3753 Cruithne
Encyclopedia
3753 Cruithne is an asteroid
in orbit
around the Sun
in approximate 1:1 orbital resonance
with the Earth. It is a periodic inclusion planetoid orbiting the Sun in an apparent horseshoe orbit
. It has been incorrectly called "Earth's second moon
", but it is only a quasi-satellite
. Cruithne never really goes around Earth, and at times it can be seen on the other side of the Sun. Cruithne's orbit takes it inwards towards the orbit of Mercury, and outside the orbit of Mars. Cruithne orbits the Sun in about 1 year, but it takes 770 years for the series to complete a horseshoe-shaped movement, with the Earth in the gap of the horseshoe.
The name Cruithne
is that of an early medieval people of Ireland.
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
or Cruthin, a people of early medieval Ireland mentioned in the Annals of Ulster
.
Cruithne is approximately 5 kilometres (3.1 mi) in diameter
, and its closest approach to Earth is approximately thirty times the separation between Earth and the Moon
(12 Gm or twelve million kilometres). From 1994 through 2015, Cruithne makes its annual closest approach to Earth every November. 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 12.5 inches (317.5 mm) reflecting telescope
to be seen.
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". However, it does not orbit the Earth and is not a moon. In 2058, Cruithne will come within 0.09 AU
(13.6 million kilometres) of Mars
. 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 centimetre (0.511811023622047 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 12500000 kilometres (7,767,159.2 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.
More resonant
near-Earth object
s (NEOs) have since been discovered. These include 54509 YORP
, , , and 2009BD 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 orbit
s include Janus
and 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
, , , and , all at the Lagrangian point
s), and Jupiter
has many (more than 1000 known objects, the Trojan asteroid
s); 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.
, which was nominated for the Arthur C. Clarke science fiction award in 2000.
The British panel show QI
brought attention to Cruithne in 2003 when host Stephen Fry
claimed it was the "second moon". He also incorrectly claimed that it was discovered in 1994.
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...
in orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
around the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
in approximate 1:1 orbital resonance
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...
with the Earth. It is a periodic inclusion planetoid orbiting the Sun in an apparent horseshoe orbit
Horseshoe orbit
A horseshoe orbit is a type of co-orbital motion of a small orbiting body relative to a larger orbiting body . The orbital period of the smaller body is very nearly the same as for the larger body, and its path appears to have a horseshoe shape in a rotating reference frame as viewed from the...
. It has been incorrectly called "Earth's second moon
Earth's second moon
Claims have existed for many centuries that the Earth might possess other natural satellites besides the Moon. Several candidates have been proposed, but all such claims have proven false...
", but it is only a quasi-satellite
Quasi-satellite
A quasi-satellite is an object in a 1:1 orbital resonance with its planet that stays close to the planet over many orbital periods.A quasi-satellite's orbit around the Sun takes exactly the same time as the planet's, but has a different eccentricity , as shown in the diagram on the right...
. Cruithne never really goes around Earth, and at times it can be seen on the other side of the Sun. Cruithne's orbit takes it inwards towards the orbit of Mercury, and outside the orbit of Mars. Cruithne orbits the Sun in about 1 year, but it takes 770 years for the series to complete a horseshoe-shaped movement, with the Earth in the gap of the horseshoe.
The name Cruithne
Cruthin
The Cruthin were a people of early Ireland, who occupied parts of Counties Down, Antrim and Londonderry in the early medieval period....
is that of an early medieval people of Ireland.
Discovery
Cruithne was discovered on October 10, 1986, by Duncan WaldronDuncan Waldron
Duncan Waldron is a photographer and amateur astronomer.Duncan Waldron was born in Glasgow, Scotland. His first job was creating high quality reproductions of astronomical plates for the Royal Observatory in Edinburgh...
on a photographic plate taken with the UK Schmidt Telescope
UK Schmidt Telescope
The 1.2 metre UK Schmidt Telescope is operated by the Australian Astronomical Observatory , and located adjacent to the 3.9 metre Anglo-Australian Telescope at Siding Spring Observatory, Australia...
at Siding Spring Observatory
Siding Spring Observatory
Siding Spring Observatory near Coonabarabran, New South Wales, Australia, part of the Research School of Astronomy & Astrophysics at the Australian National University , incorporates the Anglo-Australian Telescope along with a collection of other telescopes owned by the Australian National...
, Coonabarabran, Australia. The 1983 apparition (1983 UH) is credited to Giovanni de Sanctis
Giovanni de Sanctis
Giovanni de Sanctis is an Italian astronomer at the Osservatorio Astronomico di Torino in Turin, Italy. His name is sometimes spelt DeSanctis, particularly in the Minor Planet Circulars.- Reference :...
and Richard M. West of the European Southern Observatory
European Southern Observatory
The European Southern Observatory is an intergovernmental research organisation for astronomy, supported by fifteen countries...
in Chile
Chile
Chile ,officially the Republic of Chile , is a country in South America occupying a long, narrow coastal strip between the Andes mountains to the east and the Pacific Ocean to the west. It borders Peru to the north, Bolivia to the northeast, Argentina to the east, and the Drake Passage in the far...
. It was not until 1997 that its unusual orbit was determined by Paul Wiegert and Kimmo Innanen, working at York University
York University
York University is a public research university in Toronto, Ontario, Canada. It is Canada's third-largest university, Ontario's second-largest graduate school, and Canada's leading interdisciplinary university....
in Toronto
Toronto
Toronto is the provincial capital of Ontario and the largest city in Canada. It is located in Southern Ontario on the northwestern shore of Lake Ontario. A relatively modern city, Toronto's history dates back to the late-18th century, when its land was first purchased by the British monarchy from...
, and Seppo Mikkola
Seppo Mikkola
Seppo Mikkola is a Finnish astronomer. He is a senior lecturer at the University of Turku and staff member at Tuorla Observatory.Mikkola is a leading expert in celestial mechanics. He has made fundamental contributions to the theory of regularization of motion in the gravitational N-body...
, working at the University of Turku
University of Turku
The University of Turku , located in Turku in southwestern Finland, is the second largest university in the country as measured by student enrollment, after University of Helsinki. It was established in 1920 and also has faculties at Rauma, Pori and Salo...
in Finland.
The asteroid is named after the Cruithne
Cruthin
The Cruthin were a people of early Ireland, who occupied parts of Counties Down, Antrim and Londonderry in the early medieval period....
or Cruthin, a people of early medieval Ireland mentioned in the Annals of Ulster
Annals of Ulster
The Annals of Ulster are annals of medieval Ireland. The entries span the years between AD 431 to AD 1540. The entries up to AD 1489 were compiled in the late 15th century by the scribe Ruaidhrí Ó Luinín, under his patron Cathal Óg Mac Maghnusa on the island of Belle Isle on Lough Erne in the...
.
Dimensions and orbit
Diameter
In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...
, and its closest approach to Earth is approximately thirty times the separation between Earth and the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
(12 Gm or twelve million kilometres). From 1994 through 2015, Cruithne makes its annual closest approach to Earth every November. 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
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
of +15.8, is fainter than Pluto and would require at least a 12.5 inches (317.5 mm) reflecting telescope
Reflecting telescope
A reflecting telescope is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from...
to be seen.
Cruithne is in a normal elliptic orbit
Elliptic orbit
In astrodynamics or celestial mechanics an elliptic orbit is a Kepler orbit with the eccentricity less than 1; this includes the special case of a circular orbit, with eccentricity equal to zero. In a stricter sense, it is a Kepler orbit with the eccentricity greater than 0 and less than 1 . In a...
around the Sun. Its period of revolution
Orbital period
The orbital period is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.There are several kinds of...
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". However, it does not orbit the Earth and is not a moon. In 2058, Cruithne will come within 0.09 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
(13.6 million kilometres) of Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
. Due to a high orbital eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...
, 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
Horseshoe orbit
A horseshoe orbit is a type of co-orbital motion of a small orbiting body relative to a larger orbiting body . The orbital period of the smaller body is very nearly the same as for the larger body, and its path appears to have a horseshoe shape in a rotating reference frame as viewed from the...
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 centimetre (0.511811023622047 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 12500000 kilometres (7,767,159.2 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
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...
near-Earth object
Near-Earth object
A near-Earth object is a Solar System object whose orbit brings it into close proximity with the Earth. All NEOs have a perihelion distance less than 1.3 AU. They include a few thousand near-Earth asteroids , near-Earth comets, a number of solar-orbiting spacecraft, and meteoroids large enough to...
s (NEOs) have since been discovered. These include 54509 YORP
54509 YORP
54509 YORP is an Apollo Near-Earth Object discovered on August 3, 2000 by the Lincoln Laboratory Near-Earth Asteroid Research Team at Socorro. Measurements of the rotation rate of this object provided the first observational evidence of the YORP effect, hence the name of the asteroid...
, , , and 2009BD which exist in resonant orbits similar to Cruithne's. 2010 TK7
2010 TK7
2010 TK7 is the first Earth trojan asteroid to be discovered; it precedes Earth in its orbit around the Sun. Trojan objects are most easily conceived as orbiting at a Lagrangian point, a dynamically stable location 60 degrees ahead of or behind a massive orbiting body, in a type of 1:1 orbital...
is the first and so far only identified Earth trojan
Earth Trojan asteroid
Earth trojans are asteroids that orbit in the vicinity of the Earth-Sun Lagrangian points and . They are named after the Trojan asteroids that are associated with the analogous Lagrangian points of Jupiter....
.
Other examples of natural bodies known to be in horseshoe orbit
Horseshoe orbit
A horseshoe orbit is a type of co-orbital motion of a small orbiting body relative to a larger orbiting body . The orbital period of the smaller body is very nearly the same as for the larger body, and its path appears to have a horseshoe shape in a rotating reference frame as viewed from the...
s include Janus
Janus (moon)
Janus is an inner satellite of Saturn. It is also known as Saturn X . It is named after the mythological Janus.-Discovery and orbit:Janus occupies practically the same orbit as the moon Epimetheus...
and Epimetheus
Epimetheus (moon)
Epimetheus is an inner satellite of Saturn. It is also known as Saturn XI. It is named after the mythological Epimetheus, brother of Prometheus.-Discovery:Epimetheus occupies essentially the same orbit as the moon Janus...
, natural satellite
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....
s of Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.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
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
has four known co-orbital asteroids (5261 Eureka
5261 Eureka
5261 Eureka is the first asteroid discovered that turned out to be a Mars trojan asteroid. It was discovered by David H. Levy and Henry Holt at Palomar Observatory on June 20, 1990. It trails Mars at a distance varying by only 0.3 AU during each revolution...
, , , and , all at the Lagrangian point
Lagrangian point
The Lagrangian points are the five positions in an orbital configuration where a small object affected only by gravity can theoretically be stationary relative to two larger objects...
s), and Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
has many (more than 1000 known objects, the Trojan asteroid
Trojan asteroid
The Jupiter Trojans, commonly called Trojans or Trojan asteroids, are a large group of objects that share the orbit of the planet Jupiter around the Sun. Relative to Jupiter, each Trojan librates around one of the planet's two Lagrangian points of stability, and , that respectively lie 60° ahead...
s); there are also other small co-orbital moons in the Saturnian system: Telesto
Telesto (moon)
Telesto is a moon of Saturn. It was discovered by Smith, Reitsema, Larson and Fountain in 1980 from ground-based observations, and was provisionally designated '. In the following months, several other apparitions were observed: , , and ....
and Calypso
Calypso (moon)
Calypso is a moon of Saturn. It was discovered in 1980, from ground-based observations, by Dan Pascu, P. Kenneth Seidelmann, William A. Baum, and Douglas G. Currie, and was provisionally designated ' . Several other apparitions of it were recorded in the following months: , , , and...
with Tethys
Tethys (moon)
Tethys or Saturn III is a mid-sized moon of Saturn about across. It was discovered by G. D. Cassini in 1684 and is named after titan Tethys of Greek mythology. Tethys is pronounced |Odysseus]] is about 400 km in diameter, while the largest graben—Ithaca Chasma is about 100 km wide and...
, and Helene
Helene (moon)
Helene is a moon of Saturn. It was discovered by Pierre Laques and Jean Lecacheux in 1980 from ground-based observations at Pic du Midi Observatory, and was designated . In 1988 it was officially named after Helen of Troy, who was the granddaughter of Cronus in Greek mythology...
and Polydeuces
Polydeuces (moon)
Polydeuces is a very small natural satellite of Saturn that is co-orbital with Dione and librates around the trailing Lagrangian point . Its diameter is estimated to be about 3.5 km....
with Dione
Dione (moon)
Dione is a moon of Saturn discovered by Cassini in 1684. It is named after the titan Dione of Greek mythology. It is also designated Saturn IV.- Name :...
. However, none of these follow horseshoe orbits.
Cruithne in popular culture
Cruithne plays a major role in Steven Baxter's novel Manifold: TimeManifold: Time
Manifold: Time is a 1999 science fiction novel by Stephen Baxter. It is the first of Baxter's Manifold trilogy , although the books can be read in any order because the series takes place in a multiverse.The book was nominated for the 2000 Arthur C...
, which was nominated for the Arthur C. Clarke science fiction award in 2000.
The British panel show QI
QI
QI is a British comedy panel game television quiz show created and co-produced by John Lloyd, hosted by Stephen Fry, and featuring permanent panellist Alan Davies. Most of the questions are extremely obscure, making it unlikely that the correct answer will be given...
brought attention to Cruithne in 2003 when host Stephen Fry
Stephen Fry
Stephen John Fry is an English actor, screenwriter, author, playwright, journalist, poet, comedian, television presenter and film director, and a director of Norwich City Football Club. He first came to attention in the 1981 Cambridge Footlights Revue presentation "The Cellar Tapes", which also...
claimed it was the "second moon". He also incorrectly claimed that it was discovered in 1994.
See also
- Asteroids in fiction: Other asteroids, Asteroids in fiction: Colonization, and Asteroids in fiction: Mining
- Co-orbital configurationCo-orbital configurationIn astronomy, a co-orbital configuration refers to two or more celestial objects that orbit at the same, or very similar, distance from their parent object as each other, i.e. they are in a 1:1 mean motion resonance....
- Earth's second moon
- Natural satelliteNatural satelliteA natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....
- Quasi-satelliteQuasi-satelliteA quasi-satellite is an object in a 1:1 orbital resonance with its planet that stays close to the planet over many orbital periods.A quasi-satellite's orbit around the Sun takes exactly the same time as the planet's, but has a different eccentricity , as shown in the diagram on the right...