Kirkwood gap
Encyclopedia
Asteroid belt
The 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...
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...
s with semi-major axis
Semi-major axis
The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...
(or equivalently their 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...
), as seen in the histogram below. They correspond to the location of 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...
s with 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,...
.
For example, there are very few asteroids with semimajor axis near 2.50 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
, 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
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...
.
The gaps were first noticed in 1857 by Daniel Kirkwood
Daniel Kirkwood
Daniel Kirkwood was an American astronomer.Born in Harford County, Maryland, he was graduated in mathematics from the York County Academy in York, Pennsylvania in 1838...
, who also correctly explained their origin in the orbital resonances with Jupiter while a professor at Jefferson College
Washington & Jefferson College
Washington & Jefferson College, also known as W & J College or W&J, is a private liberal arts college in Washington, Pennsylvania, in the United States, which is south of Pittsburgh...
in Canonsburg, Pennsylvania
Canonsburg, Pennsylvania
Canonsburg is a borough in Washington County, Pennsylvania, southwest of Pittsburgh. Canonsburg was laid out by Colonel John Canon in 1789 and incorporated in 1802....
.
More recently, a relatively small number of asteroids have been found to possess high eccentricity orbits which do lie within the Kirkwood gaps. Examples include the Alinda family
Alinda family
The Alinda asteroids are a group of asteroids with a semi-major axis of about 2.5 AU and an orbital eccentricity approximately between 0.4 and 0.65. The namesake is 887 Alinda, discovered by Max Wolf in 1918....
and the Griqua family
Griqua family
The Griqua asteroids are a group of asteroids in the Main Belt orbiting the sun between 3.1 and 3.27 AU. Asteroids in this group have eccentricities greater than 0.35. The group derives its name from the asteroid 1362 Griqua....
. 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.
The most prominent Kirkwood gaps (see diagram) are located at mean orbital radii of:
- 2.06 AUAstronomical unitAn astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
(4:1 resonance) - 2.5 AU (3:1 resonance), home to the Alinda familyAlinda familyThe Alinda asteroids are a group of asteroids with a semi-major axis of about 2.5 AU and an orbital eccentricity approximately between 0.4 and 0.65. The namesake is 887 Alinda, discovered by Max Wolf in 1918....
of asteroids - 2.82 AU (5:2 resonance)
- 2.95 AU (7:3 resonance)
- 3.27 AU (2:1 resonance), home to the Griqua familyGriqua familyThe Griqua asteroids are a group of asteroids in the Main Belt orbiting the sun between 3.1 and 3.27 AU. Asteroids in this group have eccentricities greater than 0.35. The group derives its name from the asteroid 1362 Griqua....
of asteroids
Weaker and/or narrower gaps are also found at:
- 1.9 AU (9:2 resonance)
- 2.25 AU (7:2 resonance)
- 2.33 AU (10:3 resonance)
- 2.71 AU (8:3 resonance)
- 3.03 AU (9:4 resonance)
- 3.075 AU (11:5 resonance)
- 3.47 AU (11:6 resonance)
- 3.7 AU (5:3 resonance)