View source for Astronomy:ʻOumuamua

{{Short description|Interstellar object discovered in 2017}}
{{Infobox planet
 | minorplanet     = yes
 | name            = {{okina}}Oumuamua
 | background      = #FFE0C2
 | image           = A2017U1 5gsmoothWHT enhanced.jpg
 | image_scale      =
 | discovery_ref   = <ref name="NASA171026"/><ref name="Discovery"/>
 | discoverer      = Robert Weryk using {{nowrap|[[Astronomy:Pan-STARRS|Pan-STARRS]] 1}}
 | discovery_site  = [[Astronomy:Haleakalā Observatory|Haleakalā Obs.]], Hawaii
 | discovered      = 19 October 2017
 | mpc_name        = 1I/2017 U1<ref name="MPEC2017-V17"/>
 | alt_names       = {{flatlist|
* 1I
* 1I/{{okina}}Oumuamua
* 1I/2017&nbsp;U1&nbsp;({{okina}}Oumuamua)
* A/2017&nbsp;U1<ref name="A2017U1"/>
* C/2017&nbsp;U1<ref name="Discovery"/>
* P10Ee5V<ref name="spectrum"/>
}}
 | pronounced      = {{IPAc-en|oʊ|ˌ|m|uː|ə|ˈ|m|uː|ə}}, {{IPA-haw|ʔowˈmuwəˈmuwə|lang|Haw-oumuamua.oga}}
 | named_after     = [[Social:Hawaiian language|Hawaiian term]] for ''scout''<ref name="MPEC2017-V17"/>
 | mp_category     = {{ubl|[[Astronomy:Interstellar object|Interstellar object]]<ref name="MPEC2017-V17"/>|[[Astronomy:Hyperbolic asteroid|Hyperbolic asteroid]]<ref name="jpldata"/><ref name="ARX-20180920"/><ref name="QM-20181010"/>}}
 | orbit_ref       = <ref name="jpldata"/>
 | epoch           = 23 November 2017 ([[Astronomy:Julian day|JD]] 2458080.5)
 | observation_arc = 80 days
 | perihelion      = {{val|0.255916|0.000007|fmt=none|ul=AU}}
 | semimajor       = {{val|−1.2723|0.0001|fmt=none|u=AU}}{{efn|1=Objects on hyperbolic trajectories have negative semimajor axis, giving them a positive orbital energy.}}
 | eccentricity    = {{val|1.20113|0.00002|fmt=none}}
 | inclination     = 122.74°
 | asc_node        = 24.597°
 | arg_peri        = 241.811°
 | mean_anomaly    = 51.158[[Degree (angle)|°]]
 | mean_motion     = {{Deg2DMS|0.6867|sup=ms}} / day
 | moid            = {{hlist|0.0958 AU|37.3 [[Astronomy:Lunar distance|LD]]}}
 | jupiter_moid    = 1.454 AU
 | avg_speed       = {{ubl|{{val|26.33|0.01|ul=km/s}} (interstellar)<ref name="pseudoMPEC"/>|5.55 AU/year}}
 | dimensions      = {{ubl|{{val|115|x|111|x|19|u=m}}<ref name="Maschenko2019-09">{{cite journal |last= Mashchenko|first= S.|s2cid= 182952355|title= Modeling the light curve of 'Oumuamua: evidence for torque and disc-like shape|journal= Monthly Notices of the Royal Astronomical Society|year= 2019|volume= 489|issue= 3|pages= 3003–3021|doi= 10.1093/mnras/stz2380|arxiv= 1906.03696|bibcode= 2019MNRAS.489.3003M}}</ref><ref name="JewittSeligman2022"/>|{{val|100|-|1000|u=m}} long<ref name="NASA20181114">{{cite web |last=Cofield |first=Calia |title=NASA Learns More About Interstellar Visitor 'Oumuamua |url=https://www.jpl.nasa.gov/news/news.php?feature=7280 |date=14 November 2018 |work=[[Organization:NASA|NASA]] |access-date=14 November 2018 |archive-date=15 April 2020 |archive-url=https://web.archive.org/web/20200415092634/https://www.jpl.nasa.gov/news/news.php?feature=7280 |url-status=live }}</ref><ref name="STD-0181020">{{cite web |last=Watzke |first=Megan |title=Spitzer Observations of Interstellar Object {{okina}}Oumuamua |url=https://scitechdaily.com/spitzer-observations-of-interstellar-object-oumuamua/ |date=20 October 2018 |work=SciTechDaily.com |access-date=20 October 2018 |archive-date=16 October 2019 |archive-url=https://web.archive.org/web/20191016101409/https://scitechdaily.com/spitzer-observations-of-interstellar-object-oumuamua/ |url-status=live }}</ref><ref name="PHYS-20181022">{{cite web |url=https://www.cfa.harvard.edu/news/su201842 |title='Oumuamua |publisher=Smithsonian Astrophysical Observatory |date=19 October 2018 |access-date=24 October 2019 |archive-date=1 February 2021 |archive-url=https://web.archive.org/web/20210201045549/https://www.cfa.harvard.edu/news/su201842 |url-status=live }}</ref>|{{val|230|x|35|x|35|u=m}}<ref name="Jewitt2017"/><ref name="NOAO2017"/><br />(est. at [[Physics:Geometric albedo|albedo]] 0.10)<ref name="Jewitt2017" /><ref name="NOAO2017"/>}}
 | rotation        = [[Astronomy:List of tumblers (small Solar System bodies)|Tumbling]] (non-principal axis rotation)<ref name="Fraser2018a" /><br />Reported values include:{{plainlist|
*{{val|8.10|0.02}} [[Hour|h]]<ref name="Bolin2017" />
*{{val|8.10|0.42}} h<ref name="Bannister2017"/>
*{{val|6.96|1.45|-0.39}} h<ref name="Feng2017">{{Cite journal |title={{okina}}Oumuamua as a messenger from the Local Association |journal=The Astrophysical Journal |first1=F. |last1=Feng |first2=H. R. A. |last2=Jones |s2cid=56197486 |name-list-style=amp |volume=852 |issue=2 |pages=L27 |date=23 November 2017 |arxiv=1711.08800 |doi=10.3847/2041-8213/aaa404 |bibcode=2018ApJ...852L..27F |doi-access=free }}</ref>}}
 | albedo          = {{plainlist|
*0.1 (spectral est.)<ref name="Jewitt2017" />
*0.06–0.08 (spectral est.)<ref name="Bannister2017" />}}
 | spectral_type   = {{plainlist|
*[[Astronomy:D-type asteroid|D]]?<ref name="Jewitt2017" />
*B–V = {{val|0.7|0.06}}<ref name="Jewitt2017" />
*V-R = {{val|0.45|0.05}}<ref name="Jewitt2017" />
*g-r = {{val|0.47|0.04}}<ref name="Bannister2017"/>
*r-i = {{val|0.36|0.16}}<ref name="Bannister2017" />
*r-J = {{val|1.20|0.11}}<ref name="Bannister2017"/>}}
 | abs_magnitude   = {{val|22.08|0.45}}<ref name="jpldata"/>
 | magnitude       = 19.7 to >27.5<ref name="pseudoMPEC"/><ref name="HubbleProposal" />{{efn|1=Range at which the object was expected to be observable. Brightness peaked at 19.7 mag on 18 October 2017, and faded below 27.5 mag (the limit of [[Astronomy:Hubble Space Telescope|Hubble Space Telescope]] for fast-moving objects) around 1 January 2018. By late 2019, it should have dimmed to 34 mag.}}
}}

'''{{okina}}Oumuamua''' is the first [[Astronomy:Interstellar object|interstellar object]] detected passing through the [[Astronomy:Solar System|Solar System]].<ref>{{Cite web |last=Osborne |first=Hannah |date=2019-04-16 |title=First meteor of interstellar origin discovered by scientists |url=https://www.newsweek.com/first-interstellar-meteor-discovered-1397886 |access-date=2022-04-11 |website=Newsweek |language=en}}</ref> [[Astronomy:Naming of comets#Current system|Formally designated]] '''1I/2017&nbsp;U1''', it was discovered by Robert Weryk using the [[Astronomy:Pan-STARRS|Pan-STARRS]] telescope at [[Astronomy:Haleakalā Observatory|Haleakalā Observatory]], Hawaii, on 19 October 2017, approximately 40 days after it passed its [[Astronomy:Apsis#Perihelion and aphelion|closest point to the Sun]] on 9 September. When it was first observed, it was about {{convert|{{#expr: 85*384400 round -6}}|km|e6km e6mi AU|lk=on|abbr=unit|order=out}} from Earth (about 85 times as far away as the Moon) and already heading away from the Sun.

{{okina}}Oumuamua is a small object estimated to be between {{convert|100|and|1000|m|ft|sigfig=1}} long, with its width and thickness both estimated between {{convert|35|and|167|m|ft}}.<ref name="NASA20181114" /> It has a red color, like objects in the outer Solar System. Despite its close approach to the Sun, it showed no signs of having a [[Astronomy:Coma (cometary)|coma]]. It exhibited [[Physics:Gravitational acceleration|non{{nbh}}gravitational acceleration]], potentially due to [[Physics:Outgassing|outgassing]] or a push from solar [[Astronomy:Radiation pressure|radiation pressure]].<ref name="SLN-20190312">{{cite web |last=Carlisle |first=Camille M. |date=12 March 2019 |title='Oumuamua sped up as it left the inner solar system. This may be why – Astronomers think a jet-powered rocking motion could solve the puzzle |work=Salon |url=https://www.salon.com/2019/03/12/oumuamua-sped-up-as-it-left-the-inner-solar-system-this-might-be-why_partner/ |access-date=12 March 2019 |archive-date=19 March 2020 |archive-url=https://web.archive.org/web/20200319173919/https://www.salon.com/2019/03/12/oumuamua-sped-up-as-it-left-the-inner-solar-system-this-might-be-why_partner/ |url-status=live }}</ref><ref name="Nature 27-6-2018" /> It has a rotation rate similar to that of Solar System asteroids, but many valid models permit it to be more elongated than all but a few other natural bodies. Its light curve, assuming little systematic error, presents its motion as "tumbling" rather than "spinning", and moving sufficiently fast relative to the Sun that it is likely of an extrasolar origin. Extrapolated and without further deceleration, its path cannot be captured into a solar orbit, so it will eventually leave the Solar System and continue into interstellar space. Its [[Astronomy:Planetary system|planetary system]] of origin and age are unknown.

{{okina}}Oumuamua would be remarkable for its extrasolar origin, high obliqueness, and observed acceleration without an apparent coma. By July 2019, most astronomers concluded that it was a natural object, but its exact characterization is contentious given the limited observation window. While an unconsolidated object (rubble pile) would require {{okina}}Oumuamua to be of a density similar to rocky asteroids,<ref>{{Cite journal |last1=McNeill |first1=Andrew |last2=Trilling |first2=David E. |last3=Mommert |first3=Michael |s2cid=56163074 |date=1 April 2018 |title=Constraints on the Density and Internal Strength of 1I/'Oumuamua |bibcode=2018ApJ...857L...1M |journal=The Astrophysical Journal Letters |volume=857 |issue=1 |pages=L1 |doi=10.3847/2041-8213/aab9ab |issn=0004-637X |arxiv=1803.09864 |doi-access=free }}</ref> a small amount of internal strength similar to icy comets<ref>{{Cite journal |last1=Shi |first1=X. |last2=Vincent |first2=J-B. |last3=Tubiana |first3=C. |last4=Toth |first4=I. |last5=Pajola |first5=M. |last6=Oklay |first6=N. |last7=Naletto |first7=G. |last8=Mottola |first8=S. |last9=Marzari |first9=F. |s2cid=44120504 |date=1 March 2018 |title=Tensile strength of 67P/Churyumov–Gerasimenko nucleus material from overhangs |journal=Astronomy & Astrophysics |language=en |volume=611 |pages=A33 |doi=10.1051/0004-6361/201732155 |issn=0004-6361 |bibcode=2018A&A...611A..33A |arxiv=1712.07508}}</ref> would allow it to have a relatively low density. Proposed explanations of its origin include the remnant of a disintegrated [[Astronomy:Exocomet|rogue comet]],<ref name="UT-20190201" /><ref name="ARX-20190201" /> or a piece of an [[Astronomy:Exoplanet|exoplanet]] rich in nitrogen ice, similar to [[Astronomy:Pluto|Pluto]].<ref name="JGRP-20210316a">{{cite journal |last=Jackson |first=Alan P. |display-authors=et al |title=1I/'Oumuamua as an N<sub>2</sub> ice fragment of an exo-Pluto surface: I. Size and Compositional Constraints |date=16 March 2021 |journal=Journal of Geophysical Research: Planets |volume=126 |issue=5 |doi=10.1029/2020JE006706 |arxiv=2103.08788 |bibcode=2021JGRE..12606706J |doi-access=free }}</ref><ref name="JGRP-20210316b">{{cite journal |last=Desch |first=S. J. |display-authors=et al. |title=1I/'Oumuamua as an N<sub>2</sub> ice fragment of an exo-Pluto surface II: Generation of N<sub>2</sub> ice fragments and the origin of 'Oumuamua |date=16 March 2021 |journal=Journal of Geophysical Research: Planets |volume=126 |issue=5 |doi=10.1029/2020JE006807 |arxiv=2103.08812 |bibcode=2021JGRE..12606807D |doi-access=free }}</ref><ref name="NYT-20210323">{{cite news |last=Overbye |first=Dennis |title=Why Oumuamua, the Interstellar Visitor, Looks Eerily Familiar – A piece of an extrasolar Pluto may have passed through our cosmic neighborhood, a new study suggests |url=https://www.nytimes.com/2021/03/23/science/astronomy-oumuamua-comet.html |date=23 March 2021 |work={{wipe|The New York Times}} |access-date=23 March 2021 }}</ref> On 22 March 2023, astronomers proposed the observed acceleration was "due to the release of entrapped molecular hydrogen that formed through energetic processing of an H<sub>2</sub>O-rich icy body",<ref name="NAT-20230322">{{cite journal |last1=Bergner |first1=Jennifer |last2=Seligman |first2=Darryl Z. |title=Acceleration of 1I/'Oumuamua from radiolytically produced H2 in H2O ice |url=https://www.nature.com/articles/s41586-022-05687-w |date=22 March 2023 |journal=Nature |volume=615 |issue=7953 |pages=610–613 |doi=10.1038/s41586-022-05687-w |pmid=36949336 |arxiv=2303.13698 |s2cid=257668585 |access-date=23 March 2023 }}</ref> consistent with 'Oumuamua being an interstellar comet, "originating as a [[Astronomy:Planetesimal|planetesimal]] relic broadly similar to solar system comets".<ref name="NYT-20230322">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=Oumuamua Was a Comet After All, a Study Suggests – Astronomers offer 'a surprisingly simple explanation' for the curious behavior of the interstellar visitor in 2017. |url=https://www.nytimes.com/2023/03/22/science/astronomy-oumuamua-comet.html |date=22 March 2023 |work={{wipe|The New York Times}} |access-date=23 March 2023 }}</ref>

Avi Loeb has suggested that it could be a product of extraterrestrial technology,<ref name="SA-20210622">{{cite news |last=Loeb |first=Avi |title=A Possible Link between 'Oumuamua and Unidentified Aerial Phenomena – If some UAP turn out to be extraterrestrial technology, they could be dropping sensors for a subsequent craft to tune into. What if 'Oumuamua is such a craft? |url=https://www.scientificamerican.com/article/a-possible-link-between-oumuamua-and-unidentified-aerial-phenomena/ |date=22 June 2021 |work=Scientific American |access-date=22 June 2021 }}</ref> but there is insufficient evidence to support any hypotheses, "despite all [its] strangeness".<ref name="NAT-20190701">{{cite journal |author=The 'Oumuamua ISSI Team |s2cid=195791768 |title=The natural history of 'Oumuamua |date=1 July 2019 |journal=[[Astronomy:Nature Astronomy|Nature Astronomy]] |volume=3 |issue=7 |pages=594–602 |doi=10.1038/s41550-019-0816-x |arxiv=1907.01910 |bibcode=2019NatAs...3..594O |url=https://authors.library.caltech.edu/97422/2/1907.01910.pdf |access-date=7 December 2019 |archive-date=9 March 2020 |archive-url=https://web.archive.org/web/20200309150920/https://authors.library.caltech.edu/97422/2/1907.01910.pdf |url-status=live }}</ref><ref name="SA-20190701">{{cite news |last=Starr |first=Michelle |title=Astronomers Have Analysed Claims 'Oumuamua's an Alien Ship, And It's Not Looking Good |url=https://www.sciencealert.com/astronomers-have-determined-oumuamua-is-really-truly-not-an-alien-lightsail |date=1 July 2019 |work=Science Alert.com |access-date=1 July 2019 |archive-date=1 July 2019 |archive-url=https://web.archive.org/web/20190701213237/https://www.sciencealert.com/astronomers-have-determined-oumuamua-is-really-truly-not-an-alien-lightsail |url-status=live }}</ref> In January 2022, researchers proposed [[Astronomy:Project Lyra|Project Lyra]], where a spacecraft launched from Earth could catch up to 'Oumuamua in 26 years for closer studies.<ref name="UT-20220120">{{cite news |last=Williams |first=Matt |title=If Launched by 2028, a Spacecraft Could Catch up With Oumuamua in 26 Years |url=https://www.universetoday.com/154028/if-launched-by-2028-a-spacecraft-could-catch-up-with-oumuamua-in-26-years/ |date=20 January 2022 |work=Universe Today |access-date=27 January 2022 }}</ref><ref name="AR-2022011">{{cite journal|author=Hibberd, Adam|display-authors=et al.|date=11 January 2022|title=Project Lyra: A mission to 1I/'Oumuamua without Solar Oberth Manoeuvre|journal=Acta Astronautica |volume=199 |pages=161–165 |doi=10.1016/j.actaastro.2022.07.032 |arxiv=2201.04240|bibcode=2022AcAau.199..161H |s2cid=245877397 }}</ref>

== Naming ==
{{stack|[[File:Oumuamua orbit at perihelion.png|thumb|[[Astronomy:Hyperbolic trajectory|Hyperbolic trajectory]] of {{okina}}Oumuamua through the inner Solar System with the Sun at the [[Focus (geometry)|focus]].<ref>{{cite web |url=https://photojournal.jpl.nasa.gov/archive/PIA22357_JPL-20180620-ASTRDSf-0007-Interstellar%20Asteroid%20animation-720p.mp4 |title=animation |author=NASA }}</ref>]]}}

As the first known object of its type, {{okina}}Oumuamua presented a unique case for the [[Organization:International Astronomical Union|International Astronomical Union]], which assigns designations for astronomical objects. Originally classified as comet C/2017 U1, it was later reclassified as asteroid A/2017 U1 due to the absence of a coma. Once it was unambiguously identified as coming from outside the Solar System, a [[Astronomy:Naming of comets#Current system|new designation]] was created: I, for Interstellar object. As the first object so identified, {{okina}}Oumuamua was designated 1I, with rules for the eligibility of objects for I-numbers and the names to be assigned to these interstellar objects yet to be codified. The object may be called 1I; 1I/2017 U1; 1I/{{okina}}Oumuamua; or 1I/2017 U1 ({{okina}}Oumuamua).<ref name="MPEC2017-V17"/>

The name comes {{ety|haw|{{okina}}oumuamua|scout}}<ref name="Pukui-Elbert" /> ({{ety||{{okina}}ou|reach out for||mua{{noitalic|, reduplicated for emphasis}}|first, in advance of}}<ref name="MPEC2017-V17" />), and reflects the way the object is like a scout or messenger sent from the distant past to reach out to humanity. It roughly translates to 'first distant messenger'.<ref name="MPEC2017-V17" /><ref name="Outerplaces">{{cite web |url=https://www.outerplaces.com/science/item/17035-interstellar-object-asteroid-oumuamua |title=Our Solar System's First Interstellar Asteroid is Named {{okina}}Oumuamua' |last=Kesh |first=Johnathan |date=8 November 2017 |website=Outer Places |access-date=23 November 2017 |archive-date=1 December 2017 |archive-url=https://web.archive.org/web/20171201032243/https://www.outerplaces.com/science/item/17035-interstellar-object-asteroid-oumuamua |url-status=live }}</ref> The first character (not a diacritic) is a Hawaiian {{okina}}okina, not an apostrophe, and is pronounced as a glottal stop; the Pan-STARRS team chose the name<ref>{{Cite magazine |last=Wall |first=Mike |date=16 November 2017 |title=Meet {{okina}}Oumuamua, the First-Ever Asteroid from Another Star |url=http://www.scientificamerican.com/article/meet-oumuamua-the-first-ever-asteroid-from-another-star |magazine=Scientific American |language=en |via=Space.com |access-date=24 November 2017 |archive-date=22 November 2017 |archive-url=https://web.archive.org/web/20171122224917/https://www.scientificamerican.com/article/meet-oumuamua-the-first-ever-asteroid-from-another-star/ |url-status=live }}</ref> in consultation with Ka'iu Kimura and Larry Kimura of the University of Hawai{{okina}}i at Hilo.<ref>{{Cite news |url=http://www.hawaii.edu/news/2017/11/20/an-interstellar-visitor-unmasked |title=An interstellar visitor unmasked |last=Gal |first=Roy |date=20 November 2017 |work=University of Hawai{{okina}}i System News |access-date=22 November 2017 |language=en-US |archive-date=24 November 2017 |archive-url=https://web.archive.org/web/20171124124145/http://www.hawaii.edu/news/2017/11/20/an-interstellar-visitor-unmasked/ |url-status=live }}</ref>

Before the official name was decided, ''Rama'' was suggested, the name given to an alien spacecraft discovered under similar circumstances in the 1973 science fiction novel ''Rendezvous with Rama'' by [[Biography:Arthur C. Clarke|Arthur C. Clarke]].<ref>{{cite magazine |url=https://www.economist.com/news/science-and-technology/21730863-rendezvous-rama-first-visitor-another-solar-system-has-just-been |title=The first visitor from another solar system has just been spotted: Rendezvous with Rama? |magazine=The Economist |date=2 November 2017 |url-access=subscription |access-date=6 December 2017 |archive-date=6 December 2017 |archive-url=https://web.archive.org/web/20171206212306/https://www.economist.com/news/science-and-technology/21730863-rendezvous-rama-first-visitor-another-solar-system-has-just-been |url-status=live }}</ref>

== Observations ==
Observations and conclusions concerning {{okina}}Oumuamua's trajectory were primarily obtained with data from the [[Astronomy:Pan-STARRS#Instruments|Pan-STARRS1 Telescope]], part of the [[Astronomy:Spaceguard|Spaceguard Survey]],<ref>{{cite journal |last1=Morrison |first1=David |title=Interstellar Visitor: The Strange Asteroid from a Faraway System |journal=Skeptical Inquirer |date=March–April 2018 |volume=42 |issue=2 |page=9}}</ref> and the [[Astronomy:Canada–France–Hawaii Telescope|Canada–France–Hawaii Telescope]] (CFHT), and its composition and shape from the [[Astronomy:Very Large Telescope|Very Large Telescope]] and the Gemini South telescope in Chile,<ref name="Gemini">{{cite press release |website=Gemini Observatory |url=http://www.gemini.edu/node/12729 |title=First Known Interstellar Visitor is an 'Oddball' |date=20 November 2017 |access-date=28 November 2017 |archive-date=23 November 2017 |archive-url=https://web.archive.org/web/20171123071154/http://www.gemini.edu/node/12729 |url-status=live }}</ref> and the [[Astronomy:W. M. Keck Observatory|Keck II]] telescope in Hawaii. These were collected by Karen J. Meech, Robert Weryk, and their colleagues and published in ''Nature'' on 20 November 2017.<ref name="NAT-20171120"/><ref name="Rincon2017">{{cite news |url=https://www.bbc.com/news/science-environment-42053634 |title=Bizarre shape of interstellar asteroid |journal=BBC News |first=Paul |last=Rincon |date=20 November 2017 |access-date=20 November 2017 |archive-date=8 April 2020 |archive-url=https://web.archive.org/web/20200408062702/https://www.bbc.com/news/science-environment-42053634 |url-status=live }}</ref> After the announcement, the space-based telescopes [[Astronomy:Hubble Space Telescope|Hubble]] and [[Astronomy:Spitzer Space Telescope|Spitzer]] joined in the observations.<ref name="jpl-news"/>

{{stack|[[File:Oumuamua magnitude 2015-2019.png|thumb|{{okina}}Oumuamua had faded into the 34th magnitude by 2020.]]}}
{{okina}}Oumuamua is small and not very luminous. It was not seen in [[Astronomy:STEREO|STEREO HI-1A]] observations near its perihelion on 9 September 2017, limiting its brightness to approximately 13.5 [[Astronomy:Apparent magnitude|mag]].<ref name="Bannister2017" /> By the end of October, it had already faded to about [[Astronomy:Apparent magnitude|apparent magnitude]] 23,<ref name="MPC-object" /> and in mid-December 2017, it was too faint and fast moving to be studied by even the largest ground-based telescopes.<ref name="Gemini"/>

{{okina}}Oumuamua was compared to the fictional alien spacecraft ''Rama'' due to its interstellar origin. Adding to the coincidence, both the real and the fictional objects are unusually elongated.<ref name="Koren-20171211"/> {{okina}}Oumuamua has a reddish hue and unsteady brightness, which are typical of asteroids.<ref name=wenz2017>{{cite magazine |first=John |last=Wenz |magazine=Astronomy |url=http://www.astronomy.com/news/2017/11/interstellar-asteroid-is-a-quarter-mile-long-red-beast |title=The first discovered interstellar asteroid is a quarter-mile long red beast |date=22 November 2017 |access-date=6 December 2017 |archive-date=4 June 2019 |archive-url=https://web.archive.org/web/20190604023605/http://www.astronomy.com/news/2017/11/interstellar-asteroid-is-a-quarter-mile-long-red-beast |url-status=live }}</ref><ref name="NYT-20171122">{{cite news |last=Overbye |first=Dennis |title=An Interstellar Visitor Both Familiar and Alien |date=22 November 2017 |work={{wipe|The New York Times}} |url=https://www.nytimes.com/2017/11/22/science/oumuamua-space-asteroid.html |access-date=23 November 2017 |archive-date=17 April 2020 |archive-url=https://web.archive.org/web/20200417202638/https://www.nytimes.com/2017/11/22/science/oumuamua-space-asteroid.html |url-status=live }}</ref><ref>{{cite news |first=Seth |last=Shostak |title=Is this mysterious space rock actually an alien spaceship? |url=https://www.nbcnews.com/mach/science/mysterious-space-rock-actually-alien-spaceship-ncna829501 |work=NBC News |date=14 December 2017 |access-date=20 December 2017 |archive-date=19 December 2017 |archive-url=https://web.archive.org/web/20171219145459/https://www.nbcnews.com/mach/science/mysterious-space-rock-actually-alien-spaceship-ncna829501 |url-status=live }}</ref>

The [[Organization:SETI Institute|SETI Institute]]'s [[Astronomy:Radio telescope|radio telescope]], the [[Astronomy:Allen Telescope Array|Allen Telescope Array]], examined {{okina}}Oumuamua, but detected no unusual radio emissions.<ref name=SETI/> More detailed observations, using the [[Astronomy:Breakthrough Listen|Breakthrough Listen]] hardware and the [[Astronomy:Green Bank Telescope|Green Bank Telescope]], were performed;<ref name="Koren-20171211"/><ref name=SETI/><ref>{{Cite magazine |url=https://www.wired.co.uk/article/oumuamua-is-not-an-interstellar-spacecraft |title=It isn't an alien spacecraft, but we should still study {{okina}}Oumuamua |last=Beall |first=Abigail |date=12 December 2017 |magazine=Wired UK |access-date=12 December 2017 |archive-date=12 December 2017 |archive-url=https://web.archive.org/web/20171212163216/http://www.wired.co.uk/article/oumuamua-is-not-an-interstellar-spacecraft |url-status=live }}</ref> the data were searched for narrowband signals and none were found. Given the close proximity to this interstellar object, limits were placed to putative transmitters with the extremely low [[Effective radiated power|effective isotropically radiated power]] of 0.08 watts.<ref name="No narrowband signals found.">{{Cite journal |arxiv=1801.02814 |title=Breakthrough Listen Observations of 1I/{{okina}}Oumuamua with the GBT |journal=Research Notes of the American Astronomical Society |volume=2 |issue=1 |page=9 |last=Enriquez |first=J. E. |date=9 January 2018 |bibcode=2018RNAAS...2....9E |doi=10.3847/2515-5172/aaa6c9|s2cid=119435272 |doi-access=free }}</ref>

=== Trajectory ===
{{stack|[[File:Oumuamua-skypath.png|thumb|Seen from Earth, the [[Astronomy:Apparent place|apparent trajectory]] makes annual retrograde loops in the sky, with its origin in Lyra, temporarily moving south of the ecliptic between 2 September and 22 October 2017, and moving northward again towards its destination in Pegasus.]]}}
{{stack|[[File:Oumuamua-solar system-ecliptic-normals.png|thumb|{{okina}}Oumuamua's hyperbolic trajectory over the Solar System.]]}}
{{okina}}Oumuamua appears to have come from roughly the direction of [[Astronomy:Vega|Vega]] in the constellation Lyra.<ref name=wenz2017/><ref name="NYT-20171122"/><ref name="skyandtelescope"/><ref>{{cite news |first=Jamie |last=Seidel |title='Alien' object excites astronomers. Is it a 'visitor' from nearby star? |url=http://www.nzherald.co.nz/technology/news/article.cfm?c_id=5&objectid=11937034 |publisher=The New Zealand Herald |date=26 October 2017 |access-date=29 October 2017 |archive-date=24 September 2018 |archive-url=https://web.archive.org/web/20180924131335/https://www.nzherald.co.nz/technology/news/article.cfm?c_id=5&objectid=11937034 |url-status=live }}</ref> Its incoming direction of motion is 6° from the [[Astronomy:Solar apex|solar apex]] (the direction of the Sun's movement relative to local stars), the most likely direction from which objects from outside the Solar System would come.<ref name="skyandtelescope" /><ref name="Hein2017"/> On 26 October, two [[Astronomy:Precovery|precovery]] observations from the [[Astronomy:Catalina Sky Survey|Catalina Sky Survey]] were found dated 14 and 17 October.<ref name="precoveryobs" /><ref name="MPC-object" /> A two-week [[Astronomy:Observation arc|observation arc]] had verified a strongly [[Astronomy:Hyperbolic trajectory|hyperbolic trajectory]].<ref name="jpldata"/><ref name="NAT-20171120"/> It has a hyperbolic excess velocity (velocity at infinity, <math>v_{\infty}\!</math>) of {{cvt|26.33|km/s|kph mph|abbr=|lk=on}}, its speed relative to the Sun when in interstellar space.{{efn|1=For comparison, comet C/1980 E1 will only be moving 4.2&nbsp;km/s when it is 500&nbsp;AU from the Sun.}}

{| class="wikitable floatright" style="text-align:center; font-size:0.9em"
|+ {{okina}}Oumuamua speed relative to the Sun
|-
! Distance !! Date !! Velocity<br />km/s
|-
| 2300 AU || 1606 || 26.41<ref>{{Cite web|url=https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%271I%27&START_TIME=%271606-02-01%27&STOP_TIME=%271606-04-01%27&STEP_SIZE=%27120%27&QUANTITIES=%2719,22%27&CENTER=%27@Sun%27|title=Inbound 2300 AU in 1606}}</ref>
|-
| 1000 AU || 1839 || 26.42
|-
| 100 AU || 2000 || 26.73
|-
| 10 AU || 2016 || 29.56
|-
| 1 AU || 9 August 2017 || 49.70<ref>[https://ssd.jpl.nasa.gov/api/horizons.api?format=text&COMMAND=%271I%27&START_TIME=%272017-08-09%2012:00%27&STOP_TIME=%272017-08-09%2013:00%27&STEP_SIZE=%27120%27&QUANTITIES=%2719,22%27&CENTER=%27@Sun%27 Inbound 1 AU] (passing Earth's orbit)</ref>
|-
| [[Astronomy:Perihelion and aphelion|Perihelion]] || 9 September 2017 || 87.71<ref name="pseudoMPEC" />
|-
| 1 AU || 10 October 2017 || 49.70{{efn|1=The solar escape velocity from Earth's orbit (1&nbsp;AU from the Sun) is [https://www.quora.com/What-is-the-escape-velocity-from-the-sun-And-from-the-solar-system 42.1&nbsp;km/s]. For comparison, even 1P/Halley moves at 41.5&nbsp;km/s when 1&nbsp;AU from the Sun, according to the formula {{nowrap|1=''v'' = 42.1219 {{radic|1/''r'' − 0.5/''a''}}}}, where ''r'' is the distance from the Sun, and ''a'' is the major semi-axis. Near-Earth asteroid [[Astronomy:2062 Aten|2062 Aten]] only moves at 29&nbsp;km/s when 1&nbsp;AU from the Sun because of the much smaller semi-major axis.}}
|-
| 10 AU || 2019 || 29.58
|-
| 100 AU || 2034 || 26.73<ref>{{Cite web|url=https://ssd.jpl.nasa.gov/api/horizons.api?format=text&COMMAND=%271I%27&START_TIME=%272034-01-01%27&STOP_TIME=%272034-12-31%27&STEP_SIZE=%27120%27&QUANTITIES=%2719,22%27&CENTER=%27@Sun%27|title=Outbound 100 AU in 2034}}</ref>
|-
| 1000 AU || 2195 || 26.44
|-
| 2300 AU || 2429 || 26.40<ref>[https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%271I%27&START_TIME=%272429-02-01%27&STOP_TIME=%272429-04-01%27&STEP_SIZE=%27120%27&QUANTITIES=%2719,22%27&CENTER=%27@Sun%27 Outbound 2300 AU in 2429]</ref>
|}

By mid-November, astronomers were certain that it was an interstellar object.<ref>{{cite news |url=https://www.theguardian.com/science/across-the-universe/2017/nov/20/interstellar-object-confirmed-to-be-from-another-solar-system |title=Mysterious object confirmed to be from another solar system |date=20 November 2017 |access-date=21 November 2017 |first=Stuart |last=Clark |journal=The Guardian |quote=Astronomers are now certain that the mysterious object detected hurtling past our Sun last month is indeed from another solar system. They have named it 1I/2017 U1 ({{okina}}Oumuamua) and estimate it could be one of 10,000 others lurking undetected in our cosmic neighbourhood. |language=en-GB |archive-date=25 April 2020 |archive-url=https://web.archive.org/web/20200425034932/https://www.theguardian.com/science/across-the-universe/2017/nov/20/interstellar-object-confirmed-to-be-from-another-solar-system |url-status=live }}</ref> Based on observations spanning 80 days, {{okina}}Oumuamua's [[Astronomy:Orbital eccentricity|orbital eccentricity]] is 1.20, the highest ever observed<ref name="JPL-ecc" /><ref name="pseudoMPEC" /> until 2I/Borisov was discovered in August 2019. An [[Hyperbola|eccentricity exceeding 1.0]] means an object exceeds the Sun's [[Astronomy:Escape velocity|escape velocity]], is not bound to the Solar System, and may escape to interstellar space. While an eccentricity slightly above 1.0 can be obtained by encounters with planets, as happened with the previous record holder, C/1980 E1,<ref name="JPL-ecc" /><ref name="orientation" />{{efn|1=Unlike {{okina}}Oumuamua, C/1980 E1's orbit got its high eccentricity of 1.057 due to a close encounter with [[Astronomy:Jupiter|Jupiter]]. Its inbound-orbit eccentricity was less than 1.<ref name="skyandtelescope" />}} {{okina}}Oumuamua's eccentricity is so high that it could not have been obtained through an encounter with any of the planets in the Solar System. Even undiscovered planets in the Solar System cannot account for {{okina}}Oumuamua's trajectory or boost its speed to the observed value. For these reasons, it can only be of interstellar origin.<ref name="Wright">{{cite journal |title=On Distinguishing Interstellar Objects Like {{okina}}Oumuamua From Products of Solar System Scattering |year=2018 |journal=Research Notes of the AAS |volume=1 |issue=1 |page=38 |doi=10.3847/2515-5172/aa9f23 |bibcode=2017RNAAS...1...38W |arxiv=1712.06044 |last1=Wright |first1=Jason T. |last2=Jones |first2=Hugh R. A.|s2cid=119467366 |doi-access=free }}</ref><ref name=hyperbolics>{{cite journal |last1=de la Fuente Marcos |first1=Carlos |last2=de la Fuente Marcos |first2=Raúl |last3=Aarseth |first3=Sverre J. |s2cid=119405023 |year=2018 |title=Where the Solar system meets the solar neighbourhood: patterns in the distribution of radiants of observed hyperbolic minor bodies |journal=Monthly Notices of the Royal Astronomical Society Letters |volume=476 |issue=1 |pages=L1–L5 |arxiv=1802.00778 |bibcode=2018MNRAS.476L...1D |doi=10.1093/mnrasl/sly019}}</ref>

[[File:Comet 20171025-16 gif.gif|left|thumb|Animation of {{okina}}Oumuamua passing through the Solar System]]
{| class="wikitable floatright" style="text-align:center; font-size:0.9em"
|+ Inbound velocity at 200&nbsp;AU from the Sun<br />compared to Oort cloud objects
|-
! Object !! Velocity<br />km/s || # of observations<br /> and [[Astronomy:Observation arc|obs arc]]{{efn|1=Orbits computed with only a handful of observations can be unreliable. Short arcs can result in computer generated orbits rejecting some data unnecessarily.}}
|-
| [[Astronomy:90377 Sedna|90377 Sedna]] || 2.66<ref>{{Cite web|url=https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%27Sedna%27&START_TIME=%271746-07-01%27&STOP_TIME=%271746-09-01%27&STEP_SIZE=%2730%27&QUANTITIES=%2719,22%27&CENTER=%27@Sun%27|title=Sedna inbound 200 AU in 1746}}</ref> || 483 in 11796 days
|-
| C/2010 X1 (Elenin) || 2.96 || 2222 in 235 days
|-
| C/1980 E1 (Bowell) || 2.98<ref>{{Cite web|url=https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%271980+E1%27&START_TIME=%271764-12-15%27&STOP_TIME=%271765-02-01%27&STEP_SIZE=%2730%27&QUANTITIES=%2719,22%27&CENTER=%27@Sun%27|title=Bowell (C/1980 E1) inbound 200 AU in 1765}}</ref> || 187 in 2514 days
|-
| C/1997 P2 (Spacewatch) || 2.99 || 94 in 49 days
|-
| C/2012 S1 (ISON) || 2.99<ref>{{Cite web|url=https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%272012+S1%27&START_TIME=%271801-11-01%27&STOP_TIME=%271801-12-31%27&STEP_SIZE=%2760%27&QUANTITIES=%2719,22%27&CENTER=%27@Sun%27|title=ISON inbound 200 AU in 1801}}</ref> || 6514 in 784 days
|-
| C/2008 J4 (McNaught) || 4.87<ref>[https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%272008+J4%27&START_TIME=%271830-11-01%27&STOP_TIME=%271880-01-01%27&STEP_SIZE=%2790%27&QUANTITIES=%2719,22%27&CENTER=%27@Sun%27 C/2008 J4 inbound 200 AU in 1854] (C/2008 J4 has an unreliable long-term orbit due a short arc of 15 days)</ref> || 22 in 15 days{{efn|1=JPL #10 shows that on 1855-Mar-24 C/2008 J4 was moving {{val|4.88|1.8|u=km/s}}.}}
|-
| '''1I/2017 U1 ({{okina}}Oumuamua)''' || 26.55<ref>{{Cite web|url=https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%271I%27&START_TIME=%271982-10-10%27&STOP_TIME=%271982-11-05%27&STEP_SIZE=%2730%27&QUANTITIES=%2719,22%27&CENTER=%27@Sun%27|title=Inbound 200 AU in 1982}}</ref> || 207 in 80 days
|-
| 2I/Borisov || 32.43<ref>{{Cite web|url=https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%272I%27&START_TIME=%271991-01-01%27&STOP_TIME=%271991-03-01%27&STEP_SIZE=%2730%27&QUANTITIES=%2719,22%27&CENTER=%27@Sun%27|title=2I/Borisov inbound 200 AU in 1991}}</ref> || 1428 in 311 days
|}

{{okina}}Oumuamua entered the Solar System from north of the plane of the [[Astronomy:Ecliptic|ecliptic]]. The pull of the Sun's gravity caused it to speed up until it reached its maximum speed of {{cvt|87.71|km/s|kph mph}} as it passed south of the ecliptic on 6 September, where the Sun's gravity bent its orbit in a sharp turn northward at its closest approach (perihelion) on 9 September at a distance of {{cvt|0.255|AU|km mi|lk=on}} from the Sun, i.e., about 17% closer than [[Astronomy:Mercury (planet)|Mercury]]'s closest approach to the Sun.<ref name="NASA-FAQ" /><ref name="pseudoMPEC" />{{efn|1=Comet C/2012 S1 (ISON) peaked at {{cvt|377|km/s|kph}} at perihelion<ref>{{cite web |url=http://www.isoncampaign.org/karl/ison-is-just-fine |first=Karl |last=Battams |website=NASA Comet ISON Observing Campaign |title=Comet ISON is doing just fine! |date=9 October 2013 |access-date=12 December 2017 |archive-date=28 October 2017 |archive-url=https://web.archive.org/web/20171028042815/http://www.isoncampaign.org/karl/ison-is-just-fine  }}</ref> because it passed 0.0124&nbsp;AU from the Sun (20 times closer than {{okina}}Oumuamua).}} It is now heading away from the Sun toward [[Astronomy:Pegasus (constellation)|Pegasus]], toward a vanishing point 66° from the direction of its approach.{{efn|1=According to the formula: [[Astronomy:Hyperbolic trajectory#Eccentricity and angle between approach and departure|<math display=inline>2\,\theta{ \infty}=2\cos^{-1}(-1/e)</math>]]}}

On the outward leg of its journey through the Solar System, {{okina}}Oumuamua passed beyond the orbit of Earth on 14 October with a closest approach distance of approximately {{cvt|0.16175|AU|km mi}} from Earth.<ref name=jpldata/> On 16 October it moved back north of the ecliptic plane and passed beyond the orbit of Mars on 1 November.<ref name="NASA-FAQ" /><ref name="skyandtelescope"/><ref name="jpldata" /> It passed beyond Jupiter's orbit in May 2018, beyond Saturn's orbit in January 2019, and beyond Neptune's in 2022.<ref name="NASA-FAQ"/> As it leaves the Solar System it will be approximately [[Right ascension|right ascension]] 23'51" and [[Declination|declination]] +24°45', in [[Astronomy:Pegasus (constellation)|Pegasus]].<ref name="pseudoMPEC" /> It will continue to slow down until it reaches a speed of {{convert|26.33|km/s|km/h mph}} relative to the Sun, the same speed it had before its approach to the Solar System.<ref name="pseudoMPEC" />

==== Non-gravitational acceleration ====
On 27 June 2018, astronomers reported a non-gravitational acceleration to {{okina}}Oumuamua's trajectory, potentially consistent with a push from solar radiation pressure.<ref name="UT-230181031" /><ref name="ARX-20181101" /> The resulting change in velocity during the period when it was near its closest approach to the Sun summed to about 17 meters per second. Initial speculation as to the cause of this acceleration pointed to the comet-like outgassing,<ref name="Nature 27-6-2018" /> whereby volatile substances inside the object evaporate as the Sun heats its surface. Although no such tail of gasses was observed following the object,<ref name="ARX-20181108072" /> researchers estimated that enough outgassing may have increased the object's speed without the gases being detectable.<ref name="NASA-20180627nasa"/> A critical reassessment of the outgassing hypothesis argued that, instead of the observed stability of {{okina}}Oumuamua's spin, outgassing would have caused its spin to rapidly change due to its elongated shape, resulting in the object tearing apart.<ref name="ARX-20180920" />

==== Indications of origin ====
Accounting for Vega's [[Astronomy:Proper motion|proper motion]], it would have taken {{okina}}Oumuamua 600,000 years to reach the Solar System from Vega.<ref name="NAT-20171120" /> But as a nearby star, Vega was not in the same part of the sky at that time.<ref name="skyandtelescope" /> Astronomers calculate that 100 years ago the object was {{convert|561|±|0.6|AU|e9km e9mi|round=each|abbr=unit|order=flip}} from the Sun and traveling at 26.33&nbsp;km/s with respect to the Sun.<ref name="pseudoMPEC" /> This interstellar speed is very close to the mean motion of material in the Milky Way in the neighborhood of the Sun, also known as the [[Astronomy:Local standard of rest|local standard of rest]] (LSR), and especially close to the mean motion of a relatively close group of [[Astronomy:Red dwarf|red dwarf]] stars. This velocity profile also indicates an extrasolar origin, but appears to rule out the [[Astronomy:List of nearest stars and brown dwarfs|closest dozen stars]].<ref name="Mamajek2017" /> In fact, the closeness of {{okina}}Oumuamua's velocity to the local standard of rest might mean that it has circulated the Milky Way several times and thus may have originated from an entirely different part of the galaxy.<ref name="NAT-20171120" />

It is unknown how long the object has been traveling among the stars.<ref name="NASA-FAQ"/> The Solar System is likely the first [[Astronomy:Planetary system|planetary system]] {{okina}}Oumuamua has closely encountered since being ejected from its birth star system, potentially several billion years ago.<ref name="Ye2017" /><ref name="NAT-20171120" /> It has been speculated that the object may have been ejected from a stellar system in one of the local kinematic associations of young stars (specifically, [[Astronomy:Carina OB1|Carina]] or Columba) within a range of about 100 [[Astronomy:Parsec|parsec]]s,<ref>{{cite journal |year=2013 |title=Unveiling new members in five nearby young moving groups |journal=Monthly Notices of the Royal Astronomical Society |volume=435 |issue=2 |pages=1376–1388 |doi=10.1093/mnras/stt1381 |arxiv=1309.1669 |last1=Moór |first1=A. |last2=Szabó |first2=Gy. M. |last3=Kiss |first3=L. L. |last4=Kiss |first4=Cs. |last5=Ábrahám |first5=P. |last6=Szulágyi |first6=J. |last7=Kóspál |first7=Á. |last8=Szalai |first8=T. |s2cid=54584506 |bibcode=2013MNRAS.435.1376M}}</ref> 45&nbsp;million years ago.<ref name="Gaidos2017" /> The Carina and Columba associations are now very far in the sky from the [[Astronomy:Lyra|Lyra]] constellation, the direction from which {{okina}}Oumuamua came when it entered the Solar System. Others have speculated that it was ejected from a white dwarf system and that its volatiles were lost when its parent star became a red giant.<ref name="white dwarf">{{cite journal |title=Ejection of Material—'Jurads'—from Post-main-sequence Planetary Systems |journal=Research Notes of the American Astronomical Society |first1=Brad |last1=Hansen |first2=Ben |last2=Zuckerman |volume=1 |issue=1 |at=55 |date=December 2017 |doi=10.3847/2515-5172/aaa3ee |bibcode=2017RNAAS...1...55H |arxiv=1712.07247|s2cid=118957210 |doi-access=free }}</ref> About 1.3&nbsp;million years ago the object may have passed within a distance of {{convert|0.16|pc|abbr=off|lk=on}} to the nearby star TYC 4742-1027-1, but its velocity is too high to have originated from that star system, and it probably just passed through the system's [[Astronomy:Oort cloud|Oort cloud]] at a relative speed of about {{cvt|15|km/s|mph kph|abbr=}}.<ref name="PortegiesZwartetal2017" />{{efn|1=This is true for the nominal position of the star. However, its actual distance is not known precisely: According to Gaia Data Release 1, the distance to TYC4742-1027-1 is {{convert|137|±|13|pc|abbr=off|lk=on}}. It is not known if an encounter actually occurred. Update: This star has new measurements in Gaia Data Release 2, and an origins study based on this by Bailer-Jones et al. (2018) shows that TYC4742-1027-1 did not come within 2&nbsp;pc of {{okina}}Oumuamua.}} An August 2018 study using Gaia Data Release 2 updated the possible past close encounters and identified four stars—HIP 3757, HD 292249, Gaia DR2 2502921019565490176, and Gaia DR2 3666992950762141312—which {{okina}}Oumuamua passed relatively close to at moderately low velocities in the past few million years.<ref name="Bailer-Jones2018">{{cite journal|last1=Bailer-Jones|first1=Coryn A. L.|display-authors=0|date=18 October 2018|title=Plausible Home Stars of the Interstellar Object {{okina}}Oumuamua Found in Gaia DR2|journal=[[Astronomy:The Astronomical Journal|The Astronomical Journal]]|volume=156|issue=5|page=205|arxiv=1809.09009|bibcode=2018AJ....156..205B|doi=10.3847/1538-3881/aae3eb|ref=Bailer-Jones2018|s2cid=119051284 |doi-access=free }}</ref> This study also identifies future close encounters of {{okina}}Oumuamua on its outgoing trajectory from the Sun.<ref>{{cite web |last1=Bailer-Jones |first1=C. A. L. |display-authors=0 |title=Plausible home stars of the interstellar object {{okina}}Oumuamua found in Gaia DR2 |url=http://www.mpia.de/homes/calj/gdr2_oumuamua.html |website=Coryn Bailer-Jones |access-date=23 October 2018 |archive-date=23 October 2018 |archive-url=https://web.archive.org/web/20181023120401/http://www.mpia.de/homes/calj/gdr2_oumuamua.html |url-status=live }}</ref>

In September 2018, astronomers described several possible home [[Astronomy:Star system|star system]]s from which {{okina}}Oumuamua may have originated.<ref name="ARX-20180924">{{Cite journal|last1=Feng|first1=Fabo|last2=Jones|first2=Hugh R. A.|year=2018|title=Plausible home stars of the interstellar object {{okina}}Oumuamua found in Gaia DR2|journal=The Astronomical Journal|volume=156|issue=5|page=205|arxiv=1809.09009|bibcode=2018AJ....156..205B|doi=10.3847/1538-3881/aae3eb|s2cid=119051284 |doi-access=free }}</ref><ref>{{cite web|last=Bartels|first=Meghan|date=25 September 2018|title={{okina}}Oumuamua Isn't from Our Solar System. Now We May Know Which Star It Came From|url=https://www.space.com/41928-where-did-oumuamua-come-from.html|url-status=live|archive-url=https://web.archive.org/web/20180925210517/https://www.space.com/41928-where-did-oumuamua-come-from.html|archive-date=25 September 2018|access-date=25 September 2018|website=Space.com}}</ref>

In April 2020, astronomers presented a new possible scenario for the object's origin.<ref name="EA-20200413">{{cite news |author=[[Organization:University of California, Santa Cruz|University of California, Santa Cruz]] |title=New formation theory explains the mysterious interstellar object 'Oumuamua – A new scenario based on computer simulations accounts for all of the observed characteristics of the first known interstellar object to visit our solar system |url=https://www.eurekalert.org/pub_releases/2020-04/uoc--nft041020.php |date=13 April 2020 |work=EurekAlert! |access-date=13 April 2020 |archive-date=14 April 2020 |archive-url=https://web.archive.org/web/20200414124012/https://www.eurekalert.org/pub_releases/2020-04/uoc--nft041020.php |url-status=live }}</ref><ref name="NA-20200413">{{cite journal |last1=Zhang |first1=Yun |last2=Lin |first2=Douglas N. C. |s2cid=215768701 |title=Tidal fragmentation as the origin of 1I/2017 U1 ({{okina}}Oumuamua) |url=https://www.nature.com/articles/s41550-020-1065-8 |date=13 April 2020 |journal=[[Astronomy:Nature Astronomy|Nature Astronomy]] |volume=254 |issue=9 |pages=852–860 |doi=10.1038/s41550-020-1065-8 |arxiv=2004.07218 |bibcode=2020NatAs...4..852Z |access-date=13 April 2020 |archive-date=14 April 2020 |archive-url=https://web.archive.org/web/20200414123834/https://www.nature.com/articles/s41550-020-1065-8 |url-status=live }}</ref> According to one hypothesis, {{okina}}Oumuamua could be a fragment from a [[Astronomy:Disrupted planet|tidally disrupted planet]].<ref>{{Cite journal|last=Ćuk|first=Matija|year=2018|title=1I/{{okina}}Oumuamua as a Tidal Disruption Fragment From a Binary Star System|journal=[[Astronomy:The Astrophysical Journal|The Astrophysical Journal]]|volume=852|issue=1|pages=L15|arxiv=1712.01823|bibcode=2018ApJ...852L..15C|doi=10.3847/2041-8213/aaa3db|s2cid=54959652 |doi-access=free }}</ref>{{efn|1=See also {{cite arXiv |eprint=1801.02658v2 |last1=Ravikov |first1=Roman R. |title=1I/2017 {{okina}}Oumuamua-like Interstellar Asteroids as Possible Messengers from the Dead Stars |class=astro-ph.EP |year=2018}}. {{okina}}Oumuamua is a fragment of a white-dwarf-star tidal-disruption-event. This easily explains its 6:1 or 10:1 elongation and its "refractory" composition; containing probably nickel-iron, possibly other metals, too.}} If true, this would make {{okina}}Oumuamua a rare object, of a type much less abundant than most extrasolar "dusty-snowball" comets or asteroids. But this scenario leads to cigar-shaped objects, whereas {{okina}}Oumuamua's lightcurve favors a disc-like shape.<ref name="Maschenko2019-09"/>

In May 2020, it was proposed that the object was the first observed member of a class of small [[Physics:Solid hydrogen|H<sub>2</sub>-ice]]-rich bodies that form at temperatures near 3 K in the cores of giant molecular clouds. The non-gravitational acceleration and high aspect ratio shape of {{okina}}Oumuamua might be explainable on this basis.<ref name="Seligman2020-05">{{cite journal |last1= Seligman|first1= D.|last2= Laughlin|first2= G.|s2cid= 218900854|title= Evidence that 1I/2017 U1 ('Oumuamua) was composed of molecular hydrogen ice|journal= The Astrophysical Journal|year= 2020|volume= 896|issue= 1|pages= L8|doi= 10.3847/2041-8213/ab963f|arxiv= 2005.12932|bibcode= 2020ApJ...896L...8S|doi-access= free}}</ref> However, it was later calculated that hydrogen icebergs cannot survive their journey through interstellar space.<ref name="Hoang2020-06">{{cite journal |last1= Hoang|first1= T.|last2= Loeb|first2= Abraham|title= Destruction of molecular hydrogen ice and Implications for 1I/2017 U1 ('Oumuamua)|journal= The Astrophysical Journal|year= 2020|volume= 899|issue= 2|pages= L23|doi= 10.3847/2041-8213/abab0c|arxiv= 2006.08088|bibcode= 2020ApJ...899L..23H|s2cid= 219687520|doi-access= free}}</ref>

=== Classification ===
<!-- This section seems to just repeat information contained in other sections. -->
Initially, {{okina}}Oumuamua was announced as comet C/2017 U1 (PANSTARRS) based on a strongly hyperbolic trajectory.<ref name="Discovery" /> In an attempt to confirm any cometary activity, very deep stacked images were taken at the [[Astronomy:Very Large Telescope|Very Large Telescope]] later the same day, but the object showed no presence of a [[Astronomy:Coma (cometary)|coma]].{{efn|1=According to [[Organization:Central Bureau for Astronomical Telegrams|Central Bureau for Astronomical Telegrams]]'s CBET 4450, none of the observers had detected any sign of cometary activity. The initial classification as a comet was based on the object's orbit.}} Accordingly, the object was renamed A/2017 U1, becoming the first comet ever to be [[Astronomy:Naming of comets#Current system|re-designated as an asteroid]].<ref name="A2017U1" /> Once it was identified as an interstellar object, it was designated 1I/2017 U1, the first member of a new class of objects.<ref name="MPEC2017-V17" /> The lack of a coma limits the amount of surface ice to a few square meters, and any [[Earth:Volatile (astrogeology)|volatiles]] (if they exist) must lie below a crust at least {{cvt|0.5|m|ft}} thick.<ref name="Jewitt2017" /> It also indicates that the object must have formed within the [[Physics:Frost line (astrophysics)|frost line]] of its parent stellar system or have been in the inner region of that stellar system long enough for all near-surface ice to [[Sublimation (phase transition)|sublimate]], as may be the case with damocloids. It is difficult to say which scenario is more likely due to the chaotic nature of small body dynamics, Oumuamua would have been expected to occur on 18 October 2017 coming from the constellation [[Astronomy:Sextans|Sextans]], but no activity was detected by the Canadian Meteor Orbit Radar.<ref name="Ye2017" />

On 27 June 2018, astronomers reported that {{okina}}Oumuamua was thought to be a mildly active [[Astronomy:Comet|comet]], and not an [[Astronomy:Asteroid|asteroid]], as previously thought. This was determined by measuring a non-gravitational boost to {{okina}}Oumuamua's acceleration, consistent with comet outgassing.<ref name="Nature 27-6-2018"/><ref name="NAT-20180627"/><ref name="NASA-20180627nasa"/><ref name="eso1820June2018Oumuamua"/> However, studies submitted in October 2018 suggest that the object is neither an asteroid nor a comet,<ref name="ARX-20180920">{{cite arXiv |last=Rafikov |first=Roman R. |title=Spin Evolution and Cometary Interpretation of the Interstellar Minor Object 1I/2017 {{okina}}Oumuamua |date=20 September 2018 |eprint=1809.06389v2 |class=astro-ph.EP}}</ref><ref name="QM-20181010">{{cite web |last=Skibba |first=Ramin |title=Interstellar Visitor Found to Be Unlike a Comet or an Asteroid |url=https://www.quantamagazine.org/interstellar-comet-oumuamua-might-not-actually-be-a-comet-20181010/ |date=10 October 2018 |work=Quanta Magazine |access-date=10 October 2018 |archive-date=27 April 2020 |archive-url=https://web.archive.org/web/20200427010524/https://www.quantamagazine.org/interstellar-comet-oumuamua-might-not-actually-be-a-comet-20181010/ |url-status=live }}</ref> although the object could be a remnant of a disintegrated interstellar comet (or [[Astronomy:Exocomet|exocomet]]), as suggested by astronomer Zdenek Sekanina.<ref name="UT-20190201">{{cite web |last=Williams |first=Matt |title=Oumuamua Could be the Debris Cloud of a Disintegrated Interstellar Comet |url=https://www.universetoday.com/141398/oumuamua-could-be-the-debris-cloud-of-a-disintegrated-interstellar-comet/#more-141398 |date=1 February 2019 |work=Universe Today |access-date=2 February 2019 |archive-date=3 February 2019 |archive-url=https://web.archive.org/web/20190203030346/https://www.universetoday.com/141398/oumuamua-could-be-the-debris-cloud-of-a-disintegrated-interstellar-comet/#more-141398 |url-status=live }}</ref><ref name="ARX-20190201">{{cite arXiv |last=Sekanina |first=Zdenek |title=1I/'Oumuamua As Debris Of Dwarf Interstellar Comet That Disintegrated Before Perihelion |date=31 January 2019 |eprint=1901.08704 |class=astro-ph.EP}}</ref>

=== Appearance, shape and composition ===
Spectra from the Hale Telescope on 25 October showed red color resembling comet nuclei or Trojans.<ref name="Ye2017" /> Higher signal to noise spectra recorded by the {{cvt|4.2|m}} [[Astronomy:William Herschel Telescope|William Herschel Telescope]] later that day showed that the object was featureless, and [[Astronomy:Trans-Neptunian object#Colors|colored red]] like [[Astronomy:Kuiper belt|Kuiper belt]] objects.<ref name="WHT" /> Spectra obtained with the {{cvt|8.2|m}} [[Astronomy:Very Large Telescope|Very Large Telescope]] the following night showed that behavior continued into near-infrared wavelengths.<ref name="Fitzsimmons2017a" /> Its spectrum is similar to that of [[Astronomy:D-type asteroid|D-type asteroid]]s.<ref name="Jewitt2017" />

[[File:Eso1737f.svg|thumb|upright=2.3|Light curve from 25 to 27 October 2017 with dotted line from a model with 10:1 elongation]]
{{okina}}Oumuamua is not rotating around its principal axis, and its motion may be a form of tumbling.<ref name="Fraser2018a">{{cite journal |last1=Fraser |first1=W. C. |last2=Pravec |first2=P. |last3=Fitzsimmons |first3=A. |last4=Lacerda |first4=P. |last5=Bannister |first5=M. T. |last6=Snodgrass |first6=C. |last7=Smolić |first7=I. |s2cid=119353074 |title=The tumbling rotational state of 1I/{{okina}}Oumuamua<!-- Deny Citation Bot--> |journal=Nature Astronomy |volume=2 |issue=5 |pages=383–386 |date=9 February 2018 |doi=10.1038/s41550-018-0398-z |arxiv=1711.11530 |bibcode=2018NatAs...2..383F |url=https://pure.qub.ac.uk/portal/en/publications/the-tumbling-rotational-state-of-1ioumuamua(cbcc0793-72c1-4f42-b78a-5bcdeff6274c).html |access-date=3 September 2018 |archive-date=3 September 2018 |archive-url=https://web.archive.org/web/20180903215215/https://pure.qub.ac.uk/portal/en/publications/the-tumbling-rotational-state-of-1ioumuamua(cbcc0793-72c1-4f42-b78a-5bcdeff6274c).html |url-status=live }}</ref><ref>{{cite arXiv |eprint=1712.00437 |title=Tumbling motion of 1I/{{okina}}Oumuamua reveals body's violent past |last1=Drahus |first1=M. |last2=Guzik |first2=P. |last3=Waniak |first3=W. |last4=Handzlik |first4=B. |last5=Kurowski |first5=S. |last6=Xu |first6=S. |class=astro-ph.EP |date=1 December 2017}}</ref> This accounts for the various rotation periods reported, such as 8.10&nbsp;hours (±0.42&nbsp;hours<ref name="Bannister2017" /> or ±0.02&nbsp;hours<ref name="Bolin2017" />) by Bannister et al. and Bolin et al. with a lightcurve amplitude of {{nowrap|1.5–2.1 magnitudes}},<ref name="Bolin2017" /> whereas Meech et al. reported a rotation period of 7.3&nbsp;hours and a lightcurve amplitude of 2.5&nbsp;magnitudes.<ref name=ESO2017 />{{efn|1=[[Astronomy:1865 Cerberus|1865 Cerberus]] has a lightcurve amplitude of 2.3&nbsp;magnitudes.}} Most likely, {{okina}}Oumuamua was set tumbling by a collision in its system of origin, and remains tumbling since the time scale for dissipation of this motion is very long, at least a billion years.<ref name="Fraser2018a"/><ref>{{cite news |url=https://www.bbc.com/news/science-environment-43018706 |title={{okina}}Oumuamua: 'space cigar's' tumble hints at violent past |work=BBC News |first=Jonathan |last=Amos |date=11 February 2018 |access-date=21 July 2018 |archive-date=24 July 2018 |archive-url=https://web.archive.org/web/20180724044444/https://www.bbc.com/news/science-environment-43018706 |url-status=live }}</ref>

[[File:Artist's impression of {{okina}}Oumuamua.jpg|thumb|left|Artist's impression of {{okina}}Oumuamua]]
[[File:{{okina}}Oumuamua light curve simulation.gif|thumb|upright=1.5|Simulation of {{okina}}Oumuamua spinning and tumbling through space, and the resultant light curve. In reality, observations of {{okina}}Oumuamua detect the object as a single pixel – its shape here has been inferred from the light curve]]

The large variations on the light curves indicate that {{okina}}Oumuamua may be anything from a highly elongated cigar-like object, comparable to or greater than the most elongated Solar System objects,<ref name="Bannister2017" /><ref name="Bolin2017" /> to an extremely flat object, a pancake or oblate [[Spheroid|spheroid]].<ref>{{cite journal |arxiv=1804.03471 |title=The Excited Spin State of 1I/2017 U1 'Oumuamua |journal=The Astrophysical Journal |volume=856 |issue=2 |pages=L21 |last1=Belton |first1=M. J. S. |s2cid=119336678 |display-authors=0 |date=10 April 2018 |quote=We find that {{okina}}Oumuamua is 'cigar-shaped', if close to its lowest rotational energy, and an extremely oblate spheroid if close to its highest energy state for its total angular momentum. |doi=10.3847/2041-8213/aab370 |bibcode=2018ApJ...856L..21B |doi-access=free }}</ref> However, the size and shape have not been directly observed as {{okina}}Oumuamua appears as nothing more than a point source of light even in the most powerful telescopes. Neither its [[Physics:Geometric albedo|albedo]] nor its triaxial ellipsoid shape is known. If cigar-shaped, the longest-to-shortest axis ratio could be 5:1 or greater.<ref name="Fraser2018a"/> Assuming an albedo of 10% (slightly higher than typical for D-type asteroids<ref>{{Cite journal |last1=Thomas |first1=C. A. |last2=Trilling |first2=D. E. |last3=Emery |first3=J. P. |last4=Mueller |first4=M. |last5=Hora |first5=J. L. |last6=Benner |first6=L. A. M. |last7=Bhattacharya |first7=B. |last8=Bottke |first8=W. F. |last9=Chesley |first9=S. |date=1 September 2011 |title=ExploreNEOs. V. Average Albedo by Taxonomic Complex in the Near-Earth Asteroid Population |bibcode=2011AJ....142...85T |journal=The Astronomical Journal |volume=142 |issue=3 |page=85 |doi=10.1088/0004-6256/142/3/85 |issn=0004-6256|doi-access=free }}</ref>) and a 6:1 ratio, {{okina}}Oumuamua has dimensions of approximately {{convert|100|-|1000|x|35|-|167|x|35|-|167|m|ft|abbr=on}}<ref name="NASA20181114" /><ref name="STD-0181020" /><ref name="PHYS-20181022" /><ref name="Jewitt2017"/><ref name="NOAO2017"/> with an average diameter of about {{cvt|110|m|ft}}.<ref name="Jewitt2017"/><ref name="NOAO2017" /> According to astronomer David Jewitt, the object is physically unremarkable except for its highly elongated shape.<ref name="NOAO2017" /> Bannister et al. have suggested that it could also be a [[Astronomy:Contact binary (small Solar System body)|contact binary]],<ref name="Bannister2017" /> although this may not be compatible with its rapid rotation.<ref name="Rincon2017" /> One speculation regarding its shape is that it is a result of a violent event (such as a collision or stellar explosion) that caused its ejection from its system of origin.<ref name="Rincon2017" /> JPL News reported that {{okina}}Oumuamua "is up to one-quarter mile (400&nbsp;meters) long and highly-elongated — perhaps 10 times as long as it is wide".<ref name="jpl-news">{{cite web |title=Solar System's First Interstellar Visitor Dazzles Scientists |url=https://www.jpl.nasa.gov/news/news.php?feature=7006 |website=Jet Propulsion Laboratory |date=20 November 2017 |access-date=20 December 2017 |archive-date=10 March 2020 |archive-url=https://web.archive.org/web/20200310024611/https://www.jpl.nasa.gov/news/news.php?feature=7006 |url-status=live }}</ref><ref name=Guarddian2017-12-11 />

A 2019 paper finds the best models as either a cigar-shape, 1:8 aspect ratio, or disc-shape, 1:6 aspect ratio, with the disc more likely since its rotation does not require a specific orientation to see the range of brightnesses observed.<ref name="Mashchenko2019">{{cite journal |title=Modeling the light curve of 'Oumuamua: evidence for torque and disc-like shape |journal=Monthly Notices of the Royal Astronomical Society |first=Sergey |last=Mashchenko |s2cid=182952355 |volume=489 |issue=3 |pages=3003–3021 |date=November 2019 |doi=10.1093/mnras/stz2380 |bibcode=2019MNRAS.489.3003M |arxiv=1906.03696}}</ref> [[Monte Carlo method|Monte Carlo simulations]] based on the available orbit determination suggest that the equatorial [[Astronomy:Axial tilt|obliquity]] of {{okina}}Oumuamua could be about 93 degrees, if it has a very prolate or cigar-like shape, or close to 16 degrees, if it is very oblate or disk-like.<ref name="rotationpole">{{cite journal |last1=de la Fuente Marcos |first1=C. |last2=de la Fuente Marcos |first2=R. |title=Constraining the orientation of the spin axes of extrasolar minor bodies 1I/2017 U1 ('Oumuamua) and 2I/Borisov |journal=Astronomy and Astrophysics |volume=643 |pages=A18 (17 pp) |date=1 November 2020 |doi=10.1051/0004-6361/202037447 |arxiv=2009.08423 |bibcode=2020A&A...643A..18D |s2cid=221761422 |url=https://www.aanda.org/articles/aa/abs/2020/11/aa37447-20/aa37447-20.html |access-date=27 October 2020 |archive-date=1 February 2021 |archive-url=https://web.archive.org/web/20210201045557/https://www.aanda.org/articles/aa/abs/2020/11/aa37447-20/aa37447-20.html |url-status=live }}</ref> A 2021 paper found that the extreme shape was likely a result of recent evaporation, and that when the object entered the Solar System it likely had an unremarkable 2:1 aspect ratio. The authors calculated that a month after perihelion, that {{okina}}Oumuamua had lost 92% of the mass it had upon entering the Solar System.<ref name="JGRP-20210316a" />

[[Astronomy:Light curve|Light curve]] observations suggest the object may be composed of dense metal-rich rock that has been reddened by millions of years of exposure to [[Astronomy:Cosmic ray|cosmic ray]]s.<ref name="Rincon2017"/><ref>{{Cite magazine |url=https://www.science.org/content/article/updated-first-time-astronomers-are-tracking-distant-visitor-streaking-through-our-solar |title=Updated: For the first time, astronomers are tracking a distant visitor streaking through our solar system |magazine=Science |first=Paul |last=Voosen |date=20 November 2017 |doi=10.1126/science.aar3433 |access-date=30 November 2017 |archive-date=21 November 2017 |archive-url=https://web.archive.org/web/20171121122550/http://www.sciencemag.org/news/2017/11/updated-first-time-astronomers-are-tracking-distant-visitor-streaking-through-our-solar |url-status=live }}</ref><ref>{{cite web |url=https://www.space.com/38838-interstellar-asteroid-oumuamua-space-cigar.html |title=Wow! 1st Interstellar Asteroid Is a Spinning Space Cigar |first=Ian |last=O'Neill |website=Space.com |date=20 November 2017 |access-date=30 November 2017 |archive-date=25 April 2020 |archive-url=https://web.archive.org/web/20200425005615/https://www.space.com/38838-interstellar-asteroid-oumuamua-space-cigar.html |url-status=live }}</ref> It is thought that its surface contains [[Astronomy:Tholin|tholin]]s, which are irradiated [[Chemistry:Organic compound|organic compound]]s that are more common in objects in the outer Solar System and can help determine the age of the surface.<ref name=MattWilliams2017-11-20a>{{cite web |first=Matt |last=Williams |url=https://www.universetoday.com/137944/interstellar-asteroid-probably-pretty-strange-looking/ |title=That Interstellar Asteroid is probably pretty strange looking |publisher=Universe Today |date=20 November 2017 |access-date=20 December 2017 |quote=Its dark and reddened surface is also an indication of tholins, which are the result of organic molecules (like methane) being irradiated by cosmic rays for millions of years. |archive-date=22 December 2017 |archive-url=https://web.archive.org/web/20171222052002/https://www.universetoday.com/137944/interstellar-asteroid-probably-pretty-strange-looking/ |url-status=live }}</ref><ref name="MattWilliams2017-11-24a">{{cite web |last=Williams |first=Matt |date=24 November 2017 |title=Project Lyra, a mission to chase down that interstellar asteroid |url=https://www.universetoday.com/137960/project-lyra-mission-chase-interstellar-asteroid-1/ |url-status=live |archive-url=https://web.archive.org/web/20171230124909/https://www.universetoday.com/137960/project-lyra-mission-chase-interstellar-asteroid-1/ |archive-date=30 December 2017 |access-date=20 December 2017 |publisher=Universe Today |quote=It was also determined to be rocky and metal rich, and to contain traces of tholins – organic molecules that have been irradiated by UV radiation.}} Also here [https://phys.org/news/2017-11-lyra-mission-interstellar-asteroid.html] {{Webarchive|url=https://web.archive.org/web/20171222052951/https://phys.org/news/2017-11-lyra-mission-interstellar-asteroid.html|date=22 December 2017}} at Phys.org</ref> This possibility is inferred from spectroscopic characterization and its reddish color,<ref name=MattWilliams2017-11-20a /><ref name="Fitzsimmons2017a">{{cite journal |last1=Fitzsimmons |first1=A. |first2=C. |last2=Snodgrass |first3=B. |last3=Rozitis |first4=B. |last4=Yang |first5=M. |last5=Hyland |first6=T. |last6=Seccull |first7=M. T. |last7=Bannister |first8=W. C. |last8=Fraser |first9=R. |last9=Jedicke |first10=P. |last10=Lacerda |s2cid=216937304 |display-authors=1 |title=Spectroscopy and thermal modelling of the first interstellar object 1I/2017 U1 {{okina}}Oumuamua |arxiv=1712.06552 |date=18 December 2017 |journal=Nature Astronomy |volume=2 |issue=2 |page=133 |doi=10.1038/s41550-017-0361-4 |quote=The discovery epoch photometry implies a highly elongated body with radii of ~200×20 m when a comet-like geometric albedo of 0.04 is assumed. Here we report spectroscopic characterisation of {{okina}}Oumuamua, finding it to be variable with time but similar to organically rich surfaces found in the outer Solar System. The observable ISO population is expected to be dominated by comet-like bodies in agreement with our spectra, yet the reported inactivity implies a lack of surface ice. We show this is consistent with predictions of an insulating mantle produced by long-term cosmic ray exposure. An internal icy composition cannot therefore be ruled out by the lack of activity, even though {{okina}}Oumuamua passed within 0.25 au of the Sun. |bibcode=2018NatAs...2..133F |url=https://pure.qub.ac.uk/portal/en/publications/spectroscopy-and-thermal-modelling-of-the-first-interstellar-object-1i2017-u1-oumuamua(433758b2-c945-4038-8c19-b4e5b52ae1e2).html |access-date=25 September 2018 |archive-date=17 October 2019 |archive-url=https://web.archive.org/web/20191017090156/https://pure.qub.ac.uk/portal/en/publications/spectroscopy-and-thermal-modelling-of-the-first-interstellar-object-1i2017-u1-oumuamua(433758b2-c945-4038-8c19-b4e5b52ae1e2).html |url-status=live }}</ref> and from the expected effects of interstellar radiation.<ref name="Fitzsimmons2017a" /> Despite the lack of any [[Astronomy:Coma (cometary)|cometary coma]] when it approached the Sun, it may still contain internal ice, hidden by "an insulating mantle produced by long-term [[Astronomy:Cosmic ray|cosmic ray]] exposure".<ref name="Fitzsimmons2017a" />

In November 2019, some astronomers noted that {{okina}}Oumuamua may be a "cosmic dust bunny", due to its "very lightweight and 'fluffy' conglomerate of dust and ice grains".<ref name="ES-20191126">{{cite news |last=Anderson |first=Paul Scott |title=Was 'Oumuamua a cosmic dust bunny? |url=https://earthsky.org/space/was-oumuamua-a-cosmic-dust-bunny |date=26 November 2019 |work=Earth & Sky |access-date=27 November 2019 |archive-date=27 November 2019 |archive-url=https://web.archive.org/web/20191127204418/https://earthsky.org/space/was-oumuamua-a-cosmic-dust-bunny |url-status=live }}</ref><ref name="TAJL-20191111">{{cite journal |last=Flekkøy |first=Eirik G. |s2cid=204734116 |display-authors=et al. |title=The Interstellar Object 'Oumuamua as a Fractal Dust Aggregate |date=11 November 2019 |journal=The Astrophysical Journal Letters |volume=885 |pages=L41 |number=2 |doi=10.3847/2041-8213/ab4f78 |arxiv=1910.07135 |bibcode=2019ApJ...885L..41F |url=https://hal-insu.archives-ouvertes.fr/insu-02366685/file/2019-FlekkoyLuuToussaint-Flekk%C3%B8y_2019_ApJL_885_L41.pdf |access-date=30 November 2019 |archive-date=1 February 2021 |archive-url=https://web.archive.org/web/20210201045530/https://hal-insu.archives-ouvertes.fr/insu-02366685/file/2019-FlekkoyLuuToussaint-Flekk%C3%B8y_2019_ApJL_885_L41.pdf |url-status=live |doi-access=free }}</ref><ref name="UT-20200908">{{cite news |last=Tomaswick |first=Andy |title=Okay, New Idea. Oumuamua is an Interstellar 'Dust Bunny' |url=https://www.universetoday.com/147675/okay-new-idea-oumuamua-is-an-interstellar-dust-bunny/ |date=8 September 2020 |work=Universe Today |access-date=9 September 2020 |archive-date=11 September 2020 |archive-url=https://web.archive.org/web/20200911021825/https://www.universetoday.com/147675/okay-new-idea-oumuamua-is-an-interstellar-dust-bunny/ |url-status=live }}</ref>  
In August 2020, astronomers reported that ʻOumuamua is not likely to have been composed of frozen [[Chemistry:Hydrogen|hydrogen]], which had been proposed earlier; the compositional nature of the object continues to be unknown.<ref name="PHY-20200817">{{cite news |author=Harvard-Smithsonian Center for Astrophysics and [[Organization:Korea Astronomy and Space Science Institute|Korea Astronomy and Space Science Institute]] |title=Scientists determine 'Oumuamua isn't made from molecular hydrogen ice after all |url=https://phys.org/news/2020-08-scientists-oumuamua-isnt-molecular-hydrogen.html |date=17 August 2020 |work=Phys.org |access-date=17 August 2020 |archive-date=17 August 2020 |archive-url=https://web.archive.org/web/20200817190804/https://phys.org/news/2020-08-scientists-oumuamua-isnt-molecular-hydrogen.html |url-status=live }}</ref><ref name="AJL-20200817">{{cite journal|last1=Hoang|first1=Thiem|last2=Loeb |first2=Abraham|date=17 August 2020|title=Destruction of Molecular Hydrogen Ice and Implications for 1I/2017 U1 ('Oumuamua)|journal=The Astrophysical Journal Letters|volume=899|pages=L23|arxiv=2006.08088 |bibcode=2020ApJ...899L..23H|doi=10.3847/2041-8213/abab0c|doi-access=free|number=2}}</ref>
<!--table goes here-->

=== Radio measurements ===
In December 2017, astronomer [[Biography:Avi Loeb|Avi Loeb]] of [[Organization:Harvard University|Harvard University]], an adviser to the [[Astronomy:Breakthrough Listen|Breakthrough Listen]] Project, cited {{okina}}Oumuamua's unusually elongated shape as one reason the [[Astronomy:Green Bank Telescope|Green Bank Telescope]] in West Virginia would listen for radio emissions from it to see if there were any unexpected signs that it might be of [[Philosophy:Extraterrestrial intelligence|artificial origin]],<ref name=Guarddian2017-12-11 /> although earlier limited observations by other radio telescopes such as the [[Organization:SETI Institute|SETI Institute]]'s [[Astronomy:Allen Telescope Array|Allen Telescope Array]] had produced no such results.<ref name=SETI /> On 13 December 2017, the Green Bank Telescope observed the object for six hours across four bands of radio frequency. No radio signals from {{okina}}Oumuamua were detected in this very limited scanning range, but more observations were planned.<ref>{{cite web |title=Breakthrough Listen Releases Initial Results and Data from Observations of {{okina}}Oumuamua<!--Deny Citation Bot--> |url=http://breakthroughinitiatives.org/news/15 |publisher=Breakthrough Listen |date=13 December 2017 |access-date=15 December 2017 |quote=No evidence of artificial signals emanating from the object so far detected by the Green Bank Telescope, but monitoring and analysis continue. Initial data are available for public inspection in the Breakthrough Listen archive |archive-date=15 December 2017 |archive-url=https://web.archive.org/web/20171215053410/http://breakthroughinitiatives.org/news/15 |url-status=live }}</ref><ref>{{cite news |url=https://www.theguardian.com/science/2017/dec/14/is-oumuamua-an-alien-spacecraft-first-scans-show-no-signs-of-technology |title=Is {{okina}}Oumuamua an alien spacecraft? Initial scans show no signs of technology<!--Deny Citation Bot--> |journal=The Guardian |author=Ian Sample |date=15 December 2017 |access-date=15 December 2017 |archive-date=15 December 2017 |archive-url=https://web.archive.org/web/20171215000127/https://www.theguardian.com/science/2017/dec/14/is-oumuamua-an-alien-spacecraft-first-scans-show-no-signs-of-technology |url-status=live }}</ref>{{update after|2018}}

==Discussion==
=== Nitrogen ice theory ===
Outgassing of nitrogen ice (N<sub>2</sub>) could explain why no outgassing was detected. Nitrogen ice the size of 'Oumuamua could survive for 500 million years in the [[Astronomy:Interstellar medium|interstellar medium]] and would reflect two-thirds of the Sun's light.<ref>{{Cite web|last=Siegel|first=Ethan|date=10 February 2021|title=New Theory Perfectly Explains 'Oumuamua Naturally: It's A Nitrogen Iceberg|url=https://www.forbes.com/sites/startswithabang/2021/02/10/new-theory-perfectly-explains-oumuamua-naturally-from-exo-pluto-collisions/|url-status=live|archive-url=https://web.archive.org/web/20210213203138/https://www.forbes.com/sites/startswithabang/2021/02/10/new-theory-perfectly-explains-oumuamua-naturally-from-exo-pluto-collisions/|archive-date=13 February 2021|access-date=13 February 2021|website={{wipe|Forbes}}|language=en}}</ref> This explanation has been further supported in March 2021 when scientists presented a theory based on nitrogen ice, and further concluded that {{okina}}Oumuamua may be a piece of an [[Astronomy:Exoplanet|exoplanet]] similar to the [[Astronomy:Dwarf planet|dwarf planet]] [[Astronomy:Pluto|Pluto]], an exo-Pluto as noted, from beyond our [[Astronomy:Solar System|solar system]].<ref name="PHYS-20210317">{{cite news|author=Staff|date=17 March 2021|title=Scientists determine the origin of extra-solar object 'Oumuamua|work=Phys.org|url=https://phys.org/news/2021-03-scientists-extra-solar-oumuamua.html |access-date=17 March 2021}}</ref><ref name="JGRP-20210316a" /><ref name="JGRP-20210316b" /><ref name="NYT-20210323" /> This theory has been criticized by Loeb.<ref>{{Cite web |website=Scientific American |url=https://www.scientificamerican.com/article/was-the-interstellar-object-oumuamua-a-nitrogen-iceberg/ |title=Was the Interstellar Object 'Oumuamua a Nitrogen Iceberg?}}</ref><ref name="NS-20211105">{{cite journal |arxiv=2103.14032 |last1=Siraj|first1=Amir|last2=Loeb|first2=Abraham|title=The mass budget necessary to explain 'Oumuamua as a nitrogen iceberg |journal=New Astronomy |date=April 2022 |volume=92 |at=101730 |doi=10.1016/j.newast.2021.101730 |bibcode=2022NewA...9201730S |s2cid=232352541}}</ref> In November 2021, theoretical studies by Siraj and Loeb hypothesized that 'Oumuamua was not a nitrogen iceberg.<ref name="LS-20211115">{{cite news |last=Hickok |first=Kimberly |title=Interstellar visitor 'Oumuamua wasn't a nitrogen iceberg, Harvard astrophysicists say - The bizarre interloper called 'Oumuamua continues to defy explanation. |url=https://www.livescience.com/oumuamua-not-nitrogen-iceberg |date=15 November 2021 |work=Live Science |access-date=15 November 2021 }}</ref><ref name="NS-20211105"/>

=== Hydrogen ice theory===
It has been proposed that ʻOumuamua contains a significant amount of [[Physics:Solid hydrogen|hydrogen ice]].<ref name="ARX-20200526">{{cite journal |last1=Seligman |first1=Darryl |last2=Laughlin |first2=Gregory |title=Evidence that 1I/2017 U1 ('Oumuamua) was Composed of Molecular Hydrogen Ice |journal=The Astrophysical Journal |date=26 May 2020 |volume=896 |issue=1 |pages=L8 |doi=10.3847/2041-8213/ab963f |arxiv=2005.12932 |bibcode=2020ApJ...896L...8S |s2cid=218900854 |doi-access=free }}</ref><ref name="NYT-20200615">{{cite news |last=Overbye |first=Dennis |title=Oumuamua: Neither Comet nor Asteroid, but a Cosmic Iceberg – A new study suggests the interloper may have arisen in an interstellar cloud, where stars are sometimes born |url=https://www.nytimes.com/2020/06/15/science/oumuamua-astronomy-comets.html |date=15 June 2020 |work={{wipe|The New York Times}} |access-date=16 June 2020 |archive-date=16 June 2020 |archive-url=https://web.archive.org/web/20200616003628/https://www.nytimes.com/2020/06/15/science/oumuamua-astronomy-comets.html |url-status=live }}</ref> This would point to it originating from the core of an interstellar [[Astronomy:Molecular cloud|molecular cloud]], where conditions for the formation of this material might exist.<ref>{{cite journal |first1=Hagai B. |last1=Perets |first2=Ofer |last2=Biham |first3=Giulio |last3=Manico |first4=Valerio |last4=Pirronello |first5=Joe |last5=Roser |first6=Sol |last6=Swords |first7=Gianfranco |last7=Vidali |date=29 March 2005 |title=Molecular Hydrogen Formation on Ice Under Interstellar Conditions |arxiv=astro-ph/0412202 |journal=The Astrophysical Journal |volume=627 |issue=2 |pages=850–860 |url=https://www.researchgate.net/publication/230998087 |doi=10.1086/430435 |bibcode=2005ApJ...627..850P |s2cid=56368174 |archive-url=https://web.archive.org/web/20200606012206/https://arxiv.org/abs/astro-ph/0412202 |archive-date=6 June 2020 }}</ref> The Sun's heat would cause the hydrogen to [[Sublimation (phase transition)|sublime]], which would in turn propel the body. The hydrogen [[Astronomy:Coma (cometary)|coma]] formed by this process would be difficult to detect from Earth-based telescopes, as the atmosphere blocks those wavelengths.<ref name="usra">{{cite web|url=http://www.lpi.usra.edu/education/explore/comets/background/|publisher=lpi.usra.edu|title=About Comets|access-date=2020-06-06|archive-date=16 November 2020|archive-url=https://web.archive.org/web/20201116204514/https://www.lpi.usra.edu/education/explore/comets/background/|url-status=live}}</ref> Regular water-ice comets undergo this as well, however to a much lesser extent and with a visible coma. This may explain the significant non-gravitational acceleration that ʻOumuamua underwent without showing signs of coma formation. Significant mass loss caused by the sublimation would also explain the unusual cigar-like shape, comparable to how a bar of soap becomes more elongated as it is used up.

However, it was later shown that hydrogen icebergs cannot form out of small grains and that, to not evaporate during their journey in interstellar space, they would have had to have been formed about 40 million years ago, in the close neighborhood of the solar system.<ref name="HoangLoeb2020-06">{{cite journal|last1=Hoang|first1=Thiem|last2=Loeb|first2=Abraham|year=2020|title=Destruction of molecular hydrogen ice and Implications for 'Oumuamua|journal=The Astrophysical Journal Letters|volume=899|issue=2|arxiv=2006.08088|doi=10.3847/2041-8213/abab0c|bibcode=2020ApJ...899L..23H |s2cid=219687520 |doi-access=free }}</ref><ref name="LS-20200819">{{cite news |last=Letzer |first=Ran |title=Interstellar visitor 'Oumuamua could still be alien technology, new study hints – Aliens? Or a chunk of solid hydrogen? Which idea makes less sense? |url=https://www.livescience.com/oumuamua-interstellar-hydrogen-or-aliens.html |date=19 August 2020 |work=Live Science |access-date=6 January 2021 |archive-date=9 January 2021 |archive-url=https://web.archive.org/web/20210109012743/https://www.livescience.com/oumuamua-interstellar-hydrogen-or-aliens.html |url-status=live }}</ref>

===Hydrogen-laden water ice theory===
In 2023, it was proposed the observed non-gravitational acceleration and spectrum of ʻOumuamua can be best explained by hydrogen outgassing from the water ice matrix. The buildup of the hydrogen in the water ice is expected to happen in the interstellar comets, due to low-temperature water ice [[Physics:Radiolysis|radiolysis]] by cosmic ray particles while ʻOumuamua or similar cometary body was in interstellar space.<ref>{{citation|arxiv=2303.13698|year=2023|title=Acceleration of 1I/'Oumuamua from radiolytically produced H2 in H2O ice|doi=10.1038/s41586-022-05687-w |last1=Bergner |first1=Jennifer B. |last2=Seligman |first2=Darryl Z. |journal=Nature |volume=615 |issue=7953 |pages=610–613 |pmid=36949336 |s2cid=257668585 }}</ref><ref name="NAT-20230322"/>

=== Hypothetical space missions ===
{{Main|Astronomy:Project Lyra}}
The [[Organization:Initiative for Interstellar Studies|Initiative for Interstellar Studies]] (i4is) launched [[Astronomy:Project Lyra|Project Lyra]] to assess the feasibility of a mission to {{okina}}Oumuamua.<ref name=ProjectLyra>{{cite web |title=Project Lyra – A Mission to {{okina}}Oumuamua |url=https://i4is.org/what-we-do/technical/project-lyra/ |website=I4IS |publisher=Initiative for Interstellar Studies |access-date=3 December 2017 |archive-date=3 December 2017 |archive-url=https://web.archive.org/web/20171203224457/https://i4is.org/what-we-do/technical/project-lyra/ |url-status=live }}</ref> Several options for sending a spacecraft to {{okina}}Oumuamua within a time-frame of 5 to 25 years were suggested.<ref>{{Cite journal |last1=Hein |first1=Andreas M. |last2=Perakis |first2=Nikolaos |last3=Eubanks |first3=T. Marshall |last4=Hibberd |first4=Adam |last5=Crowl |first5=Adam |last6=Hayward |first6=Kieran |last7=Kennedy III |first7=Robert G. |last8=Osborne |first8=Richard |title=Project Lyra: Sending a spacecraft to 1I/'Oumuamua (former A/2017 U1), the interstellar asteroid |journal=Acta Astronautica |date=7 January 2019 |volume=161 |page=552 |doi=10.1016/j.actaastro.2018.12.042 |arxiv=1711.03155 |bibcode= 2019AcAau.161..552H|s2cid=119474144 }}</ref><ref>{{cite journal |last1=Hibberd |first1=Adam |last2=Hein |first2=Andreas M. |last3=Eubanks |first3=T. Marshall |s2cid=119078436 |title=Project Lyra: Catching 1I/'Oumuamua – Mission Opportunities After 2024 |journal=Acta Astronautica |volume= 170|pages= 136–144|arxiv=1902.04935 |doi=10.1016/j.actaastro.2020.01.018 |bibcode=2020AcAau.170..136H |year=2020 }}</ref> Different mission durations and their velocity requirements were explored with respect to the launch date, assuming direct impulsive transfer to the intercept trajectory.{{citation needed|date=February 2021}}

The Space Launch System (also being looked at for "interstellar precursor missions") would be even more capable.<ref>{{cite conference |url=https://www.hou.usra.edu/meetings/lpsc2015/pdf/2346.pdf |title=The Space Launch System and Missions to the Outer Solar System |conference=46th Lunar and Planetary Science Conference. 16–20 March 2015. The Woodlands, Texas. |first=K. |last=Klaus |date=2015 |access-date=5 June 2019 |archive-date=26 October 2020 |archive-url=https://web.archive.org/web/20201026105533/https://www.hou.usra.edu/meetings/lpsc2015/pdf/2346.pdf |url-status=live }}</ref><ref>{{cite conference |url=https://www.researchgate.net/publication/285199699 |title=Enabling interstellar probe with the Space Launch System (SLS) |conference=65th International Astronautical Congress. 29 September-3 October 2014. Toronto, Canada. |first1=R. L. Jr. |last1=McNutt |first2=M. S. |last2=Elsperman |first3=Mike |last3=Gruntman |first4=K. K. |last4=Klaus |first5=S. M. |last5=Krimigis |first6=E. C. |last6=Roelof |first7=D. B. |last7=Smith |first8=S. R. |last8=Vernon |first9=R. F. |last9=Wimmer-Schweingruber |display-authors=1 |date=2014 |access-date=5 June 2019 |archive-date=1 February 2021 |archive-url=https://web.archive.org/web/20210201045618/https://www.researchgate.net/publication/285199699_Enabling_interstellar_probe_with_the_Space_Launch_System_SLS |url-status=live }}</ref> Such an interstellar precursor could easily pass by {{okina}}Oumuamua on its way out of the Solar System, at speeds of {{convert|63|km/s|mi/s|abbr=on}}.<ref>{{cite web |url=https://studylib.net/doc/18163873/sls-mission-booklet |title=Space Launch System: Mission Booklet |publisher=Boeing |website=Studylib.net |date=2013 |access-date=5 June 2019 |archive-date=5 June 2019 |archive-url=https://web.archive.org/web/20190605183847/https://studylib.net/doc/18163873/sls-mission-booklet |url-status=live }}</ref><ref>{{cite web |url=https://www.kiss.caltech.edu/workshops/ism/presentations/KISS%20How%20Fast%20How%20Far_strange_final.pdf |title=An Architectural Framework for the design of missions to explore the ISM |publisher=NASA/Jet Propulsion Laboratory |first1=Nitin |last1=Arora |first2=Nathan |last2=Strange |first3=Robert |last3=Cesarone |first4=Leon |last4=Alkalai |display-authors=1 |date=2014 |access-date=25 October 2019 |archive-date=1 September 2020 |archive-url=https://web.archive.org/web/20200901100315/https://www.kiss.caltech.edu/workshops/ism/presentations/KISS%20How%20Fast%20How%20Far_strange_final.pdf |url-status=live }}</ref>

More advanced options of using solar, laser electric, and laser sail propulsion, based on [[Astronomy:Breakthrough Starshot|Breakthrough Starshot]] technology, have also been considered. The challenge is to get to the interstellar object in a reasonable amount of time (and so at a reasonable distance from Earth), and yet be able to gain useful scientific information. To do this, decelerating the spacecraft at {{okina}}Oumuamua would be "highly desirable, due to the minimal science return from a hyper-velocity encounter".<ref name="Hein2017" /> If the investigative craft goes too fast, it would not be able to get into orbit or land on the object and would fly past it. The authors conclude that, although challenging, an encounter mission would be feasible using near-term technology.<ref name="Hein2017" /><ref name=ProjectLyra /> Seligman and Laughlin adopt a complementary approach to the Lyra study but also conclude that such missions, though challenging to mount, are both feasible and scientifically attractive.<ref name="ARX-20180412">{{Cite journal |last1=Seligman |first1=Darryl |last2=Laughlin |first2=Gregory |s2cid=73656586 |title=The Feasibility and Benefits of in situ Exploration of {{okina}}Oumuamua-like Objects |journal=The Astronomical Journal |volume=155 |issue=5 |page=217 |date=12 April 2018 |arxiv=1803.07022 |doi=10.3847/1538-3881/aabd37 |bibcode=2018AJ....155..217S |doi-access=free }}</ref>

=== Technosignature hypothesis ===
On 26 October 2018, Loeb and his [[Social:Postdoctoral researcher|postdoc]] Shmuel Bialy submitted a paper exploring the possibility of {{okina}}Oumuamua being an artificial thin [[Physics:Solar sail|solar sail]]<ref>{{Cite news |url=https://www.haaretz.com/world-news/.premium.MAGAZINE-if-true-this-could-be-one-of-the-greatest-discoveries-in-human-history-1.6828318 |title=If True, This Could Be One of the Greatest Discoveries in Human History |last=Carmeli |first=Oded |date=14 January 2019 |work=Haaretz |language=en |access-date=14 January 2019 |archive-date=14 January 2019 |archive-url=https://web.archive.org/web/20190114154442/https://www.haaretz.com/world-news/.premium.MAGAZINE-if-true-this-could-be-one-of-the-greatest-discoveries-in-human-history-1.6828318 |url-status=live }}</ref><ref name="CT-20190204">{{cite news |last=Selik |first=Avi |title=Alien ship may be among us, Harvard astronomer insists, despite grumbling and criticism from peers |url=https://www.chicagotribune.com/news/nationworld/science/ct-harvard-astronomer-aliens-20190204-story.html |date=4 February 2019 |work=ChicagoTribune |access-date=5 February 2019 |archive-date=5 February 2019 |archive-url=https://web.archive.org/web/20190205085137/https://www.chicagotribune.com/news/nationworld/science/ct-harvard-astronomer-aliens-20190204-story.html |url-status=live }}</ref> accelerated by solar radiation pressure, in an effort to help explain the object's comet-like non-gravitational acceleration.<ref name="UT-230181031">{{cite web |last=Williams |first=Matt |title=Could Oumuamua Be an Extra-Terrestrial Solar Sail? |url=https://www.universetoday.com/140391/could-oumuamua-be-an-extra-terrestrial-solar-sail/ |date=2 November 2018 |work=Universe Today |access-date=2 November 2018 |archive-date=3 November 2018 |archive-url=https://web.archive.org/web/20181103042615/https://www.universetoday.com/140391/could-oumuamua-be-an-extra-terrestrial-solar-sail/ |url-status=live }}</ref><ref name="ARX-20181101">{{cite journal |last1=Bialy |first1=Shmuel |last2=Loeb |first2=Abraham |s2cid=118956077 |title=Could Solar Radiation Explain {{okina}}Oumuamua's Peculiar Acceleration? |journal=The Astrophysical Journal |volume=868 |pages=L1 |date=26 October 2018 |issue=1 |arxiv=1810.11490 |doi=10.3847/2041-8213/aaeda8 |bibcode=2018ApJ...868L...1B |doi-access=free }}</ref><ref name="SA-201809287">{{cite web |last=Loeb |first=Abraham |title=How to Search for Dead Cosmic Civilizations |url=https://blogs.scientificamerican.com/observations/how-to-search-for-dead-cosmic-civilizations/ |date=26 September 2018 |work=Scientific American |access-date=26 September 2018 |archive-date=27 April 2020 |archive-url=https://web.archive.org/web/20200427025843/https://blogs.scientificamerican.com/observations/how-to-search-for-dead-cosmic-civilizations/ |url-status=live }}</ref> Other scientists have stated that the available evidence is insufficient to consider such a premise,<ref>{{Cite web|last=Sheridan|first=Kerry|date=7 November 2018|title=Scientists push back against Harvard 'alien spacecraft' theory|url=https://phys.org/news/2018-11-scientists-harvard-alien-spacecraft-theory.html|url-status=live|archive-url=https://web.archive.org/web/20210123055305/https://phys.org/news/2018-11-scientists-harvard-alien-spacecraft-theory.html|archive-date=23 January 2021|access-date=14 February 2021|website=Phys.org}}</ref><ref>{{Cite web |url=https://www.yahoo.com/news/oumuamua-oh-interstellar-object-actually-225825249.html |title='Oumuamua, oh my! Was interstellar object actually an alien solar sail? Not so fast |last=Boyle |first=Alan |date=6 November 2018 |website=Yahoo! |access-date=8 November 2018 |archive-date=8 November 2018 |archive-url=https://web.archive.org/web/20181108224820/https://www.yahoo.com/news/oumuamua-oh-interstellar-object-actually-225825249.html |url-status=live }}</ref><ref>{{Cite news|last=Schadwinkel|first=Alina|date=8 November 2018|title=Glaubt dieser Harvard-Professor selbst, was er sagt?|language=de|work=Zeit Online|url=https://www.zeit.de/wissen/2018-11/oumuamua-lichtsegel-ausserirdische-aliens|url-status=live|access-date=8 November 2018|archive-url=https://web.archive.org/web/20181108172742/https://www.zeit.de/wissen/2018-11/oumuamua-lichtsegel-ausserirdische-aliens|archive-date=8 November 2018}}</ref> and that a tumbling solar sail would not be able to accelerate.<ref>{{cite news |url=https://www.wpsdlocal6.com/archive/cigar-shaped-interstellar-object-may-have-been-an-alien-probe/article_24c3cd57-36e8-580a-87e1-f139ab01d53d.html |title=Cigar-shaped interstellar object may have been an alien probe, Harvard paper claims |publisher=WPSD Local 6 |agency=CNN |date=6 November 2018 |access-date=25 October 2019 |archive-date=25 October 2019 |archive-url=https://web.archive.org/web/20191025043903/https://www.wpsdlocal6.com/archive/cigar-shaped-interstellar-object-may-have-been-an-alien-probe/article_24c3cd57-36e8-580a-87e1-f139ab01d53d.html |url-status=live }}</ref> In response, Loeb wrote an article detailing six anomalous properties of {{okina}}Oumuamua that make it unusual, unlike any comets or asteroids seen before.<ref name="SA-20181120">{{cite web |last=Loeb |first=Abraham |title=6 Strange Facts about the Interstellar Visitor 'Oumuamua |url=https://blogs.scientificamerican.com/observations/6-strange-facts-about-the-interstellar-visitor-oumuamua/ |date=20 November 2018 |work=Scientific American |access-date=20 November 2018 |archive-date=20 November 2018 |archive-url=https://web.archive.org/web/20181120233035/https://blogs.scientificamerican.com/observations/6-strange-facts-about-the-interstellar-visitor-oumuamua/ |url-status=live }}</ref><ref name="TNY-20190116">{{cite magazine |last=Chotiner |first=Isaac |title=Have Aliens Found Us? A Harvard Astronomer on the Mysterious Interstellar Object 'Oumuamua |url=https://www.newyorker.com/news/the-new-yorker-interview/have-aliens-found-us-a-harvard-astronomer-on-the-mysterious-interstellar-object-oumuamua |date=16 January 2019 |magazine=The New Yorker |access-date=16 January 2019 |archive-date=16 January 2019 |archive-url=https://web.archive.org/web/20190116185121/https://www.newyorker.com/news/the-new-yorker-interview/have-aliens-found-us-a-harvard-astronomer-on-the-mysterious-interstellar-object-oumuamua |url-status=live }}</ref> A subsequent report on observations by the Spitzer Space Telescope set a tight limit on cometary outgassing of any carbon-based molecules and indicated that {{okina}}Oumuamua is at least ten times shinier than a typical comet.<ref name="ARX-20181108072">{{cite journal |last1=Trilling |first1=David |last2=al. |first2=et |s2cid=119444117 |title=Spitzer Observations of Interstellar Object 1I/'Omumuamua |journal=The Astronomical Journal |volume=156 |issue=6 |page=261 |date=20 November 2018 |arxiv=1811.08072 |doi=10.3847/1538-3881/aae88f |bibcode=2018AJ....156..261T |doi-access=free }}</ref> The solar sail [[Astronomy:Technosignature|technosignature]] hypothesis is considered unlikely by many experts owing to [[Occam's razor|available simpler explanations]] that align with the expected characteristics of interstellar asteroids and comets.<ref name="MD-20230718">{{cite news |last1=Wright |first1=Jason T. |last2=Desch |first2=Steven |last3=Raymond |first3=Sean |date=18 July 2023 |title='Oumuamua: Natural or Artificial? |work=[[Software:Medium (website)|Medium]] |url=https://medium.com/@astrowright/oumuamua-natural-or-artificial-f744b70f40d5 |url-status=live |access-date=27 July 2023 |archive-url=https://archive.today/20230721033228/https://medium.com/@astrowright/oumuamua-natural-or-artificial-f744b70f40d5 |archive-date=21 July 2023}}</ref><ref name="LS-20200819"/><ref>{{cite arXiv |eprint=2102.07871 |title='Oumuamua is not Artificial |last=Katz |first=J. I. |date= 15 February 2021 |class=physics.pop-ph }}</ref>

===Other interstellar objects===
{{Main|Astronomy:Interstellar object}}
2I/Borisov was discovered on 30 August 2019, and was soon confirmed to be an interstellar comet. Arriving from the direction of [[Astronomy:Cassiopeia (constellation)|Cassiopeia]], the object arrived at perihelion (closest point to the Sun) on 8 December 2019.

Other proposed interstellar objects include the meteors [[Astronomy:CNEOS 2014-01-08|CNEOS 2014-01-08]]<ref name="SPC-20221103">{{cite news |last=Pultarova |first=Tereza |title=Confirmed! A 2014 meteor is Earth's 1st known interstellar visitor - Interstellar space rocks might be falling to Earth every 10 years. |url=https://www.space.com/2014-meteor-first-interstellar-visitor-oumuamua |date=3 November 2022 |work=Space.com |access-date=4 November 2022 }}</ref> and CNEOS 2017-03-09 that impacted Earth in 2014<ref name="VICE-202209407">{{cite news |last=Ferreira |first=Becky |title=Secret Government Info Confirms First Known Interstellar Object on Earth, Scientists Say - A small meteor that hit Earth in 2014 was from another star system, and may have left interstellar debris on the seafloor. |url=https://www.vice.com/en/article/dyp9ez/secret-government-info-confirms-first-known-interstellar-object-on-earth-scientists-say |date=7 April 2022 |work=Vice News |access-date=9 April 2022 }}</ref><ref name="INV-20220411">{{cite news |last=Wenz |first=John |title="It Opens A New Frontier Where You're Using The Earth As A Fishing Net For These Objects." - Harvard Astronomer Believes An Interstellar Meteor (or Craft) Hit Earth In 2014 |url=https://www.inverse.com/science/interstellar-meteor-2014-discovery |date=11 April 2022 |work=Inverse |access-date=11 April 2022 }}</ref><ref name="ARX-20190604">{{cite arXiv |last1=Siraj |first1=Amir |last2=Loeb |first2=Abraham |title=Discovery of a Meteor of Interstellar Origin |date=4 June 2019 |class=astro-ph.EP |eprint=1904.07224 }}</ref><ref name="NASA-20220408">{{cite news |last1=Handal |first1=Josh |last2=Fox |first2=Karen |last3=Talbert |first3=Tricia |title=U.S. Space Force Releases Decades of Bolide Data to NASA for Planetary Defense Studies |url=https://www.nasa.gov/feature/us-space-force-releases-decades-of-bolide-data-to-nasa-for-planetary-defense-studies |date=8 April 2022 |work=[[Organization:NASA|NASA]] |access-date=11 April 2022 }}</ref> and 2017, respectively,<ref name="TDB-20220923">{{cite news |last=Loeb |first=Avi |title=The discovery of a second interstellar meteor |url=https://thedebrief.org/the-discovery-of-a-second-interstellar-meteor/ |date=23 September 2022 |work=TheDebrief.org |access-date=24 September 2022 }}</ref> although these claims have been met with skepticism.

== See also ==
* 2I/Borisov an interstellar comet and the second interstellar interloper discovered
* C/1980 E1 (Bowell) – the most eccentric comet known in the Solar System with an eccentricity of 1.057
* 514107 Ka{{okina}}epaoka{{okina}}awela, an asteroid of  possible interstellar origin
* C/2017 U7, a non-interstellar hyperbolic comet discovered 10 days after {{okina}}Oumuamua, announced in March 2018
* C/2018 C2, another non-interstellar hyperbolic comet, announced in March 2018
* [[Astronomy:CNEOS 2014-01-08|CNEOS 2014-01-08]], interstellar meteor 1 (IM1), announced in June 2019; confirmed in April 2022
* ''Extraterrestrial: The First Sign of Intelligent Life Beyond Earth'', a 2021 book by Avi Loeb describing the ʻOumuamua technosignature hypothesis
* ''Rendezvous with Rama'', a 1973 Arthur C. Clarke science-fiction novel about intercepting a large cylindrical spacecraft transiting the Solar System

== Notes ==
{{Notelist}}

== References ==
{{Reflist|refs=

<ref name="Discovery">{{cite web |title=MPEC 2017-U181: COMET C/2017 U1 (PANSTARRS) |url=http://www.minorplanetcenter.net/mpec/K17/K17UI1.html |website=[[Organization:Minor Planet Center|Minor Planet Center]] |publisher=[[Organization:International Astronomical Union|International Astronomical Union]] |date=25 October 2017 |access-date=25 October 2017 |archive-date=25 October 2017 |archive-url=https://web.archive.org/web/20171025184710/http://www.minorplanetcenter.net/mpec/K17/K17UI1.html |url-status=live }} (CK17U010)</ref>

<ref name="spectrum">{{cite web |last1=Antier |first1=K. |title=A/2017 U1, first interstellar asteroid ever detected! |url=https://www.imo.net/a2017-u1-first-interstellar-asteroid-ever-detected/ |website=International Meteor Organization |date=30 October 2017 |access-date=7 November 2017 |archive-date=7 November 2017 |archive-url=https://web.archive.org/web/20171107004829/https://www.imo.net/a2017-u1-first-interstellar-asteroid-ever-detected/ |url-status=live }}</ref>

<ref name="pseudoMPEC">{{cite web |title=Pseudo-MPEC for A/2017 U1 (FAQ File) |url=https://projectpluto.com/temp/2017u1.htm |publisher=Bill Gray of Project Pluto |date=26 October 2017 |access-date=26 October 2017 |archive-date=26 October 2017 |archive-url=https://web.archive.org/web/20171026214746/https://projectpluto.com/temp/2017u1.htm |url-status=live }} [https://projectpluto.com/temp/2017u1_a.htm#elements (Orbital elements)] {{Webarchive|url=https://web.archive.org/web/20180930231741/https://projectpluto.com/temp/2017u1_a.htm#elements |date=30 September 2018 }}</ref>

<ref name="jpldata">{{cite web |title=JPL Small-Body Database Browser: {{okina}}Oumuamua (A/2017 U1) |url=https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=1I&view=OPC |url-status=live |website=[[Astronomy:JPL Small-Body Database|JPL Small-Body Database]] |publisher=[[Organization:Jet Propulsion Laboratory|Jet Propulsion Laboratory]] |access-date=19 March 2021 |archive-url=https://web.archive.org/web/20210121065013/https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=1I |archive-date=21 January 2021}}<br />
JPL 1 (Solution date: 2017-Oct-24)<br />
JPL 10 (Solution date: 2017-Nov-03)<br />
JPL 14 (Solution date: 2017-Nov-21)<br />
JPL 16 (Solution date: 2018-Jun-26</ref>

<ref name="Pukui-Elbert">{{cite web |url=http://wehewehe.org/gsdl2.85/cgi-bin/hdict?e=q-11000-00---off-0hdict--00-1----0-10-0---0---0direct-10-ED--4-------0-1lp0--11-en-Zz-1---Zz-1-home-%CA%BBoumuamua--00-3-1-00-0--4----0-0-11-00-0utfZz-8-00&a=d&d=D15888 |title=Hawaiian Dictionary |last1=Pukui |first1=M.K. |last2=Elbert |first2=S.H. |website=Ulukau: Hawaiian Electronic Library |publisher=University of Hawai{{okina}}i Press |date=2003 |access-date=21 November 2017 |archive-date=1 February 2021 |archive-url=https://web.archive.org/web/20210201045614/http://wehewehe.org/gsdl2.85/cgi-bin/hdict?e=q-11000-00---off-0hdict--00-1----0-10-0---0---0direct-10-ED--4-------0-1lp0--11-en-Zz-1---Zz-1-home-%CA%BBoumuamua--00-3-1-00-0--4----0-0-11-00-0utfZz-8-00&a=d&d=D15888 |url-status=live }}</ref>

<ref name="MPC-object">{{cite web |title=1I/{{okina}}Oumuamua = A/2017 U1 Orbit |website=Minor Planet Center |publisher=International Astronomical Union |url=https://www.minorplanetcenter.net/db_search/show_object?object_id=1I |access-date=9 November 2017 |archive-date=4 January 2018 |archive-url=https://web.archive.org/web/20180104073303/https://www.minorplanetcenter.net/db_search/show_object?object_id=1I |url-status=live }}</ref>

<ref name="skyandtelescope">{{cite web |title=Astronomers Spot First-Known Interstellar Comet |url=http://www.skyandtelescope.com/astronomy-news/astronomers-spot-first-known-interstellar-comet/ |publisher=Sky & Telescope |last=Beatty |first=Kelly |date=25 October 2017 |access-date=25 October 2017 |archive-date=26 October 2017 |archive-url=https://wayback.archive-it.org/all/20171026001610/http://www.skyandtelescope.com/astronomy-news/astronomers-spot-first-known-interstellar-comet/ |url-status=live }}</ref>

<ref name="A2017U1">{{cite web |title=MPEC 2017-U183: A/2017 U1 |url=http://www.minorplanetcenter.net/mpec/K17/K17UI3.html |website=Minor Planet Center |publisher=International Astronomical Union |date=25 October 2017 |access-date=25 October 2017 |archive-date=26 October 2017 |archive-url=https://web.archive.org/web/20171026054257/http://www.minorplanetcenter.net/mpec/K17/K17UI3.html |url-status=live }} (AK17U010)</ref>

<ref name="precoveryobs">{{cite web |title=MPEC 2017-U185: A/2017 U1 |url=http://www.minorplanetcenter.net/mpec/K17/K17UI5.html |website=Minor Planet Center |publisher=International Astronomical Union |date=26 October 2017 |access-date=26 October 2017 |archive-date=1 November 2017 |archive-url=https://web.archive.org/web/20171101170918/http://www.minorplanetcenter.net/mpec/K17/K17UI5.html |url-status=live }}</ref>

<ref name="JPL-ecc">{{cite web
 | title=JPL Small-Body Database Search Engine – Constraints: {{nowrap|e > 1}}
 | website=[[Astronomy:JPL Small-Body Database|JPL Small-Body Database]]
 | publisher=[[Organization:Jet Propulsion Laboratory|Jet Propulsion Laboratory]]
 | url=http://ssd.jpl.nasa.gov/sbdb_query.cgi?obj_group=all;obj_kind=all;obj_numbered=all;OBJ_field=0;ORB_field=0;c1_group=ORB;c1_item=Bg;c1_op=%3E;c1_value=1;table_format=HTML;max_rows=50;format_option=comp;c_fields=AcBhBgBjBiCk;.cgifields=format_option;.cgifields=ast_orbit_class;.cgifields=table_format;.cgifields=obj_kind;.cgifields=obj_group;.cgifields=obj_numbered;.cgifields=com_orbit_class&query=1&c_sort=BgD
 | access-date=26 October 2017
 | archive-date=9 December 2019
 | archive-url=https://web.archive.org/web/20191209165519/http://ssd.jpl.nasa.gov/sbdb_query.cgi?obj_group=all;obj_kind=all;obj_numbered=all;OBJ_field=0;ORB_field=0;c1_group=ORB;c1_item=Bg;c1_op=%3E;c1_value=1;table_format=HTML;max_rows=50;format_option=comp;c_fields=AcBhBgBjBiCk;.cgifields=format_option;.cgifields=ast_orbit_class;.cgifields=table_format;.cgifields=obj_kind;.cgifields=obj_group;.cgifields=obj_numbered;.cgifields=com_orbit_class&query=1&c_sort=BgD
 | url-status=live
 }}</ref>

<ref name="NASA171026">{{cite web |title=Small Asteroid or Comet 'Visits' from Beyond the Solar System |url=https://www.nasa.gov/feature/jpl/small-asteroid-or-comet-visits-from-beyond-the-solar-system |publisher=[[Organization:NASA|NASA]] |date=26 October 2017 |access-date=29 October 2017 |archive-date=2 December 2017 |archive-url=https://web.archive.org/web/20171202092239/https://www.nasa.gov/feature/jpl/small-asteroid-or-comet-visits-from-beyond-the-solar-system/ |url-status=live }}</ref>

<ref name="NASA-FAQ">{{cite web |title=Interstellar Asteroid FAQs |url=https://www.nasa.gov/planetarydefense/faq/interstellar |publisher=[[Organization:NASA|NASA]] |date=20 November 2017 |access-date=21 November 2017 |archive-date=18 December 2019 |archive-url=https://web.archive.org/web/20191218124241/https://www.nasa.gov/planetarydefense/faq/interstellar/ |url-status=live }}</ref>

<ref name="Mamajek2017">{{Cite arXiv |last=Mamajek |first=Eric |year=2017 |title=Kinematics of the Interstellar Vagabond A/2017 U1 |eprint=1710.11364 |class=astro-ph.EP}}</ref>

<ref name="PortegiesZwartetal2017">{{Cite journal |last1=Portegies Zwart |first1=S. |first2=I. |last2=Pelupessy |first3=J. |last3=Bedorf |first4=M. |last4=Cai |first5=S. |last5=Torres |s2cid=56249057 |title=The origin of interstellar asteroidal objects like 1I/2017 U1 |journal=Monthly Notices of the Royal Astronomical Society: Letters |volume=479 |issue=1 |pages=L17–L22 |date=9 November 2017 |arxiv=1711.03558 |doi=10.1093/mnrasl/sly088 |bibcode=2018MNRAS.479L..17P}}</ref>

<ref name="Gaidos2017">{{cite journal |last1=Gaidos |first1=E. |first2=J.P. |last2=Williams |first3=A. |last3=Kraus |year=2017 |title=Origin of Interstellar Object A/2017 U1 in a Nearby Young Stellar Association? |journal=[[Astronomy:Research Notes of the AAS|Research Notes of the AAS]] |volume=1 |issue=1 |page=13 |arxiv=1711.01300 |bibcode=2017RNAAS...1...13G |doi=10.3847/2515-5172/aa9851|s2cid=119091790 |doi-access=free }}</ref>

<ref name="Hein2017">{{cite arXiv |first1=A.M. |last1=Hein |first2=N. |last2=Perakis |first3=K.F. |last3=Long |first4=A. |last4=Crowl |first5=M. |last5=Eubanks |first6=R.G. III |last6=Kennedy |first7=R. |last7=Osborne |year=2017 |title=Project Lyra: Sending a Spacecraft to 1I/{{okina}}Oumuamua (former A/2017 U1), the Interstellar Asteroid |eprint=1711.03155 |class=physics.space-ph}}</ref>

<ref name="MPEC2017-V17">{{cite web |title=MPEC 2017-V17: New Designation Scheme for Interstellar Objects |url=http://www.minorplanetcenter.net/mpec/K17/K17V17.html |website=Minor Planet Center |publisher=International Astronomical Union |date=6 November 2017 |access-date=6 November 2017 |archive-date=8 January 2020 |archive-url=https://web.archive.org/web/20200108040032/https://www.minorplanetcenter.net/mpec/K17/K17V17.html |url-status=live }}</ref>

<ref name="Bolin2017">{{Cite journal |last1=Bolin |first1=B.T. |first2=H.A. |last2=Weaver |first3=Y.R. |last3=Fernandez |first4=C.M. |last4=Lisse |first5=D. |last5=Huppenkothen |first6=R.L. |last6=Jones |first7=M. |last7=Juric |first8=J. |last8=Moeyens |first9=C.A. |last9=Schambeau |first10=C.T. |last10=Slater |first11=Z. |last11=Ivezic |first12=A.J. |last12=Connolly |s2cid=118894742 |display-authors=1 |year=2017 |title=APO Time Resolved Color Photometry of Highly-Elongated Interstellar Object 1I/{{okina}}Oumuamua |journal=The Astrophysical Journal |volume=852 |issue=1 |pages=L2 |arxiv=1711.04927 |doi=10.3847/2041-8213/aaa0c9 |bibcode=2018ApJ...852L...2B |doi-access=free }}</ref>

<ref name="Jewitt2017">{{Cite journal |last1=Jewitt |first1=D. |first2=J. |last2=Luu |first3=J. |last3=Rajagopal |first4=R. |last4=Kotulla |first5=S. |last5=Ridgway |first6=W. |last6=Liu |first7=T. |last7=Augusteijn |s2cid=32684355 |title=Interstellar Interloper 1I/2017 U1: Observations from the NOT and WIYN Telescopes<!-- |url=http://www2.ess.ucla.edu/~jewitt/papers/2017/JLR17.pdf --> |journal=The Astrophysical Journal Letters |volume=850 |issue=2 |pages=L36 |arxiv=1711.05687 |date=30 November 2017 |doi=10.3847/2041-8213/aa9b2f |bibcode=2017ApJ...850L..36J |doi-access=free }}</ref>

<ref name="JewittSeligman2022">{{Cite arXiv |last1=Jewitt |first1=D. |first2=D. |last2=Seligman |title=Interstellar Interlopers |eprint=2209.08182 |date=September 2022 |class=astro-ph.EP }}</ref>

<ref name="NOAO2017">{{cite press release |title=A Familiar-Looking Messenger from Another Solar System |publisher=National Optical Astronomy Observatory |url=https://www.noao.edu/news/2017/pr1706.php |date=15 November 2017 |id=NOAO 17-06 |access-date=15 November 2017 |archive-date=16 November 2017 |archive-url=https://web.archive.org/web/20171116133909/https://www.noao.edu/news/2017/pr1706.php |url-status=live }}</ref>

<ref name="Ye2017">{{Cite journal |last1=Ye |first1=Q.-Z. |last2=Zhang |first2=Q. |s2cid=119392232 |date=5 December 2017 |title=1I/{{okina}}Oumuamua is Hot: Imaging, Spectroscopy and Search of Meteor Activity |journal=The Astrophysical Journal Letters |volume=851 |issue=1 |pages=L5 |arxiv=1711.02320 |doi=10.3847/2041-8213/aa9a34 |bibcode=2017ApJ...851L...5Y |url=https://authors.library.caltech.edu/83727/1/Ye_2017_ApJL_851_L5.pdf |access-date=3 November 2018 |archive-date=23 July 2018 |archive-url=https://web.archive.org/web/20180723033917/https://authors.library.caltech.edu/83727/1/Ye_2017_ApJL_851_L5.pdf |url-status=live |doi-access=free }}</ref>

<ref name="Bannister2017">{{Cite journal |last1=Bannister |first1=M.T. |last2=Schwamb |first2=M.E. |s2cid=56264680 |year=2017 |title=Col-OSSOS: Colors of the Interstellar Planetesimal 1I/2017 U1 in Context with the Solar System |journal=The Astrophysical Journal |volume=851 |issue=2 |pages=L38 |arxiv=1711.06214 |quote=As its albedo is unknown, we do not describe 1I/{{okina}}Oumuamua as consistent with Tholen (1984) P type. |doi=10.3847/2041-8213/aaa07c |bibcode=2017ApJ...851L..38B |doi-access=free }}</ref>

<ref name=ESO2017>{{cite web |website=ESO |first=Karen |last=Meech |display-authors=et al |title=Light curve of interstellar asteroid {{okina}}Oumuamua |date=20 November 2017 |url=https://www.eso.org/public/images/eso1737f/ |publisher=European Southern Observatory |access-date=21 November 2017 |archive-date=19 December 2019 |archive-url=https://web.archive.org/web/20191219152200/https://www.eso.org/public/images/eso1737f/ |url-status=live }}</ref>

<ref name="orientation">{{cite journal |last1=de la Fuente Marcos |first1=C. |last2=de la Fuente Marcos |first2=R.úl |date=1 November 2017 |title=Pole, Pericenter, and Nodes of the Interstellar Minor Body A/2017 U1 |journal=[[Astronomy:Research Notes of the AAS|Research Notes of the AAS]] |volume=1 |issue=1 |page=5 |arxiv=1711.00445 |bibcode=2017RNAAS...1....5D |doi=10.3847/2515-5172/aa96b4|s2cid=119537175 |doi-access=free }}</ref>

<!--The following would be a useful reference for additional content-->
<ref name="HubbleProposal">{{cite web |title=Proposal 15405 – Which way home? Finding the origin of our Solar System's first interstellar visitor |first=Karen |last=Meech |display-authors=et al |website=STScI – Space Telescope Science Institute |url=http://www.stsci.edu/hst/phase2-public/15405.pdf |date=8 November 2017 |access-date=15 November 2017 }}</ref>

<ref name="NAT-20171120">{{Cite journal |display-authors=1 |last1=Meech |first1=K.J. |last2=Weryk |first2=R. |last3=Micheli |first3=M. |last4=Kleyna |first4=J.T. |last5=Hainaut |first5=O.R. |last6=Jedicke |first6=R. |last7=Wainscoat |first7=R.J. |last8=Chambers |first8=K.C. |last9=Keane |first9=J.V. |first10=A. |last10=Petric |first11=L. |last11=Denneau |first12=E. |last12=Magnier |first13=T. |last13=Berger |first14=M.E. |last14=Huber |first15=H. |last15=Flewelling |first16=C. |last16=Waters |first17=E. |last17=Schunova-Lilly |first18=S. |last18=Chastel |s2cid=4393243 |date=20 November 2017 |title=A brief visit from a red and extremely elongated interstellar asteroid |journal=Nature |volume=552 |issue=7685 |pages=378–381 |doi=10.1038/nature25020 |pmid=29160305 |pmc=8979573 |bibcode=2017Natur.552..378M}}</ref>

<ref name=Guarddian2017-12-11>{{cite news |url=https://www.theguardian.com/science/2017/dec/11/astronomers-to-check-interstellar-body-for-signs-of-alien-technology |title=Astronomers to check interstellar body for signs of alien technology |author=Ian Sample |journal=The Guardian |date=11 December 2017 |quote=Green Bank telescope in West Virginia will listen for radio signals from {{okina}}Oumuamua, an object from another solar system ... "Most likely it is of natural origin, but because it is so peculiar, we would like to check if it has any sign of artificial origin, such as radio emissions," said Avi Loeb, professor of astronomy at Harvard University and an adviser to the Breakthrough Listen project. "If we do detect a signal that appears artificial in origin, we'll know immediately." ... While many astronomers believe the object is an interstellar asteroid, its elongated shape is unlike anything seen in the asteroid belt in our own solar system. Early observations of {{okina}}Oumuamua show that it is about 400m long but only one tenth as wide. "It's curious that the first object we see from outside the solar system looks like that," said Loeb. |access-date=12 December 2017 |archive-date=25 April 2020 |archive-url=https://web.archive.org/web/20200425025003/https://www.theguardian.com/science/2017/dec/11/astronomers-to-check-interstellar-body-for-signs-of-alien-technology |url-status=live }}</ref>

<ref name="WHT">{{Cite tweet |number=923928561722982400 |user=FitzsimmonsAlan |title=Spectrum of A/2017 U1 obtained on Wednesday night with the @INGLaPalma 4.2m WHT. Colour is red like Kuiper Belt Objects, featureless. |date=27 October 2017 |first=Alan |last=Fitzsimmons |language=en-GB}}</ref>

<ref name="Koren-20171211">{{cite web |url=https://www.theatlantic.com/science/archive/2017/12/yuri-milner-oumuamua-interstellar-asteroid/547985/ |title=Astronomers to Check Mysterious Interstellar Object for Signs of Technology |publisher=The Atlantic |first=Marina |last=Koren |date=11 December 2017 |access-date=11 December 2017 |archive-date=11 December 2017 |archive-url=https://web.archive.org/web/20171211201521/https://www.theatlantic.com/science/archive/2017/12/yuri-milner-oumuamua-interstellar-asteroid/547985/ |url-status=live }}</ref>

<ref name=SETI>{{cite magazine |url=https://www.scientificamerican.com/article/alien-probe-or-galactic-driftwood-seti-tunes-in-to-oumuamua/ |title=Alien Probe or Galactic Driftwood? SETI Tunes In to {{okina}}Oumuamua |magazine=Scientific American |first=Lee |last=Billings |date=11 December 2017 |access-date=12 December 2017 |quote=So far limited observations of {{okina}}Oumuamua, using facilities such as the SETI Institute's Allen Telescope Array, have turned up nothing. |archive-date=14 December 2017 |archive-url=https://web.archive.org/web/20171214003318/https://www.scientificamerican.com/article/alien-probe-or-galactic-driftwood-seti-tunes-in-to-oumuamua/ |url-status=live }}</ref>

<ref name="eso1820June2018Oumuamua">{{cite web |url=http://www.eso.org/public/news/eso1820/ |title=ESO's VLT Sees {{okina}}Oumuamua Getting a Boost – New results indicate interstellar nomad {{okina}}Oumuamua is a comet<!--Deny Citation Bot--> |date=27 June 2018 |website=www.eso.org |language=en-GB |quote=Such outgassing is a behaviour typical for comets and contradicts the previous classification of {{okina}}Oumuamua as an interstellar asteroid. "We think this is a tiny, weird comet," commented Marco Micheli. "We can see in the data that its boost is getting smaller the farther away it travels from the Sun, which is typical for comets." |access-date=28 June 2018 |archive-date=3 July 2018 |archive-url=https://web.archive.org/web/20180703210038/http://eso.org/public/news/eso1820/ |url-status=live }}</ref>

<ref name="Nature 27-6-2018">{{cite journal |title=Non-gravitational acceleration in the trajectory of 1I/2017 U1 ({{okina}}Oumuamua) |journal=Nature |volume=559 |issue=7713 |pages=223–226 |year=2018 |doi=10.1038/s41586-018-0254-4 |pmid=29950718 |last1=Micheli |first1=M. |last2=Farnocchia |first2=D. |last3=Meech |first3=K.J. |last4=Buie |first4=M.W. |last5=Hainaut |first5=O.R. |last6=Prialnik |first6=D. |last7=Schörghofer |first7=N. |last8=Weaver |first8=H.A. |last9=Chodas |first9=P.W. |last10=Kleyna |first10=J.T. |last11=Weryk |first11=R. |last12=Wainscoat |first12=R.J. |last13=Ebeling |first13=H. |last14=Keane |first14=J.V. |last15=Chambers |first15=K.C. |last16=Koschny |first16=D. |last17=Petropoulos |first17=A.E. |s2cid=49477508 |display-authors=1 |bibcode=2018Natur.559..223M}}</ref>

<ref name="NAT-20180627">{{cite journal |last=Witze |first=Alexandra |title=Mysterious interstellar visitor is a comet – not an asteroid – Quirks in {{okina}}Oumuamua's path through the Solar System helped researchers solve a case of mistaken identity |url=https://www.nature.com/articles/d41586-018-05552-9 |date=27 June 2018 |journal=Nature |doi=10.1038/d41586-018-05552-9 |s2cid=126317359 |access-date=27 June 2018 |archive-date=27 June 2018 |archive-url=https://web.archive.org/web/20180627181835/https://www.nature.com/articles/d41586-018-05552-9 |url-status=live }}</ref>

<ref name="NASA-20180627nasa">{{cite web |last1=Cofield |first1=Calla |last2=Chou |first2=Felicia |last3=Wendel |first3=JoAnna |last4=Weaver |first4=Donna |last5=Villard |first5=Ray |title=Our Solar System's First Known Interstellar Object Gets Unexpected Speed Boost |url=https://www.jpl.nasa.gov/news/news.php?feature=7173 |date=27 June 2018 |website=[[Organization:NASA|NASA]] |access-date=27 June 2018 |archive-date=27 June 2018 |archive-url=https://web.archive.org/web/20180627194643/https://www.jpl.nasa.gov/news/news.php?feature=7173 |url-status=live }}</ref>

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<ref name="Laughlin2017">{{cite arXiv |last1=Laughlin |first1=G. |last2=Batygin |first2=K. |title=On the Consequences of the Detection of an Interstellar Asteroid |year=2017 |eprint=1711.02260}} http://oklo.org/wp-content/papers/interstellar_asteroids.pdf</ref>

<ref name="Schneider2017">
{{cite arXiv |last=Schneider |first=Jean |title=Is 1I/2017 U1 really of interstellar origin ? |date=15 November 2017 |eprint=1711.05735 |class=astro-ph.EP}}</ref>

<ref name="Knight2017">
{{cite arXiv |last=Knight |first=M. |first2=S. |last2=Protopapa |first3=M.S.P. |last3=Kelley |first4=T.L. |last4=Farnham |first5=J.M. |last5=Bauer |first6=D. |last6=Bodewits |first7=L.M. |last7=Feaga |first8=J.M. |last8=Sunshine |title=The rotation period and shape of the hyperbolic asteroid A/2017 U1 from its lightcurve |year=2017 |eprint=1711.01402 |class=astro-ph.EP}}</ref>

<ref name="SizeEstimator">{{cite web |title=Asteroid Size Estimator |publisher=CNEOS – NASA/JPL |url=https://cneos.jpl.nasa.gov/tools/ast_size_est.html |access-date=9 November 2017}}</ref>

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}} <!-- end of reflist -->

== External links ==
* {{cite web |url=https://solarsystem.nasa.gov/small-bodies/asteroids/oumuamua/in-depth/ |title=Oumuamua |publisher=NASA.gov}}
* Where is 'Oumuamua in the sky: [https://theskylive.com/oumuamua-info The Sky Live]
* Talk about [https://www.facebook.com/SETIInstitute/videos/10155677978165535/ A/2017 U1] from 31 October 2017. [[Organization:SETI Institute|SETI Institute]] at Facebook Live.
* {{YouTube|id=k7y65DyrsbY|title=Interstellar Asteroid A/2017 U1 ''(update 7 November 2017)''}} {{small|(time 3:31 min.)}}
* {{cite web |url=http://ssc.spitzer.caltech.edu/warmmission/scheduling/approvedprograms/ddt/13249.txt |title=Spitzer DDT observations of the interstellar comet A/2017 U1 |publisher=Caltech.edu}} Proposal #13249
* {{cite web |url=http://exoplanet.eu/catalog/1i/2017_u1/ |title=Planet 1I/2017 U1 |access-date=8 November 2017 |archive-date=8 November 2017 |work=[[Astronomy:Extrasolar Planets Encyclopaedia|Extrasolar Planets Encyclopaedia]] |archive-url=https://web.archive.org/web/20171108035255/http://exoplanet.eu/catalog/1i/2017_u1/  }}
* {{JPL small body}}
* [https://www.nytimes.com/2021/03/23/science/astronomy-oumuamua-comet.html "A piece of an extrasolar Pluto may have passed through our cosmic neighborhood, a new study suggests."], article by Dennis Overbye, New York Times, Updated March 29, 2021
* {{cite news |url=https://www.nytimes.com/video/science/100000005592208/glimpse-of-oumuamua.html |medium=Video – 2:53 |title=A Glimpse of {{okina}}Oumuamua |others=Narrated by Dennis Overbye; Produced by Jonathan Corum and Jason Drakeford |work={{wipe|The New York Times}} |date=12 December 2017}}
* [https://wyldblood.com/the-listener-between-worlds/ Science fiction story about Oumuamua by astronomer Andrew Fraknoi]
* [https://gravitysimulator.org/solar-system/oumuamua Interactive 3D gravity simulation of Oumuamua's Solar System flyby] {{Webarchive|url=https://web.archive.org/web/20200903042335/https://gravitysimulator.org/solar-system/oumuamua/ |date=3 September 2020 }}

{{2017 in space}}

{{DEFAULTSORT:Oumuamua}}
[[Category:Hyperbolic asteroids]]
[[Category:Milky Way]]

{{Sourceattribution|ʻOumuamua}}