3753 Cruithne

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3753 Cruithne
Asteroid 3753 Cruithne
Discovered by Duncan Waldron
Discovery date October 10, 1986
Alternate name 1983 UH; 1986 TO
Minor planet
Near-Earth asteroid,
Venus-crosser asteroid,
Mars-crosser asteroid
Epoch May 14, 2008 (JD 2454600.5)
Aphelion 226.105 Gm (1.51 AU)
Perihelion 72.415 Gm (0.484 AU)
Semi-major axis 149.260 Gm (0.998 AU)
Eccentricity 0.515
Orbital period 363.99 d (1.00 a)
Average orbital speed 27.73 km/s
Mean anomaly 134.76°
Inclination 19.81°
Longitude of ascending node 126.28°
Argument of perihelion 43.74°
Physical characteristics
Dimensions ~5 km
Mass 1.3×1014 kg
Mean density 2 ? g/cm³
Equatorial surface gravity 0.0014 m/s²
Escape velocity 0.0026 km/s
Rotation period 27.44 h[1]
Albedo 0.15 ?
Temperature ~275 K
Spectral type ?
Absolute magnitude (H) 15.1[1]

3753 Cruithne (pronounced /ˈkrɪnjə/, from Old Irish IPA[ˈkrɪθnɛ]; Modern Irish IPA[ˈkrɪhnʲə] or [ˈkrɪnʲə]) is an asteroid in orbit around the Sun in 1:1 orbital resonance with that of the Earth. It is a periodic inclusion planetoid orbiting the Sun in a horseshoe orbit.[2] It has been called "Earth's second moon", although it is only a quasi-satellite.[2]


[edit] Discovery

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. 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.

The asteroid is named after the Cruithne people (also known as the Priteni or the Picts) who inhabited Scotland and parts of Ireland and the Isle of Man between 800 BCE and 1000 CE[citation needed]; the name may specifically refer to their legendary first leader, also called Cruithne.

[edit] Dimensions and orbit

Cruithne and Earth seem to follow each other in their orbits.
Cruithne appears to make a bean-shaped orbit from the perspective of Earth.
Cruithne's distance to Earth and the Sun plotted over 500 years (top) and 10 years (bottom).

Cruithne is approximately 5 km in diameter, and its closest approach to Earth is approximately 30 times the separation between Earth and the Moon (12 Gm or million kilometres). Although Cruithne's orbit is not thought to be stable over the long term, calculations by Wiegert and Innanen showed that it has likely 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 opposition magnitude of +15.8, is fainter than Pluto and would require at least a 12.5-inch (320 mm) reflecting telescope to be seen.[3][4]

Cruithne is in a normal elliptic orbit around the Sun. However, because its period of revolution around the Sun is almost exactly equal to that of the Earth, they appear to "follow" each other in their paths around the Sun. This is why Cruithne is sometimes called "Earth's second moon".[2]However, it does not orbit the Earth and is not a moon.[5] 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 has fallen behind far enough that Cruithne is now actually "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 (whilst Earth's orbit is altered by about 1.3 centimetres) so that its period of revolution around the Sun is slightly more than a year. The kidney bean then starts 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 km.

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.

[edit] Similar minor planets

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

More near-Earth asteroids (NEAs) have since been discovered. These include 54509 YORP, (85770) 1998 UP1, 2002 AA29, and 2009BD which exist in resonant orbits similar to Cruithne's.

Other examples of natural bodies known to be in horseshoe orbits 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 (more than 1000 known objects, the Trojan asteroids); 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.

[edit] See also

[edit] References

  1. ^ a b c "JPL Small-Body Database Browser: 3753 Cruithne (1986 TO)". 2007-10-20 last obs. http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=3753. Retrieved on 2008-07-23. 
  2. ^ a b c Lloyd, Robin. "More Moons Around Earth?". Space.com. http://www.space.com/scienceastronomy/solarsystem/second_moon_991029.html. 
  3. ^ "This month Pluto's apparent magnitude is m=14.1. Could we see it with an 11" reflector?". Singapore Science Centre. http://www.science.edu.sg/ssc/detailed.jsp?artid=1950&type=6&root=6&parent=6&cat=66. Retrieved on 2007-03-25. 
  4. ^ "The astronomical magnitude scale.". The ICQ Comet Information Website. http://www.cfa.harvard.edu/icq/MagScale.html. Retrieved on 2007-09-26. 
  5. ^ 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".

[edit] Further reading

  • Wiegert, Paul A.; Innanen, Kimmo A.; Mikkola, Seppo (1997). "An asteroidal companion to the Earth". Nature 387 (6634): 685–686. doi:10.1038/42662. 
  • Meeus, Jean (2002). "Cruithne, an asteroid with a remarkable orbit". More Mathematical Astronomy Morsels. Richmond, VA: Willmann-Bell. ISBN 0943396743. 

[edit] External links

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