.
It had already made its closest approach to the Sun in early September, inside the orbit of Mercury, and had passed Earth in mid-October at something like a hundred times the distance of the Moon. It was outbound and fading fast. Every night it was dimmer.
The light it did give off was strange. Its brightness swung by roughly a factor of ten as it turned, over a period of about seven or eight hours. To produce a swing that deep, an object has to be extraordinarily elongated, at least five or six times longer than it is wide. Nothing in the solar system is shaped like that. It was also tumbling rather than spinning cleanly, and its color was red, the deep red of a surface cooked by cosmic rays for a very long time.
Off by more than the error bars
Then the telescopes did their last work on it.
An international team led by Marco Micheli, of the European Space Agency's near-Earth object center, gathered everything: ground-based positions from October and November 2017, and Hubble Space Telescope images taken as late as the first days of January 2018. By then the object was a smudge at the edge of what Hubble could hold.
They fitted the whole arc to gravity. Gravity alone would not fit it.
The object kept turning up in the wrong place. Not by a random amount, and not in a random direction. It was consistently ahead of where the Sun's pull said it should be, as though something had been pressing on it, outward, along the line from the Sun.
Their paper appeared in Nature on 27 June 2018, under a title that hides how strange it is: "Non-gravitational acceleration in the trajectory of 1I/2017 U1 ('Oumuamua)." The team reported the excess at very high statistical significance, around thirty sigma in their fit. Whatever else it was, it was not noise.
The push itself was gentle. Roughly a thousandth of the Sun's gravitational grip on the object at that distance, weakening as it receded. Over weeks, that thousandth adds up. It is the difference between a trajectory that fits and a trajectory that does not.
Something was pushing an object from another star system, and it was doing it in full view of the largest telescopes on and above this planet.
The ordinary answer, stated fairly
Comets do this. That is the first thing to say, and it is a strong point.
Ice on a comet's surface warms near the Sun and turns straight to gas. The gas jets off, and the comet recoils. Orbit specialists have been putting non-gravitational terms into cometary solutions since the 1970s, because without them the predictions drift. On this reading, Oumuamua is simply a comet from somewhere else, and the standard account fits it without inventing anything.
The trouble is the missing evidence.
A comet that outgasses hard enough to shift its own path normally announces it. There is dust, a coma, a tail, and emission lines you can pick out in a spectrum. Deep imaging found no coma. Searches for the usual cometary gases came back empty. Infrared observations with the Spitzer Space Telescope, reported in 2018, did not detect the object at all, which put a hard ceiling on how much dust it could be shedding.
So the conventional case has to argue for outgassing that leaves no visible trace. Various forms have been proposed: coarse grains too large and too few to see, or a driver like carbon monoxide, or hydrogen. In March 2023, Jennifer Bergner and Darryl Seligman published work in Nature arguing that molecular hydrogen, made inside water ice by radiation over millions of years in interstellar space and then released as the ice warmed, could supply the thrust while producing almost nothing a telescope would catch.
Others took a different road. Earlier proposals had it as a fragment of nitrogen ice broken off a Pluto-like world, and, in a paper by Shmuel Bialy and Abraham Loeb in the Astrophysical Journal Letters in late 2018, as something thin enough for sunlight itself to move. Radiation pressure alone works, they showed, if the object is a sheet a fraction of a millimeter thick. Their paper noted that such a structure could be a lightsail, and that a lightsail might be artificial. Loeb went on to argue the case at book length in Extraterrestrial in 2021.
The Breakthrough Listen project pointed the Green Bank Telescope at it in December 2017. Nothing came back.
The part that will not go away
Here is the objection the outgassing picture has never fully shaken, and it did not come from anyone hunting for aliens.
Jets torque. A vent firing off one side of a small, wildly elongated, tumbling body does not just push it along, it spins it. In 2018 the astrophysicist Roman Rafikov argued that thrust of the size reported should have altered Oumuamua's rotation dramatically over the observed period, and could have torn a weakly bound body apart. The rotation, as measured, did not do that.
So the ordinary answer requires outgassing strong enough to move the object, gentle enough to leave no coma, and balanced enough not to wreck its spin. Each condition is defensible on its own. Together they describe something nobody has ever watched happen.
And there is no going back for a second look.
That is the hardest fact in the whole affair. Oumuamua was found on its way out, six weeks past its closest approach. It was observable for a matter of weeks. It is now far beyond the giant planets, dark and cold and gone, and no instrument that will ever be built can catch it. Every argument about what pushed it has to be made from eleven weeks of images that nobody can add to.
What is left is a trajectory
Strip away everything anyone has said about it and this is the residue.
An object from another star came through, moved on a path that gravity alone does not account for, and showed no tail while it did so.
Either it was a comet of a kind we had never seen, venting something invisible, and the next interstellar visitor will show us how that works.
Or the first thing to reach us from outside was not the sort of thing we expected at all.
It is not coming back to settle it.