# 1I/ʻOumuamua <!-- SOLSIM:BEGIN g31 — Solar System explorer state (hand-built on wt-core, specs/solar/); do not hand-edit inside --> **Microsim — three.js (Wikitube framework):** *1I/ʻOumuamua in the Solar System explorer* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?obj=Oumuamua&embed=1" data-title="1I/ʻOumuamua in the Solar System explorer"></div> *The Solar System explorer locked on this article's state (`?obj=Oumuamua`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.* <!-- SOLSIM:END --> *Try: set the year slider to 2017 and the speed to 1 day/s, and watch ʻOumuamua drop in from above the planets' plane, whip round the Sun inside Mercury's orbit in early September and head out again; press l to label the planets and see it cross Earth's orbit in mid-October; then drag to an edge-on view to follow its steep, open path through the plane of the planets.* **1I/ʻOumuamua** (formally 1I/2017 U1) is the first object known to have passed through the [[PORTAL_Solar_System|Solar System]] from interstellar space. Robert Weryk found it on 19 October 2017 with the Pan-STARRS1 telescope on Haleakalā, Hawaii, about 40 days after it had passed closest to the [[Sun]] on 9 September.[^meech2017][^nasa-faq] It was then about 0.2 AU from [[Earth]] and already leaving. Its speed of 26.3 km/s far from the Sun, on a hyperbolic [[Orbit|orbit]] of eccentricity 1.20, shows that it is not bound to the Sun and will not return.[^gray2017][^jpl-sbdb] ʻOumuamua is small, probably a few hundred metres long, reddish like many outer Solar System bodies, and was never seen with a coma. Its brightness swung by a factor of about ten as it tumbled, implying an unusually elongated or flattened shape, and its path showed a small push away from the Sun that gravity alone does not explain.[^meech2017][^fraser2018][^micheli2018] These traits led to proposals that it is a fragment of nitrogen ice, a body of hydrogen ice, an ordinary icy planetesimal releasing trapped hydrogen, or even an artificial light sail; most astronomers regard it as a natural object, and the [[Interstellar_object|interstellar object]] class it opened now includes 2I/Borisov and 3I/ATLAS.[^issi2019][^bergner2023] The explorer at the top of this page draws ʻOumuamua's hyperbolic path through the inner Solar System in 2017 among the planets. ## Name As the first object of its kind, ʻOumuamua needed a new kind of name. It was first announced as a comet, C/2017 U1 (PANSTARRS), and then, when deep images showed no coma, redesignated A/2017 U1, the first comet ever re-classed as an asteroid.[^mpec2017u181][^mpec2017u183] Once its interstellar origin was clear, the Minor Planet Center created the designation I, for interstellar, and numbered it 1I; it can be cited as 1I, 1I/2017 U1, 1I/ʻOumuamua or 1I/2017 U1 (ʻOumuamua).[^mpec2017v17] The name is Hawaiian. It joins *ʻou*, to reach out for, and *mua*, first or in advance, with *mua* repeated for emphasis, and the Minor Planet Center renders it as "a messenger from afar arriving first".[^pukui2003][^mpec2017v17] The first character is an ʻokina, a letter marking a glottal stop, not an apostrophe. The Pan-STARRS team chose the name with Kaʻiu Kimura and Larry Kimura of the University of Hawaiʻi at Hilo.[^gal2017] Before the name was settled, some suggested Rama, after the alien spacecraft found in similar circumstances in Arthur C. Clarke's 1973 novel *Rendezvous with Rama*.[^economist2017] ## Observations Karen Meech, Robert Weryk and colleagues combined data from Pan-STARRS1 and the Canada–France–Hawaii Telescope, which fixed its orbit, with measurements from the Very Large Telescope, Gemini South and Keck II, which constrained its colour, shape and spin, and published the first full study in *Nature* on 20 November 2017.[^meech2017] The Hubble and Spitzer space telescopes followed.[^jpl2017] The object faded fast: it was about magnitude 23 by the end of October and by mid-December was too faint for the largest ground-based telescopes.[^mpc-1i][^gemini2017] ### Trajectory ʻOumuamua came from the direction of Vega in Lyra, only about 6° from the solar apex, the direction toward which the Sun moves through the nearby stars and from which interlopers are most likely to come.[^mamajek2017] Precovery images from the Catalina Sky Survey on 14 and 17 October extended the arc, and within two weeks the hyperbolic orbit was secure.[^mpec2017u185][^jpl-sbdb] Its speed changed as the Sun's [[Gravity|gravity]] acted on it. It approached at 26.33 km/s, sped up to 87.71 km/s at perihelion, 0.255 AU from the Sun and closer than [[Mercury_(planet)|Mercury]] ever comes, and is now slowing back toward 26.33 km/s.[^gray2017] The vis-viva equation reproduces the perihelion speed from the fitted orbit to within about half a percent (derived). An eccentricity of 1.20 is far beyond anything a planetary encounter could produce: the previous record, C/1980 E1 at 1.057, was set by a close pass of [[Jupiter]], but no planet, known or unknown, can impart ʻOumuamua's speed.[^wright2017][^jpl-sbdb] It entered from north of the [[Ecliptic|ecliptic]], passed south of it near perihelion, and crossed back on 16 October. It passed 0.162 AU from Earth on 14 October, crossed the orbits of [[Mars]] on 1 November 2017, Jupiter in May 2018 and [[Saturn]] in January 2019, and is heading toward Pegasus.[^nasa-faq][^jpl-sbdb] In June 2018 Marco Micheli and colleagues reported a small non-gravitational acceleration directed away from the Sun, about 5 × 10⁻⁶ m/s² at 1 AU and falling with distance.[^micheli2018] That is less than a thousandth of the Sun's pull at the same distance, 5.9 × 10⁻³ m/s² (derived), but it was measured clearly. They attributed it to outgassing, although no gas or dust was seen, and Roman Rafikov argued that jets strong enough to push it would also have changed its spin quickly, which was not observed.[^micheli2018][^rafikov2018] Its origin is unknown. Its velocity before it met the Sun is close to the average motion of stars in the Sun's neighbourhood, so it may have wandered the galaxy for a long time.[^mamajek2017] Candidate birthplaces include young stellar associations about 45 million years ago, and a search of Gaia data found four stars it passed at fairly low speed within the last few million years, none a convincing source.[^gaidos2017][^bailerjones2018] ### Classification The absence of a coma limits any exposed ice to a few square metres, so any volatiles must lie under a crust at least about half a metre thick.[^jewitt2017] That suggests that ʻOumuamua formed inside the [[Frost_line_(astrophysics)|frost line]] of its home system or lost its surface ices there. No meteors were detected when Earth passed near its path on 18 October 2017.[^ye2017] After the acceleration was reported it was considered a mildly active comet.[^micheli2018] In 2024 a class of "dark comets", asteroids with non-gravitational acceleration but no visible coma, was defined with 14 members, and ʻOumuamua resembles them.[^seligman2024] ### Appearance, shape and composition Its spectrum is red and featureless, like those of D-type asteroids and many [[Kuiper_belt|Kuiper belt]] objects, which suggests a surface reddened by cosmic rays.[^fitzsimmons2018][^jewitt2017] It is not spinning about a single axis but tumbling, with a period near 8 hours; a collision may have set it tumbling, and such motion takes more than a billion years to damp out.[^fraser2018][^bolin2018] Its brightness varied by up to about 2.5 magnitudes.[^meech2017] For a body seen side-on, a range of Δm magnitudes corresponds to an axis ratio of 10^(Δm/2.5), so 2.5 magnitudes implies about 10:1 (derived), though viewing angle can exaggerate the effect. Models allow either a cigar about 6:1 to 8:1 or a flattened disc about 6:1, with the disc favoured in one analysis.[^mashchenko2019] At 10% reflectivity the mean diameter is about 110 m; NASA gives up to about 400 m in length, and later analyses allow 100–1,000 m by 35–167 m.[^jewitt2017][^jpl2017][^jpl2018] ### Radio measurements The Allen Telescope Array found no radio emission. On 13 December 2017 Breakthrough Listen used the Green Bank Telescope for six hours across four frequency bands and found no narrowband signals, ruling out a transmitter on the object radiating as little as 0.08 W.[^billings2017][^enriquez2018] ## Discussion ### Nitrogen ice theory Alan Jackson and Steven Desch proposed in 2021 that ʻOumuamua is a fragment of nitrogen ice knocked from the surface of an exo-Pluto. Sublimating [[Nitrogen|nitrogen]] would push it without a visible coma, a nitrogen body would reflect much more light, and its extreme shape could be the result of evaporation, having started near 2:1 and lost about 92% of its mass by a month after perihelion.[^jackson2021][^desch2021] Amir Siraj and Avi Loeb argued that too few such fragments could be produced.[^hickok2021] ### Hydrogen ice theory Darryl Seligman and Gregory Laughlin suggested in 2020 that ʻOumuamua was made of molecular [[Hydrogen|hydrogen]] ice formed in a dense molecular cloud; hydrogen outgassing would be hard to detect from the ground.[^seligman2020] Thiem Hoang and Loeb showed that such a body would evaporate in interstellar space long before arriving unless it formed very recently and nearby.[^hoang2020] ### Hydrogen-laden water ice theory In 2023 Jennifer Bergner and Seligman proposed that cosmic rays had split [[Water|water]] ice in the body over its long journey, trapping molecular hydrogen that escaped as the Sun warmed it. That makes ʻOumuamua an ordinary icy planetesimal, broadly like a Solar System [[Comet|comet]].[^bergner2023] ### Hypothetical space missions Project Lyra, run by the Initiative for Interstellar Studies, found that a spacecraft could still catch ʻOumuamua, using flybys of Jupiter and a close pass of the Sun, with flight times of about 5–25 years; one scenario launches by 2028 and arrives about 26 years later.[^hein2019][^hibberd2020] Seligman and Laughlin reached a similar conclusion for future interlopers.[^seligman2018] ### Technosignature hypothesis In October 2018 Shmuel Bialy and Loeb suggested that radiation pressure could explain the acceleration if ʻOumuamua were an extremely thin sheet, possibly an artificial light sail.[^bialy2018] Loeb later listed its anomalies in popular articles.[^loeb2018] Most researchers consider natural explanations sufficient, and the ʻOumuamua ISSI Team concluded in 2019 that nothing about it requires an artificial origin.[^issi2019] ### Other interstellar objects 2I/Borisov, found on 30 August 2019, arrived from the direction of Cassiopeia and was plainly a comet; it passed perihelion on 8 December 2019. 3I/ATLAS, found on 1 July 2025, arrived at about 58 km/s.[^jpl-sbdb] Claims that two fireballs, in 2014 and 2017, were interstellar remain disputed.[^brown2023] ## See also - [[Interstellar_object]] · [[Comet]] · [[Interstellar_medium]] - [[Oort_cloud]] · [[Local_Interstellar_Cloud]] ## Notes Derived values: with a = −1.272 AU and q = 0.2553 AU, the vis-viva equation v² = GM☉(2/r − 1/a) gives about 87.4 km/s at perihelion, against 87.71 km/s from the fitted orbit. The solar gravitational acceleration at 1 AU is GM☉/r² ≈ 5.9 × 10⁻³ m/s². An amplitude Δm of 2.5 magnitudes corresponds to a brightness ratio of 10^(Δm/2.5) = 10. ## References [^meech2017]: Meech, K. J.; Weryk, R.; Micheli, M.; Kleyna, J. T.; Hainaut, O. R.; Jedicke, R.; et al. (2017). "A brief visit from a red and extremely elongated interstellar asteroid". *Nature* 552: 378–381. https://doi.org/10.1038/nature25020 [^nasa-faq]: NASA Planetary Defense (20 November 2017). "Interstellar asteroid FAQs". https://www.nasa.gov/planetarydefense/faq/interstellar [^gray2017]: Gray, B. (26 October 2017). "Pseudo-MPEC for A/2017 U1 (FAQ file)". *Project Pluto*. https://projectpluto.com/temp/2017u1.htm [^jpl-sbdb]: JPL Solar System Dynamics. "Small-Body Database Lookup": ʻOumuamua (A/2017 U1), 2I/Borisov, 3I/ATLAS. https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=1I (elements and close approaches fetched 2026-09-18). [^fraser2018]: Fraser, W. C.; Pravec, P.; Fitzsimmons, A.; Lacerda, P.; Bannister, M. T.; Snodgrass, C.; et al. (2018). "The tumbling rotational state of 1I/ʻOumuamua". *Nature Astronomy* 2: 383–386. https://doi.org/10.1038/s41550-018-0398-z [^micheli2018]: Micheli, M.; Farnocchia, D.; Meech, K. J.; Buie, M. W.; Hainaut, O. R.; Prialnik, D.; et al. (2018). "Non-gravitational acceleration in the trajectory of 1I/2017 U1 (ʻOumuamua)". *Nature* 559: 223–226. https://doi.org/10.1038/s41586-018-0254-4 [^issi2019]: 'Oumuamua ISSI Team; Bannister, M. T.; Bhandare, A.; Dybczyński, P. A.; Fitzsimmons, A.; Guilbert-Lepoutre, A.; et al. (2019). "The natural history of ʻOumuamua". *Nature Astronomy* 3: 594–602. https://doi.org/10.1038/s41550-019-0816-x [^bergner2023]: Bergner, J. B.; Seligman, D. Z. (2023). "Acceleration of 1I/ʻOumuamua from radiolytically produced H2 in H2O ice". *Nature* 615: 610–613. https://doi.org/10.1038/s41586-022-05687-w [^mpec2017u181]: Minor Planet Center (25 October 2017). "MPEC 2017-U181: Comet C/2017 U1 (PANSTARRS)". http://www.minorplanetcenter.net/mpec/K17/K17UI1.html [^mpec2017u183]: Minor Planet Center (25 October 2017). "MPEC 2017-U183: A/2017 U1". http://www.minorplanetcenter.net/mpec/K17/K17UI3.html [^mpec2017v17]: Minor Planet Center (6 November 2017). "MPEC 2017-V17: New designation scheme for interstellar objects". http://www.minorplanetcenter.net/mpec/K17/K17V17.html [^pukui2003]: Pukui, M. K.; Elbert, S. H. (2003). *Hawaiian Dictionary*. Ulukau: Hawaiian Electronic Library. http://wehewehe.org/ [^gal2017]: Gal, R. (20 November 2017). "An interstellar visitor unmasked". *University of Hawaiʻi System News*. http://www.hawaii.edu/news/2017/11/20/an-interstellar-visitor-unmasked [^economist2017]: *The Economist* (2 November 2017). "The first visitor from another solar system has just been spotted". https://www.economist.com/news/science-and-technology/21730863-rendezvous-rama-first-visitor-another-solar-system-has-just-been [^jpl2017]: NASA Jet Propulsion Laboratory (20 November 2017). "Solar System's first interstellar visitor dazzles scientists". https://www.jpl.nasa.gov/news/news.php?feature=7006 [^mpc-1i]: Minor Planet Center. "1I/ʻOumuamua = A/2017 U1 orbit". https://www.minorplanetcenter.net/db_search/show_object?object_id=1I [^gemini2017]: Gemini Observatory (20 November 2017). "First known interstellar visitor is an 'oddball'". http://www.gemini.edu/node/12729 [^mamajek2017]: Mamajek, E. (2017). "Kinematics of the interstellar vagabond 1I/ʻOumuamua (A/2017 U1)". *Research Notes of the AAS* 1: 21. https://doi.org/10.3847/2515-5172/aa9bdc [^mpec2017u185]: Minor Planet Center (26 October 2017). "MPEC 2017-U185: A/2017 U1". http://www.minorplanetcenter.net/mpec/K17/K17UI5.html [^wright2017]: Wright, J. T. (2017). "On distinguishing interstellar objects like ʻOumuamua from products of Solar System scattering". *Research Notes of the AAS* 1: 38. https://doi.org/10.3847/2515-5172/aa9f23 [^rafikov2018]: Rafikov, R. R. (2018). "Spin evolution and cometary interpretation of the interstellar minor object 1I/2017 ʻOumuamua". *The Astrophysical Journal Letters* 867: L17. https://doi.org/10.3847/2041-8213/aae977 [^gaidos2017]: Gaidos, E.; Williams, J. P.; Kraus, A. (2017). "Origin of interstellar object A/2017 U1 in a nearby young stellar association?". *Research Notes of the AAS* 1: 13. https://doi.org/10.3847/2515-5172/aa9851 [^bailerjones2018]: Bailer-Jones, C. A. L.; Farnocchia, D.; Meech, K. J.; Brasser, R.; Micheli, M.; Chakrabarti, S.; Buie, M. W.; Hainaut, O. R. (2018). "Plausible home stars of the interstellar object ʻOumuamua found in Gaia DR2". *The Astronomical Journal* 156: 205. https://doi.org/10.3847/1538-3881/aae3eb [^jewitt2017]: Jewitt, D.; Luu, J.; Rajagopal, J.; Kotulla, R.; Ridgway, S.; Liu, W.; Augusteijn, T. (2017). "Interstellar interloper 1I/2017 U1: observations from the NOT and WIYN telescopes". *The Astrophysical Journal Letters* 850: L36. https://doi.org/10.3847/2041-8213/aa9b2f [^ye2017]: Ye, Q.-Z.; Zhang, Q.; Kelley, M. S. P.; et al. (2017). "1I/2017 U1 (ʻOumuamua) is hot: imaging, spectroscopy, and search of meteor activity". *The Astrophysical Journal Letters* 851: L5. https://doi.org/10.3847/2041-8213/aa9a34 [^seligman2024]: Seligman, D. Z.; Farnocchia, D.; Micheli, M.; et al. (2024). "Two distinct populations of dark comets delineated by orbits and sizes". *Proceedings of the National Academy of Sciences* 121: e2406424121. https://doi.org/10.1073/pnas.2406424121 [^fitzsimmons2018]: Fitzsimmons, A.; Snodgrass, C.; Rozitis, B.; Yang, B.; Hyland, M.; Seccull, T.; et al. (2018). "Spectroscopy and thermal modelling of the first interstellar object 1I/2017 U1 ʻOumuamua". *Nature Astronomy* 2: 133–137. https://doi.org/10.1038/s41550-017-0361-4 [^bolin2018]: Bolin, B. T.; Weaver, H. A.; Fernandez, Y. R.; Lisse, C. M.; Huppenkothen, D.; Jones, R. L.; et al. (2018). "APO time-resolved color photometry of highly elongated interstellar object 1I/ʻOumuamua". *The Astrophysical Journal Letters* 852: L2. https://doi.org/10.3847/2041-8213/aaa0c9 [^mashchenko2019]: Mashchenko, S. (2019). "Modelling the light curve of ʻOumuamua: evidence for torque and disc-like shape". *Monthly Notices of the Royal Astronomical Society* 489: 3003–3021. https://doi.org/10.1093/mnras/stz2380 [^jpl2018]: Cofield, C. (14 November 2018). "NASA learns more about interstellar visitor ʻOumuamua". NASA Jet Propulsion Laboratory. https://www.jpl.nasa.gov/news/news.php?feature=7280 [^billings2017]: Billings, L. (11 December 2017). "Alien probe or galactic driftwood? SETI tunes in to ʻOumuamua". *Scientific American*. https://www.scientificamerican.com/article/alien-probe-or-galactic-driftwood-seti-tunes-in-to-oumuamua/ [^enriquez2018]: Enriquez, J. E.; Siemion, A.; Lazio, T. J. W.; et al. (2018). "Breakthrough Listen observations of 1I/ʻOumuamua with the GBT". *Research Notes of the AAS* 2: 9. https://doi.org/10.3847/2515-5172/aaa6c9 [^jackson2021]: Jackson, A. P.; Desch, S. J. (2021). "1I/ʻOumuamua as an N2 ice fragment of an exo-Pluto surface: I. Size and compositional constraints". *Journal of Geophysical Research: Planets* 126: e2020JE006706. https://doi.org/10.1029/2020JE006706 [^desch2021]: Desch, S. J.; Jackson, A. P. (2021). "1I/ʻOumuamua as an N2 ice fragment of an exo-Pluto surface II: Generation of N2 ice fragments and the origin of ʻOumuamua". *Journal of Geophysical Research: Planets* 126: e2020JE006807. https://doi.org/10.1029/2020JE006807 [^hickok2021]: Hickok, K. (15 November 2021). "Interstellar visitor ʻOumuamua wasn't a nitrogen iceberg, Harvard astrophysicists say". *Live Science*. https://www.livescience.com/oumuamua-not-nitrogen-iceberg [^seligman2020]: Seligman, D.; Laughlin, G. (2020). "Evidence that 1I/2017 U1 (ʻOumuamua) was composed of molecular hydrogen ice". *The Astrophysical Journal Letters* 896: L8. https://doi.org/10.3847/2041-8213/ab963f [^hoang2020]: Hoang, T.; Loeb, A. (2020). "Destruction of molecular hydrogen ice and implications for 1I/2017 U1 (ʻOumuamua)". *The Astrophysical Journal Letters* 899: L23. https://doi.org/10.3847/2041-8213/abab0c [^hein2019]: Hein, A. M.; Perakis, N.; Eubanks, T. M.; Hibberd, A.; Crowl, A.; Hayward, K.; et al. (2019). "Project Lyra: sending a spacecraft to 1I/ʻOumuamua (former A/2017 U1), the interstellar asteroid". *Acta Astronautica* 161: 552–561. https://doi.org/10.1016/j.actaastro.2018.12.042 [^hibberd2020]: Hibberd, A.; Hein, A. M.; Eubanks, T. M. (2020). "Project Lyra: catching 1I/ʻOumuamua – mission opportunities after 2024". *Acta Astronautica* 170: 136–144. https://doi.org/10.1016/j.actaastro.2020.01.018 [^seligman2018]: Seligman, D.; Laughlin, G. (2018). "The feasibility and benefits of in situ exploration of ʻOumuamua-like objects". *The Astronomical Journal* 155: 217. https://doi.org/10.3847/1538-3881/aabd37 [^bialy2018]: Bialy, S.; Loeb, A. (2018). "Could solar radiation pressure explain ʻOumuamua's peculiar acceleration?". *The Astrophysical Journal Letters* 868: L1. https://doi.org/10.3847/2041-8213/aaeda8 [^loeb2018]: Loeb, A. (20 November 2018). "6 strange facts about the interstellar visitor ʻOumuamua". *Scientific American* (Observations blog). https://blogs.scientificamerican.com/observations/6-strange-facts-about-the-interstellar-visitor-oumuamua/ [^brown2023]: Brown, P. G.; Borovička, J. (2023). "On the proposed interstellar origin of the USG 20140108 fireball". *The Astrophysical Journal* 953: 167. https://doi.org/10.3847/1538-4357/ace421 ## External links - NASA Planetary Defense: interstellar asteroid FAQs. https://www.nasa.gov/planetarydefense/faq/interstellar - JPL Small-Body Database: ʻOumuamua. https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=1I - Minor Planet Center: MPEC 2017-V17. https://www.minorplanetcenter.net/mpec/K17/K17V17.html - ESO: first interstellar asteroid (eso1737). https://www.eso.org/public/news/eso1737/ ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/1I%2F%CA%BBOumuamua) : [Wikitube](https://en.wikitube.io/wiki/1I/ʻOumuamua) · pinned revision [1371353553](https://en.wikipedia.org/w/index.php?oldid=1371353553) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. 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