In the autumn of 2017, astronomers tracked a small, fast-moving object cutting across the inner solar system on a path that no comet or asteroid born near the Sun could follow. Its trajectory was open rather than looping, meaning it was not bound to the Sun at all. It had arrived from the depths of interstellar space, swung around the Sun, and was already on its way back out, the first object of confirmed interstellar origin ever detected passing through the solar system.
The visitor was faint, tumbling, and available for study for only a matter of weeks before it faded beyond reach of even the largest telescopes. In that narrow window it displayed a set of properties strange enough that a handful of researchers openly floated the possibility of an artificial origin, while most concluded it was a natural body of an unfamiliar kind. It has not been seen since, and it never will be again.
A confirmed interstellar origin
The object was discovered on 19 October 2017 by Robert Weryk using the Pan-STARRS survey telescope in Hawaii, and it was given a Hawaiian name meaning “scout.” According to the observational record for the object, measurements spanning about 80 days yielded an orbital eccentricity of roughly 1.20, the highest ever recorded at the time. An eccentricity above 1.0 means the path is hyperbolic: the object was moving faster than the Sun’s escape velocity, was never captured into orbit, and was simply passing through. It approached and departed at about 26 kilometers per second relative to the Sun, a speed inherited from its long journey between the stars.
An unusual shape
The object’s brightness varied dramatically and rhythmically as it tumbled, swinging by a factor of many times over a period of hours. The most straightforward reading of that light curve is that the body is highly elongated, far more so than nearly any asteroid observed in the solar system, with estimates suggesting a length several times its width. Whether it resembled a long cigar or a flattened disk was never fully resolved, because the object was too faint and too fleeting to image directly. Its reddish surface color and irregular brightness are, in themselves, typical of certain asteroids, which supported a natural interpretation of its makeup. The reddening is generally attributed to long exposure to cosmic radiation, which darkens and colors the surfaces of small bodies over vast spans of time, consistent with an object that had wandered between the stars for a very long journey before reaching the Sun.
The mysterious acceleration
The property that most complicated a simple explanation emerged in mid-2018, when astronomers reported that the object had experienced a slight non-gravitational acceleration, a small extra push that gravity alone could not account for. In comets, such acceleration comes from outgassing, as ices vaporize in sunlight and act like a weak thruster. But no coma, tail, or obvious gas emission was detected around this object. That mismatch, an apparent push without a visible cause, is at the heart of why the object resisted tidy classification.
The megastructure hypothesis
The combination of an extreme shape and an unexplained acceleration led a small number of scientists to raise, and publicly argue for, the idea that the object might be an artifact, potentially a thin, sail-like structure driven by sunlight. That proposal drew wide attention, but it remained a minority view. Most researchers favored natural explanations, including a highly porous body, an exotic nitrogen-ice fragment shed from another planetary system, or a loosely bound object composed of frozen hydrogen. Each natural model has its own weaknesses, and none has been universally accepted, which is precisely why the object remains a subject of debate rather than a closed case. A frequently cited natural resolution holds that the object was outgassing a substance that produced little or no visible dust, so that a gentle jet could push it without creating a detectable coma. Whether the vented material was an exotic ice or a more familiar volatile has not been settled, and the amount of acceleration observed constrains, but does not uniquely identify, the mechanism.
A single fleeting pass
The object was detected only after it had already rounded the Sun and begun to recede, so the entire study period was compressed into a few weeks in late 2017. It brightened briefly, then dimmed rapidly as it retreated, quickly becoming too faint to observe. There was no opportunity to send a spacecraft, capture a close image, or gather a sample. Everything known about it comes from that short burst of ground-based observation, which is one reason the ambiguities have never been resolved.
The first of a new class
Whatever its true nature, the object confirmed something long suspected but never proven: that fragments from other planetary systems drift through the galaxy and occasionally cross paths with the Sun. That expectation was reinforced less than two years later, in August 2019, when a second and unmistakably cometary interstellar object, 2I/Borisov, was discovered. The 2017 visitor thus stands as the opening entry in a genuinely new category of solar system science, an emissary from another star that appeared without warning, defied easy description, and vanished back into the dark before its secrets could be settled.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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