1I/ʻOumuamua

• 1. Introduction

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  On the 19th of October 2017, Canadian astronomer Robert Weryk was working with the Pan-STARRS telescope at Haleakalā Observatory in Hawaii when he found ʻOumuamua, the first confirmed interstellar object ever detected passing through the Solar System. It was about 33 million kilometres from Earth, roughly 85 times as far as the Moon, and it was already heading away from the Sun. Within weeks it would fade beyond the reach of even the largest ground-based telescopes. Astronomers had only a short window to study a visitor from another star. They wanted to know what it looked like, how it moved, and where it began. They also wanted to understand why it behaved unlike any comet or asteroid they had seen. A few scientists went further and asked whether it might be something that someone had built.

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• 2. A Name For The First

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  The first label ʻOumuamua received was C/2017 U1, the code for a comet, chosen because its path was strongly hyperbolic. Deep stacked images from the Very Large Telescope, taken later the same day, showed no coma. A coma is the cloud of gas and dust that a comet normally grows near the Sun. The object was renamed A/2017 U1, which made it the first comet ever re-designated as an asteroid.

  Once astronomers were sure it came from outside the Solar System, the International Astronomical Union had to invent a new category. It added the letter I, for Interstellar object, and ʻOumuamua became 1I. The rules for future I-numbers and names had not yet been written. The object can be called 1I, 1I/2017 U1, or 1I/Oumuamua.

  The Pan-STARRS team chose a Hawaiian name after consulting Kaiu Kimura and Larry Kimura of the University of Hawaii at Hilo. It roughly translates as first distant messenger, an image of a scout sent from the distant past to reach out to humanity. The opening mark is a Hawaiian okina, not an apostrophe, and it is pronounced as a glottal stop.

  Someone had proposed a different name before that choice was made. It was Rama, borrowed from the alien spacecraft in Arthur C. Clarke's 1973 novel Rendezvous with Rama, which is discovered under similar circumstances.

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• 3. Path Through The Sun

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  On the 9th of September 2017, ʻOumuamua swept past the Sun at a distance of 0.255 astronomical units. That is about 17% closer than Mercury ever comes. The Sun's gravity had accelerated it to a top speed of 87.71 kilometres per second, and it bent the object's path sharply northward. It had arrived from north of the ecliptic, the plane in which the planets orbit, and crossed to the south side on the 6th of September. Astronomers did not spot it until roughly 40 days after this closest pass.

  On the way out, ʻOumuamua passed beyond Earth's orbit on the 14th of October, coming within about 0.16175 astronomical units of our planet. It crossed the orbit of Mars on the 1st of November, then Jupiter's in May 2018 and Saturn's in January 2019. It passed beyond Neptune's orbit in 2022. It is now bound for the constellation Pegasus and is slowing toward 26.33 kilometres per second, the same speed it had before it arrived.

  The object came from roughly the direction of Vega, in the constellation Lyra. Its inbound motion lay only 6 degrees from the solar apex, the direction in which the Sun moves relative to nearby stars. That is the most likely direction for any outside object to arrive from.

  The decisive clue was its orbital eccentricity, which measures how stretched an orbit is. Any value above 1.0 means the object is not bound to the Sun. Across 80 days of observation, ʻOumuamua's came to 1.20, the highest on record until 2I/Borisov appeared in August 2019. The previous record holder, C/1980 E1, owed its slightly elevated figure to encounters with planets. No planet in the Solar System, not even an undiscovered one, could produce a value like 1.20.

  By mid-November astronomers were certain. On the 20th of November, a team that included Karen J. Meech and Robert Weryk published the findings in the journal Nature.

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• 4. Shape Without A Picture

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  The Nature paper rested on several instruments. Pan-STARRS1 and the Canada-France-Hawaii Telescope supplied the trajectory. The Very Large Telescope, Gemini South in Chile and the Keck II telescope in Hawaii measured composition and shape. After the announcement, the space telescopes Hubble and Spitzer joined in.

  Time was against them. The Sun-watching STEREO HI-1A instrument did not see the object near its closest approach, which limits its brightness then to about magnitude 13.5. By the end of October it had faded to apparent magnitude 23. By mid-December it was too faint and fast-moving for even the largest ground-based telescopes.

  On the 25th of October, spectra from the Hale Telescope showed a red colour like that of comet nuclei or Trojan asteroids. The 4.2-metre William Herschel Telescope found the spectrum featureless and red, like Kuiper belt objects. The 8.2-metre Very Large Telescope confirmed the pattern the next night in the near-infrared. Overall, the spectrum resembles that of D-type asteroids.

  No telescope ever resolved ʻOumuamua as more than a single point of light. Its shape has to be inferred from how its brightness rose and fell. Bannister and colleagues, and Bolin and colleagues, reported a rotation period near 8.10 hours, with swings of 1.5 to 2.1 magnitudes. Meech's team reported 7.3 hours and 2.5 magnitudes. The mismatch makes sense if the object is tumbling instead of spinning around one axis. A collision in its home system probably set it tumbling, and the motion could last at least a billion years.

  The brightness swings allow anything from a very elongated cigar to a flat pancake. Assuming a 10% albedo and a 6:1 ratio, the dimensions work out to roughly 100-1000 by 35-167 by 35-167 metres, with an average diameter near 110 metres. JPL News described it as up to 400 metres long and perhaps ten times as long as it is wide. A 2019 paper preferred either a cigar at 1:8 or a disc at 1:6, and judged the disc more likely. Bannister and colleagues suggested it might be a contact binary, though its fast rotation may not fit that idea. Astronomer David C. Jewitt called it physically unremarkable apart from its elongation.

  A 2021 paper offered a stranger answer. If ʻOumuamua is made of nitrogen ice, it may have entered the Solar System as an unremarkable object with a 2:1 aspect ratio. One month after perihelion it would have lost 92% of its mass.

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• 5. A Push Without A Tail

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  On the 27th of June 2018, astronomers reported that ʻOumuamua's motion could not be explained by gravity alone. Near its closest approach to the Sun, an extra nongravitational push changed its velocity by about 17 metres per second. Pressure from sunlight was one possible cause.

  The usual cause of such a push on a comet is outgassing, where volatile substances inside the body evaporate as the Sun heats its surface. No tail of gas was seen. Researchers estimated that enough gas could escape undetected to speed the object up. A critical reassessment disagreed. Because of the elongated shape, outgassing would have changed its spin quickly and torn it apart, yet its spin looked stable.

  The same June announcement called ʻOumuamua a mildly active comet instead of an asteroid. Studies submitted in October 2018 suggested it was neither. Astronomer Zdenek Sekanina proposed it could be the remnant of a disintegrated interstellar comet. By 2024, astronomers had found 14 asteroids showing the same unusual behaviour. That made ʻOumuamua the first member of another new class, called dark comets.

  The missing coma still carries information. It limits the amount of surface ice to a few square metres, and any volatiles must lie beneath a crust at least 0.5 metres thick. The object must have formed inside the frost line of its parent star's system, or lingered there long enough for near-surface ice to sublimate, as may be the case with damocloids. Its spectrum favours the second story if it formed like Solar System objects. The Canadian Meteor Orbit Radar also looked for a meteor shower expected on the 18th of October 2017 from the constellation Sextans. It detected nothing.

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• 6. Icebergs And Dust Bunnies

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  Nitrogen ice offered one tidy explanation for the lack of visible outgassing. Nitrogen ice the size of ʻOumuamua could survive 500 million years in interstellar space and would reflect two-thirds of the Sun's light. In March 2021, scientists presented a theory built on this idea. They suggested ʻOumuamua was a fragment of an exoplanet similar to Pluto, an exo-Pluto. Avi Loeb criticised the theory, and in November 2021 Siraj and Loeb argued that it was not a nitrogen iceberg.

  A rival idea appeared in May 2020. It proposed that ʻOumuamua was the first seen member of a class of small bodies rich in hydrogen ice. These would form at temperatures near 3 kelvin in the cores of giant molecular clouds. The Sun's heat would turn the hydrogen to gas and push the body along. A hydrogen coma would be hard to see from the ground, because the atmosphere blocks those wavelengths. Steady mass loss would also stretch the body, the way a bar of soap grows longer as it is used up.

  The hydrogen iceberg idea ran into trouble. Later work showed such icebergs cannot form from small grains. To survive the journey, they would have to have formed about 40 million years ago, near the Solar System. In August 2020, astronomers reported that ʻOumuamua was probably not frozen hydrogen.

  On the 22nd of March 2023, astronomers proposed a third version. They attributed the acceleration to the release of "entrapped molecular hydrogen that formed through energetic processing of an H2O-rich icy body". Cosmic rays would have broken down water ice during the object's long years between the stars, trapping hydrogen inside it. On that view, ʻOumuamua is an interstellar comet.

  Other analyses of its light suggest dense, metal-rich rock reddened by millions of years of cosmic rays. Its surface may hold tholins, which are irradiated organic compounds. Ice might still hide inside under an insulating mantle formed by long exposure to cosmic rays. In November 2019, some astronomers floated the picture of a cosmic dust bunny, a very light and fluffy mix of dust and ice grains.

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• 7. Home Unknown

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  Tracing the object backward from Vega gives a startling number. Allowing for Vega's own motion, the trip would have taken ʻOumuamua 600,000 years, though Vega was not in that part of the sky then. A century ago it was 561 astronomical units from the Sun, moving at 26.33 kilometres per second. That speed is very close to the mean motion of material near the Sun, known as the local standard of rest, and especially close to a nearby group of red dwarf stars. It appears to rule out the closest dozen stars. It also hints that the object may have circled the Milky Way several times and come from a different part of the galaxy.

  Nobody knows how long it has wandered. The Solar System is probably the first planetary system it has closely met since being thrown out of its birth system, possibly several billion years ago. One proposal points to the young star groups called Carina and Columba, within about 100 parsecs, with an ejection 45 million years ago. Both now sit far across the sky from where ʻOumuamua appeared. Others suggested a white dwarf system, where the object lost its volatiles when its parent star swelled into a red giant.

  About 1.3 million years ago, it may have passed within 0.16 parsecs of the star TYC 4742-1027-1. Its speed is too high for that to be home. It probably just crossed that system's Oort cloud at roughly 15 kilometres per second. In August 2018, a study using Gaia Data Release 2 named four stars it passed fairly close to, at moderately low speeds, within the past few million years. They were HIP 3757, HD 292249, Gaia DR2 2502921019565490176 and Gaia DR2 3666992950762141312. A September 2018 study described several possible home systems.

  In April 2020, astronomers raised a more violent idea. ʻOumuamua might be a fragment of a planet torn apart by tides. That would make it rare, but such a process tends to produce cigar-shaped fragments, while the light curve leans toward a disc.

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• 8. Ship Or Rock

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  The fictional Rama was long and thin, and ʻOumuamua turned out to be unusually elongated as well. That coincidence fed speculation that the visitor might be a spacecraft. The SETI Institute's Allen Telescope Array checked and found no unusual radio emissions.

  In December 2017, Harvard astronomer Avi Loeb, an adviser to the Breakthrough Listen Project, cited the elongated shape as one reason to listen more closely. On the 13th of December, the Green Bank Telescope in West Virginia observed the object for six hours across four radio bands. It found no signals. Because the object was so close, the searches could rule out any transmitter above an extremely low power of 0.08 watts.

  On the 26th of October 2018, Loeb and his postdoctoral researcher Shmuel Bialy submitted a paper on a bolder idea. They asked whether ʻOumuamua might be an artificial thin solar sail pushed along by sunlight. Other scientists answered that the evidence was insufficient, and that a tumbling sail could not accelerate. Loeb replied with an article listing six anomalous properties and a public science book in 2021. By July 2019, most astronomers had concluded that it was a natural object. Many experts consider the solar sail idea unlikely, because simpler explanations fit what is expected of interstellar asteroids and comets.

  Some researchers have asked whether anyone could ever visit it. The Initiative for Interstellar Studies launched Project Lyra to test the idea. It studied ways of reaching ʻOumuamua within 5 to 25 years. NASA's Space Launch System could pass it at 63 kilometres per second. Solar, laser electric and laser sail options based on Breakthrough Starshot technology were also weighed. The hard part is slowing down. A craft that flew past at hypervelocity would return little science and could neither orbit nor land. The Lyra authors concluded that an encounter mission would be difficult but feasible with near-term technology. Seligman and Laughlin reached a similar verdict.

  Meanwhile, the Solar System has had other callers. 2I/Borisov was discovered on the 30th of August 2019 and soon confirmed as an interstellar comet. It arrived from the direction of Cassiopeia and reached perihelion on the 8th of December 2019. Then 3I/ATLAS was discovered on the 1st of July 2025, arriving at 58 kilometres per second, more than twice ʻOumuamua's pace.

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