View source for Interstellar object

{{short description|Astronomical object not gravitationally bound to a star}}
{{Not to be confused with|Extrasolar object}}
{{multiple image
| direction = vertical
| header    = Interstellar objects passing through the Solar System
| image1    = First Known Interstellar Visitor is an “Oddball” (gemini1710b).tiff
| alt1      = ʻOumuamua in 2017
| caption1  = [[ʻOumuamua]]
| image2    = Comet_2I_Borisov_at_Perihelion_in_December_2019.tif
| alt2      = Comet 2I/Borisov at perihelion in December 2019
| caption2  = [[2I/Borisov]]
}}

An '''interstellar object''' is an [[astronomical object]] in [[interstellar space]] that is not [[gravitationally bound]] to a [[star]]. Applicable objects include [[asteroid]]s, [[comet]]s, and [[rogue planet]]s, but not a star or [[stellar remnant]]. 

This term can also be applied to an object that is on an interstellar trajectory but is temporarily passing close to a star, such as certain [[asteroid]]s and comets (that is, [[exoasteroid]]s and [[exocomet]]s<ref name="Valtonen" /><ref name="Francis" />). In the latter case, the object may be called an '''interstellar interloper'''.<ref>{{cite journal|first1=Dimitri|last1=Veras|title=Creating the first interstellar interloper|journal=Nature Astronomy|date=13 April 2020|volume=4|issue=9|issn=2397-3366|pages=835–836|doi=10.1038/s41550-020-1064-9|bibcode=2020NatAs...4..835V|doi-access=free}}</ref>

The first interstellar objects discovered were [[rogue planet]]s, planets ejected from their original stellar system (e.g., [[OTS 44]] or [[Cha 110913−773444]]), though they are difficult to distinguish from [[sub-brown dwarf]]s, planet-mass objects that formed in interstellar space as stars do.

The first interstellar object which was discovered traveling through the [[Solar System]] was [[ʻOumuamua|1I/ʻOumuamua]] in 2017. The second was [[2I/Borisov]] in 2019. They both possess significant [[Hyperbolic trajectory#Semi-major axis, energy and hyperbolic excess velocity|hyperbolic excess velocity]], indicating they did not originate in the Solar System. The discovery of ʻOumuamua inspired the tentative identification of [[CNEOS 2014-01-08]], also known as the Manus Island fireball, as an interstellar object that impacted the Earth by astronomers [[Amir Siraj]] and [[Avi Loeb]] in 2019.<ref name="ARX-20190604" /> This was supported by the [[United States Space Command|U.S. Space Command]] in 2022 based on the object's velocity relative to the Sun,<ref name="TWR-20220406"/><ref name="VICE-20220407" /><ref name="INV-20220411" /><ref name="NASA-20220408" /><ref name="SA-20220412" /><ref name="NYT-20220415" /> In May 2023, astronomers reported the possible capture of other interstellar objects in Near Earth Orbit (NEO) over the years.<ref name="UT-20230517" /><ref name="ARX-20230517" /> however, [[NASA]] and Other astronomers doubt this,<ref name=Vaubaillon2022>{{cite journal |last1=Vaubaillon |first1=J. |date=October 2022 |title=Hyperbolic meteors: is CNEOS 2014-01-08 interstellar? |journal=WGN, Journal of the International Meteor Organization |volume=50 |issue=5 |pages=140–143 |arxiv=2211.02305 |bibcode=2022JIMO...50..140V}}</ref><ref name=Brown2023>{{cite journal
  |last1=Brown |first1=Peter G.
  |last2=Borovička |first2=Jiří 
  |date=August 2023
  |title=On the Proposed Interstellar Origin of the USG 20140108 Fireball
  |journal=[[The Astrophysical Journal]]
  |volume=953
  |issue=2
  |page=167
  |doi=10.3847/1538-4357/ace421
  |arxiv=2306.14267 |doi-access=free
  |bibcode=2023ApJ...953..167B
}}</ref><ref name=Gallardo2023>{{cite journal |last1=Gallardo |first1=Patricio A. |date=October 2023 |title=Anthropogenic Coal Ash as a Contaminant in a Micro-meteoritic Underwater Search |journal=[[Research Notes of the AAS]] |volume=7 |issue=10 |pages=220 |doi=10.3847/2515-5172/ad03f9 |doi-access=free |bibcode=2023RNAAS...7..220G}}</ref><ref name=Desch2023>{{cite arXiv |last1=Desch |first1=Steve |last2=Jackson |first2=Alan |date=November 2023 |title=Critique of arXiv submission 2308.15623, "Discovery of Spherules of Likely Extrasolar Composition in the Pacific Ocean Site of the CNEOS 2014-01-08 (IM1) Bolide", by A. Loeb et al |eprint=2311.07699 |class=astro-ph.EP}}</ref><ref name=Fernando2024>{{cite arXiv |last1=Fernando |first1=Benjamin |last2=Mialle |first2=Pierrick |display-authors=etal |date=March 2024 |title=Seismic and acoustic signals from the 2014 'Interstellar Meteor' |eprint=2403.03966 |class=astro-ph.EP}}</ref><ref name=Desch2024>{{cite arXiv |last1=Desch |first1=Steve |date=March 2024 |title=Be,La,U-rich spherules as microtektites of terrestrial laterites: What goes up must come down |eprint=2403.05161 |class=astro-ph.EP}}</ref> and still other experts found Earth-related explanations for the purported meteorite impact instead.<ref name="NYT-20240311">{{cite news |last=Richtel |first=Matt |title=Surprise: An ‘Extraterrestrial’ Gadget Was Something More Familiar - In 2014 a fireball from outer space was posited to be an alien artifact. A recent study suggests otherwise. |url=https://www.nytimes.com/2024/03/11/science/meteor-avi-loeb.html |date=11 March 2024 |work=[[The New York Times]] |url-status=live |archive-url=https://archive.today/20240311121317/https://www.nytimes.com/2024/03/11/science/meteor-avi-loeb.html |archive-date=11 March 2024 |access-date=11 March 2024}}</ref>

The interstellar objects were once bound to a host star  and have become unbound since. Different processes can cause planets and smaller objects (planetesimals) to become unbound from their host star.<ref>{{Cite journal |last=The ‘Oumuamua ISSI Team |last2=Bannister |first2=Michele T. |last3=Bhandare |first3=Asmita |last4=Dybczyński |first4=Piotr A. |last5=Fitzsimmons |first5=Alan |last6=Guilbert-Lepoutre |first6=Aurélie |last7=Jedicke |first7=Robert |last8=Knight |first8=Matthew M. |last9=Meech |first9=Karen J. |last10=McNeill |first10=Andrew |last11=Pfalzner |first11=Susanne |last12=Raymond |first12=Sean N. |last13=Snodgrass |first13=Colin |last14=Trilling |first14=David E. |last15=Ye |first15=Quanzhi |date=2019-07-01 |title=The natural history of ‘Oumuamua |url=https://www.nature.com/articles/s41550-019-0816-x |journal=Nature Astronomy |language=en |volume=3 |issue=7 |pages=594–602 |doi=10.1038/s41550-019-0816-x |issn=2397-3366|arxiv=1907.01910 }}</ref>

==Nomenclature==
With the first discovery of an interstellar object in the Solar System, the IAU has proposed a new series of small-body designations for interstellar objects, the ''I numbers,'' similar to the comet numbering system. The [[Minor Planet Center]] will assign the numbers. Provisional designations for interstellar objects will be handled using the C/ or A/ prefix (comet or asteroid), as appropriate.<ref name="NewDesignation" />

==Overview==
{| class="wikitable floatright" style="text-align:center"; 
|+Interstellar velocity inbound {{nowrap|(<math>v_\infty</math>)}}
|-
! Object !! Velocity
|-
| [[Comet ISON|C/2012 S1 (ISON)]]<br />(weakly hyperbolic<br />Oort Cloud comet) || {{cvt|0.2|km/s|au/years|2|disp=br}}<ref name="Note-1" />
|-
| [[Voyager 1]]<br />(For comparison) || {{cvt|16.9|km/s|au/years|2|disp=br}}<ref name="Voyager" />
|-
| [[ʻOumuamua|1I/2017 U1 (ʻOumuamua)]] || {{cvt|26.33|km/s|au/years|2|disp=br}}<ref name="pseudoMPEC" />
|-
| [[2I/Borisov]] || {{cvt|32.1|km/s|au/years|2|disp=br}}<ref name="Gray-FAQ" />
|-
| [[CNEOS 2014-01-08|2014Jan08 bolide]]<br />(in [[peer review]],<br /> no widespread consensus) || {{cvt|43.8|km/s|au/years|2|disp=br}}<ref name="Siraj2019b" />
|}

Astronomers estimate that several interstellar objects of extrasolar origin (like ʻOumuamua) pass inside the [[orbit of Earth]] each year,<ref name="NASA-FAQ">{{cite web |title=Interstellar Asteroid FAQs |url=https://www.nasa.gov/planetarydefense/faq/interstellar |publisher=[[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=dead }}</ref> and that 10,000 are passing inside the [[orbit of Neptune]] on any given day.<ref>{{Cite interview |last=Fraser |first=Wesley |url=https://www.bbc.co.uk/iplayer/episode/b09rvpts/the-sky-at-night-the-mystery-of-oumuamua |title=The Sky at Night: The Mystery of ʻOumuamua |date=11 February 2018 |publisher=BBC |interviewer=[[Chris Lintott]]}}</ref>

Interstellar comets occasionally pass through the inner [[Solar System]]<ref name="Valtonen" /> and approach with random velocities, mostly from the direction of the constellation [[Hercules (constellation)|Hercules]] because the Solar System is moving in that direction, called the [[solar apex]].<ref name="Struve" /> Until the discovery of [['Oumuamua]], the fact that no comet with a speed greater than the [[Sun]]'s escape velocity<ref name="MacRobert2008" /> had been observed was used to place upper limits to their density in interstellar space. A paper by Torbett indicated that the density was no more than 10<sup>13</sup> (10 [[Orders of magnitude (numbers)#1012|trillion]]) comets per cubic [[parsec]].<ref name="Torbett" /> Other analyses, of data from [[LINEAR]], set the upper limit at 4.5{{e|−4}}/[[Astronomical unit|AU]]<sup>3</sup>, or 10<sup>12</sup> (1&nbsp;trillion) comets per cubic [[parsec]].<ref name="Francis" /> A more recent estimate by [[David C. Jewitt]] and colleagues, following the detection of [['Oumuamua]], predicts that "The steady-state population of similar, ~100 m scale interstellar objects inside the orbit of Neptune is ~1{{e|4}}, each with a residence time of ~10 years."<ref name="Jewitt2017" />

Current models of [[Oort cloud]] formation predict that more comets are ejected into interstellar space than are retained in the Oort cloud, with estimates varying from 3 to 100 times as many.<ref name="Francis" /> Other simulations suggest that 90–99% of comets are ejected.<ref name="Choi" /> There is no reason to believe comets formed in other star systems would not be similarly scattered.<ref name="Valtonen" /> [[Amir Siraj]] and [[Avi Loeb]] demonstrated that the Oort Cloud could have been formed from ejected planetesimals from other stars in the Sun's birth cluster.<ref>{{Cite journal|last1=Siraj|first1=Amir|last2=Loeb|first2=Abraham|date=2020-08-18|title=The Case for an Early Solar Binary Companion|journal=The Astrophysical Journal|language=en|volume=899|issue=2|pages=L24|doi=10.3847/2041-8213/abac66|issn=2041-8213|arxiv=2007.10339|bibcode=2020ApJ...899L..24S|s2cid=220665422 |doi-access=free }}</ref><ref>{{Cite web|last=Carter|first=Jamie|title=Was Our Sun A Twin? If So Then 'Planet 9' Could Be One Of Many Hidden Planets In Our Solar System|url=https://www.forbes.com/sites/jamiecartereurope/2020/08/18/was-our-sun-a-twin-if-so-then-planet-9-could-be-one-of-many-hidden-planets-in-our-solar-system/|access-date=2020-11-14|website=Forbes|language=en}}</ref><ref>{{Cite web|date=2020-08-20|title=Did the Sun have an early binary companion?|url=https://cosmosmagazine.com/space/astrophysics/did-the-sun-have-an-early-binary-companion/|access-date=2020-11-14|website=Cosmos Magazine|language=en-AU|archive-date=2020-11-16|archive-url=https://web.archive.org/web/20201116034154/https://cosmosmagazine.com/space/astrophysics/did-the-sun-have-an-early-binary-companion/}}</ref> Both researchers proposed a search for ʻOumuamua-like objects which are trapped in the Solar System as a result of losing orbital energy through a close encounter with Jupiter.<ref name="ARX-20181109632">{{cite journal |last1=Siraj |first1=Amir |last2=Loeb |first2=Abraham |date=2019 |title=Identifying Interstellar Objects Trapped in the Solar System through Their Orbital Paramteters |journal=The Astrophysical Journal |volume=872 |issue=1 |pages=L10 |arxiv=1811.09632 |bibcode=2019ApJ...872L..10S |doi=10.3847/2041-8213/ab042a |s2cid=119198820 |doi-access=free}}</ref><ref name="TA-20190123">{{cite web |last=Koren |first=Marina |date=23 January 2019 |title=When a Harvard Professor Talks About Aliens – News about extraterrestrial life sounds better coming from an expert at a high-prestige institution. |url=https://www.theatlantic.com/science/archive/2019/01/oumuamua-interstellar-harvard-astrophysicist/580948/ |access-date=23 January 2019 |work=[[The Atlantic]]}}</ref>

It is possible for objects orbiting a star to be ejected due to interaction with a third massive body, thereby becoming interstellar objects.  Such a process was initiated in the early 1980s when [[C/1980 E1]], initially gravitationally bound to the Sun, passed near Jupiter and was accelerated sufficiently to reach escape velocity from the Solar System.  This changed its orbit from elliptical to hyperbolic and made it the most eccentric known object at the time, with an [[Orbital eccentricity|eccentricity]] of 1.057.<ref name="JPL-Data" />  It is heading for interstellar space.[[File:96P 20070403 000500 HI1A.png|thumb|[[Machholz 1|Comet Machholz 1 (96P/Machholz)]] as viewed by [[STEREO|STEREO-A]] (April 2007)]]Due to present observational difficulties, an interstellar object can usually only be detected if it passes through the [[Solar System]], where it can be distinguished by its strongly [[hyperbolic trajectory]] and [[hyperbolic excess velocity]] of more than a few km/s, proving that it is not gravitationally bound to the Sun.<ref name="Francis" /><ref name="de la Fuente Marcos 2018" /> In contrast, gravitationally bound objects follow [[elliptic orbit]]s around the Sun. (There are [[List of hyperbolic comets|a few objects]] whose orbits are so close to parabolic that their gravitationally bound status is unclear.)

An interstellar comet can probably, on rare occasions, be captured into a [[heliocentric]] orbit while passing through the [[Solar System]]. Computer simulations show that [[Jupiter]] is the only planet massive enough to capture one, and that this can be expected to occur once every sixty million years.<ref name="Torbett" /> Comets [[Machholz 1]] and [[Comet Hyakutake|Hyakutake C/1996 B2]] are possible examples of such comets. They have atypical chemical makeups for comets in the Solar System.<ref name="MacRobert2008" /><ref name="Mumma1996" />

Recent research suggests that asteroid [[514107 Kaʻepaokaʻawela]] may be a former interstellar object, captured some 4.5&nbsp;billion years ago, as evidenced by its co-orbital motion with Jupiter and its retrograde orbit around the Sun.<ref name="Clery2018" /> In addition, comet [[C/2018 V1 (Machholz-Fujikawa-Iwamoto)]] has a significant probability (72.6%) of having an extrasolar provenance although an origin in the Oort cloud cannot be excluded.<ref name="de la Fuente Marcos 2019" /> [[Harvard University|Harvard]] astronomers suggest that matter—and potentially dormant [[spore]]s—can be exchanged across vast distances.<ref>{{Cite news |url=https://news.harvard.edu/gazette/story/2019/07/harvard-study-suggests-asteroids-might-play-key-role-in-spreading-life/ |title=Harvard study suggests asteroids might play key role in spreading life |work=Harvard Gazette |last=Reuell |first=Peter |date=8 July 2019 |access-date=29 September 2019}}</ref> The detection of ʻOumuamua crossing the inner Solar System confirms the possibility of a material link with exoplanetary systems.
[[File:Uncertain size-frequency distribution of interstellar visitors.jpg|thumb|Interstellar visitors in the Solar System cover the whole range of sizes – from kilometer large objects down to submicron particles. Also, interstellar dust and meteoroids carry with them valuable information from their parent systems. Detection of these objects along the continuum of sizes is, however, not evident.<ref>{{Cite journal|date=2020-11-01|title=The challenge of identifying interstellar meteors|journal=Planetary and Space Science|language=en|volume=192|page=105060|doi=10.1016/j.pss.2020.105060|issn=0032-0633|doi-access=free|last1=Hajdukova|first1=Maria|last2=Sterken|first2=Veerle|last3=Wiegert|first3=Paul|last4=Kornoš|first4=Leonard|bibcode=2020P&SS..19205060H|hdl=20.500.11850/432235|hdl-access=free}}</ref>]]
Interstellar visitors in the Solar System cover the whole range of sizes – from kilometer large objects down to submicron particles. Also, interstellar dust and meteoroids carry with them valuable information from their parent systems. Detection of these objects along the continuum of sizes is, however, not evident (see Figure).<ref name="sciencedirect.com">{{Cite journal|last1=Hajdukova|first1=M.|last2=Sterken|first2=V.|last3=Wiegert|first3=P.|last4=Kornoš|first4=L.|date=2020-11-01|title=The challenge of identifying interstellar meteors|journal=Planetary and Space Science|language=en|volume=192|page=105060|doi=10.1016/j.pss.2020.105060|bibcode=2020P&SS..19205060H|issn=0032-0633|doi-access=free|hdl=20.500.11850/432235|hdl-access=free}}</ref> The smallest interstellar dust particles are filtered out of the solar system by electromagnetic forces, while the largest ones are too sparse to obtain good statistics from in situ spacecraft detectors. Discrimination between interstellar and interplanetary populations can be a challenge for intermediate (0.1–1 micrometer) sizes. These can vary widely in velocity and directionality.<ref>{{Cite journal|last1=Sterken|first1=V. J.|last2=Altobelli|first2=N.|last3=Kempf|first3=S.|last4=Schwehm|first4=G.|last5=Srama|first5=R.|last6=Grün|first6=E.|date=2012-02-01|title=The flow of interstellar dust into the solar system|url=https://www.aanda.org/articles/aa/abs/2012/02/aa17119-11/aa17119-11.html|journal=Astronomy & Astrophysics|language=en|volume=538|pages=A102|doi=10.1051/0004-6361/201117119|bibcode=2012A&A...538A.102S|issn=0004-6361|doi-access=free}}</ref> The identification of interstellar meteoroids, observed in the Earth's atmosphere as meteors, is highly challenging and requires high accuracy measurements and appropriate error examinations.<ref>{{Cite journal|last1=Hajduková|first1=Mária|last2=Kornoš|first2=Leonard|date=2020-10-01|title=The influence of meteor measurement errors on the heliocentric orbits of meteoroids|url=http://www.sciencedirect.com/science/article/pii/S0032063319304623|journal=Planetary and Space Science|language=en|volume=190|page=104965|doi=10.1016/j.pss.2020.104965|bibcode=2020P&SS..19004965H|s2cid=224927095|issn=0032-0633|url-access=subscription}}</ref> Otherwise, measurement errors can transfer near-parabolic orbits over the parabolic limit and create an artificial population of hyperbolic particles, often interpreted as of interstellar origin.<ref name="sciencedirect.com"/> 
Large interstellar visitors like asteroids and comets were detected the first time in the solar system in 2017 (1I/'Oumuamua) and 2019 (2I/Borisov) and are expected to be detected more frequently with new telescopes, e.g. the Vera Rubin Observatory. Amir Siraj and Avi Loeb have predicted that the [[Vera C. Rubin Observatory]] will be capable of detecting an anisotropy in the distribution of interstellar objects due to the Sun's motion relative to the [[Local standard of rest|Local Standard of Rest]] and identify the characteristic ejection speed of interstellar objects from their parent stars.<ref>{{Cite journal|last1=Siraj|first1=Amir|last2=Loeb|first2=Abraham|date=2020-10-29|title=Observable Signatures of the Ejection Speed of Interstellar Objects from Their Birth Systems|journal=The Astrophysical Journal|language=en|volume=903|issue=1|pages=L20|doi=10.3847/2041-8213/abc170|issn=2041-8213|arxiv=2010.02214|bibcode=2020ApJ...903L..20S|s2cid=222141714 |doi-access=free }}</ref><ref>{{Cite web|last=Williams|first=Matt|date=2020-11-07|title=Vera Rubin Should be Able to Detect a Couple of Interstellar Objects a Month|url=https://www.universetoday.com/148696/vera-rubin-should-be-able-to-detect-a-couple-of-interstellar-objects-a-month/|access-date=2020-11-14|website=Universe Today|language=en-US}}</ref><ref>{{Cite web|last=Clery|first=Daniel|date=2021-07-26|title=Project launched to look for extraterrestrial visitors to our Solar System|url=https://www.science.org/content/article/project-launched-look-extraterrestrial-visitors-our-solar-system|access-date=2021-10-22|website=www.science.org|language=en}}</ref>

In May 2023, astronomers reported the possible capture of other interstellar objects in Near Earth Orbit (NEO) over the years.<ref name="UT-20230517">{{cite news |last=Gough |first=Evan |title=A Few Interstellar Objects Have Probably Been Captured |url=https://www.universetoday.com/161434/a-few-interstellar-objects-have-probably-been-captured/ |date=18 May 2023 |work=[[Universe Today]] |access-date=19 May 2023 }}</ref><ref name="ARX-20230517">{{cite journal |arxiv=2305.08915 |last1=Mukherjee |first1=Diptajyoti |last2=Siraj |first2=Amir |last3=Trac |first3=Hy |last4=Loeb |first4=Abraham |title=Close Encounters of the Interstellar Kind: Exploring the Capture of Interstellar Objects in Near Earth Orbit |journal=Monthly Notices of the Royal Astronomical Society |year=2023 |volume=525 |pages=908–921 |doi=10.1093/mnras/stad2317 |doi-access=free }}</ref>

==Confirmed objects==
===1I/2017 U1 (ʻOumuamua)===
{{main|ʻOumuamua}}
[[File:Oumuamua-solar system 2018.png|thumb|Path of the hyperbolic, extrasolar object [[ʻOumuamua]], the first confirmed interstellar object, discovered in 2017]]
A dim object was discovered on October 19, 2017, by the [[Pan-STARRS]] telescope, at an apparent magnitude of 20. The observations showed that it follows a strongly hyperbolic trajectory around the Sun at a speed greater than the solar escape velocity, in turn meaning that it is not gravitationally bound to the Solar System and likely to be an interstellar object.<ref name="C2017U1discovery" /> It was initially named C/2017 U1 because it was assumed to be a comet, and was renamed to A/2017 U1 after no cometary activity was found on October 25.<ref name="A2017U1announcement" /><ref name="IFLScience" /> After its interstellar nature was confirmed, it was renamed to 1I/ʻOumuamua – "1" because it is the first such object to be discovered, "I" for interstellar, and "'Oumuamua" is a Hawaiian word meaning "a messenger from afar arriving first".<ref name="GeekWire" />

The lack of [[Coma (cometary)|cometary activity]] from ʻOumuamua suggests an origin from the inner regions of whatever stellar system it came from, losing all surface volatiles within the [[Frost line (astrophysics)|frost line]], much like the rocky asteroids, [[extinct comets]] and [[damocloids]] we know from the Solar System. This is only a suggestion, as ʻOumuamua might very well have lost all surface volatiles to eons of [[cosmic radiation]] exposure in interstellar space, developing a thick crust layer after it was expelled from its parent system.

ʻOumuamua has an [[Orbital eccentricity|eccentricity]] of 1.199, which was the highest eccentricity ever observed for any non-artificial object in the Solar System by a wide margin prior to the discovery of comet [[2I/Borisov]] in August 2019.

In September 2018, astronomers described several possible home [[star system]]s from which ʻOumuamua may have begun its interstellar journey.<ref name="Feng2018" /><ref name="Space" />

===2I/Borisov===
{{main|2I/Borisov}}
The object was discovered on 30 August 2019 at MARGO, Nauchnyy, [[Crimea]] by [[Gennadiy Borisov]] using his custom-built 0.65-meter telescope.<ref name="S&T" /> On 13 September 2019, the [[Gran Telescopio Canarias]] obtained a low-resolution visible spectrum of [[2I/Borisov]] that revealed that this object has a surface composition not too different from that found in typical [[Oort Cloud]] comets.<ref name="first_spectrum" /><ref name="deLeon2019" /><ref name="deLeón2020"/> The IAU Working Group for Small Body Nomenclature kept the name Borisov, giving the comet the interstellar designation of 2I/Borisov.<ref name="MPEC 2019-S72" /> On 12 March 2020, astronomers reported observational evidence of "ongoing nucleus fragmentation" from Borisov.<ref name="AT-20200312">{{cite news |author=Drahus, Michal |display-authors=et al. |title=ATel#1349: Multiple Outbursts of Interstellar Comet 2I/Borisov |url=http://www.astronomerstelegram.org/?read=13549 |date=12 March 2020 |work=[[The Astronomer's Telegram]] |access-date=13 March 2020 }}</ref>

==Candidates==
[[File:Hyakutake Color.jpg|thumb|[[Comet Hyakutake]] (C/1996 B2) might be a former interstellar object captured by the Solar System]]
In 2007, Afanasiev et al. reported the likely detection of a multi-centimeter intergalactic meteor hitting the atmosphere above the [[Special Astrophysical Observatory]] of the [[Russian Academy of Sciences]] on July 28, 2006.<ref>{{cite journal|first1=V. L.|last1=Afanasiev|first2=V. V.|last2=Kalenichenko|first3=I. D.|last3=Karachentsev|title=Detection of an intergalactic meteor particle with the 6-m telescope|journal=Astrophysical Bulletin|date=1 December 2007|issn=1990-3421|pages=301–310|volume=62|issue=4|doi=10.1134/S1990341307040013|arxiv=0712.1571 |bibcode=2007AstBu..62..301A |s2cid=16340731 }}</ref>

In November 2018, Harvard astronomers Amir Siraj and Avi Loeb reported that there should be hundreds of 'Oumuamua-size interstellar objects in the Solar System, based on calculated orbital characteristics, and presented several [[Centaur (minor planet)|centaur]] candidates such as {{mp|2017 SV|13}} and {{mp|2018 TL|6}}. These are all orbiting the Sun, but may have been captured in the distant past.<ref name="Siraj2019a" /> Both researchers have proposed methods for increasing the discovery rate of interstellar objects that include [[stellar occultation]]s, optical signatures from impacts with the moon or the Earth's atmosphere, and radio flares from collisions with [[neutron star]]s.<ref>{{Cite journal|last1=Siraj|first1=Amir|last2=Loeb|first2=Abraham|date=2020-02-26|title=Detecting Interstellar Objects through Stellar Occultations|journal=The Astrophysical Journal|language=en|volume=891|issue=1|pages=L3|doi=10.3847/2041-8213/ab74d9|issn=2041-8213|arxiv=2001.02681|bibcode=2020ApJ...891L...3S|s2cid=210116475 |doi-access=free }}</ref><ref>{{Cite journal|last1=Siraj|first1=Amir|last2=Loeb|first2=Abraham|date=2020-08-01|title=A real-time search for interstellar impacts on the moon|url=http://www.sciencedirect.com/science/article/pii/S0094576520302058|journal=Acta Astronautica|language=en|volume=173|pages=53–55|doi=10.1016/j.actaastro.2020.04.006|issn=0094-5765|arxiv=1908.08543|bibcode=2020AcAau.173...53S|s2cid=201645069}}</ref><ref>{{Cite journal|last1=Siraj|first1=Amir|last2=Loeb|first2=Abraham|date=2019-09-19|title=Radio Flares from Collisions of Neutron Stars with Interstellar Asteroids|journal=Research Notes of the AAS|language=en|volume=3|issue=9|page=130|doi=10.3847/2515-5172/ab43de|issn=2515-5172|arxiv=1908.11440|bibcode=2019RNAAS...3..130S|s2cid=201698501 |doi-access=free }}</ref><ref>{{Cite web|last=August 2019|first=Mike Wall 30|title=A Telescope Orbiting the Moon Could Spy 1 Interstellar Visitor Per Year|url=https://www.space.com/interstellar-meteor-moon-orbiting-telescope.html|access-date=2020-11-14|website=Space.com|date=30 August 2019|language=en}}</ref>

===2014 interstellar meteor===
{{main|CNEOS 2014-01-08}}
[[CNEOS 2014-01-08]] (also known as Interstellar meteor 1; IM1),<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><ref name="ARX-20220920">{{cite journal |last1=Siraj |first1=Amir |last2=Loeb |first2=Avi |title=Interstellar Meteors are Outliers in Material Strength |journal=The Astrophysical Journal |date=20 September 2022 |volume=941 |issue=2 |pages=L28 |doi=10.3847/2041-8213/aca8a0 |arxiv=2209.09905v1|bibcode=2022ApJ...941L..28S |s2cid=252407502 |doi-access=free }}</ref><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> a meteor with a mass of 0.46 tons and width of {{convert|0.45|m|ft|abbr=on}}, burned up in the Earth's atmosphere on January 8, 2014.<ref name="ARX-20190604" /><ref name="NYT-20220415">{{cite news |last=Roulette |first=Joey |title=Military Memo Deepens Possible Interstellar Meteor Mystery – The U.S. Space Command seemed to confirm a claim that a meteor from outside the solar system had entered Earth's atmosphere, but other scientists and NASA are still not convinced. (+ Comment) |url=https://www.nytimes.com/2022/04/15/science/interstellar-meteor-debate.html#permid=117852708 |date=15 April 2022 |work=[[The New York Times]] |access-date=15 April 2022 }}</ref> A 2019 [[preprint]] suggested this meteor had been of interstellar origin.<ref name="SA-20190423">{{cite news |last=Billings |first=Lee |title=Did a Meteor from Another Star Strike Earth in 2014? – Questionable data cloud the potential discovery of the first known interstellar fireball |url=https://www.scientificamerican.com/article/did-a-meteor-from-another-star-strike-earth-in-2014/ |date=23 April 2019 |work=[[Scientific American]] |access-date=12 April 2022 }}</ref><ref name="SP-20190416">{{cite news |last=Choi |first=Charles Q. |title=The First Known Interstellar Meteor May Have Hit Earth in 2014 – The 3-foot-wide rock rock visited us three years before 'Oumuamua. |url=https://www.space.com/second-interstellar-object-a-meteor-discovery.html |date=16 April 2019 |work=[[Space.com]] |access-date=12 April 2022 }}</ref><ref name="VICE-20220407">{{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/secret-government-info-confirms-first-known-interstellar-object-on-earth-scientists-say/ |date=7 April 2022 |work=[[Vice (magazine)|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 (website)|Inverse]] |access-date=11 April 2022 }}</ref><ref name="SA-20220412">{{cite news |last=Siraj |first=Amir |title=Spy Satellites Confirmed Our Discovery of the First Meteor from beyond the Solar System - A high-speed fireball that struck Earth in 2014 looked to be interstellar in origin, but verifying this extraordinary claim required extraordinary cooperation from secretive defense programs |url=https://www.scientificamerican.com/article/spy-satellites-confirmed-our-discovery-of-the-first-meteor-from-beyond-the-solar-system/ |date=12 April 2022 |work=[[Scientific American]] |access-date=14 April 2022 }}</ref> It had a heliocentric speed of {{convert|60|km/s|mi/s|abbr=on}} and an asymptotic speed of {{convert|42.1 ± 5.5|km/s|mi/s|abbr=on}}, and it exploded at 17:05:34 UTC near [[Papua New Guinea]] at an altitude of {{convert|18.7|km|ft|abbr=on}}.<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> After declassifying the data in April 2022,<ref name="LS-20220411">{{cite news |last=Specktor |first=Brandon |title=An interstellar object exploded over Earth in 2014, declassified government data reveal – Classified data prevented scientists from verifying their discovery for 3 years.|url=https://www.livescience.com/first-interstellar-object-detected |date=11 April 2022 |work=[[Live Science]] |access-date=12 April 2022 }}</ref> the [[U.S. Space Command]], based on information collected from its [[planetary defense]] sensors, confirmed the velocity of the potential interstellar meteor.<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=[[NASA]] |access-date=11 April 2022 }}</ref><ref name="TWR-20220406">{{cite news |author=[[United States Space Command]] |title=I had the pleasure of signing a memo with @ussfspoc's Chief Scientist, Dr. Mozer, to confirm that a previously-detected interstellar object was indeed an interstellar object, a confirmation that assisted the broader astronomical community. |url=https://twitter.com/US_SpaceCom/status/1511856370756177921 |date=6 April 2022 |work=[[Twitter]] |access-date=12 April 2022 }}</ref> In 2023, [[The Galileo Project]] completed an expedition to retrieve small fragments of the apparently peculiar<ref>{{cite web |last1=Loeb |first1=Avi |author1-link=Avi Loeb |title=The First Interstellar Meteor Had a Larger Material Strength Than Iron Meteorites |url=https://avi-loeb.medium.com/the-first-interstellar-meteor-had-a-larger-material-strength-than-iron-meteorites-bd8680ffaeb2 |website=Medium |access-date=21 August 2022 |language=en |date=18 April 2022}}</ref><ref>{{cite news |last1=Fuschetti |first1=Ray |last2=Johnson |first2=Malcolm |last3=Strader |first3=Aaron |title=Harvard Professor Believes Alien Tech Could Have Crashed Into Pacific Ocean — And He Wants to Find It |url=https://www.nbcboston.com/news/local/harvard-professor-believes-alien-tech-could-have-crashed-into-the-pacific-ocean-and-he-wants-to-find-it/2805992/ |access-date=2 September 2022 |work=NBC Boston}}</ref><ref name="arXiv:2208.00092"/> meteor.<ref>{{cite news |last1=Carter |first1=Jamie |title=Astronomers plan to fish an interstellar meteorite out of the ocean using a massive magnet |url=https://www.livescience.com/interstellar-asteroid-fishing-expedition |access-date=21 August 2022 |work=livescience.com |date=9 August 2022 |language=en}}</ref><ref name="arXiv:2208.00092">{{cite arXiv|last1=Siraj |first1=Amir |last2=Loeb |first2=Abraham |last3=Gallaudet |first3=Tim |title=An Ocean Expedition by the Galileo Project to Retrieve Fragments of the First Large Interstellar Meteor CNEOS 2014-01-08 |eprint=2208.00092 |date=5 August 2022|class=astro-ph.EP }}</ref> Claims about their findings have been doubted by their peers according to a report in ''[[The New York Times]]''.<ref name="NYT-20230724">{{cite news |last=Miller |first=Katrina |title=Scientist's Deep Dive for Alien Life Leaves His Peers Dubious - Avi Loeb, a Harvard astrophysicist, says that material recovered from the seafloor could be from an extraterrestrial spacecraft. His peers are skeptical. + comment |url=https://www.nytimes.com/2023/07/24/science/avi-loeb-extraterrestrial-life.html#permid=126537672 |date=24 July 2023 |work=[[The New York Times]] |url-status=live |archive-url=https://ghostarchive.org/archive/20230725130754/https://www.nytimes.com/2023/07/24/science/avi-loeb-extraterrestrial-life.html#permid=126537672 |archive-date=25 July 2023 |access-date=24 July 2023 }}</ref> Further related studies were reported on 1 September 2023.<ref name="SA20230901">{{cite news |last=McRae |first=Mike |title=Material Found in Ocean Is Not From This Solar System, Study Claims |url=https://www.sciencealert.com/material-found-in-ocean-is-not-from-this-solar-system-study-claims |date=1 September 2023 |url-status=live |archive-url=https://archive.today/20230901161613/https://www.sciencealert.com/material-found-in-ocean-is-not-from-this-solar-system-study-claims |archive-date=1 September 2023 |access-date=1 September 2023 }}</ref><ref name="AXV-20230829">{{cite journal |author=Loeb, Avi |display-authors=et al. |author-link=Avi Loeb |title=Discovery of Spherules of Likely Extrasolar Composition in the Pacific Ocean Site of the CNEOS 2014-01-08 (IM1) Bolide |date=29 August 2023 |arxiv=2308.15623 }}</ref>

Other astronomers doubt the interstellar origin because the meteoroid catalog used does not report [[Error bar|uncertainties]] on the incoming velocity.<ref name="ScientificAmerican" /> The validity of any single data point (especially for smaller meteoroids) remains questionable. In November 2022, a paper was published, claiming the anomalous properties (including its high strength and strongly hyperbolic trajectory) of [[CNEOS 2014-01-08]] are better described as measurement error rather than genuine parameters. Successful retrieval of any meteoroid fragments is highly unlikely.<ref>{{citation|arxiv=2211.02305|year=2022|title=Hyperbolic meteors: is CNEOS 2014-01-08 interstellar?|last1=Vaubaillon |first1=J. |journal=WGN |volume=50 |issue=5 |page=140 |bibcode=2022JIMO...50..140V }}</ref> Common micrometeorites would be indistinguishable from one another.

===2017 interstellar meteor===
CNEOS 2017-03-09 (aka Interstellar meteor 2; IM2),<ref name="ARX-20220920"/><ref name="TDB-20220923"/> a meteor with a mass of roughly 6.3 tons, burned up in the Earth's atmosphere on March 9, 2017. Similar to IM1, it has a high mechanical strength.<ref>{{cite news |title=Alien-Hunting Astronomer Says There May Be a Second Interstellar Object on Earth in New Study |url=https://www.vice.com/en/article/alien-hunting-astronomer-says-there-may-be-a-second-interstellar-object-on-earth-in-new-study/ |access-date=3 November 2022 |work=Vice |language=en}}</ref><ref name="ARX-20220920"/> In September 2022, astronomers Amir Siraj and Avi Loeb reported the discovery of a candidate interstellar meteor, CNEOS 2017-03-09, that impacted Earth in 2017 and is considered, based in part on the high [[material strength]] of the meteor, to be a possible interstellar object.<ref name="ARX-20220920" /><ref name="TDB-20220923" />

==Hypothetical missions==
With current space technology, close visits and orbital missions are challenging due to their high speeds, though not impossible.<ref name="ARX-20180412">{{Cite journal |last1=Seligman |first1=Darryl |last2=Laughlin |first2=Gregory |title=The Feasibility and Benefits of in situ Exploration of ʻOumuamua-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|s2cid=73656586 |doi-access=free }}</ref><ref name="VC-20221108">{{cite news |last=Ferreira |first=Becky |title=We Need to Intercept Our Next Interstellar Visitor to See If It's Artificial, Astronomers Say in New Study - A new study games out a mission to intercept an interstellar object in space and get a close-up look to see just what its made of. |url=https://www.vice.com/en/article/we-need-to-intercept-our-next-interstellar-visitor-to-see-if-its-artificial-astronomers-say-in-new-study/ |date=8 November 2022 |work=[[Vice (magazine)|Vice]] |access-date=8 November 2022 }}</ref>

The [[Initiative for Interstellar Studies]] (i4is) launched in 2017 [[Project Lyra]] to assess the feasibility of a mission to [[ʻOumuamua]].<ref name="ProjectLyra" /> Several options for sending a spacecraft to ʻOumuamua within a time-frame of 5 to 25 years were suggested.<ref name="Hein2019" /><ref name="Hibberd2019" /> One option is using first a Jupiter flyby followed by a close solar flyby at {{convert|3|solar radius|e6km e6mi|sigfig=2|lk=in}} in order to take advantage of the [[Oberth effect]].<ref name="Hein2017" /> Different mission durations and their velocity requirements were explored with respect to the launch date, assuming direct impulsive transfer to the intercept trajectory.

The ''[[Comet Interceptor]]'' spacecraft by [[ESA]] and [[JAXA]], planned to launch in 2029, will be positioned at the [[Lagrangian point|Sun-Earth L<sub>2</sub> point]] to wait for a suitable [[long-period comet]] to intercept and flyby for study.<ref>{{cite web |url=https://www.esa.int/Science_Exploration/Space_Science/Ariel_moves_from_blueprint_to_reality |title=Ariel moves from blueprint to reality |work=[[ESA]] |date=12 November 2020 |access-date=12 November 2020}}</ref> In case that no suitable comet is identified during its 3-year wait, the spacecraft could be tasked to intercept an interstellar object in short notice, if reachable.<ref name="OCallaghan2019" />

== See also ==
* {{annotated link|Global catastrophic risk}}
* {{annotated link|Hyperbolic asteroid}}
* {{annotated link|List of artificial objects leaving the Solar System}}
* {{annotated link|List of hyperbolic comets}}
* {{annotated link|List of Solar System objects by greatest aphelion}}
* [[Rogue black hole]]

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|last12      = Popescu             |first12 = M.
|last13      = Scarpa              |first13 = R.
|last14      = Font Serra          |first14 = J.
|last15      = Geier               |first15 = S.
|last16      = Lorenzi             |first16 = V.
|last17      = Harutyunyan         |first17 = A.
|last18      = Cabrera-Lavers      |first18 = A.
|date        = 30 April 2020
|title       = Visible and near-infrared observations of interstellar comet 2I/Borisov with the 10.4-m GTC and the 3.6-m TNG telescopes
|volume      = 495
|number      = 2
|pages       = 2053–2062
|arxiv       = 2005.00786v1
|url         = https://academic.oup.com/mnras/article-abstract/495/2/2053/5827545
|journal     = [[Monthly Notices of the Royal Astronomical Society]] 
|doi         = 10.1093/mnras/staa1190
|doi-access = free
|bibcode     = 2020MNRAS.495.2053D
|s2cid = 218486809
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<ref name="Siraj2019a">
{{cite journal
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<ref name="Siraj2019b">
{{cite arXiv
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<ref name="ScientificAmerican">
{{cite web
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<ref name="ProjectLyra">
{{cite web
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<ref name="Hein2017">
{{cite arXiv
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<ref name="Clery2018">
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<ref name="NewDesignation">
{{cite web
 |title=MPEC 2017-V17: NEW DESIGNATION SCHEME FOR INTERSTELLAR OBJECTS
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<ref name="IFLScience">
{{cite news
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<ref name="GeekWire">
{{cite web
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<ref name="Note-1">[[C/2012 S1 (ISON)]] had an epoch 1600 barycentric semi-major axis of [https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%272012+S1%27&TABLE_TYPE=%27ELEMENTS%27&START_TIME=%271600-01-01%27&STOP_TIME=%272400-01-01%27&STEP_SIZE=%27900%20years%27&CENTER=%27@0%27&OUT_UNITS=%27AU-D%27 −145127] and would have an inbound v_infinite of 0.2 km/s at 50000 au:<br/>''v''=42.1219 {{radic |1/''50000'' − 0.5/''−145127''}}</ref>

<ref name="Voyager">[https://voyager.jpl.nasa.gov/frequently-asked-questions/fast-facts/ Voyager Fast Facts]</ref>

<ref name="Space">[https://www.space.com/41928-where-did-oumuamua-come-from.html 'Oumuamua Isn't from Our Solar System. Now We May Know Which Star It Came From]</ref>

<ref name="S&T">
{{Cite web
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<ref name="MPEC 2019-S72">
{{cite web
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<ref name="Hein2019">
{{Cite journal
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 |last2=Perakis |first2=Nikolaos
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{{cite journal
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<ref name="OCallaghan2019">
{{cite web 
 |last=O'Callaghan |first=Jonathan
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}}

== External links ==
{{Wiktionary|Interstellar comet}}
*{{cite journal |arxiv=1702.02237|doi=10.3847/1538-3881/aa5c8a|title=An Observational Upper Limit on the Interstellar Number Density of Asteroids and Comets|journal=The Astronomical Journal|volume=153|issue=3|page=133|year=2017|last1=Engelhardt|first1=Toni|last2=Jedicke|first2=Robert|last3=Vereš|first3=Peter|last4=Fitzsimmons|first4=Alan|last5=Denneau|first5=Larry|last6=Beshore|first6=Ed|last7=Meinke|first7=Bonnie|bibcode=2017AJ....153..133E|s2cid=54893830 |doi-access=free }}

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