Near-Earth Asteroid 3753 Cruithne Frequently Asked Questions

This FAQ answers some of the more common questions about asteroid 3753 Cruithne. For a more complete description of the motion of Cruithne relative to the Earth, please see the 3753 Cruithne main page. Try your local library for more information on asteroids, the Earth, our Solar System and astronomy in general.


  • What is an asteroid?

    Simply put, an asteroid is an airless chunk of rock and/or iron orbiting the Sun. Asteroids vary in size from almost 1000 kilometers (625 miles) in diameter (for Ceres, the largest asteroid) down to a few meters or less for the smallest ones. Most asteroids orbit the Sun at distances between the orbits of Mars and Jupiter, in what is known as the asteroid belt. Because of the gravitational "jostling" of the planets, some asteroids leave the asteroid belt and wander about the Solar system. Some of these are currently orbiting near the Earth and are called Near-Earth Asteroids (NEAs). For more on-line information on asteroids, check out Wikipedia's asteroid page.

  • What does 3753 Cruithne stand for?

    The number 3753 is just its official number in the catalogue of known asteroids. The name "Cruithne" was given to it by its discoverers (see below), and refers the first Celtic racio-tribal group to come to the British Isles, appearing between about 800 and 500 B.C., and coming from the European continent. They were also known as the Picts. BKW informs me that Cruithne was also the name of a legendary king of the Picts. The correct pronunciation for 'Cruithne' is The word is pronounced "krooy-nyuh" which can also be written as "KROOee-nyuh" and in many other ways. My point is that the stress is on the first syllable, which contains both the OO and the ee sounds. The word has only two syllables, "cruith" and "ne". The stress is not on the "ee" sound. The OOee or ooy (ui) diphthong is very common in the Celtic languages. Moran taing (many thanks) to DKC for the above guide to the pronunciation! For more on how asteroids and comets are named, see the International Astronomical Union - Minor Planet Center's explanation.

  • When/where was Cruithne discovered?

    Asteroid 3753 Cruithne was officially discovered on October 10, 1986 by D. Waldron, working with R. McNaught, M. Hartley and M. Hawkins at Siding Spring Observatory, Coonabarabran, Australia. Later, C. Bardwell realized that this asteroid had been observed three years earlier at the European Southern Observatory in Chile by G. de Sanctis and R. West. However, the title of ``discoverer'' traditionally goes to the person(s) who observes the asteroid over a sufficient period of time to allow a good orbit to be computed (otherwise the asteroid will be quickly lost again). Since this is not always practical, the first person to see the asteroid may not be considered its official discoverer. For more information on discovery priority, see the International Astronomical Union Minor Planet Center's explanation .

  • Who first noticed this asteroid's special relationship with the Earth?

    The strange nature of the asteroid's orbit was uncovered by Paul Wiegert and Kimmo Innanen, working at York University in Canada, and Seppo Mikkola, working at the University of Turku in Finland. The fact that Cruithne accompanied the Earth in its orbit was revealed by computer simulations of the near-Earth asteroids performed during early 1997.

  • If the asteroid was discovered in 1986, why wasn't the unusual nature of its orbit noticed earlier?

    The interaction of Cruithne with the Earth is unusual, and takes place over several hundred years. Because of this, fairly long and careful simulations of its orbit are required to make its true nature clear. In addition, there are more technical reasons: the asteroid's high inclination (20 degrees) and eccentricity (0.51) seemed to make it a very unlikely candidate for such a delicate relationship with the Earth, as horseshoe orbits had previously only been thought possible if an asteroid was on an orbit with a very low eccentricity and inclination.

  • Does asteroid 3753 Cruithne orbit the Earth like a moon or satellite?

    No. The asteroid's behaviour is more complicated than that: it doesn't orbit the Earth, but rather it shares the Earth's orbit. The relationship of a moon to its planet is called a two-body because there are only two important players (ie. the moon and the planet). However, in the case of Cruithne, the Earth and the asteroid both share the same orbit about the Sun, but are choreographed in such a away as to remain stable and avoid colliding with each other. This is called a three-body relationship as there are three main players: the Earth, the asteroid and the Sun. Please see the 3753 Cruithne main page for more details on the asteroid's motion.

  • When the asteroid gets close to the Earth, it starts moving away again. Why doesn't the Earth's gravity pull it right in?

    Non-technical answer: Any orbit is a balance between two competing forces: gravity, which pulls things together, and the speed of the bodies, which (if it is in the right direction) tends to make them move away from each other. Think of a satellite in orbit: if it was not moving, it would fall down onto the planet, but its speed keeps it in orbit. However, gravity and speed are not completely independent of each other: gravity makes things go faster as they fall, and a moving object traveling in the wrong direction will not stay in orbit. It is an interplay between these two which allows Cruithne to avoid the Earth. As Cruithne gets closer to our planet, the Earth's gravity does pull it towards it; but because of the trajectory of the asteroid, the Earth's gravity just makes its speed change in such a way that its orbit starts to move away from the Earth. Gravity never repels the asteroid in the strict sense but the result is the same: the asteroid reverses direction and begins to move away from our planet.

    Technical answer: Conservation of energy and angular momentum.

    Very technical answer: The three-dimensional restricted three-body problem has been studied very little compared to the circular planar restricted three-body problem: to the best of our knowledge, no mention of the existence of horseshoe orbits with high eccentricities and inclinations has been made before this. After our discovery, an analytical theory of such objects was constructed by Namouni, Christou and Murray (see eg. Namouni, 1999, Icarus, 137,293). For more information on low e, i co-orbital trajectories, see Dermott and Murray, 1981, "The dynamics of tadpole and horseshoe orbits. I. Theory", Icarus , vol. 48, p. 1-11, or Brown, 1911, "On a new family of periodic orbits in the problem of three bodies", Monthly Notices of the Royal Astronomical Society , vol. 71, p. 438-454.

  • What are the physical characteristics of Cruithne?

    We know very little about the physical properties of Cruithne, because very few observations have been made of it. Its diameter is estimated at 5 kilometres (3 miles), quite large for a near-Earth asteroid, but not exceptional for asteroids in general. Little else is known.

  • How close does Cruithne get to the Earth?

    The closest approach is only to within 0.1 astronomical units (about 15 million kilometers (10 million miles) or 40 times the Earth-Moon distance), and it only gets this close every few hundred years. Over the next year, Cruithne won't get any closer than 0.3 astronomical units, and at this point it will be almost directly beneath the Earth's South Pole (this is possible due to the asteroid's inclined orbit). This asteroid is also moving away from the Earth along its horseshoe, so its minimum yearly approach distance is slowing increasing. In June and July, this asteroid is about 1.5 astronomical units (over 500 times the Earth-Moon distance) from us.

  • Is there any danger that Cruithne will hit the Earth?

    No. The relationship between the Earth and Cruithne, particularly the asteroid's high (20 degrees) inclination, helps avoid a collision. Though Cruithne is unlikely to remain stable in its current orbit indefinitely, the possibility of a collision over at least the next ten thousand years is nil. For more information on the possibility of impacts on the Earth, see the NASA impact hazards page

  • Are there many other near-Earth asteroids/comets around?

    As of Feb 2000, there are over 1000 known objects with orbits that pass close to that of the Earth. Currently, none of these are known to be on a collision course with our planet. However, it is estimated that there are tens of thousands of objects over 100 meters across that cross the Earth's orbit. Despite this fact, many astronomers looking for these objects are facing serious budget cuts in these hard economic times. For more information on searches for near-Earth objects, see

  • the Spacewatch Project

  • Will we be able to see asteroid Cruithne like the recent comets?

    No. Unfortunately, Cruithne is very faint (peak visual magnitude about 15.5) and will be far in the southern sky when at its brightest in the fall. Thus its a challenging object for amateur astronomers and cannot be seen at all with the naked eye. Asteroids do not produce the large tails and gas clouds that make comets so bright.

  • What is the difference between an asteroid, a comet and a minor planet?

    A minor planet is a term used for any small body in the Solar System which is not itself the moon of a planet: both asteroids and comets are minor planets. An asteroid is composed primarily of rock and iron, whereas comets contain a large proportion of ice. It is this difference in composition which accounts for the different appearances of these two objects. When comets get near the Sun, they get hot enough for the ice to evaporate and produce large amounts of gas, which become a bright tail and make comets conspicuous. Asteroids have no such volatile elements, and thus remain very much fainter. Some asteroids may be comets which have, over time, lost all their ice.

  • What are the asteroid's orbital elements?

    The following elements are for the epoch JD 2450500.5 (Feb 21 1997) and relative to the equinox J2000.0. Semimajor axis = 0.99778030 astronomical units, eccentricity 0.51478431, inclination i=19.812285 degrees, longitude of the ascending node = 126.373212 degrees, argument of perihelion = 43.640637 degrees, mean anomaly = 40.048932 degrees, and epoch of perihelion passage = 1997 Jan 11.50 UT = JD 2450460.00. These elements are derived from the Asteroid Orbital Elements Database maintained by Ted Bowell at Lowell Observatory. You can also find them at Minor Planet Center.

  • What does it mean for the asteroid to be in a "horseshoe" orbit?

    There is no short answer to this one. Please see the 3753 Cruithne main web page for more details.

  • Are there any other known horseshoe orbits?

    There is only one other known case, involving the small moons Janus and Epimetheus of the planet Saturn. Janus plays the role of the Earth in that case, and Epimetheus that of Cruithne. However, this system's behaviour produces a much simpler horseshoe than that of the Earth and Cruithne. There are other asteroids which are known to be co-orbital with planets (being co-orbital means that they share their planet's orbit. This is a necessary first ingredient for horseshoe behaviour). Mars has one co-orbital asteroid (its name is 5261 Eureka), and Jupiter has many (about 400 objects); there are also other small co-orbital moons in the Saturnian system: Telesto and Calypso with Tethys, and Helene with Dione. However, none of these systems display horseshoe orbits.

  • Does the Earth have any other companions or moons?

    Yes, the Moon. Apart from that, there are now a few natural objects known to be in close dynamical relationships with the Earth. Asteroid 2002 AA29 is one of them. We are also currently (23 Jan 2004) working on publishing results on 3 more asteroids. Look for more info here soon.


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