'Oumuamua: The Interstellar Object Unlike Anything We've Seen

On October 19, 2017, a telescope on a Hawaiian mountaintop caught a faint smudge of light moving the wrong way. Within days, astronomers realized they were not looking at another comet or asteroid born in our own solar system. They were looking at a tourist — an object that had fallen in from interstellar space, swung around the Sun, and was already on its way back out, never to return. They named it 'Oumuamua. And the more they studied it, the stranger it got.

This is the story of the first confirmed interstellar visitor — what we actually know, what remains genuinely unexplained, and the theories scientists have proposed to make sense of it.

The Documented Facts

It was discovered leaving, not arriving. 'Oumuamua was first spotted on October 19, 2017, by the Pan-STARRS1 telescope at Haleakalā Observatory in Hawaii, funded by NASA's Near-Earth Object Observations Program (NASA Science). It had already passed its closest point to the Sun (perihelion) on September 9, 2017, so we caught it on the way out. Its formal designation is 1I/2017 U1 — the "1I" marking it as the first interstellar object ever catalogued. The name is Hawaiian for "a messenger from afar arriving first" (NASA Science).

Its orbit proved it came from outside the solar system. The decisive evidence was mathematical. 'Oumuamua's orbit had an eccentricity of about 1.20 — well above 1.0, the threshold that separates a closed loop around the Sun from an open, hyperbolic path that is not gravitationally bound (Wikipedia summary of orbital data). It approached the Sun with an interstellar velocity of roughly 26 km/s and made its perihelion pass inside the orbit of Mercury, at about 0.255 AU (Wikipedia). NASA's Jet Propulsion Laboratory clocked it streaking past the Sun at around 87 km/s (about 196,000 mph) (NASA JPL). It entered from the direction of the constellation Lyra and is now headed outward toward Pegasus.

It was small, reddish, and oddly proportioned. Estimates of its size depend on assumptions about how reflective it is, but it was roughly a few hundred meters in its longest dimension (NASA Science). Its brightness varied by a factor of about ten as it tumbled — too dramatic a swing for a roughly round body — which told astronomers it was extremely elongated, perhaps as much as ten times longer than it was wide (NASA JPL). It rotated, or rather tumbled, on a period of roughly 8 hours in a non-principal-axis spin (Wikipedia). Its surface was reddened, consistent with long exposure to cosmic radiation over millions of years in deep space.

It looked like a comet's cousin but showed no comet's tail. Here is where 'Oumuamua broke the mold. Despite passing close to the Sun, it produced no visible coma — no glowing halo of gas, no dust tail — and direct searches detected no outgassed molecules or dust around it (Nature, 2018). By every visible sign, it behaved like an inert rock.

The Genuine Open Question

And yet it did not move like an inert rock.

In June 2018, a team publishing in Nature reported that 'Oumuamua's path could not be explained by gravity alone. After accounting for the pull of the Sun and planets, the object showed a small but real extra push directed away from the Sun — a non-gravitational acceleration that changed its velocity by about 17 meters per second near perihelion (Nature, 2018).

For a comet, this would be unremarkable. Comets get nudged by jets of gas boiling off their surface — the recoil acts like a tiny thruster. But that mechanism normally produces a visible cloud of gas and dust. 'Oumuamua showed none. The implied gas-to-dust ratio would have to be at least a hundred times higher than that of any comet ever measured in our solar system (Nature, 2018).

So the central mystery is simple to state and hard to answer: What gave 'Oumuamua an extra push, when it showed none of the outgassing that normally explains such a push? The object is now far too distant to observe and far too fast to chase, so we will likely never gather new data on this particular visitor (Astronomy.com). The answer has to come from modeling — and from the next interstellar object we catch.

Theories and Interpretations

Everything in this section is interpretation — scientists' best attempts to reconcile the facts above. None is settled.

A hidden, dust-free outgassing (the leading natural explanation)

The most widely accepted line of thinking keeps 'Oumuamua firmly in the family of natural icy bodies, just an unusual one. In a 2023 paper in Nature, Jennifer Bergner (then at the University of Chicago, now UC Berkeley) and Darryl Seligman (then at UChicago) proposed that during 'Oumuamua's long journey through interstellar space, cosmic radiation converted some of its water ice into molecular hydrogen, trapping the gas inside the body (University of Chicago News; Nature, 2023).

When the object warmed near the Sun, that trapped hydrogen escaped and provided thrust — but because the process released hydrogen rather than sublimating dusty ice, it produced no visible coma. Crucially, the authors argue, 'Oumuamua was small enough that even this gentle hydrogen venting could measurably alter its course (University of Chicago News). Speculative caveat: this elegant model has been debated — critics question whether the object could retain hydrogen at the relevant temperatures — and the authors themselves note it may not explain similar "dark comets" closer to home.

A different kind of ice

Related natural proposals suggest 'Oumuamua was a fragment of something more exotic — for example, a chip off a nitrogen-ice surface, like the frozen plains of Pluto. A body made largely of vaporizing nitrogen or hydrogen ice could, in principle, accelerate without leaving an obvious dusty trail. These ideas remain plausible but unproven, limited by the simple fact that we cannot resample the object.

Was it flat rather than cigar-shaped?

The familiar artist's renderings show a dark, cigar-like rock. But the light curve is also consistent with a flattened, pancake or disk shape (Sky & Telescope). Because we never resolved 'Oumuamua as more than a point of light, its true shape is itself a labeled uncertainty — the elongation is well supported; "cigar versus pancake" is not resolved.

The artificial-origin idea — and why most scientists set it aside

The thin, oddly proportioned shape and the unexplained push led Harvard astrophysicist Avi Loeb to publish a hypothesis that 'Oumuamua could be artificial — a thin, light-sail-like structure pushed by sunlight (arXiv preprint; labeled here as a preprint). It is an interpretation, not a finding, and the broader astronomical community regards it as unlikely. Detailed analyses found that a tumbling light sail should have been pushed sideways in a way 'Oumuamua was not, and that such a thin sheet would periodically appear edge-on and vanish from view — which never happened (Big Think summary). The mainstream consensus by 2021 was that 'Oumuamua's properties are consistent with a natural object (Wikipedia). We include this theory because it shaped the public conversation — and because separating it from the evidence is exactly the discipline 'Oumuamua demands.

Why It Still Matters

'Oumuamua is gone, but it kicked open a door. In 2019 came 2I/Borisov, an unmistakable interstellar comet with a proper tail (Wikipedia). In July 2025, astronomers confirmed a third visitor, 3I/ATLAS (NASA Science). With the NSF–DOE Vera C. Rubin Observatory now surveying the sky and ESA's Comet Interceptor mission designed to chase a future visitor (Scientific American), the era of catching these wanderers has only begun.

The first messenger from afar left us a riddle we could not finish reading. The next one may let us read the whole sentence.

Sources and Further Reading

• NASA Science — 'Oumuamua overview
• NASA JPL — Solar System's First Interstellar Visitor Dazzles Scientists
• Nature (2018) — Non-gravitational acceleration in the trajectory of 1I/2017 U1
• Nature (2023) — Acceleration of 1I/'Oumuamua from radiolytically produced H2 in H2O ice
• University of Chicago News — A surprisingly simple explanation for 'Oumuamua's weird orbit
• Sky & Telescope — Is 'Oumuamua an Interstellar Pancake?
• NASA Science — Comet 3I/ATLAS Facts and FAQs
• Scientific American — European Comet Interceptor Could Visit an Interstellar Object

Sources & further reading

• https://science.nasa.gov/solar-system/comets/oumuamua/
• https://www.jpl.nasa.gov/news/solar-systems-first-interstellar-visitor-dazzles-scientists/
• https://www.nature.com/articles/s41586-018-0254-4
• https://www.nature.com/articles/s41586-022-05687-w
• https://news.uchicago.edu/story/surprisingly-simple-explanation-interstellar-visitor-oumuamuas-weird-orbit
• https://en.wikipedia.org/wiki/1I/%CA%BBOumuamua
• https://skyandtelescope.org/astronomy-news/could-oumuamua-be-an-interstellar-pancake/
• https://science.nasa.gov/solar-system/comets/3i-atlas/3i-atlas-facts-and-faqs/
• https://www.scientificamerican.com/article/european-comet-interceptor-could-visit-an-interstellar-object/
• https://arxiv.org/abs/2110.15213
• https://bigthink.com/13-8/oumuamua/
• https://www.astronomy.com/science/can-we-catch-oumuamua-interstellar-interloper/