# Interstellar object <!-- SOLSIM:BEGIN g31 — Solar System explorer state (hand-built on wt-core, specs/solar/); do not hand-edit inside --> **Microsim — three.js (Wikitube framework):** *2I/Borisov in the Solar System explorer* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?obj=Borisov&embed=1" data-title="2I/Borisov in the Solar System explorer"></div> *The Solar System explorer locked on this article's state (`?obj=Borisov`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.* <!-- SOLSIM:END --> *Try: drag the year slider from 2019 to 2020 and watch 2I/Borisov sweep past the Sun at about 2 AU in December 2019 and head out again on a path that never curves back; set the speed to 1 month/s to see how quickly it crosses the orbits of Mars and Jupiter; then drag to an edge-on view and compare its steeply tilted track with the flat plane of the planets.* An **interstellar object** is a body in the [[Interstellar_medium|space between the stars]] that is not gravitationally bound to any star. The term covers [[Asteroid|asteroids]], [[Comet|comets]] and rogue planets, but not stars or stellar remnants. Most such objects probably formed around a star and were later ejected, for example by close encounters with giant planets such as [[Jupiter]].[^issi2019] When one passes through a planetary system on an unbound, hyperbolic path it is called an interstellar interloper.[^veras2020] Three have been identified in the [[PORTAL_Solar_System|Solar System]]: [[1I/ʻOumuamua]] in 2017, 2I/Borisov in 2019 and 3I/ATLAS in 2025, each recognised by an orbital speed too high for the [[Sun]] to hold.[^jpl-sbdb] The "I" in their designations stands for interstellar and the number for the order of discovery.[^mpec2017v17] The first interstellar bodies recognised were free-floating planetary-mass objects, though these are hard to tell apart from very small brown dwarfs that formed on their own. Speculation that interlopers might be alien spacecraft has not been supported by observation.[^sample2017] At the smallest sizes, a stream of dust from the young star Beta Pictoris may be the main source of interstellar micrometeoroids reaching the Solar System.[^baggaley2000] The explorer at the top of this page follows 2I/Borisov, the first interstellar comet, on its open orbit through the inner Solar System in 2019, among the planets on their [[Orbit|orbits]]. ## Nomenclature Before [[1I/ʻOumuamua|ʻOumuamua]] there was no category for such objects. After its discovery, the Minor Planet Center announced in November 2017 a new series of permanent designations for interstellar objects, modelled on the numbering of periodic [[Comet|comets]]: a sequence number followed by the letter I.[^mpec2017v17] Until an object's interstellar nature is confirmed, it receives an ordinary provisional designation, with C/ if it shows cometary activity and A/ if it looks asteroidal.[^mpec2017v17] The first case shows how the scheme works in practice. Pan-STARRS found the object on 19 October 2017, and it was first announced as comet C/2017 U1, then redesignated A/2017 U1 on 25 October when deep images showed no coma.[^mpec2017u181][^mpec2017u183] Once its hyperbolic orbit was secure it became 1I/2017 U1, with the Hawaiian name ʻOumuamua, "a messenger from afar arriving first".[^geekwire2017] Gennadiy Borisov's comet of August 2019 was first designated C/2019 Q4 (Borisov) and then renumbered 2I/Borisov, keeping the discoverer's name as comets traditionally do.[^mpec2019s72][^iau2019] The third, found by the ATLAS survey in July 2025, was catalogued as comet C/2025 N1 (ATLAS) and numbered 3I/ATLAS.[^jpl-sbdb] ## Overview A body falling toward the [[Sun]] from far away arrives with at least the local escape speed. What marks an interloper is its hyperbolic excess speed v∞, the speed it had before the Sun's [[Gravity|gravity]] accelerated it and will have again once it leaves. Objects from the [[Oort_cloud|Oort cloud]] arrive with v∞ close to zero, whereas interlopers carry the random motions of stars in the Sun's neighbourhood, tens of kilometres per second.[^francis2005] | Object | v∞ (km/s) | |---|---| | C/2012 S1 (ISON), a weakly hyperbolic Oort cloud comet | 0.2[^jpl-sbdb] | | [[Voyager_1|Voyager 1]], for comparison (speed leaving the Solar System) | about 17, i.e. 3.6 AU per year[^nasa-voyager] | | 1I/ʻOumuamua | 26.33[^gray2017] | | 2I/Borisov | 32.3[^gray2019] | | 3I/ATLAS | 58.0[^mpml2025] | The speed at infinity fixes the shape of the path. For perihelion distance q, the eccentricity of a hyperbolic orbit is e = 1 + q v∞²/GM☉. ʻOumuamua, with q = 0.256 AU and v∞ = 26.3 km/s, gives e ≈ 1.20; Borisov, with q = 2.01 AU and v∞ = 32.3 km/s, gives e ≈ 3.36; and 3I/ATLAS, with q = 1.36 AU and 58 km/s, gives e ≈ 6.1, all matching the fitted orbits (derived).[^jpl-sbdb] Interlopers are expected from all directions, but more often from the solar apex in Hercules, the direction in which the Sun moves through the local stars.[^struve1959][^delafuente2018] The numbers were once only limits. Before 2017 the absence of any such comet capped their density in space: Torbett estimated no more than about 10¹³ per cubic parsec in 1986, and an analysis of LINEAR survey data lowered that to about 10¹² per cubic parsec.[^torbett1986][^francis2005] After ʻOumuamua, David Jewitt and colleagues estimated that about 10,000 objects of its size are inside [[Neptune]]'s orbit at any time, each spending about 10 years there, and NASA estimates that several pass inside [[Earth]]'s orbit each year.[^jewitt2017][^nasa-faq] Models of the Oort cloud predict that planetary systems eject between 3 and 100 times as many comets as they keep, so interstellar space should be full of them.[^francis2005][^valtonen1992] Objects can also move between the categories. [[Jupiter]] ejected the Solar System comet C/1980 E1 (Bowell) onto a hyperbolic orbit with e = 1.057 in 1980, and, in reverse, Jupiter could in principle capture a passing interloper, an event simulations suggest might happen once in about 60 million years.[^jpl-sbdb][^torbett1986] Comet Hyakutake's unusual chemistry, the retrograde co-orbital asteroid of Jupiter 514107 Kaʻepaokaʻāwela, and comet C/2018 V1 have all been suggested as possible captured or interstellar bodies.[^mumma1996][^namouni2018][^delafuente2019] The explorer shows only Borisov's path and the planets from JPL elements; the hyperbolic track is computed as a simple two-body orbit.[^jpl-sbdb] ## Solar System interlopers ### Confirmed [[1I/ʻOumuamua|ʻOumuamua]], found by Pan-STARRS at magnitude 20 on 19 October 2017, was already leaving: it had passed perihelion at 0.26 AU the previous month.[^mpec2017u181][^jpl-sbdb] It showed no coma, but its motion revealed a small non-gravitational acceleration. Its lack of visible activity may mean it formed inside the [[Frost_line_(astrophysics)|frost line]] of its home system, or lost its surface volatiles during a long journey.[^issi2019] Astronomers using Gaia data identified several stars that it may have passed closely in the past few million years, none of them a certain origin.[^bailerjones2018] Borisov was discovered on 30 August 2019 by Gennadiy Borisov with a 0.65 m telescope he built himself at MARGO observatory, Nauchnyy, in Crimea.[^king2019] Unlike ʻOumuamua it was plainly a [[Comet|comet]], with a coma and tail, and a spectrum taken on 13 September 2019 with the 10.4 m Gran Telescopio Canarias showed a composition much like that of Solar System comets.[^deleon2019] Its perihelion, at 2.01 AU, came in December 2019, and in March 2020 observers reported outbursts and signs that its nucleus was fragmenting.[^jpl-sbdb][^drahus2020] 3I/ATLAS was found on 1 July 2025 at about 4.5 AU from the Sun, just inside Jupiter's orbit. It reached perihelion on 29 October 2025 at 1.36 AU, and with an eccentricity of 6.14 and v∞ of about 58 km/s it is the fastest of the three.[^jpl-sbdb][^mpml2025] ### Unconfirmed Other claims are less secure. In 2007 a team reported what they took to be a centimetre-sized intergalactic meteor over the Special Astrophysical Observatory in Russia on 28 July 2006.[^afanasiev2007] Amir Siraj and Avi Loeb argued in 2019 that some objects already orbiting the Sun, including a few [[Centaur_(small_Solar_System_body)|centaurs]], have orbits better explained as captured interstellar objects, and that several such bodies should exist.[^siraj2019b] A 2023 study argued that a few interlopers may have been captured into [[Near-Earth_object|near-Earth orbits]].[^mukherjee2023] The most publicised case is CNEOS 2014-01-08, a fireball about half a metre across that exploded near Papua New Guinea on 8 January 2014. A 2019 preprint by Siraj and Loeb argued from its catalogued speed of about 60 km/s relative to the Sun that it came from interstellar space.[^siraj2019a] In April 2022 the US Space Command stated that the velocity estimate was accurate enough to support that conclusion.[^nasa-bolide2022] Other researchers are unconvinced. The government catalogue gives no uncertainties, and analyses by Jérémie Vaubaillon, by Peter Brown and Jiří Borovička, and of seismic records by Benjamin Fernando and colleagues all found that the speed and trajectory are poorly constrained or inconsistent with the claim.[^vaubaillon2022][^brown2023][^fernando2024] Spherules recovered from the seafloor in 2023 and proposed as fragments are attributed by critics to terrestrial sources such as coal ash.[^gallardo2023][^miller2023] A second fireball, CNEOS 2017-03-09, has been proposed by the same group on the basis of its high strength.[^vice2017] ### Micrometeoroids Dust from interstellar space is easier to catch, as small [[Meteoroid|meteoroids]] and grains mixed into the [[Interplanetary_dust_cloud|interplanetary dust]]. In 2000 the Advanced Meteor Orbit Radar in New Zealand detected a stream of particles arriving from the direction of Beta Pictoris, possibly the main source of interstellar micrometeoroids in the Solar System.[^baggaley2000] The grains are larger than 20 micrometres and must have left their star at about 25 km/s; migrating giant planets or radiation pressure could have ejected them, so the stream may be a sign that Beta Pictoris is building its own [[Oort_cloud|Oort cloud]].[^krivova2003][^krivov2004] Identifying interstellar meteors in general is difficult, because small errors in measured speed can push a near-parabolic orbit over the escape limit and create a false population.[^hajdukova2020] The smallest grains are also filtered out by the Sun's magnetic field in the [[Heliosphere|heliosphere]] before they reach the inner system.[^sterken2012] ## Hypothetical missions Reaching an interloper is hard because they are fast and are found late, usually close to perihelion. Studies argue that a close flyby is nonetheless possible with current technology if a spacecraft can be launched quickly.[^seligman2018] The Initiative for Interstellar Studies began Project Lyra in 2017 to study a mission to ʻOumuamua. Its options, with flight times of 5–25 years, include a [[Jupiter]] flyby followed by a close pass of the [[Sun]] at about 3 solar radii, where a rocket burn gains the most speed by the Oberth effect.[^hein2019][^hibberd2020] Because the [[Kinetic_energy|kinetic energy]] added by a burn grows with the speed at which it is made, a burn deep in the Sun's gravity well gives a far larger speed at infinity than the same burn made far away. The most concrete plan is ESA's Comet Interceptor, built with JAXA, which is due to launch in 2029 and wait at the Sun–[[Earth]] L2 point for a suitable newly found long-period comet. If none appears within about three years, it could be sent to a reachable interstellar object instead.[^esa-ariel][^ocallaghan2019] The Vera C. Rubin Observatory is expected to find interlopers far more often than earlier surveys, giving such missions more targets and warning.[^siraj2020][^williams2020] ## See also - [[1I/ʻOumuamua]] · [[Comet]] · [[Interstellar_medium]] - [[Oort_cloud]] · [[Hills_cloud]] · [[Local_Interstellar_Cloud]] - Rogue planet · exocomet ## References [^issi2019]: 'Oumuamua ISSI Team; Bannister, M. T.; Bhandare, A.; Dybczyński, P. A.; Fitzsimmons, A.; Guilbert-Lepoutre, A.; et al. (2019). "The natural history of ʻOumuamua". *Nature Astronomy* 3: 594–602. https://doi.org/10.1038/s41550-019-0816-x [^veras2020]: Veras, D. (2020). "Creating the first interstellar interloper". *Nature Astronomy* 4: 835–836. https://doi.org/10.1038/s41550-020-1064-9 [^jpl-sbdb]: JPL Solar System Dynamics. "Small-Body Database Lookup": 1I/ʻOumuamua, 2I/Borisov (C/2019 Q4), 3I/ATLAS (C/2025 N1), C/1980 E1 (Bowell), C/2012 S1 (ISON). https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html (elements fetched 2026-09-18). [^mpec2017v17]: Minor Planet Center (6 November 2017). "MPEC 2017-V17: New designation scheme for interstellar objects". https://www.minorplanetcenter.net/mpec/K17/K17V17.html [^sample2017]: Sample, I. (14 December 2017). "Is ʻOumuamua an alien spacecraft? Initial scans show no signs of technology". *The Guardian*. https://www.theguardian.com/science/2017/dec/14/is-oumuamua-an-alien-spacecraft-first-scans-show-no-signs-of-technology [^baggaley2000]: Baggaley, W. J. (2000). "Advanced Meteor Orbit Radar observations of interstellar meteoroids". *Journal of Geophysical Research* 105: 10353–10361. https://doi.org/10.1029/1999JA900383 [^mpec2017u181]: Minor Planet Center (25 October 2017). "MPEC 2017-U181: Comet C/2017 U1 (PANSTARRS)". https://www.minorplanetcenter.net/mpec/K17/K17UI1.html [^mpec2017u183]: Meech, K.; et al. (25 October 2017). "MPEC 2017-U183: A/2017 U1". Minor Planet Center. https://www.minorplanetcenter.net/mpec/K17/K17UI3.html [^geekwire2017]: *GeekWire* (20 November 2017). "Aloha, ʻOumuamua! Scientists confirm that interstellar asteroid is a cosmic oddball". *GeekWire*. https://www.geekwire.com/2017/aloha-oumuamua-interstellar-asteroid-gets-hawaiian-name-first-kind/ [^mpec2019s72]: Minor Planet Center (24 September 2019). "MPEC 2019-S72: 2I/Borisov = C/2019 Q4 (Borisov)". https://minorplanetcenter.net/mpec/K19/K19S72.html [^iau2019]: Christensen, L. L. (24 September 2019). "Naming of new interstellar visitor: 2I/Borisov" (press release iau1910). International Astronomical Union. https://www.iau.org/news/pressreleases/detail/iau1910/ [^francis2005]: Francis, P. J. (2005). 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V.; Krivova, N. A.; Solanki, S. K.; Titov, V. B. (2004). "Towards understanding the β Pictoris dust stream". *Astronomy & Astrophysics* 417: 341–352. https://doi.org/10.1051/0004-6361:20034379 [^hajdukova2020]: Hajduková, M.; Sterken, V.; Wiegert, P.; Kornoš, L. (2020). "The challenge of identifying interstellar meteors". *Planetary and Space Science* 192: 105060. https://doi.org/10.1016/j.pss.2020.105060 [^sterken2012]: Sterken, V. J.; Altobelli, N.; Kempf, S.; Schwehm, G.; Srama, R.; Grün, E. (2012). "The flow of interstellar dust into the solar system". *Astronomy & Astrophysics* 538: A102. https://doi.org/10.1051/0004-6361/201117119 [^seligman2018]: Seligman, D.; Laughlin, G. (2018). "The feasibility and benefits of in situ exploration of ʻOumuamua-like objects". *The Astronomical Journal* 155: 217. https://doi.org/10.3847/1538-3881/aabd37 [^hein2019]: Hein, A. M.; Perakis, N.; Eubanks, T. M.; Hibberd, A.; Crowl, A.; Hayward, K.; et al. (2019). "Project Lyra: sending a spacecraft to 1I/ʻOumuamua (former A/2017 U1), the interstellar asteroid". *Acta Astronautica* 161: 552–561. https://doi.org/10.1016/j.actaastro.2018.12.042 [^hibberd2020]: Hibberd, A.; Hein, A. M.; Eubanks, T. M. (2020). "Project Lyra: catching 1I/ʻOumuamua – mission opportunities after 2024". *Acta Astronautica* 170: 136–144. https://doi.org/10.1016/j.actaastro.2020.01.018 [^esa-ariel]: European Space Agency (12 November 2020). "Ariel moves from blueprint to reality". https://www.esa.int/Science_Exploration/Space_Science/Ariel_moves_from_blueprint_to_reality [^ocallaghan2019]: O'Callaghan, J. (24 June 2019). "European Comet Interceptor could visit an interstellar object". *Scientific American*. https://www.scientificamerican.com/article/european-comet-interceptor-could-visit-an-interstellar-object/ [^williams2020]: Williams, M. (7 November 2020). "Vera Rubin should be able to detect a couple of interstellar objects a month". *Universe Today*. https://www.universetoday.com/148696/vera-rubin-should-be-able-to-detect-a-couple-of-interstellar-objects-a-month/ [^vice2017]: *Vice*. "Alien-hunting astronomer says there may be a second interstellar object on Earth in new study". https://www.vice.com/en/article/alien-hunting-astronomer-says-there-may-be-a-second-interstellar-object-on-earth-in-new-study/ [^siraj2020]: Siraj, A.; Loeb, A. (2020). "Observable signatures of the ejection speed of interstellar objects from their birth systems". *The Astrophysical Journal Letters* 903: L20. https://doi.org/10.3847/2041-8213/abc170 ## External links - NASA Planetary Defense: interstellar asteroid FAQs. https://www.nasa.gov/planetarydefense/faq/interstellar - Minor Planet Center: designation scheme for interstellar objects (MPEC 2017-V17). https://www.minorplanetcenter.net/mpec/K17/K17V17.html - JPL Small-Body Database: 2I/Borisov. https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=2I - ESA Comet Interceptor. https://www.esa.int/Science_Exploration/Space_Science/Comet_Interceptor ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Interstellar_object) : [Wikitube](https://en.wikitube.io/wiki/Interstellar_object) · pinned revision [1373958509](https://en.wikipedia.org/w/index.php?oldid=1373958509) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Solar_System]]. --- *Solar System portal child articles, wave 1 · 2026-09-18 · drafted · row SOL-085 · explorer state `?obj=Borisov`.* <!-- hub_tags: Life_Physics · PORTAL_Solar_System -->