3753 Cruithne

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3753 Cruithne
Asteroid 3753 Cruithne
Discovered by Duncan Waldron
Discovery date October 10, 1986
Named after
1983 UH; 1986 TO
Aten asteroid,[1]
bean-shaped horseshoe orbit
Near-Earth asteroid,
Venus-crosser asteroid,
Mars-crosser asteroid
Orbital characteristics[1]
Epoch 2014-Dec-09 (JD 2457000.5)
Aphelion 1.511 AU (226 Gm)
Perihelion 0.4840 AU (72.4 Gm)
0.9977 AU (149.3 Gm)
Eccentricity 0.5149
(500,000 w.r.t. Earth)
1.00 a (363.99 d)
27.73 km/s
Inclination 19.81°
Physical characteristics
Dimensions ~5 km
27.31 hours[1]

3753 Cruithne (/krˈnjə/[2] or /ˈkrʊnjə/)[3] is a Q-type, Aten asteroid in orbit around the Sun in 1:1 orbital resonance with Earth, making it a co-orbital object. It is a minor planet in solar orbit that, relative to Earth, orbits in a bean-shaped orbit that ultimately effectively describes a horseshoe, and which can transition into a quasi-satellite orbit.[4] It has been incorrectly called "Earth's second moon".[2][5] Cruithne does not orbit Earth and at times it is on the other side of the Sun.[6] Its orbit takes it inside the orbit of Mercury and outside the orbit of Mars.[6] Cruithne orbits the Sun in about 1 year but it takes 770 years for the series to complete a horseshoe-shaped movement around the Earth.[6]

The name Cruithne is from Old Irish and refers to the early Picts (Irish: Cruthin) in the Annals of Ulster[6] and their eponymous king ("Cruidne, son of Cinge") in the Pictish Chronicle.


Cruithne was discovered on October 10, 1986, by Duncan Waldron on a photographic plate taken with the UK Schmidt Telescope at Siding Spring Observatory, Coonabarabran, Australia. The 1983 apparition (1983 UH) is credited to Giovanni de Sanctis and Richard M. West of the European Southern Observatory in Chile.[7]

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.[8]

Dimensions and orbit

Cruithne and Earth seem to follow each other because of a 1:1 orbital resonance.
Cruithne appears to make a bean-shaped orbit from the perspective of Earth.
Cruithne's distance to Earth (blue) and the Sun (yellow) plotted over 500 years (top) and 10 years (bottom).

Cruithne is approximately 5 kilometres (3 mi) in diameter, and its closest approach to Earth is 12 million kilometres (0.080 AU; 7,500,000 mi), approximately thirty times the separation between Earth and the Moon. From 1994 through 2015, Cruithne makes its annual closest approach to Earth every November.[9]

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 the Earth by 19.8°. Cruithne, having a maximum near-Earth magnitude of +15.8, is fainter than Pluto and would require at least a 320-millimetre (12.5 in) reflecting telescope to be seen.[10][11]

Cruithne is in a normal elliptic orbit around the Sun. Its period of revolution around the Sun, approximately 364 days at present, is almost equal to that of the 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".[12] However, it does not orbit the Earth and is not a moon.[13] In 2058, Cruithne will come within 0.09 AU (13.6 million kilometres or 8.5 million miles) of Mars.[9]

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

After many years, the Earth will have fallen so far behind that Cruithne will then actually be "catching up" on the Earth from "behind". When it eventually does catch up, Cruithne will make a series of annual close approaches to the Earth and 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 1.3 centimetres (0.51 in)—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 the Earth again in the opposite direction – instead of the Earth "falling behind" the bean, the Earth 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 12.5 million kilometres (0.084 AU; 7,800,000 mi).

After 380 to 390 years or so, the kidney-bean-shaped orbit approaches Earth again from the other side, and the 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 centred on 1902, and will next happen with a series centered on 2676). The pattern then repeats itself.

Similar minor planets

Figure 1. Plan showing possible orbits along gravitational contours (not to scale)

More near-resonant near-Earth objects (NEOs) have since been discovered. These include 54509 YORP, (85770) 1998 UP1, 2002 AA29, and 2009 BD which exist in resonant orbits similar to Cruithne's. 2010 TK7 is the first and so far only identified Earth trojan.

Other examples of natural bodies known to be in horseshoe orbits (with respect to each other) include Janus and Epimetheus, natural satellites 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, 1999 UJ7, 1998 VF31, and 2007 NS2, all at the Lagrangian points), and Jupiter has many (an estimated one million greater than 1 km in diameter, the Jovian trojans); there are also other small co-orbital moons in the Saturnian system: Telesto and Calypso with Tethys, and Helene and Polydeuces with Dione. However, none of these follow horseshoe orbits.

Cruithne in popular culture

Cruithne plays a major role in Stephen Baxter's novel Manifold: Time, which was nominated for the Arthur C. Clarke science fiction award in 2000.

Cruithne is mentioned on the QI season 1 episode "Astronomy", in which it is incorrectly described as a second moon of Earth.

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 before Brand destroys it.[14]

In Peter Cawdron's novel XENOPHOBIA, a spacecraft named Orion was supposedly scheduled to explore Cruithne, but was actually tasked to intercept an alien spacecraft before its launch was scrubbed.

See also


  1. 1.0 1.1 1.2 1.3 "JPL Small-Body Database Browser: 3753 Cruithne (1986 TO)" (2014-09-13 last obs). Retrieved 2014-02-26.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  2. 2.0 2.1 For instance, on the British television show Q.I. (Episode 2, Season 1; aired 11 Sept 2003).
  3. Modern Irish [ˈkrɪhnʲə], [ˈkrɪnʲə], or [ˈkrʊnʲə]; Old Irish [ˈkruθʲnʲe]
  4. Lua error in Module:Citation/CS1/Identifiers at line 47: attempt to index field 'wikibase' (a nil value).
  5. "More Mathematical Astronomy Morsels" (2002) ISBN 0-943396-74-3, Jean Meeus, chapter 38: Cruithne, an asteroid with a remarkable orbit
  6. 6.0 6.1 6.2 6.3 Cruithne: Asteroid 3753. Western Washington University Planetarium. Retrieved January 27, 2011.
  7. Lua error in Module:Citation/CS1/Identifiers at line 47: attempt to index field 'wikibase' (a nil value).
  8. Lua error in Module:Citation/CS1/Identifiers at line 47: attempt to index field 'wikibase' (a nil value).
  9. 9.0 9.1 "JPL Close-Approach Data: 3753 Cruithne (1986 TO)" (2008-10-25 last obs). Retrieved 2009-06-28.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  10. "This month Pluto's apparent magnitude is m=14.1. Could we see it with an 11" reflector?". Singapore Science Centre. Archived from the original on 2007-09-30. Retrieved 2007-03-25.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  11. "The astronomical magnitude scale". The ICQ Comet Information Website. Retrieved 2007-09-26.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  12. Lloyd, Robin. "More Moons Around Earth?". Space.com.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  13. 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".
  14. Astonishing X-Men vol. 3 #31

Further reading

  • Lua error in Module:Citation/CS1/Identifiers at line 47: attempt to index field 'wikibase' (a nil value).
  • Meeus, Jean (2002). "Cruithne, an asteroid with a remarkable orbit". More Mathematical Astronomy Morsels. Richmond, Virginia: Willmann-Bell. ISBN 0-943396-74-3.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>

External links