Editing 3753 Cruithne

{{Short description|Aten asteroid co-orbital with Earth}}
{{Use dmy dates|date=October 2019}}
{{Infobox planet
| minorplanet = yes
| background = #FFC2E0
| name = 3753 Cruithne
| image = Cruithne.jpg
| image_scale =
| discoverer = [[Duncan Waldron]]
| discovered = 10 October 1986
| mpc_name = (3753) Cruithne
| alt_names = 1983 UH; 1986 TO
| named_after = [[Cruthin]]
| pronounced = {{IPAc-en|lang|k|r|u|ˈ|iː|n|j|ə}} {{respell|kroo|EEN|yə}}<br/>{{IPA|ga|ˈkɾˠɪ(h)nʲə, ˈkɾˠʊnʲə|lang}}
| mp_category = {{Ubl
 | [[Near-Earth asteroid|NEO]]
 | [[Aten asteroid|Aten]]&thinsp;<ref name=jpldata/>
 | [[horseshoe orbit|horseshoe]]
 | [[Venus-crosser]]
 | [[Mars-crosser]]
 }}
| orbit_ref = <ref name=jpldata />
| epoch = 4 September 2017 ([[Julian day|JD]]&nbsp;2458000.5)
| observation_arc = 16087 days (44.04&nbsp;yr)
| uncertainty = 0
| aphelion = {{Convert|1.5114|AU|km|abbr=on|lk=on}}
| perihelion = {{Convert|0.48405|AU|km|abbr=on}}
| semimajor = {{Convert|0.99774|AU|km|abbr=on}}
| eccentricity = 0.51485<br />(213000 wrt Earth)<ref name="geocentric"/>
| period = 1.00&nbsp;[[Julian year (astronomy)|yr]] (364.02&nbsp;[[Julian year (astronomy)|d]])
| inclination = 19.805°
| asc_node = 126.23°
| arg_peri = 43.831°
| mean_anomaly = 257.46[[Degree (angle)|°]]
| mean_motion = {{Deg2DMS|0.98901|sup=ms}} / day
| moid = {{Convert|0.07119|AU|km|abbr=on}}
| avg_speed = 27.73&nbsp;[[kilometer per second|km/s]]
| mean_diameter = ~5&nbsp;km
| mass = {{Val|1.3e14|u=kg}}
| rotation = {{Convert|27.30990|h|d|abbr=on|lk=on}}<ref name=jpldata/>
| albedo = 0.15
| spectral_type = Q
| abs_magnitude = 15.6<ref name=jpldata/>
}}

'''3753 Cruithne''' is a [[Q-type asteroid|Q-type]], [[Aten asteroid]] in [[orbit]] around the [[Sun]] in 1:1 [[orbital resonance]] with [[Earth]], making it a [[Co-orbital configuration|co-orbital object]]. It is an [[asteroid]] that, relative to Earth, orbits the Sun in a bean-shaped orbit that effectively describes a [[horseshoe orbit|horseshoe]], and that can change into a [[quasi-satellite]] orbit.<ref>{{cite journal|doi=10.1111/j.1365-2966.2011.18595.x|title=A long-lived horseshoe companion to the Earth|date=2011|last1=Christou|first1=A. A.|last2=Asher|first2=D. J.|journal=[[Monthly Notices of the Royal Astronomical Society]]|volume=414|issue=4|pages=2965|doi-access=free |arxiv = 1104.0036 |bibcode = 2011MNRAS.414.2965C |s2cid=13832179}}</ref> Cruithne does not orbit Earth and at times it is on the other side of the Sun,<ref name=WWU/> placing Cruithne well outside of Earth's [[Hill sphere]]. Its orbit takes it near the orbit of [[Mercury (planet)|Mercury]] and outside the orbit of [[Mars]].<ref name=WWU/> Cruithne orbits the Sun in about one Earth year, but it takes 770 years for the series to complete a horseshoe-shaped movement around Earth.<ref name=WWU/>

The asteroid takes its name from the [[Cruthin|Cruithne]], a people mentioned in early Irish annals.<ref name=WWU>[http://www.wwu.edu/depts/skywise/a101_cruithne.html Cruithne: Asteroid 3753] {{Webarchive|url=https://web.archive.org/web/20120302001141/http://www.wwu.edu/depts/skywise/a101_cruithne.html |date=2012-03-02 }}. Western Washington University Planetarium. Retrieved 27 January 2011.</ref>

== Discovery ==
Cruithne was discovered on 10 October 1986 by [[Duncan Waldron]] on a photographic plate taken with the [[UK Schmidt Telescope]] at [[Siding Spring Observatory]], [[Coonabarabran]], [[Australia]]. A 1983 [[precovery]] (1983 UH) is credited to [[Giovanni de Sanctis]] and [[Richard M. West]] of the [[European Southern Observatory]] in [[Chile]].<ref>{{cite journal |title=The Orbital Evolution of Near-Earth Asteroid 3753 |author=Wiegert, Paul A. |author2=Innanen, Kimmo |name-list-style=amp |date=June 1998 |journal=The Astronomical Journal |volume=115 |issue=6 |pages=2604–2613 |doi=10.1086/300358|bibcode = 1998AJ....115.2604W |s2cid=121795378 |doi-access=free }}</ref>

It was not until 1997 that its unusual orbit was determined by [[Paul Wiegert]] and [[Kimmo Innanen]], working at [[York University]] in [[Toronto]], and [[Seppo Mikkola]], working at the [[University of Turku]] in [[Finland]].<ref>{{cite journal |url=http://www.astro.uwo.ca/~wiegert/papers/1997Nature.387.685.pdf |archive-url=https://web.archive.org/web/20131203004924/http://www.astro.uwo.ca/~wiegert/papers/1997Nature.387.685.pdf |archive-date=2013-12-03 |url-status=live |title=An asteroidal companion to the Earth (letter) |author=Wiegert, Paul A. |journal=Nature |date=12 June 1997 |volume=387 |issue=6634 |pages=685–86 |access-date=25 November 2013 |doi=10.1038/42662|s2cid=4305272 |display-authors=etal}}</ref>

== Dimensions and orbit ==
[[File:Animation of 3753 Cruithne orbit.gif|thumb|left|Animation of 3753 Cruithne orbit from 1600 to 2500<br />{{legend2| Yellow| Sun}}{{·}}{{legend2| RoyalBlue| Earth}}{{·}}{{legend2|Magenta| 3753 Cruithne}}]]
[[Image:Orbits of Cruithne and Earth.gif|frame|left|Cruithne and Earth seem to follow each other because of a 1:1 [[orbital resonance]].]]
[[Image:Horseshoe orbit of Cruithne from the perspective of Earth.gif|frame|left|Cruithne appears to make a bean-shaped orbit from the perspective of Earth.]]
Cruithne is approximately {{convert|5|km|0}} in [[diameter]], and its closest approach to Earth is {{convert|12|e6km|AU mi}}, approximately thirty times the separation between Earth and the [[Moon]]. From 1994 through 2015, Cruithne made its annual closest approach to Earth every November.<ref name=jpl-close>{{cite web |type=2008-10-25 last obs |title=JPL Close-Approach Data: 3753 Cruithne (1986 TO) |url=https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=3753;cad=1#cad |access-date=2009-06-28}}</ref>

Although Cruithne's orbit is not thought to be stable over the long term, calculations by Wiegert and Innanen showed that it has probably been synchronized with Earth's orbit for a long time. There is no danger of a collision with Earth for millions of years, if ever. Its orbital path and Earth's do not cross, and its orbital plane is currently tilted to that of Earth by 19.8°. Cruithne, having a maximum near-Earth [[apparent magnitude|magnitude]] of +15.8, is fainter than [[Pluto]] and would require at least a {{convert|12.5|in|mm|adj=on|order=flip}} [[reflecting telescope]] to be seen.<ref>{{cite web |url=http://www.science.edu.sg/ssc/detailed.jsp?artid=1950&type=6&root=6&parent=6&cat=66 |title=This month Pluto's apparent magnitude is m=14.1. Could we see it with an 11" reflector? |publisher=Singapore Science Centre |access-date=2007-03-25 |archive-url=https://web.archive.org/web/20070930160349/http://www.science.edu.sg/ssc/detailed.jsp?artid=1950&type=6&root=6&parent=6&cat=66 |archive-date=2007-09-30 |url-status=dead }}</ref><ref>{{cite web |url = http://www.icq.eps.harvard.edu/MagScale.html |title = The astronomical magnitude scale. |publisher = The ICQ Comet Information Website |access-date = 2007-09-26}}</ref>

Cruithne is in a normal [[elliptic orbit]] around the Sun. Its [[orbital period|period of revolution]] around the Sun, approximately 364 days in the early 21st century, is almost equal to that of Earth. Because of this, Cruithne and Earth appear to "follow" each other in their paths around the Sun. This is why Cruithne is sometimes called "Earth's second moon".<ref name="space.com">{{Cite web |last=Lloyd |first=Robin |publisher=[[Space.com]] |url=http://utstaging.space.com/scienceastronomy/solarsystem/second_moon_991029.html |archive-url=https://archive.today/20121208143923/http://utstaging.space.com/scienceastronomy/solarsystem/second_moon_991029.html |url-status=dead |archive-date=2012-12-08 |title=More Moons Around Earth? }}</ref> However, it does not orbit Earth and is not a moon.<ref>Meeus, reference above, writes "we may not deduce that Cruithne is a "companion" of the Earth, as some authors wrote, and certainly it is not a satellite! The object simply ''cannot'' be a satellite of the Earth, as it moves from nearly the orbit of Mercury to outside that of Mars, and because sometimes it is in ''superior'' conjunction, at the far side of the Sun as seen from the Earth".</ref> In 2058, Cruithne will come within 0.09&nbsp;[[Astronomical unit|AU]] ({{convert|13.6|e6km|e6mi|disp=or|abbr=off}}) of Mars.<ref name=jpl-close/>

Due to a high [[orbital eccentricity]], Cruithne's distance from the Sun and orbital speed vary a lot more than Earth's, so from Earth's point of view Cruithne actually follows a [[kidney bean|kidney-bean]]-shaped [[horseshoe orbit]] ahead of Earth, taking slightly less than one year to complete a circuit of the "bean". Because it takes slightly less than a year, Earth "falls behind" the bean a little more each year, and so, from the point of view of an observer on Earth, the circuit is not quite closed, but rather like a spiral loop that moves slowly away from Earth.{{citation needed|date=April 2021}}

After many years, Earth will have fallen so far behind that Cruithne will then actually be "catching up" on Earth from "behind". When it eventually does catch up, Cruithne will make a series of annual close approaches to Earth and [[gravity assist|gravitationally exchange orbital energy]] with Earth; this will alter Cruithne's orbit by a little over half a million kilometres—while Earth's orbit is altered by about {{convert|1.3|cm}}—so that its period of revolution around the Sun will then become slightly more than a year. The kidney bean will then start to migrate away from Earth again in the opposite direction—instead of Earth "falling behind" the bean, it is "pulling away from" the bean. The next such series of close approaches will be centred on the year 2292—in July of that year, Cruithne will approach Earth to about {{convert|12.5|e6km|AU mi}}.{{citation needed|date=April 2021}}

After 380 to 390 years or so, the kidney-bean-shaped orbit approaches Earth again from the other side, and Earth, once more, alters the orbit of Cruithne so that its period of revolution around the Sun is again slightly less than a year (this last happened with a series of close approaches centered on 1902, and will next happen with a series centered on 2676). The pattern then repeats itself.<ref>{{Cite web |date=2022-12-28 |title=3753 Cruithne, la «deuxième Lune» dont vous ignoriez l'existence |url=https://korii.slate.fr/et-caetera/astronomie-3753-cruithne-deuxieme-lune-espace-terre-satellite-orbite-ellipse-soleil |access-date=2024-10-17 |website=korii. |language=fr-FR}}</ref>

== Similar minor planets ==
More near-[[Orbital resonance|resonant]] [[near-Earth object]]s (NEOs) have since been discovered. These include [[54509 YORP]], {{mpl|(85770) 1998 UP|1}}, {{mpl|2002 AA|29}}, and {{mpl|2009 BD|}} which exist in resonant orbits similar to Cruithne's. {{mpl|706765|2010 TK|7}} is the first identified [[Earth trojan]] (out of only two known {{asof|2024|lc=y}}).

Other examples of natural bodies known to be in [[horseshoe orbit]]s (with respect to each other) include [[Janus (moon)|Janus]] and [[Epimetheus (moon)|Epimetheus]], [[natural satellite]]s of [[Saturn]]. The orbits these two moons follow around Saturn are much simpler than the one Cruithne follows, but operate along the same general principles.

Mars has four known co-orbital asteroids ([[5261 Eureka]], {{mpl|1999 UJ|7}}, {{mpl|1998 VF|31}}, and {{mpl|2007 NS|2}}, all at the [[Lagrangian point]]s), and [[Jupiter]] has many (an estimated one million greater than 1&nbsp;km in diameter, the [[Jovian trojans]]); there are also other small co-orbital moons in the Saturnian system: [[Telesto (moon)|Telesto]] and [[Calypso (moon)|Calypso]] with [[Tethys (moon)|Tethys]], and [[Helene (moon)|Helene]] and [[Polydeuces (moon)|Polydeuces]] with [[Dione (moon)|Dione]]. However, none of these follow horseshoe orbits.

== In popular culture ==
Cruithne plays a major role in Stephen Baxter's novel ''[[Manifold: Time]]'', which was nominated for the [[Arthur C. Clarke Award]] for best science fiction in 2000.

Cruithne is mentioned on the ''[[QI]]'' season 1 episode "Astronomy", in which it is described as a second moon of Earth.<ref>{{cite web |title=QI (2003–…): SEASON 1, EPISODE 2 – ASTRONOMY – FULL TRANSCRIPT |url=https://subslikescript.com/series/QI-380136/season-1/episode-2-Astronomy}}</ref> In a later episode, it was added that, if using the same definition, Earth has over 18,000 "mini-moons".

In ''[[Astonishing X-Men]]'', Cruithne is the site of a secret lab assaulted by [[Abigail Brand]] and her S.W.O.R.D. team. It contains many [[Brood (comics)|Brood]] before Brand destroys it.<ref>''Astonishing X-Men'' vol. 3 #31</ref>

In the [[Insignia trilogy]], 3753 Cruithne has been moved into an orbit around Earth to serve as a training ground for the Intrasolar Forces.  In the third novel, ''Catalyst'', it is intentionally directed at Earth.  While it is destroyed before impact, its fragments rain down on Earth's surface, killing nearly 800 million people across the world.

{{clear left}}

==Gallery==
<gallery widths="200" heights="200">
Cruithnes distance to Earth and Sun.jpg|Cruithne's distance to Earth (blue) and the Sun (yellow) plotted over 500 years (top) and 10 years (bottom)
Lagrange Horseshoe Orbit.jpg|Plan showing possible orbits along gravitational contours (not to scale)
Animation of (419624) 2010 SO16 orbit.gif|An example of a horseshoe orbit<br />{{legend2| Yellow| Sun}}{{·}}{{legend2| RoyalBlue| Earth}}{{·}}{{legend2|Magenta| [[(419624) 2010 SO16]]}}
</gallery>

== See also ==
* {{mpl|2006 RH|120}}{{Snd}} temporary Earth satellite discovered in 2006
* {{mpl|2010 SO|16}}
* [[Co-orbital configuration]]
* [[Claimed moons of Earth|Earth's second moon]]
* [[Natural satellite]]
* [[Quasi-satellite]]
* [[List of Earth-crossing asteroids]]

== References ==
{{Reflist|refs=

<ref name=jpldata>{{cite web
   |type=2017-11-02 last obs
   |title=JPL Small-Body Database Browser: 3753 Cruithne (1986 TO)
   |url=https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=3753
   |access-date=14 April 2016}}</ref>

<ref name="geocentric">{{cite web
  |title      = JPL Horizons On-Line Ephemeris for Cruithne orbit of Earth (geocentric) at epoch 2017-Sep-04
  |url        = https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%27Cruithne%27&TABLE_TYPE=%27ELEMENTS%27&START_TIME=%272017-Sep-04%27&STOP_TIME=%272100-01-01%27&STEP_SIZE=%27150%20years%27&CENTER=%27@399%27&OUT_UNITS=%27AU-D%27
  |work       = [[JPL Horizons On-Line Ephemeris System]]
  |publisher  = [[Jet Propulsion Laboratory]]
  |access-date = 19 July 2021}} Geocentric solution. Ephemeris Type: Orbital Elements / Center: @399 / Time Span: 2017-Sep-04 (to match infobox epoch)</ref>

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

== Further reading ==
*{{cite journal |last=Wiegert |first=Paul A. |author2=Innanen, Kimmo A.|author3= Mikkola, Seppo |date=1997 |title=An asteroidal companion to the Earth |journal=[[Nature (journal)|Nature]] |volume=387 |issue=6634 |pages=685–686 |doi=10.1038/42662 |s2cid=4305272 |doi-access=free }}
*{{cite book |chapter=Cruithne, an asteroid with a remarkable orbit |title=More Mathematical Astronomy Morsels |last=Meeus |first=Jean |date=2002 |publisher=Willmann-Bell |location=Richmond, Virginia |isbn=978-0-943396-74-3 }}

== External links ==
* [http://www.astro.uwo.ca/~wiegert/3753/3753.html Paul Wiegert's page about Cruithne, with movies]
* [http://burtleburtle.net/bob/physics/cruithne.html Java-applet based animations showing Cruithne's orbit]
* [http://www.orbitsimulator.com/gravity/articles/cruithne.html A simulation of Cruithne's orbit with animation] (Gravity Simulator)
* [https://web.archive.org/web/20071013190453/http://curious.astro.cornell.edu/question.php?number=578 Curious About Astronomy: Have astronomers discovered Earth's second moon?]
*[https://gravitysimulator.org/solar-system/cruithne-co-orbital-object-of-earth Interactive 3D gravity simulation of Cruithne's orbit]
* {{NeoDys|3753}}
* {{ESA-SSA|3753|Cruithne}}
* {{JPL small body}}

{{Minor planets navigator |3752 Camillo |number=3753 |3754 Kathleen}}
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{{DEFAULTSORT:Cruithne}}
[[Category:Aten asteroids|003753]]
[[Category:Discoveries by Duncan Waldron]]
[[Category:Named minor planets]]
[[Category:Earth-crossing asteroids]]
[[Category:Earth co-orbital asteroids|003753]]
[[Category:Q-type asteroids (SMASS)|003753]]
[[Category:Astronomical objects discovered in 1986|19861010]]
[[Category:Horseshoe orbit]]