2002 AA29
Encyclopedia
is a small near-Earth asteroid that was discovered on January 9, 2002 by the LINEAR
(Lincoln Near Earth Asteroid Research) automatic sky survey. The diameter of the asteroid
is only about 50 to 110 metres (160 to 360 feet). It revolves about the Sun
on an almost circular orbit very similar to that of the Earth
. This lies for the most part inside the Earth's orbit, which it crosses near the asteroid's furthest point from the Sun, the aphelion. Because of this orbit, the asteroid is classified as Aten type
, named after the asteroid (2062) Aten
.
A further characteristic is that its mean orbital period
about the Sun is exactly one sidereal year
. This means that it is locked into a relationship with the Earth, since such an orbit is only stable under particular conditions. As yet only a few asteroids of this sort are known, locked into a 1:1 resonance
with the Earth. The first was (3753) Cruithne
, discovered in 1986.
Asteroids that have a 1:1 orbital resonance with a planet are also called co-orbital objects
, because they follow the orbit of the planet. The most numerous known co-orbital asteroids are the so-called Trojans
, which occupy the L4 and L5 Lagrangian points of the relevant planet. However, 2002 AA29 does not belong to these. Instead, it follows a so-called horseshoe orbit
along the path of the Earth.
(JPL), the Athabasca University
(Canada
), the Queen’s University in Kingston (Ontario, Canada)
, the York University
in Toronto
and the Tuorla Observatory
of the University of Turku
in Finland
determined the unusual orbit of 2002 AA29, and through further observations at the Canada-France-Hawaii Telescope
in Hawaii it was confirmed that:
If one looks at the orbit of 2002 AA29 from a point moving with the Earth around the Sun (the reference frame
of the Earth-Sun system), it describes over the course of 95 years an arc of almost 360°, which during the next 95 years it retraces in reverse. The shape of this arc is reminiscent of a horseshoe, from which comes the name “horseshoe orbit
”. As it moves along the Earth's orbit, it winds in a spiral about it, in which each loop of the spiral takes one year. This spiral motion (in the Earth-Sun reference frame) arises from the slightly lower eccentricity and the tilt of the orbit: the inclination relative to the Earth's orbit is responsible for the vertical component of the spiral loop, and the difference in eccentricity for the horizontal component.
When 2002 AA29 is approaching the Earth from in front (i.e. it is moving slightly slower, and the Earth is catching it up), the gravitational attraction of the Earth shifts it onto a slightly faster orbit, a little nearer the Sun. It now hurries ahead of the Earth along its new orbit, until after 95 years it has almost lapped the Earth and is coming up from behind. Again it comes under the Earth's gravitational influence; this time it is lifted onto a slower orbit, further from the Sun. On this orbit it can no longer keep pace with the Earth, and it falls behind until in 95 years it is once again approaching the Earth from in front. The Earth and 2002 AA29 chase each other in turn around the Sun, but do not get close enough to break the pattern.
On 8 January 2003 the asteroid approached the Earth from in front to a distance of 5.9 million kilometres (3.7 million miles), its closest approach for nearly a century. Since that date it has been hurrying ahead, and will continue to do so until it has reached its closest approach from behind. As a result of this subtle exchange with the Earth, unlike other Earth orbit crossing asteroids, we need have no fear that it could ever collide with the Earth. Calculations indicate that in the next few thousand years it will never come closer than 4.5 million kilometres, or about twelve times the distance from the Earth to the Moon.
Because of its orbital inclination of 10.739° to the ecliptic, 2002 AA29 is not always forced by the Earth on its horseshoe orbit however but can sometimes slip out of this pattern. It is then caught for a while in the neighbourhood of the Earth. This will next happen in about 600 years i.e. in 26th century
. It will then stay within the small gap in the Earth’s orbit which it does not reach in its previous horseshoe orbit, and will be no further than 0.2 astronomical unit
s (30 million km) away from the Earth. There it will slowly circle the Earth almost like a second Moon, although it takes one year for a circuit. After 45 years it finally switches back into the horseshoe orbit, until it again stays near the Earth for 45 years around the year 3750 and again in 6400. In these phases in which it stays outside its horseshoe orbit it oscillates in the narrow region along the Earth’s orbit where it is caught, moving back and forth in 15 years. Because it is not bound to the Earth like the Moon but is mainly under the gravitational influence of the Sun, it belongs to the bodies called quasi-satellite
s. This is somewhat analogous to two cars
travelling side by side at the same speed and repeatedly overtaking one another but which are however not attached to each other. Orbital calculations show that 2002 AA29 was in this quasi-satellite orbit for 45 years from about 520 A.D. but because of its tiny size was too dim to have been seen. It switches approximately cyclically between the two orbital forms, but always stays for 45 years in the quasi-satellite orbit. Outside the time frame from about 520-6500 A.D. the calculated orbits become chaotic
i.e. not predictable, and thus for periods outside this time frame no exact statements can be made. 2002 AA29 was the first known heavenly body
that switches between horseshoe and quasi-satellite orbits.
s, and can only be observed using highly sensitive CCD
cameras. At the time of its closest approach on 8 January 2003 it had an apparent brightness
in the visible region of magnitude 20.4.
So far nothing concrete is known about the composition of 2002 AA29. Because of its nearness to the Sun, it cannot however consist of volatile substances
such as water ice
, since these would evaporate
or sublime; one can clearly observe this happening to a comet
as this forms the visible tail. Presumably it will have a dark, carbon
-bearing or somewhat lighter silicate
-rich surface; in the former case the albedo
would be around 0.05, in the latter somewhat higher at 0.15 to 0.25. It is due to this uncertainty that the figures for its diameter cover such a wide range.
A further uncertainty arises from radar
echo measurements at the Arecibo
Radio Telescope
, which could only pick up an unexpectedly weak radar echo, implying that 2002 AA29 is either smaller than estimated or reflects radio waves only weakly. In the former case it would have to have an unusually high albedo. This would be evidence in support of the speculation that it, or at least the material of which it is composed, is different from most other asteroids so far discovered on near-Earth orbits, or represents a fragment thrown off by the collision of a medium-sized asteroid with the Earth or the Moon.
of known wavelength
are emitted from a radio telescope aimed at an asteroid. There they are reflected, and because of the Doppler effect
the part of the surface that is moving towards the observer (because of the asteroid's rotation) shortens the wavelength of the reflected waves, whilst the other part which is turning away from the observer lengthens the reflected wavelength. As a result the wavelength of the reflected waves is “smeared out”. The extent of the wavelength smearing and the diameter of the asteroid allow the rotational period to be narrowed down. 33 minutes is thus calculated as the upper limit of the rotational period for 2002 AA29; it probably rotates more quickly. This rapid rotation together with the small diameter and therefore low mass leads to some interesting conclusions:
. Furthermore it is assumed that there are small Trojan companions of the Earth with diameters in the region of 100 metres located at the L4 and L5 Lagrangian points of the Earth-Sun system.
Lincoln Near-Earth Asteroid Research
The Lincoln Near-Earth Asteroid Research project is a cooperative project between the United States Air Force, NASA, and MIT's Lincoln Laboratory for the systematic discovery and tracking of near-Earth asteroids. LINEAR was responsible for the majority of asteroid detections since 1998 until...
(Lincoln Near Earth Asteroid Research) automatic sky survey. The diameter of the asteroid
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...
is only about 50 to 110 metres (160 to 360 feet). It revolves about 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...
on an almost circular orbit very similar to that of the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
. This lies for the most part inside the Earth's orbit, which it crosses near the asteroid's furthest point from the Sun, the aphelion. Because of this orbit, the asteroid is classified as Aten type
Aten asteroid
The Aten asteroids are a group of near-Earth asteroids, named after the first of the group to be discovered . They are defined by having semi-major axes of less than one astronomical unit...
, named after the asteroid (2062) Aten
2062 Aten
2062 Aten is an asteroid that was discovered at the Palomar Mountain Observatory by Eleanor F. Helin, who is now the principal scientist for the NEAT project. It is named after Aten, the Egyptian god of the sun....
.
A further characteristic is that its mean orbital period
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...
about the Sun is exactly one sidereal year
Sidereal year
A sidereal year is the time taken by the Earth to orbit the Sun once with respect to the fixed stars. Hence it is also the time taken for the Sun to return to the same position with respect to the fixed stars after apparently travelling once around the ecliptic. It was equal to at noon 1 January...
. This means that it is locked into a relationship with the Earth, since such an orbit is only stable under particular conditions. As yet only a few asteroids of this sort are known, locked into a 1:1 resonance
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
with the Earth. The first was (3753) Cruithne
3753 Cruithne
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...
, discovered in 1986.
Asteroids that have a 1:1 orbital resonance with a planet are also called co-orbital objects
Co-orbital configuration
In 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....
, because they follow the orbit of the planet. The most numerous known co-orbital asteroids are the so-called Trojans
Trojan (astronomy)
In astronomy, a Trojan is a minor planet or natural satellite that shares an orbit with a larger planet or moon, but does not collide with it because it orbits around one of the two Lagrangian points of stability , and , which lie approximately 60° ahead of and behind the larger body,...
, which occupy the L4 and L5 Lagrangian points of the relevant planet. However, 2002 AA29 does not belong to these. Instead, it follows a so-called 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...
along the path of the Earth.
Orbital data
Shortly after the discovery by LINEAR, Scientists at the Jet Propulsion LaboratoryJet Propulsion Laboratory
Jet Propulsion Laboratory is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. The facility is headquartered in the city of Pasadena on the border of La Cañada Flintridge and Pasadena...
(JPL), the Athabasca University
Athabasca University
Athabasca University is a Canadian university in Athabasca, Alberta. It is an accredited research institution which also offers distance education courses and programs. Courses are offered primarily in English with some French offerings. Each year, 32,000 students attend the university. It offers...
(Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
), the Queen’s University in Kingston (Ontario, Canada)
Kingston, Ontario
Kingston, Ontario is a Canadian city located in Eastern Ontario where the St. Lawrence River flows out of Lake Ontario. Originally a First Nations settlement called "Katarowki," , growing European exploration in the 17th Century made it an important trading post...
, the 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 the Tuorla Observatory
Tuorla Observatory
Tuorla Observatory is the Department of Astronomy at the University of Turku, southwest Finland. Currently it is the largest astronomical research institute in Finland...
of 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
Finland
Finland , officially the Republic of Finland, is a Nordic country situated in the Fennoscandian region of Northern Europe. It is bordered by Sweden in the west, Norway in the north and Russia in the east, while Estonia lies to its south across the Gulf of Finland.Around 5.4 million people reside...
determined the unusual orbit of 2002 AA29, and through further observations at the Canada-France-Hawaii Telescope
Canada-France-Hawaii Telescope
The Canada–France–Hawaii Telescope is located near the summit of Mauna Kea mountain on Hawaii's Big Island at an altitude of 4,204 meters , and is one of the observatories that comprise the Mauna Kea Observatory...
in Hawaii it was confirmed that:
- Its orbit lies for the most part inside the Earth’s orbit. The orbits of most asteroids lie in the so-called asteroid beltAsteroid beltThe asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...
between MarsMarsMars 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...
and JupiterJupiterJupiter 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,...
. Through orbital disturbances by the gas giant planets, mainly Jupiter, and through the Yarkovsky effectYarkovsky effectThe Yarkovsky effect is a force acting on a rotating body in space caused by the anisotropic emission of thermal photons, which carry momentum...
(force due to asymmetrical absorption and emission of infra-red radiation) asteroids are diverted into the inner solar systemSolar SystemThe Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
, where their orbits further influenced by close approaches with the inner planets. 2002 AA29 has probably been brought in the same way from the outer solar system into the Earth’s influence. However, it is also suggested that the asteroid has always been on a near-Earth orbit and thus that it or a precursor body was formed near the Earth’s orbit. In this case one possibility is that it could be a fragment from a collision of a middle-sized asteroid with the Earth or the MoonMoonThe 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...
.
- Its mean orbital period is one sidereal yearSidereal yearA sidereal year is the time taken by the Earth to orbit the Sun once with respect to the fixed stars. Hence it is also the time taken for the Sun to return to the same position with respect to the fixed stars after apparently travelling once around the ecliptic. It was equal to at noon 1 January...
. After it was diverted into the inner solar system – or formed on a path near the Earth’s orbit – the asteroid must have been moved into an orbit corresponding with the Earth. In this orbit it was repeatedly pulled by the Earth in such a way that its own orbital period became the same as that of the Earth. In the current orbit, the Earth thus holds the asteroid in synchronicity with its own orbit.
- The orbit of the asteroid is almost circular, with an eccentricityOrbital eccentricityThe 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...
of 0.012 which is even lower than that of the Earth at 0.0167. The other near-Earth asteroids have on average a significantly higher eccentricity of 0.29. Also, all other asteroids in 1:1 resonance with the Earth known before 2002 have very strongly elliptical orbits – e.g. the eccentricity of (3753) Cruithne is 0.515. At the time of its discovery the orbit of 2002 AA29 was unique, because of which the asteroid is often called the first true co-orbital companion of the Earth, since the paths of previously discovered asteroids are not very similar to the Earth’s orbit. The very low orbital eccentricity of 2002 AA29 is also an indication that it must always have been on a near-Earth orbit, or the Yarkovsky effect must have comparatively strongly caused it to spiral into the inner solar system over billions of years, since as a rule asteroids which have been steered by planets have orbits with higher eccentricity.
- The orbital inclination with respect to the eclipticEclipticThe ecliptic is the plane of the earth's orbit around the sun. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun...
(orbital plane of the Earth) of 2002 AA29 is a moderate 10.739°. Hence its orbit is slightly tilted compared with that of the Earth; if it were not tilted at all, the orbits would lie right on top of each other.
Shape of the orbit
Frame of reference
A frame of reference in physics, may refer to a coordinate system or set of axes within which to measure the position, orientation, and other properties of objects in it, or it may refer to an observational reference frame tied to the state of motion of an observer.It may also refer to both an...
of the Earth-Sun system), it describes over the course of 95 years an arc of almost 360°, which during the next 95 years it retraces in reverse. The shape of this arc is reminiscent of a horseshoe, from which comes the name “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...
”. As it moves along the Earth's orbit, it winds in a spiral about it, in which each loop of the spiral takes one year. This spiral motion (in the Earth-Sun reference frame) arises from the slightly lower eccentricity and the tilt of the orbit: the inclination relative to the Earth's orbit is responsible for the vertical component of the spiral loop, and the difference in eccentricity for the horizontal component.
When 2002 AA29 is approaching the Earth from in front (i.e. it is moving slightly slower, and the Earth is catching it up), the gravitational attraction of the Earth shifts it onto a slightly faster orbit, a little nearer the Sun. It now hurries ahead of the Earth along its new orbit, until after 95 years it has almost lapped the Earth and is coming up from behind. Again it comes under the Earth's gravitational influence; this time it is lifted onto a slower orbit, further from the Sun. On this orbit it can no longer keep pace with the Earth, and it falls behind until in 95 years it is once again approaching the Earth from in front. The Earth and 2002 AA29 chase each other in turn around the Sun, but do not get close enough to break the pattern.
On 8 January 2003 the asteroid approached the Earth from in front to a distance of 5.9 million kilometres (3.7 million miles), its closest approach for nearly a century. Since that date it has been hurrying ahead, and will continue to do so until it has reached its closest approach from behind. As a result of this subtle exchange with the Earth, unlike other Earth orbit crossing asteroids, we need have no fear that it could ever collide with the Earth. Calculations indicate that in the next few thousand years it will never come closer than 4.5 million kilometres, or about twelve times the distance from the Earth to the Moon.
Because of its orbital inclination of 10.739° to the ecliptic, 2002 AA29 is not always forced by the Earth on its horseshoe orbit however but can sometimes slip out of this pattern. It is then caught for a while in the neighbourhood of the Earth. This will next happen in about 600 years i.e. in 26th century
26th century
The 26th century of the anno Domini era will span from January 1, 2501–December 31, 2600 of the Gregorian calendar.-List of the long total solar eclipses:* June 14, 2504 : Solar eclipse,, , of saros 145....
. It will then stay within the small gap in the Earth’s orbit which it does not reach in its previous horseshoe orbit, and will be no further than 0.2 astronomical unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
s (30 million km) away from the Earth. There it will slowly circle the Earth almost like a second Moon, although it takes one year for a circuit. After 45 years it finally switches back into the horseshoe orbit, until it again stays near the Earth for 45 years around the year 3750 and again in 6400. In these phases in which it stays outside its horseshoe orbit it oscillates in the narrow region along the Earth’s orbit where it is caught, moving back and forth in 15 years. Because it is not bound to the Earth like the Moon but is mainly under the gravitational influence of the Sun, it belongs to the bodies called 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...
s. This is somewhat analogous to two cars
CARS
Cars, or automobiles, motor cars, are wheeled motor vehicles used for transporting passengers.Cars or CARS may also refer to:-Entertainment:* Cars , a Disney/Pixar film series...
travelling side by side at the same speed and repeatedly overtaking one another but which are however not attached to each other. Orbital calculations show that 2002 AA29 was in this quasi-satellite orbit for 45 years from about 520 A.D. but because of its tiny size was too dim to have been seen. It switches approximately cyclically between the two orbital forms, but always stays for 45 years in the quasi-satellite orbit. Outside the time frame from about 520-6500 A.D. the calculated orbits become chaotic
Chaos theory
Chaos theory is a field of study in mathematics, with applications in several disciplines including physics, economics, biology, and philosophy. Chaos theory studies the behavior of dynamical systems that are highly sensitive to initial conditions, an effect which is popularly referred to as the...
i.e. not predictable, and thus for periods outside this time frame no exact statements can be made. 2002 AA29 was the first known heavenly body
Heavenly Body
is a yaoi manga anthology by Takashi Kanzaki and published by Daitosha. It is licensed in English by Aurora Publishing, which released the manga in August 2008.-Reception:...
that switches between horseshoe and quasi-satellite orbits.
Brightness and size
Relatively little is known about 2002 AA29 itself. With a size of about 50 to 110 metres it is very small, on account of which it is seen from the Earth as a small point even with large telescopeTelescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
s, and can only be observed using highly sensitive CCD
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...
cameras. At the time of its closest approach on 8 January 2003 it had an apparent brightness
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...
in the visible region of magnitude 20.4.
So far nothing concrete is known about the composition of 2002 AA29. Because of its nearness to the Sun, it cannot however consist of volatile substances
Volatiles
In planetary science, volatiles are that group of chemical elements and chemical compounds with low boiling points that are associated with a planet's or moon's crust and/or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, and methane, all compounds of C, H, O...
such as water ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...
, since these would evaporate
Evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
or sublime; one can clearly observe this happening to a comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
as this forms the visible tail. Presumably it will have a dark, carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
-bearing or somewhat lighter silicate
Silicate minerals
The silicate minerals make up the largest and most important class of rock-forming minerals, constituting approximately 90 percent of the crust of the Earth. They are classified based on the structure of their silicate group...
-rich surface; in the former case the albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
would be around 0.05, in the latter somewhat higher at 0.15 to 0.25. It is due to this uncertainty that the figures for its diameter cover such a wide range.
A further uncertainty arises from radar
Radar astronomy
Radar astronomy is a technique of observing nearby astronomical objects by reflecting microwaves off target objects and analyzing the echoes. This research has been conducted for six decades. Radar astronomy differs from radio astronomy in that the latter is a passive observation and the former an...
echo measurements at the Arecibo
Arecibo Observatory
The Arecibo Observatory is a radio telescope near the city of Arecibo in Puerto Rico. It is operated by SRI International under cooperative agreement with the National Science Foundation...
Radio Telescope
Radio telescope
A radio telescope is a form of directional radio antenna used in radio astronomy. The same types of antennas are also used in tracking and collecting data from satellites and space probes...
, which could only pick up an unexpectedly weak radar echo, implying that 2002 AA29 is either smaller than estimated or reflects radio waves only weakly. In the former case it would have to have an unusually high albedo. This would be evidence in support of the speculation that it, or at least the material of which it is composed, is different from most other asteroids so far discovered on near-Earth orbits, or represents a fragment thrown off by the collision of a medium-sized asteroid with the Earth or the Moon.
Rotational period
Using radar echo measurements at the Arecibo radio telescope the rotational period of 2002 AA29 could be determined. In this radar astronomy procedure radio wavesRadio waves
Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies from 300 GHz to as low as 3 kHz, and corresponding wavelengths from 1 millimeter to 100 kilometers. Like all other electromagnetic waves,...
of known wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
are emitted from a radio telescope aimed at an asteroid. There they are reflected, and because of the Doppler effect
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
the part of the surface that is moving towards the observer (because of the asteroid's rotation) shortens the wavelength of the reflected waves, whilst the other part which is turning away from the observer lengthens the reflected wavelength. As a result the wavelength of the reflected waves is “smeared out”. The extent of the wavelength smearing and the diameter of the asteroid allow the rotational period to be narrowed down. 33 minutes is thus calculated as the upper limit of the rotational period for 2002 AA29; it probably rotates more quickly. This rapid rotation together with the small diameter and therefore low mass leads to some interesting conclusions:
- The asteroid rotates so quickly that the centrifugal forceCentrifugal forceCentrifugal force can generally be any force directed outward relative to some origin. More particularly, in classical mechanics, the centrifugal force is an outward force which arises when describing the motion of objects in a rotating reference frame...
on its surface exceeds its gravitational pull. It is therefore under tension and so cannot be composed of an agglomeration of loosely-bound debris or of fragments circling each other – as is supposed for several other asteroids and for example has been determined for the asteroid (69230) Hermes69230 Hermes-External links:* * *...
. Instead the body must be made of a single relatively strong block of rock or of pieces baked together. However, its tensile strength is probably considerably lower than terrestrial rockRock (geology)In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
and the asteroid also very porous.
- 2002 AA29 cannot possibly have been built up from individual small pieces, as these would be thrown apart by the rapid rotation. Therefore it must be a fragment blown off in the collision of two heavenly bodies. J. Richard GottJ. Richard GottJohn Richard Gott III is a professor of astrophysical sciences at Princeton University. He is known for developing and advocating two cosmological theories with the flavor of science fiction: Time travel and the Doomsday argument.- Exotic matter time travel theories :Paul Davies's bestseller How...
and Edward BelbrunoEdward BelbrunoEdward Belbruno is a mathematician whose interests are in celestial mechanics, dynamical systems, dynamical astronomy, and aerospace engineering....
from Princeton UniversityPrinceton UniversityPrinceton University is a private research university located in Princeton, New Jersey, United States. The school is one of the eight universities of the Ivy League, and is one of the nine Colonial Colleges founded before the American Revolution....
have speculated that 2002 AA29 might have formed together with Earth and Theia, the postulated planet that, according to the Giant Impact hypothesisGiant impact hypothesisThe giant impact hypothesis states that the Moon was created out of the debris left over from a collision between the young Earth and a Mars-sized body. The colliding body is sometimes called Theia for the mythical Greek Titan who was the mother of Selene, the goddess of the moon.The giant impact...
, collided with Earth in its early history.
Outlook
Because its orbit is very similar to the Earth’s, the asteroid is relatively easily reachable by space probes. 2002 AA29 would therefore be a suitable object of study for more precise research into the structure and formation of asteroids and the evolution of their orbits around the Sun. Meanwhile further co-orbital companions of the Earth of this type on horseshoe orbits or on orbits as quasi-satellites have already been found, such as the quasi-satellite 2003 YN1072003 YN107
' is a very small Near-Earth object. It was discovered by the Lincoln Near Earth Asteroid Research system in orbit around the Sun on December 20, 2003. Its diameter is approximately 10 to 30 metres, which is near the most commonly used 10-metre demarcation line between meteoroids and asteroids...
. Furthermore it is assumed that there are small Trojan companions of the Earth with diameters in the region of 100 metres located at the L4 and L5 Lagrangian points of the Earth-Sun system.
See also
- 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...
- List of asteroids
- Naming of Asteroids
Articles
- MPEC 2003-A17
- Earth coorbital asteroid
- Research paper describing horseshoe orbits. Recommend starting at page 105!
Databases
- Orbital data for 2002 AA29 from the MPEC database
- Physical data for 2002 AA29 from the EARN Database
- [ftp://ftp.lowell.edu/pub/elgb/astorb.html Asteroid Orbital Elements Database] of the Lowell ObservatoryLowell ObservatoryLowell Observatory is an astronomical observatory in Flagstaff, Arizona. Lowell Observatory was established in 1894, placing it among the oldest observatories in the United States, and was designated a National Historic Landmark in 1965....