![Kirkwood Gaps](https://upload.wikimedia.org/wikipedia/commons/d/d3/Kirkwood_Gaps.svg)
A Kirkwood gap is a gap or dip in the distribution of the
semi-major axes
In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the longes ...
(or equivalently of the
orbital period
The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...
s) of the orbits of
main-belt
The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...
asteroid
An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere.
...
s. They correspond to the locations of
orbital resonance
In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationsh ...
s with
Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
.
For example, there are very few asteroids with semimajor axis near 2.50
AU, period 3.95 years, which would make three orbits for each orbit of Jupiter (hence, called the 3:1 orbital resonance). Other orbital resonances correspond to orbital periods whose lengths are simple fractions of Jupiter's. The weaker resonances lead only to a depletion of asteroids, while spikes in the histogram are often due to the presence of a prominent
asteroid family
An asteroid family is a population of asteroids that share similar proper orbital elements, such as semimajor axis, eccentricity, and orbital inclination. The members of the families are thought to be fragments of past asteroid collisions. An a ...
''(see
List of asteroid families
An asteroid family is a population of asteroids that share similar proper orbital elements, such as semimajor axis, eccentricity (orbit), eccentricity, and orbital inclination. The members of the families are thought to be Collisional family, fra ...
)''.
The gaps were first noticed in 1866 by
Daniel Kirkwood
Daniel Kirkwood (September 27, 1814 – June 11, 1895) was an American astronomer.
Kirkwood was born in Harford County, Maryland to John and Agnes (née Hope) Kirkwood. He graduated in mathematics from the York County Academy in York, Pennsyl ...
, who also correctly explained their origin in the orbital resonances with Jupiter while a professor at
Jefferson College in
Canonsburg, Pennsylvania
Canonsburg is a Borough (Pennsylvania), borough in Washington County, Pennsylvania, southwest of Pittsburgh. Canonsburg was laid out by Colonel John Canon in 1789 and incorporated in 1802. The population was 9,735 at the 2020 census. The town li ...
.
Most of the Kirkwood gaps are depleted, unlike the
mean-motion resonance
In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationsh ...
s (MMR) of Neptune or Jupiter's 3:2 resonance, that retain objects captured during the giant planet migration of the
Nice model
The Nice () model is a scenario for the dynamical evolution of the Solar System. It is named for the location of the Côte d'Azur Observatory—where it was initially developed in 2005—in Nice, France.
It proposes the migration of the giant p ...
. The loss of objects from the Kirkwood gaps is due to the overlapping of the ν
5 and ν
6 secular resonance A secular resonance is a type of orbital resonance between two bodies with synchronized precessional frequencies. In celestial mechanics, secular refers to the long-term motion of a system, and resonance is periods or frequencies being a simple nu ...
s within the mean-motion resonances. The orbital elements of the asteroids vary chaotically as a result and evolve onto planet-crossing orbits within a few million years.
The 2:1 MMR has a few relatively stable islands within the resonance, however. These islands are depleted due to slow diffusion onto less stable orbits. This process, which has been linked to Jupiter and Saturn being near a 5:2 resonance, may have been more rapid when Jupiter's and Saturn's orbits were closer together.
More recently, a relatively small number of asteroids have been found to possess high
eccentricity
Eccentricity or eccentric may refer to:
* Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal"
Mathematics, science and technology Mathematics
* Off-center, in geometry
* Eccentricity (graph theory) of a v ...
orbits which do lie within the Kirkwood gaps. Examples include the
Alinda
Alinda ( grc, Ἄλινδα) was an inland city and bishopric in ancient Caria, in Asia Minor (Anatolia). Modern scholars identify Alinda with the Hellenistic foundation of Alexandria ad Latmum (Ἀλεξάνδρεια πρὸς τῷ Λάτμ ...
and
Griqua group A Hecuba-gap asteroid is a member of a dynamical group of resonant asteroids located in the Hecuba gap at 3.27 AU – one of the largest Kirkwood gaps in the asteroid belt, which is considered the borderline separating the outer main belt ast ...
s. These orbits slowly increase their eccentricity on a timescale of tens of millions of years, and will eventually break out of the resonance due to close encounters with a major planet. This is why asteroids are rarely found in the Kirkwood gaps.
Main gaps
The most prominent Kirkwood gaps are located at mean orbital radii of:
* 1.780
AU (5:1 resonance)
* 2.065 AU (4:1 resonance)
* 2.502 AU (3:1 resonance), home to the
Alinda group
The Alinda asteroids are a dynamical group of asteroids with a semi-major axis of about 2.5 AU and an orbital eccentricity approximately between 0.4 and 0.65.Griqua group A Hecuba-gap asteroid is a member of a dynamical group of resonant asteroids located in the Hecuba gap at 3.27 AU – one of the largest Kirkwood gaps in the asteroid belt, which is considered the borderline separating the outer main belt ast ...
of asteroids.
* 3.972 AU (3:2 resonance), home to the
Hilda asteroid
The Hilda asteroids (adj. ''Hildian'') are a dynamical group of more than 5,000 asteroids located beyond the asteroid belt but within Jupiter's orbit, in a 3:2 orbital resonance with Jupiter. The namesake is the asteroid 153 Hilda.
Hildas move i ...
s.
* 4.296 AU (4:3 resonance), home to the
Thule group of asteroids.
Weaker and/or narrower gaps are also found at:
* 1.909 AU (9:2 resonance)
* 2.258 AU (7:2 resonance)
* 2.332 AU (10:3 resonance)
* 2.706 AU (8:3 resonance)
* 3.031 AU (9:4 resonance)
* 3.077 AU (11:5 resonance)
* 3.474 AU (11:6 resonance)
* 3.517 AU (9:5 resonance)
* 3.584 AU (7:4 resonance), home to the
Cybele asteroids
The Cybele asteroids (also known as the "Cybeles") are a dynamical group of asteroids, named after the asteroid 65 Cybele. Considered by some as the last outpost of an extended asteroid belt, the group consists of just over 2000 members and ...
* 3.702 AU (5:3 resonance).
Asteroid zones
The gaps are not seen in a simple snapshot of the locations of the asteroids at any one time because asteroid orbits are elliptical, and many asteroids still cross through the radii corresponding to the gaps. The actual spatial density of asteroids in these gaps does not differ significantly from the neighboring regions.
The main gaps occur at the 3:1, 5:2, 7:3, and 2:1 mean-motion resonances with Jupiter. An asteroid in the 3:1 Kirkwood gap would orbit the Sun three times for each Jovian orbit, for instance. Weaker resonances occur at other semi-major axis values, with fewer asteroids found than nearby. (For example, an 8:3 resonance for asteroids with a semi-major axis of 2.71 AU).
The main or core population of the asteroid belt may be divided into the inner and outer zones, separated by the 3:1 Kirkwood gap at 2.5 AU, and the outer zone may be further divided into middle and outer zones by the 5:2 gap at 2.82 AU:
* 4:1 resonance (2.06 AU)
** Zone I population (inner zone)
* 3:1 resonance (2.5 AU)
** Zone II population (middle zone)
* 5:2 resonance gap (2.82 AU)
** Zone III population (outer zone)
* 2:1 resonance gap (3.28 AU)
4 Vesta
Vesta (minor-planet designation: 4 Vesta) is one of the largest objects in the asteroid belt, with a mean diameter of . It was discovered by the German astronomer Heinrich Wilhelm Matthias Olbers on 29 March 1807 and is named after Vesta, the ...
is the largest asteroid in the inner zone,
1 Ceres
Ceres (; minor-planet designation: 1 Ceres) is a dwarf planet in the asteroid belt between the orbits of Mars and Jupiter. It was the first asteroid discovered, on 1 January 1801, by Giuseppe Piazzi at Palermo Astronomical Observatory in Sici ...
and
2 Pallas
Pallas (minor-planet designation: 2 Pallas) is the second asteroid to have been discovered, after Ceres. It is believed to have a mineral composition similar to carbonaceous chondrite meteorites, like Ceres, though significantly less hyd ...
in the middle zone, and
10 Hygiea
Hygiea (minor-planet designation: 10 Hygiea) is a major asteroid and possible dwarf planet located in the main asteroid belt. With a diameter of and a mass estimated to be 3% of the total mass of the belt, it is the fourth-largest asteroid in ...
in the outer zone.
87 Sylvia
Sylvia (minor planet designation: 87 Sylvia) is the one of the largest asteroids (approximately tied for 7th place, to within measurement uncertainties). It is the parent body of the Sylvia family and member of Cybele group located beyond the m ...
is probably the largest Main Belt asteroid beyond the outer zone.
See also
*
Orbital resonance
In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationsh ...
*
Alinda group
The Alinda asteroids are a dynamical group of asteroids with a semi-major axis of about 2.5 AU and an orbital eccentricity approximately between 0.4 and 0.65.Cybele group
The Cybele asteroids (also known as the "Cybeles") are a dynamical group of asteroids, named after the asteroid 65 Cybele. Considered by some as the last outpost of an extended asteroid belt, the group consists of just over 2000 members an ...
*
Griqua group A Hecuba-gap asteroid is a member of a dynamical group of resonant asteroids located in the Hecuba gap at 3.27 AU – one of the largest Kirkwood gaps in the asteroid belt, which is considered the borderline separating the outer main belt ast ...
References
External links
Article on Kirkwood gaps at Wolfram'sscienceworld
{{Asteroids
Asteroids
Resonance with Jupiter