View source for 2I/Borisov

{{Short description|Interstellar comet passing through the Solar System, discovered in 2019}}
{{redirect|Comet Borisov|other comets discovered by Gennadiy Borisov|List of comets discovered by Gennadiy Borisov}}
{{Use dmy dates|date=March 2020}}
{{Infobox comet
| name           = 2I/Borisov
| image          = Comet-2IBorisov-HubbleST-20191016 (cropped).png
| caption        = Borisov in October 2019{{efn|A photograph of Borisov's dust trail taken on {{nowrap|12 October 2019}} with the UVIS F350LP channel of [[Wide Field Camera 3]] aboard the [[Hubble Space Telescope]].{{r|hubblesite-1|hubblesite-2}} At the time of this photograph, Borisov was {{convert|418|e6km|e6mi|abbr=unit}} from [[Earth]], similar to the average distance of the [[Ceres (dwarf planet)|asteroid Ceres]] from the [[Sun]], and traveling at a velocity of {{Convert|177,000|kph|mph}}.<ref name=hubblesite-2/>}}
| discoverer     = [[Gennadiy Borisov]]
| discovery_date = 30&nbsp;August 2019<br/>Earliest [[precovery]]:<BR />13&nbsp;December 2018
| discovery_site = [[List of observatory codes#L51|MARGO observatory]], Nauchnyy, Crimea
| epoch          = 5&nbsp;January 2020<ref name=JPL-Data/>
| observation_arc       = 389&nbsp;days
| obs            = 1191
| perihelion     = {{val|2.00662|0.00002|ul=AU}}
| eccentricity   = {{val|3.3570|0.0002}} (JPL)<br/>{{val|3.357}} (MPC)<ref name=MPC-DB />
| inclination    = {{val|44.0535|0.0001|u=deg}}
| semimajor      = {{val|-0.85132|0.00007|u=AU}}{{Efn|Objects with eccentricities greater than 1 will have a negative semimajor axis, giving them a positive orbital energy – the smaller the perihelion distance, and the larger above 1 the eccentricity is, the closer to 0 the semimajor axis value will be.}}
| asc_node       = {{val|308.1500|0.0003|u=deg}}
| arg_peri       = {{val|209.1244|0.0004|u=deg}}
| Earth_moid     = {{val|1.09302|u=AU}} 
| Jupiter_moid   = {{val|2.388|u=AU}}
| dimensions     = ≤&nbsp;0.5&nbsp;km<ref name=Jewitt2019b/>(for an [[albedo]] of 0.04)<br/>≤&nbsp;0.4&nbsp;km<ref name=Hui2020/>
| last_p         = 8&nbsp;December 2019<ref name=JPL-Data/><ref name=MPC-DB/><ref name=Gray234 />
| next_p=''interstellar object''}}

'''2I/Borisov''', originally designated '''C/2019 Q4 (Borisov)''',{{r|MPEC2019-S72}} is the first observed [[rogue comet]] and the second observed interstellar interloper after [[ʻOumuamua]].{{r|EA-20191014|Grossman2019}} It was discovered by the [[Crimean]] [[Amateur astronomy|amateur astronomer]] and telescope maker [[Gennadiy Borisov]] on 29&nbsp;August 2019 [[UTC]] (30&nbsp;August [[Moscow Time|local time]]).<ref name="NYT-20191207a" /><ref name=RIANovosti>
{{cite news
 |script-title = ru:Крымский астроном заявил, что открытая им комета изменит название
|title=Krymskiy astronom zayavil, chto otkrytaya im kometa izmenit nazvaniye
 |trans-title  = Crimean astronomer says that the comet discovered by him will be renamed
 |website      = [[RIA Novosti]] | script-website = ru:РИА Новости
 |date         = 16 September 2019
 |at           = para.&nbsp;5
 |url          = https://ria.ru/20190916/1558728005.html
 |access-date  = 16 September 2019
 |language     = ru
}}
</ref>

2I/Borisov has a [[heliocentric]] [[orbital eccentricity]] of 3.36 and is not bound to the Sun.<ref name="NYT-20191224"/> The comet passed through the [[ecliptic]] of the Solar System at the end of October&nbsp;2019, and made its closest approach to the Sun at just over {{val|2|ul=AU}} on 8&nbsp;December 2019.{{r|JPL-Data}}<ref name="NYT-20191207b"/>  The comet passed closest to Earth on 28 December 2019.<ref name="Guzik2019"/> In November&nbsp;2019, astronomers from [[Yale University]] said that the comet's tail was 14&nbsp;times the size of Earth, and stated, "It's humbling to realize how small Earth is next to this visitor from another solar system."<ref name="YALE-20191126">{{cite news |last=Shelton |first=Jim |title=New image offers close-up view of interstellar comet |url=https://news.yale.edu/2019/11/26/new-image-offers-close-view-interstellar-comet |date=26 November 2019 |work=[[Yale University]] | access-date=7 December 2019 }}</ref>

== Nomenclature ==

The comet is formally called "2I/Borisov" by the [[International Astronomical Union]] (IAU), with "2I" or "2I/2019&nbsp;Q4" being its designation and "Borisov" being its name, but is sometimes referred to as "Comet Borisov", especially in the popular press.{{Efn|This is the eighth comet discovered by [[Gennadiy Borisov]], and thus the ambiguous term "Comet Borisov" is not formally used here.}} As the second observed interstellar interloper after [[ʻOumuamua|1I/ʻOumuamua]], it was given the "2I" [[Naming of comets#Current system|designation]], where "I" stands for interstellar.<ref name=Christensen2019/><ref name=MPEC2019-S72/> The name Borisov follows the tradition of naming comets after their discoverers. Before final designation as 2I/Borisov, the object was referred to by other names:
* Early orbit solutions suggested that the comet could be a [[near-Earth object]] and was thus listed on [[International Astronomical Union|IAU]]'s [[Minor Planet Center]]'s (MPC) [[Near-Earth Object Confirmation Page]] (NEOCP) as gb00234.<ref name=Gray-NEO/>
* Further refinements after thirteen days of observation made clear the object was a hyperbolic comet, and it was given the designation C/2019&nbsp;Q4 (Borisov) by the Minor Planet Center on 11&nbsp;September 2019.<ref name=MPECK19RA6/> A number of other astronomers including [[Davide Farnocchia]], [[Bill Gray (astronomer)|Bill Gray]], and [[David Tholen]] concluded that the comet was interstellar.<ref name=MPECK19RA6/> 
* On 24&nbsp;September 2019 the IAU announced that the Working Group for Small Body Nomenclature kept the name Borisov giving the comet the interstellar designation of 2I/Borisov, formally announcing the comet was indeed interstellar.<ref name=Christensen2019/><ref name=MPEC2019-S72/>

== Characteristics ==

[[File:Comet 2I Borisov at Perihelion in December 2019.tif|thumb|In this observation, the NASA/ESA Hubble Space Telescope revisited the comet shortly after its closest approach to the Sun in December&nbsp;2019.]]
{{External media
 |width     = 200px
 |float     = right
 |video1    = [https://www.youtube.com/watch?v=_jjSTnH7xmk Interstellar comet 2I/Borisov at perihelion (8 December 2019). ]
}}
Unlike ʻOumuamua, which had an [[asteroid]]al appearance, 2I/Borisov's nucleus was surrounded by a [[Coma (cometary)|coma]], a cloud of dust and gas.<ref name=Greskno2019/>
[[File:Comet 2I Borisov and Distant Galaxy in November 2019.tif|left|thumb|Comet 2I/Borisov and a distant galaxy (2MASX J10500165-0152029<ref>{{cite web |first=Chelsea |last=Gohd |date=14 December 2019 |title=Interstellar comet Borisov shines in incredible new Hubble photos |website=[[Space.com]] | url=https://www.space.com/interstellar-comet-borisov-new-hubble-telescope-photos.html |access-date=21 January 2020}}</ref>) in November&nbsp;2019.<ref>{{cite web |title=Hubble Watches Interstellar Comet Borisov Speed Past the Sun |url=https://www.spacetelescope.org/news/heic1922/ |website=www.spacetelescope.org |access-date=16 December 2019 |language=en}}</ref>]]

=== Size and shape ===

Early estimates of nucleus 2I/Borisov diameter have ranged from {{val|1.4|to|16|u=km}}.<ref name=NASA-Initial/><ref>{{cite journal |last=Lee |first=Chien-Hsiu |date=11 December 2019 |title=FLAMINGOS-2 infrared photometry of 2I/Borisov |journal=Research Notes of the AAS |volume=3 |issue=12 |page=184 |doi=10.3847/2515-5172/ab5f69 |s2cid=209324025 |arxiv=1912.05628 |bibcode=2019RNAAS...3..184L |doi-access=free }}</ref> 2I/Borisov has, unlike Solar System comets, noticeably shrunk during Solar System flyby, losing at least 0.4% of its mass before [[Apsis|perihelion]].<ref name=Hui2020>{{cite arXiv |last1=Hui |first1=Man-To |last2=Ye |first2=Quan-Zhi |last3=Föhring |first3=Dora |last4=Hung |first4=Denise |last5=Tholen |first5=David J. |year=2020 |title=Physical characterisation of interstellar comet 2I/2019&nbsp;Q4 (Borisov) |eprint=2003.14064 |class=astro-ph.EP}}</ref> Also, the amplitude of non-gravitational acceleration place an upper limit of 0.4&nbsp;km on nucleus size, consistent with a previous [[Hubble Space Telescope]] upper limit of 0.5&nbsp;km.<ref name=Jewitt2019b/> The comet did not come much closer to Earth than 300&nbsp;million km, which prevents using radar to directly determine its size and shape. This could be done using the [[occultation]] of a star by 2I/Borisov but an occultation would be difficult to predict, requiring a precise determination of its orbit, and the detection would necessitate a network of small telescopes.<ref name=Bamberger2019/>

=== Rotation ===

A study using observations from Hubble could not find a variation in the light curve. According to this study the rotational period must be larger than 10&nbsp;hours.<ref>{{cite journal |last=Bolin |first=Bryce T. |date=13 December 2019 |title= Constraints on the spin-pole orientation, jet morphology, and rotation of interstellar comet 2I/Borisov with deep HST imaging|journal=Monthly Notices of the Royal Astronomical Society |volume=497 |issue=4 |pages=4031–4041 |doi=10.1093/mnras/staa2192 |doi-access=free |arxiv=1912.07386 |s2cid=218595761}}</ref> A study with [[Canadian Space Agency|CSA]]'s [[Near Earth Object Surveillance Satellite|NEOSSat]] found a period of 13.2&nbsp;±&nbsp;0.2&nbsp;days, which is unlikely to be the nuclear spin.<ref>{{Cite journal |last1=Gladman |first1=Brett |last2=Boley |first2=Aaron |last3=Balam |first3=Dave |date=12 December 2019 |title=The inbound light curve of 2I/Borisov |journal=Research Notes of the AAS |volume=3 |issue=12 |pages=187 |doi=10.3847/2515-5172/ab6085 |issn=2515-5172 |bibcode=2019RNAAS...3..187G |s2cid=216391537 |language=en |doi-access=free }}</ref> [[Monte Carlo method|Monte Carlo]] simulations based on the available orbit determinations suggest that the equatorial obliquity of 2I/Borisov could be about 59&nbsp;degrees or 90&nbsp;degrees, the latter is favored for the latest orbit determination.<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|page=A18 |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}}</ref>

=== Chemical makeup and nucleus structure ===

[[David C. Jewitt|David Jewitt]] and [[Jane Luu]] estimate from the size of its coma the comet is producing 2&nbsp;kg/s of dust and is losing 60&nbsp;kg/s of water. They extrapolate that it became active in June&nbsp;2019 when it was between 4 and 5&nbsp;{{smallcaps|au}} from the Sun.<ref name=Jewitt2019b/> A search of image archives found [[precovery]] observations of 2I/Borisov as early as 13&nbsp;December 2018, but not on 21&nbsp;November 2018, indicating it became active between these dates.<ref name="Ye_etal_precovery">{{cite journal |last1=Ye |first1=Quanzi |display-authors=etal |year=2020 |title=Pre-discovery activity of new interstellar comet 2I/Borisov beyond 5&nbsp;AU |journal=The Astronomical Journal |volume=159 |issue=2 |pages=77 |arxiv=1911.05902 |s2cid=208006384 |doi=10.3847/1538-3881/ab659b |bibcode=2020AJ....159...77Y |doi-access=free }}</ref>

2I/Borisov's composition appears uncommon yet not unseen in Solar System comets, being relatively depleted in water and [[diatomic carbon]] (C<sub>2</sub>), but enriched in [[carbon monoxide]] and [[amines]] (R-NH<sub>2</sub>).<ref>{{cite journal |last1=Bodewits |first1=D. |last2=Noonan |first2=J.W. |last3=Feldman |first3=P.D. |last4=Bannister |first4=M.T. |last5=Farnocchia |first5=D. |last6=Harris |first6=W.M. |last7=Li|first7=J.-Y. |last8=Mandt |first8=K.E. |last9=Parker |first9=J.Wm. |last10=Xing |first10=Z.-X. |display-authors=6 |year=2020 |title=The carbon monoxide-rich interstellar comet 2I/Borisov |journal=Nature Astronomy |volume=4 |issue=9 |pages=867–871 |arxiv=2004.08972 |s2cid=215827703 |doi=10.1038/s41550-020-1095-2 |bibcode=2020NatAs...4..867B}}</ref> The molar ratio of carbon monoxide to water in 2I/Borisov tail is 35–105%, resembling the unusual blue-tailed comet [[C/2016 R2 (PANSTARRS)]] in contrast to the average ratio of 4% for solar system comets.<ref>{{cite journal |last1=Cordiner |first1=M.A. |last2=Milam |first2=S.N. |last3=Biver |first3=N. |last4=Bockelée-Morvan |first4=D. |author4-link=Dominique Bockelée-Morvan|last5=Roth |first5=N.X. |last6=Bergin |first6=E.A. |last7=Jehin |first7=E. |last8=Remijan |first8=A.J. |last9=Charnley |first9=S.B. |last10=Mumma |first10=M.J. |last11=Boissier |first11=J. |last12=Crovisier |first12=J. |last13=Paganini |first13=L. |last14=Kuan |first14=Y.-J. |last15=Lis |first15=D.C. |display-authors=6 |year=2020 |title=Unusually high CO abundance of the first active interstellar comet |journal=Nature Astronomy |volume=4 |issue=9 |pages=861–866 |bibcode=2020NatAs...4..861C |s2cid=216036159 |arxiv=2004.09586 |doi=10.1038/s41550-020-1087-2}}</ref>

The 2I/Borisov has also produced a minor amount of neutral nickel emission attributed to an unknown volatile compound of nickel.<ref>{{citation|arxiv=2105.09305|year=2021|title=Gaseous atomic nickel in the coma of interstellar comet 2I/Borisov|doi=10.1038/s41586-021-03485-4|last1=Guzik|first1=Piotr|last2=Drahus|first2=Michał|journal=Nature|volume=593|issue=7859|pages=375–378|pmid=34012084|bibcode=2021Natur.593..375G|s2cid=234777842}}</ref> The nickel to iron abundance ratio is similar to Solar System comets.<ref>Optim et al. 2021, Manfroid et al 2021</ref>

=== Trajectory ===

[[File:Sky_plot_13.png|thumb|left|Trajectory across sky in cylindrical coordinates, with 30&nbsp;August 2019 discovery position along with precovery positions marked back to 13&nbsp;December 2018 which help narrow down the incoming trajectory]]
{| class="wikitable floatright" style="<!--{{float style|margin=1em}} -->text-align:center; font-size:0.9em;"
|+ Interstellar velocity inbound (<math>v_\infty</math>){{efn|For objects gravitationally bound to the Sun, velocities when approaching the equivalent of an infinite distance from the Sun should be less than a few kilometres per second. At its furthest distance from the Sun, Halley's comet moves {{nowrap|≈ 1 km/s}} with respect to the Sun.}}
|-
! Object !! Velocity
|-
| [[1I/2017 U1 (ʻOumuamua)]] || {{cvt|26.33|km/s|AU/years|2|disp=br}}<ref name=ProjectPluto />
|-
| '''2I/2019 Q4 (Borisov)''' || {{cvt|32.2|km/s|AU/years|2|disp=br}}<ref name=Gray-FAQ />
|}
As seen from Earth, the comet was in the northern sky from September until mid-November. It crossed the ecliptic plane on 26&nbsp;October near the star [[Regulus]],<ref>{{cite web |title=Comet 2I Borisov |website=spaceweathergallery.com |url=https://spaceweathergallery.com/indiv_upload.php?upload_id=157027}}</ref> and the [[celestial equator]] on 13&nbsp;November 2019, entering the southern sky. On 8&nbsp;December 2019, the comet reached [[Apsis|perihelion]] (closest approach to the Sun)<ref name=JPL-Data/> and was near the inner edge of the asteroid belt. In late December, it made its closest approach to Earth,<ref name="NYT-20191207a">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |date=7 December 2019 |title=The Interstellar Comet Has Arrived in Time for the Holidays |newspaper=[[The New York Times]] | quote=This weekend an ice cube from beyond our solar system will makes its closest approach to the sun, trailing mystery and dust. |url=https://www.nytimes.com/2019/12/07/science/interstellar-comet-2i-borisov.html |access-date=7 December 2019 }}</ref><ref name="NYT-20191207b">{{cite news |last=Corum |first=Jonathan |date=7 December 2019 |title=Tracking comet Borisov |newspaper=[[The New York Times]] | url=https://www.nytimes.com/interactive/2019/12/07/science/comet-borisov-tracker.html |access-date=7 December 2019 }}</ref> 1.9&nbsp;{{smallcaps|au}}, and had a solar elongation of about 80°.<ref name=Gray234/> Due to its 44° [[orbital inclination]], 2I/Borisov did not make any notable close approaches to the planets.<ref name=JPL-Data/> 2I/Borisov entered the Solar System from the direction of [[Cassiopeia (constellation)|Cassiopeia]] near the border with [[Perseus (constellation)|Perseus]]. This direction indicates that it originates from the [[galactic plane]], rather than from the [[galactic halo]].<ref name="Gray-FAQ" /><ref name=deLeón2020/> It will leave the Solar System in the direction of [[Telescopium]].<ref name=Gray-FAQ/><ref name=deLeón2020/> In interstellar space, 2I/Borisov takes roughly {{val|9000}} years to travel a [[light-year]] relative to the Sun.{{efn|1=[[Speed of light|{{val|299792.458|u=km/s}}]] / 32.2 km/s = {{#expr:299792.458 / 32.2 round 0}} times slower than light.}}

2I/Borisov's trajectory is extremely [[Hyperbolic trajectory|hyperbolic]], having an [[orbital eccentricity]] of 3.36.<ref name=JPL-Data/><ref name=MPC-DB/> This is much higher than the 300+&nbsp;known weakly [[List of hyperbolic comets|hyperbolic comets]], with heliocentric eccentricities just over 1, and even ʻOumuamua with an eccentricity of 1.2.<ref name=JPL-Hyperbolic />{{efn|A comet with a parabolic trajectory (with an eccentricity of 1) would leave the Solar System in the direction it entered, having had its path altered by 180°. 2I/Borisov, with its higher eccentricity, has a more open trajectory and will have its path altered by only 34° as it passes through the Solar System.}} 2I/Borisov also has a [[hyperbolic excess velocity]] (<math>v_\infty</math>) of {{val|32|u=km/s}}, much higher than what could be explained by perturbations, which could produce velocities when approaching an infinite distance from the Sun of less than a few km/s.<ref name=Siegel2017/> These two parameters are important indicators of 2I/Borisov's interstellar origin.<ref name=Gray-FAQ /><ref name=Atel13100/> For comparison, the ''[[Voyager 1]]'' spacecraft, which is leaving the Solar System, is traveling at {{cvt|16.9|km/s|AU/year|2}}.<ref name=Voyager/> 2I/Borisov has a much larger eccentricity than ʻOumuamua due to its higher excess velocity and its significantly higher perihelion distance. At this larger distance, the Sun's gravity is less able to alter its path as it passes through the Solar System.<ref name=Gray-FAQ/>

{{multiple image
| align    = center
| caption_align = center
| header   = 2I/Borisov — orbit diagrams
| image1   = A comparison of two interstellar objects passing through our solar system.gif
| width1   = 281
| caption1 = Trajectory of Borisov (yellow) crossing the ecliptic plane; 'Oumuamua (red) shown for comparison
| image2   = Interstellar visitors.jpg
| width2   = 260
| caption2 = Borisov approaches the ecliptic plane between the orbits of Jupiter (pink) and Mars (orange)
| image3   = Comet-2I-Borisov-Orbit-JPL.png
| width3   = 412
| caption3 = Borisov's trajectory and position (white) as of 13&nbsp;October 2019 (top view)
}}

== Observation ==

=== Discovery ===
The comet was discovered on 30&nbsp;August 2019 by [[amateur astronomer]] [[Gennadiy Borisov]] at his personal observatory [[List of observatory codes#L51|MARGO]] in Nauchnyy, [[Crimea]], using a 0.65&nbsp;meter telescope he designed and built himself.<ref name=S&T2019/> The discovery has been compared to the discovery of [[Pluto]] by [[Clyde Tombaugh]].<ref name=Fontanka/> Tombaugh was also an amateur astronomer who was building his own telescopes, although he discovered Pluto using [[Lowell Observatory]]'s [[astrograph]]. At discovery, it was inbound {{val|3|u=AU}} from the Sun, {{val|3.7|u=AU}} from Earth, and had a [[solar elongation]] of 38°.<ref name=Gray-prec/> Borisov described his discovery thus:<ref name=RIA/>
{{clear}}
{| class="wikitable floatright" style="<!--{{float style|margin=1em}} -->text-align:center; font-size:0.9em;"
|+ JPL Scout eccentricity ranges<ref name=archive-is />
! # of<br/>observations
! Observation<br/>arc (hours)
! Eccentricity<br/>range
|-
| 81 || 225 || 0.9–1.6
|-
| 99 || 272 || 2.0–4.2
|-
| 127 || 289 || 2.8–4.7
|-
| 142 || 298 || 2.8–4.5
|-
| 151 || 302 || 2.9–4.5
|}
{{blockquote|I observed it on August&nbsp;29, but it was August&nbsp;30 [[GMT|Greenwich Time]].{{Efn|This is obviously a mistake: Crimea is 3&nbsp;hours ''ahead'' of Greenwich, so it was August&nbsp;30 in Crimea when Borisov first observed the comet.<ref>{{cite web | title=Current local time in Crimea, Republic of, Russia | website=WorldTimeServer.com | date=23 May 2016 | url=https://www.worldtimeserver.com/current_time_in_RU-RC.aspx | access-date=15 August 2023}}</ref>}} I saw a moving object in the frame, it moved in a direction that was slightly different from that of [[main-belt|main asteroids]].{{efn|Of the 850,000&nbsp;objects known to orbit the Sun, 756,000 (89%) are [[main-belt]] asteroids.<ref>{{Cite web|url=https://minorplanetcenter.net/mpc/summary|title=IAU Minor Planet Center|website=minorplanetcenter.net|accessdate=15 August 2023}}</ref>}} I measured its coordinates and consulted the [[Minor Planet Center]] database. Turned out, it was a new object. Then I measured the near-Earth object rating,{{efn|The NEO rating calculates the probability that a new object is a near-Earth candidate.<ref>{{Cite web|url=https://minorplanetcenter.net//iau/NEO/PossNEO.html|title=Neo Rating|website=minorplanetcenter.net|accessdate=15 August 2023}}</ref>}} it is calculated from various parameters, and it turned out to be 100% – in other words, [[Potentially hazardous object|dangerous]]. In such cases I must immediately post the parameters to the world webpage for confirmation of dangerous asteroids.{{efn|This refers to the NEO Confirmation Page for confirming near-Earth objects and potentially hazardous objects.<ref>{{Cite web|url=https://www.minorplanetcenter.net/iau/NEO/toconfirm_tabular.html|title=The NEO Confirmation Page|website=www.minorplanetcenter.net|accessdate=15 August 2023}}</ref>}} I posted it and wrote that the object was diffuse and that it was not an asteroid, but a comet.}}

2I/Borisov's interstellar origin required a couple of weeks to confirm. Early orbital solutions based on initial observations included the possibility that the comet could be a near-Earth object 1.4&nbsp;AU from the Sun in an elliptical orbit with an orbital period of less than 1&nbsp;year.<ref name=Gray-NEO/> Later using 151 observations over 12&nbsp;days, [[NASA]] [[Jet Propulsion Laboratory]]'s [[Sentry (monitoring system)#Scout|Scout]] gave an eccentricity range of 2.9–4.5&nbsp;.<ref name=Scout/> But with an [[observation arc]] of only 12&nbsp;days, there was still some doubt that it was interstellar because the observations were at a low solar elongation, which could introduce biases in the data such as [[Atmospheric refraction|differential refraction]]. Using large non-gravitational forces on the highly eccentric orbit, a solution could be generated with an eccentricity of about 1, an Earth [[minimum orbit intersection distance]] (MOID) of {{Convert|0.34|AU|e6km|abbr=unit}}, and a perihelion at 0.90&nbsp;AU around 30&nbsp;December 2019.<ref name=Gray35237/> However, based on available observations, the orbit could only be parabolic if non-gravitational forces (thrust due to outgassing) affected its orbit more than any previous comets.<ref name=Gray234c/> Eventually with more observations the orbit converged to the hyperbolic solution that indicated an interstellar origin and non-gravitational forces could not explain the motion.<ref name=MPECK19RA6/>

=== Observation ===

[[File:Video-NASA-InterstellarComet-2IBorisov-HubbleST-20191016.webm|thumb|right|Hubble started observations of 2I/Borisov in October&nbsp;2019, two months prior to the comet's closest approach to the Sun]]
The last observations were in July 2020, seven months after perihelion.<ref name=JPL-Data/> Observation of 2I/Borisov was aided by the fact that the comet was detected while inbound towards the Solar System. ʻOumuamua had been discovered as it was leaving the system, and thus could only be observed for 80&nbsp;days before it was out of range. Because of its closest approach occurring near traditional year-end holidays, and the capability to have extended observations, some astronomers have called 2I/Borisov a "Christmas comet".<ref name="NYT-20191224">{{cite news |last=St.&nbsp;Fleur |first=Nicholas |date=24 December 2019 |title=Watching an interstellar comet and hoping for a bang |quote=Seeing comet Borisov won't be easy for the typical sky gazer, but astronomers still have a lot to learn from this extrasolar tourist. |newspaper=[[The New York Times]] | url=https://www.nytimes.com/2019/12/24/science/interstellar-comet-christmas.html |access-date=28 December 2019}}</ref><ref name=NYT-20191001/> Observations using the [[Hubble Space Telescope]] began on 12&nbsp;October, when the comet moved far enough from the Sun to be safely observed by the telescope.<ref name=Hubble100719/> Hubble is less affected by the confounding effects of the coma than ground-based telescopes, which will allow it to study the rotational [[light curve]] of 2I/Borisov's nucleus. This should facilitate an estimate of its size and shape.{{update inline|date=April 2022}}

=== Comet chemistry ===

A preliminary (low-resolution) visible spectrum of 2I/Borisov was similar to typical [[Oort Cloud]] comets.<ref name=IAC/><ref name=deLeón2019/> Its [[color index]]es also resemble the Solar System's long period comets.<ref name=Jewitt2019b/> Emissions at {{val|388|ul=nm}} indicated the presence of [[cyanide]] (formula CN), which is typically the first detected in Solar System comets including [[comet Halley]].<ref name=Fitzsimmons2019/> This was the first detection of gas emissions from an interstellar object.<ref name=CBAT4670 /> The non-detection of diatomic carbon had also been reported in October 2019, with the ratio C<sub>2</sub> to CN being less than either 0.095<ref name=Kareta2019 /> or 0.3&nbsp;.<ref name=Opitom2019 /> The diatomic carbon was positively detected in November 2019, with measured C<sub>2</sub> to CN ratio of {{val|0.2|0.1}}&nbsp;.<ref>{{cite journal |first1=Hsing Wen |last1=Lin |last2=Lee |first2=Chien-Hsiu |last3=Gerdes |first3=David W. |last4=Adams |first4=Fred C. |last5=Becker |first5=Juliette |last6=Napier |first6=Kevin |last7=Markwardt |first7=Larissa |year=2019 |title=Detection of diatomic carbon in 2I/Borisov |journal=The Astrophysical Journal |volume=889 |issue=2 |pages=L30 |doi=10.3847/2041-8213/ab6bd9 |arxiv=1912.06161 |s2cid=210472954 |doi-access=free }}</ref> This resembles a carbon-chain depleted group of comets, which are either Jupiter family comets<ref name=Kareta2019 /> or rare blue-colored carbon monoxide comets exemplified by [[C/2016 R2]].<ref>{{cite journal |last1=Venkataramani |first1=Kumar |last2=Ganesh |first2=Shashikiran |last3=Baliyan |first3=Kiran S. |year=2020 |title=Ionic emissions in comet C/2016 R2 (Pan-STARRS) |journal=Monthly Notices of the Royal Astronomical Society |volume=495 |issue=4 |pages=3559–3570 |doi=10.1093/mnras/staa1141 |doi-access=free |s2cid=216056627 |bibcode=2020MNRAS.495.3559V |arxiv=2004.10266}}</ref> By the end of November&nbsp;2019, C<sub>2</sub> production had dramatically increased, and C<sub>2</sub> to CN ratio reached 0.61, along with appearance of bright amine (NH<sub>2</sub>) bands.<ref>{{cite arXiv |last1=Bannister |first1=Michele T. |last2=Opitom |first2=Cyrielle |last3=Fitzsimmons |first3=Alan |last4=Moulane |first4=Youssef |last5=Jehin |first5=Emmanuel |last6=Seligman |first6=Darryl |last7=Rousselot |first7=Philippe|last8=Knight |first8=Matthew M. |last9=Marsset |first9=Michael |last10=Schwamb |first10=Megan E. |last11=Guilbert-Lepoutre |first11=Aurélie |last12=Jorda|first12=Laurent |last13=Vernazza |first13=Pierre |last14=Benkhaldoun |first14=Zouhair |display-authors=6 |year=2020 |title=Interstellar comet 2I/Borisov as seen by MUSE: C<sub>2</sub>, NH<sub>2</sub> and red CN detections |class=astro-ph.EP |eprint=2001.11605}}</ref> Atomic oxygen has also been detected, from this observers estimated an outgassing of water at a rate similar to Solar System comets.<ref name="SA-20191030">{{cite news |last=Starr |first=Michelle |date=30 October 2019 |title=That interstellar comet is carrying water from beyond our Solar System |website=ScienceAlert.com |url=https://www.sciencealert.com/interstellar-comet-2i-borisov-is-carrying-a-heap-of-water |access-date=30 October 2019}}</ref><ref name="ARX-20191028">{{cite journal |last1=McKay |first1=Adam J. |display-authors=etal |year=2020 |title=Detection of a Water Tracer in Interstellar Comet 2I/Borisov |journal=The Astrophysical Journal |volume=889 |issue=1 |pages=L10 |arxiv=1910.12785 |s2cid=204907108 |doi=10.3847/2041-8213/ab64ed |bibcode=2020ApJ...889L..10M |doi-access=free }}</ref> Initially, neither water nor OH lines were directly detected<ref>{{cite arXiv |last=Eubanks |first=T. M. |year=2019 |title=Is Interstellar Object 2I/Borisov a Stardust Comet? Predictions for the Post Perihelion Period |class=astro-ph.EP |eprint=1912.12730}}</ref> in September 2019. First unambiguous detection of OH lines was done 1&nbsp;November 2019, and OH production peaked in early December 2019.<ref>{{cite journal |last1=Xing |first1=Zexi |last2=Bodewits |first2=Dennis |last3=Noonan |first3=John |last4=Bannister |first4=Michele T. |year=2020 |title=Water production rates and activity of interstellar comet 2I/Borisov |journal=The Astrophysical Journal |volume=893 |issue=2 |page=L48 |doi=10.3847/2041-8213/ab86be |s2cid=210473691 |arxiv=2001.04865 |bibcode=2020ApJ...893L..48X |doi-access=free }}</ref>

=== Suspected nucleus fragmentation ===

The comet did come within about 2&nbsp;AU of the Sun, a distance at which many small comets have been found to disintegrate. The probability that a comet disintegrates strongly depends on the size of its nucleus; Guzik et al. estimated a probability of 10% that this would happen to 2I/Borisov.<ref name=Guzik2019/> Jewitt and Luu compared 2I/Borisov to [[C/2019 J2 (Palomar)]], another comet of similar size that disintegrated in May&nbsp;2019 at a distance of 1.9&nbsp;AU from the Sun.<ref name=Jewitt2019a /><ref name=Jewitt2019b/> In the event that the nucleus disintegrates, as is sometimes seen with small comets, Hubble can be used to study the evolution of the disintegration process.<ref name=STScI-16009/><ref name=Jewitt2019b/>

The severe outburst in February–March&nbsp;2020, led to suspected "ongoing nucleus fragmentation" from the comet by 12&nbsp;March.<ref name="AT-20200312">{{cite news |last1=Drahus |first1=Michal |display-authors=etal |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> Indeed, images from the Hubble Space Telescope taken on 30&nbsp;March 2020 show a non-stellar core indicating that Comet 2I/Borisov has ejected sunward a large fragment.<ref name="AT-20200402">{{cite news |last1=Jewitt |first1=David |display-authors=etal |title=ATel #13611: Interstellar Object 2I/Borisov Double |url=http://www.astronomerstelegram.org/?read=13611 |date=2 April 2020 |work=[[The Astronomer's Telegram]] | access-date=3 April 2020}}</ref><ref name="AT-20200403">{{cite news |last1=Bolin |first1=Bryce T. |display-authors=etal |title=ATel #13613: Possible fragmentation of interstellar comet 2I/Borisov |date=3 April 2020 |work=[[The Astronomer's Telegram]] | url=http://www.astronomerstelegram.org/?read=13613 |access-date=3 April 2020}}</ref><ref name="SA-20200403">{{cite news |last=Starr |first=Michelle |date=3 April 2020 |title=Interstellar Comet 2I/Borisov really is breaking apart, according to new data |website=ScienceAlert.com |url=https://www.sciencealert.com/interstellar-comet-2i-borisov-is-breaking-apart |access-date=3 April 2020}}</ref> The ejection is estimated to have begun around 7&nbsp;March,<ref name="Bolin_fragmentation">{{cite news |title=Possible fragmentation of interstellar comet 2I/Borisov |website=The Astronomer's Telegram |url=http://www.astronomerstelegram.org/?read=13613 |access-date=3 April 2020}}</ref> and may have occurred during one of the outbursts that occurred near that time.<ref name="Drahus_outburst">{{cite news |title=Multiple Outbursts of Interstellar Comet 2I/Borisov |website=The Astronomer's Telegram |url=http://www.astronomerstelegram.org/?read=13549 |access-date=3 April 2020}}</ref> The ejected fragment appeared to have vanished by 6&nbsp;April 2020.<ref name=AT-20200406/>

A followup study, reported on 6&nbsp;April 2020, observed only a single object, and noted that the fragment component had now disappeared.<ref name="AT-20200406">{{cite news |last1=Zhang |first1=Qicheng |display-authors=etal |title=ATel #16318: interstellar comet 2I/Borisov is single again |date=6 April 2020 |work=[[The Astronomer's Telegram]] | url=http://www.astronomerstelegram.org/?read=13618 |access-date=6 April 2020}}</ref><ref name="NYT-20200407">{{cite news |last=Sokol |first=Joshua |title=It came from outside our Solar system and now it's breaking up |date=7 April 2020 |newspaper=[[The New York Times]] | quote=Comet Borisov, only the second interstellar object spotted by astronomers, shed at least one big chunk as it rounded our sun. |url=https://www.nytimes.com/2020/04/07/science/comet-borisov-break-up.html |access-date=8 April 2020 }}</ref> Later analysis of the event showed the ejected dust and fragments have a combined mass of about 0.1% of total mass of nucleus, making the event a large outburst rather than fragmentation.<ref>{{cite journal |last1=Jewitt |first1=David |last2=Kim |first2=Yoonyoung |last3=Mutchler |first3=Max |last4=Weaver |first4=Harold |last5=Agarwal |first5=Jessica |last6=Hui |first6=Man-To |year=2020 |title=Outburst and splitting of interstellar comet 2I/Borisov |journal=The Astrophysical Journal |volume=896 |issue=2 |page=L39 |arxiv=2006.01242 |doi=10.3847/2041-8213/ab99cb|s2cid=219179411 |bibcode=2020ApJ...896L..39J |doi-access=free }}</ref>

== Exploration ==

The high hyperbolic excess velocity of 2I/Borisov of {{val|32|u=km/s}} makes it hard for a spacecraft to reach the comet with existing technology: according to a team of the [[Initiative for Interstellar Studies]], a 202&nbsp;kg (445&nbsp;lb) spacecraft could theoretically have been sent in July 2018 to intercept 2I/Borisov using a [[Falcon Heavy]]-class launcher, or 765&nbsp;kg (1687&nbsp;lb) on a [[Space Launch System]] (SLS)-class booster, but only if the object had been discovered much earlier than it was to meet the optimal launch date. Launches after the actual discovery date would eliminate the possibility to use Falcon Heavy-class rockets, requiring [[Oberth effect|Oberth maneuvre]]s near Jupiter and near the Sun and a larger launch vehicle. Even an SLS-class launcher would only have been able to deliver a {{convert|3|kg|lb|abbr=on}} payload (such as a [[CubeSat]]) into a trajectory that would intercept 2I/Borisov in 2045 at a relative speed of {{val|34|u=km/s}}.<ref name=Hibberd2019/> According to congressional testimony, NASA may need at least five years of preparation to launch such an [[Asteroid impact avoidance#Deflection efforts|intercepting mission]].<ref name=US-Congress-20130410/>

== See also ==
* [[ʻOumuamua]] - the first interstellar interloper discovered
* Blue (carbon monoxide rich) comets
** [[Comet Morehouse]]
** [[Comet Humason]]
** [[C/2016 R2 (PanSTARRS)|C/2016 R2]]
* [[C/1980 E1 (Bowell)]] – the most eccentric comet known in the Solar System with an eccentricity of 1.057
* [[List of Solar System objects by greatest aphelion]]

== Notes ==

{{Notelist}}

== References ==

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 |url-status=dead  |access-date=13 September 2019
 |archive-url=https://web.archive.org/web/20190910052200/https://projectpluto.com/neocp2/mpecs/gb00234.htm#elements
 |archive-date=10 September 2019
}}</ref>

<ref name=Gray35237>
{{cite web
 |last=Gray |first=Bill
 |date=10 September 2019
 |title=Is gb00234 an interstellar comet or asteroid?
 |publisher=Minor Planet Mailing List
 |url=https://groups.yahoo.com/neo/groups/mpml/conversations/messages/35237
 |archive-url=https://archive.today/20190919073304/https://groups.yahoo.com/neo/groups/mpml/conversations/messages/35237
 |url-status=dead
 |archive-date=19 September 2019
 |access-date=10 September 2019
}}</ref>

<ref name=MPC-DB>
{{cite web
 |title=2I/Borisov = C/2019&nbsp;Q4 (Borisov)
 |date=1 October 2019
 |website=[[Minor Planet Center]]
 |url=https://minorplanetcenter.net/db_search/show_object?object_id=2I
 |access-date=5 October 2019
}}</ref>

<ref name=JPL-Hyperbolic>
{{cite web
 |title=Search Engine: e > 1
 |website=JPL Small-Body Database
 |publisher=[[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=100;format_option=comp;c_fields=AcBgBiBjBhBqCiCk;.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  |access-date=25 September 2019
 |archive-url=https://archive.today/20190925183732/https://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=100;format_option=comp;c_fields=AcBgBiBjBqCiCk;.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
 |archive-date=25 September 2019
}}
</ref>

<ref name=Atel13100>
{{cite news
 |title=Interstellar Comet gb00234
 |website=Astronomer's Telegram
 |url=http://www.astronomerstelegram.org/?read=13100
 |url-status=live |access-date=12 September 2019
 |archive-url=https://web.archive.org/web/20190912164340/http://www.astronomerstelegram.org/?read=13100
 |archive-date=12 September 2019
}}
</ref>

<ref name=Gray-prec>
{{cite web
 |last=Gray  |first=Bill
 |title=Pseudo-MPEC for gb00234 (precovery-eph)
 |website=Project Pluto
 |url=https://projectpluto.com/temp/gb_prec.htm#eph
 |url-status=live |access-date=11 September 2019
 |archive-url=https://web.archive.org/web/20190912141149/https://projectpluto.com/temp/gb_prec.htm
 |archive-date=12 September 2019
}}
</ref>

<ref name=MPEC2019-S72>
{{cite web
 |title=MPEC 2019-S72 : 2I/Borisov = C/2019&nbsp;Q4 (Borisov)
 |website=[[Minor Planet Center]]
 |url=https://minorplanetcenter.net/mpec/K19/K19S72.html
 |access-date=24 September 2019
}}
</ref>

<ref name=Christensen2019>
{{cite press release
 |last1=Christensen |first1=Lars Lindberg
 |title=Naming of new interstellar visitor: 2I/Borisov
 |website=International Astronomical Union
 |url=https://www.iau.org/news/pressreleases/detail/iau1910/
 |access-date=24 September 2019
}}
</ref>

<ref name=archive-is>
{{cite web
 |title=81 obs / 225 hours
 |series=Scout: gb00234
 |website=JPL CNEOS
 |url=https://cneos.jpl.nasa.gov/scout/%23/object/gb00234
 |url-status=dead
 |archive-url=https://archive.today/20190908144030/https://cneos.jpl.nasa.gov/scout/%23/object/gb00234
 |archive-date=2019-09-08
}}<br/>
{{cite web
 |title=99 obs / 272 hours
 |series=Scout: gb00234
 |website=JPL CNEOS
 |url=https://cneos.jpl.nasa.gov/scout/%23/object/gb00234
 |url-status=dead
 |archive-url=https://archive.today/20190910180156/https://cneos.jpl.nasa.gov/scout/%23/object/gb00234
 |archive-date=2019-09-10
}}<br/>
{{cite web
 |title=127 obs / 289 hours
 |series=Scout: gb00234
 |website=JPL CNEOS
 |url=https://cneos.jpl.nasa.gov/scout/%23/object/gb00234
 |url-status=dead
 |archive-url=https://archive.today/20190911040521/https://cneos.jpl.nasa.gov/scout/%23/object/gb00234
 |archive-date=2019-09-11
}}<br/>
{{cite web
 |title=142 obs / 298 hours
 |series=Scout: gb00234
 |website=JPL CNEOS
 |url=https://cneos.jpl.nasa.gov/scout/%23/object/gb00234
 |url-status=dead
 |archive-url=https://archive.today/20190911120905/https://cneos.jpl.nasa.gov/scout/%23/object/gb00234
 |archive-date=2019-09-11
}}<br/>
{{cite web
 |title=151 obs / 302 hours
 |series=Scout: gb00234
 |website=JPL CNEOS
 |url=https://cneos.jpl.nasa.gov/scout/%23/object/gb00234
 |url-status=dead
 |archive-url=https://archive.today/20190911213623/https://cneos.jpl.nasa.gov/scout/%23/object/gb00234
 |archive-date=2019-09-11
}}
</ref>

<ref name=NASA-Initial>
{{cite web
 |title=Newly discovered comet is likely interstellar visitor
 |date=12 September 2019
 |website=JPL / NASA
 |url=https://www.jpl.nasa.gov/news/news.php?feature=7498
}}
</ref>

<ref name=IAC>
{{cite web
 |title=The Gran Telescopio Canarias (GTC) obtains the visible spectrum of C/2019&nbsp;Q4 (Borisov), the first confirmed interstellar comet
 |website=[[Instituto de Astrofísica de Canarias]]
 |date=14 September 2019
 |url=https://www.iac.es/en/outreach/news/gran-telescopio-canarias-obtains-visible-spectrum-c2019-q4-borisov-first-confirmed-interstellar-comet
 |access-date=14 September 2019
}}
</ref>

<ref name=CBAT4670>{{cite news
 |title=IAU Electronic Telegram No.&nbsp;4670
 |publisher=Central Bureau for Astronomical Telegrams
 |url=https://pdssbn.astro.umd.edu/cbet/2019/4670.html
 |url-status=dead  |access-date=25 September 2019
 |archive-url=https://web.archive.org/web/20190925201509/https://pdssbn.astro.umd.edu/cbet/2019/4670.html
 |archive-date=25 September 2019
 }}
</ref>

<ref name=ProjectPluto>
{{cite web
 |last=Gray |first=Bill
 |date=26 October 2017
 |title=Pseudo-MPEC for A/2017&nbsp;U1 (FAQ File)
 |website=Project Pluto
 |url=https://projectpluto.com/temp/2017u1.htm
 |access-date=26 October 2017
}}
</ref>

<ref name=US-Congress-20130410>
{{cite report
 |collaboration=U.S. Congress
 |date=19 March 2013
 |title=Threats from Space: A review of U.S. Government efforts to track and mitigate asteroids and meteors
 |volume=(Part&nbsp;I and Part&nbsp;II)  |page=147
 |department=Committee on Science, Space, and Technology
 |publisher=House of Representatives, One Hundred Thirteenth Congress, First Session
 |url=http://www.gpo.gov/fdsys/pkg/CHRG-113hhrg80552/pdf/CHRG-113hhrg80552.pdf
 |access-date=14 September 2019
}}
</ref>

<ref name=Grossman2019>
{{Cite web
 |last=Grossman |first=Lisa
 |date=12 September 2019
 |title=Astronomers have spotted a second interstellar object
 |website=Science News
 |language=en-US
 |url=https://www.sciencenews.org/article/astronomy-interstellar-comet-space
 |access-date=16 September 2019
}}
</ref>

<ref name=Greskno2019>
{{cite web
 |last=Greskno |first=Michael
 |date=12 September 2019
 |title=Bizarre comet from another star system just spotted
 |website=[[National Geographic]]
 |url=https://www.nationalgeographic.com/science/2019/09/bizarre-interstellar-comet-from-another-star-system-just-spotted/
 |archive-url=https://web.archive.org/web/20190912150200/https://www.nationalgeographic.com/science/2019/09/bizarre-interstellar-comet-from-another-star-system-just-spotted/
 |url-status=dead
 |archive-date=12 September 2019
 |access-date=25 September 2019
}}
</ref>

<ref name=Voyager>
{{cite web
 |title=Fast Facts
 |series=Voyager
 |website=Jet Propulsion Laboratory
 |url=https://voyager.jpl.nasa.gov/frequently-asked-questions/fast-facts/
 |access-date=27 September 2019
}}
</ref>

<ref name=S&T2019>
{{cite news
 |last=King |first=Bob
 |date=11 September 2019
 |title=Is another interstellar visitor headed our way?
 |website=[[Sky & Telescope]]
 |url=https://www.skyandtelescope.com/astronomy-news/possible-interstellar-comet-headed-our-way/
 |access-date=12 September 2019
}}
</ref>

<ref name=RIA>
{{cite news
 |script-title=ru:Крымский астроном заявил, что открытая им комета изменит название
 |trans-title=Crimean astronomer says that the comet discovered by him will be renamed.
 |date=16 September 2019
 |at=para.&nbsp;5
 |language=ru
 |script-website=ru:РИА Новости
 |website=[[RIA Novosti]]
 |url=https://ria.ru/20190916/1558728005.html
 |access-date=16 September 2019
}}
</ref>

<ref name=Fontanka>
{{cite web
 |last=Нелюбин |first=Николай
 |date=13 September 2019
 |script-title=ru:«Готовы открывать кометы бесплатно, лишь бы имя осталось». Российский астроном Геннадий Борисов о первой в истории человечества межзвёздной комете
 |trans-title="We'll discover comets without pay, what matters is that the name lives on" — Russian astronomer Gennadiy Borisov on the first interstellar comet in the history of humankind.
 |script-website=ru:Фонтанка.ру
 |website=[[Fontanka.ru]]
 |language=ru
 |url=https://www.fontanka.ru/2019/09/13/052/
 |access-date=16 September 2019
}}
</ref>

<ref name=STScI-16009>
{{cite web
 |last1=Jewitt   |first1=D.
 |last2=Weaver   |first2=H. A.
 |last3=Mutchler |first3=M.
 |last4=Agarwal  |first4=J.
 |last5=Kim      |first5=Y.
 |last6=Hui      |first6=M.-T.
 |date=25 September 2019
 |title=Proposal&nbsp;16009 - Interstellar Object C/2019&nbsp;Q4
 |website=Space Telescope Science Institute
 |url=http://www.stsci.edu/hst/phase2-public/16009.pdf
 |access-date=27 September 2019
}}
</ref>

<ref name=NYT-20191001>
{{cite news
 |last=Overbye  |first=Dennis   |author-link=Dennis Overbye
 |date=1 October 2019
 |title=An interstellar comet, in time for the holidays
 |quote=On Dec.&nbsp;7, the extrasolar comet now known as 2I/Borisov will make its closest approach to the sun.
 |newspaper=[[The New York Times]]
 |url=https://www.nytimes.com/2019/10/01/science/interstellar-comet-astronomy-borisov.html
 |access-date=2 October 2019
}}
</ref>

<ref name=Hubble100719>{{cite report
 |date=7 October 2019
 |title=HST Weekly Observing Timeline
 |publisher=Space Telescope Science Institute
 |url=http://www.stsci.edu/ftp/observing/weekly_timeline/timeline_10_07_19
 |access-date=8 October 2019
 |archive-url=https://web.archive.org/web/20191105193414/http://www.stsci.edu/ftp/observing/weekly_timeline/timeline_10_07_19
 |archive-date=5 November 2019
 |url-status=dead
 }}</ref>

<ref name=EA-20191014>
{{cite press release
 |title=Interstellar comet with a familiar look
 |publisher=Astronomical Observatory, [[Jagiellonian University]]
 |date=14 October 2019
 |website=[[EurekAlert!]]
 |url=https://www.eurekalert.org/pub_releases/2019-10/aoju-icw101119.php
 |access-date=14 October 2014
}}
</ref>

<ref name=Siegel2017>
{{cite web
 |last=Siegel  |first=Ethan
 |date=16 December 2017
 |title=Why don't comets orbit the same way planets do?
 |series=Ask Ethan
 |url=https://medium.com/starts-with-a-bang/ask-ethan-why-dont-comets-orbit-the-same-way-planets-do-75872f5d5ec0
 |url-status=live |access-date=1 October 2019
 |archive-url=https://web.archive.org/web/20191001175343/https://medium.com/starts-with-a-bang/ask-ethan-why-dont-comets-orbit-the-same-way-planets-do-75872f5d5ec0
 |archive-date=1 October 2019
}}
</ref>

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

== External links ==
{{commons category}}
* {{JPL small body|id=1003639}}
* [https://news.yale.edu/sites/default/files/styles/featured_media/public/comet-side-by-side.jpg?itok=SqC8WoFN&c=07307e7d6a991172b9f808eb83b18804 Image of 2I/Borisov] – [https://news.yale.edu/2019/11/26/new-image-offers-close-view-interstellar-comet comet is "14 times the size of Earth"] ([[Yale University]]; November 2019)
* [https://www.gemini.edu/node/21240 Image of 2I/Borisov] from the [[Gemini Observatory]], Hawaii
* [http://lesia.obspm.fr/comets/lib/display-all-pict.php?Code=CK19Q040&Username=&CodeObs=&Config=&y1=2019&m1=08&y2=2021&m2=10&typ=pict Pictures of 2I/Borisov] from [[Paris Observatory#Meudon|Paris Observatory (LESIA)]]
* [https://in-space.ru/wp-content/uploads/2019/09/gb00234.gif Discovery animation]
* [https://projectpluto.com/temp/2i.htm FAQ] at ProjectPluto (Bill Gray)
* [https://web.archive.org/web/20190918205957/http://exoplanet.eu/catalog/2i/borisov/ Extrasolar Planetary Encyclopedia] – 2I/Borisov
* [http://orbitsimulator.com/gravitySimulatorCloud/simulations/1568147419153_gb00234.html Hyperbolic orbit simulation] – 2I/Borisov (Tony Dunn)
* [https://minorplanetcenter.net/mpec/K19/K19TB6.html Minor Planet Center] MPEC 2019-T116 : COMET 2I/Borisov
* [http://aerith.net/comet/catalog/0002I/mag.gif Magnitude plot] by Seiichi Yoshida @ aerith.net (with predicted brightness)
* {{YouTube|C1IVdfFSPww|Video (31:13) – 2I/Borisov talk}} (MBusch/[[Seth Shostak|SShostak]]) – [[SETI]] (19 September 2019)
* {{YouTube|NNOb3xrjOhE|Video (02:07) – 2I/Borisov chemical composition}} – [[NASA]] (20 April 2020)
* {{YouTube|vqMJo3DHOfg|Video (01:00) – 2I/Borisov orbit animations}}
* [https://gravitysimulator.org/solar-system/interstellar-comet-borisov Interactive 3D gravity simulation of Borisov's Solar System flyby]

{{2019 in space}}
{{Comets}}
{{Portal bar|Astronomy|Stars|Spaceflight|Solar System}}
{{Authority control}}

[[Category:Astronomical objects discovered in 2019|20190830]]
[[Category:Comets in 2019|002I]]
[[Category:Discoveries by amateur astronomers]]
[[Category:Discoveries by the Crimean Astrophysical Observatory]]
[[Category:Hyperbolic comets]]
[[Category:Interstellar objects]]
[[Category:Milky Way]]