Kirkwood Gap

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Distribution of the major orbit half axes of the asteroids in the main belt. The arrows mark distances at which objects are in orbit resonance with Jupiter, with the first digit indicating the number of asteroid orbits.

The Kirkwood gaps in the asteroid belt correspond to orbits with orbital times that are in an integer ratio to the orbit time of Jupiter . Asteroids with these orbits cannot exist there for long periods of time because there are resonances with Jupiter.

The integer ratio of the orbital times means that the great gravity of Jupiter can periodically act on such an asteroid. As a result, the asteroid is deflected from this orbit inwards or outwards; a gap arises in the distribution of the major orbit half-axes or cycle times.

For example, a ratio of 5: 2 means that the duration of five orbits of the asteroid is equal to the duration of two orbits of Jupiter. Since Jupiter needs 11.86 years for one solar orbit, the conditions for an asteroid with 4.74 years orbit are repeated every 23.72 years.

The gaps were named after Daniel Kirkwood , who discovered them in 1866 after statistical studies of the major orbit half-axes.

Noticeable gaps

Noticeable gaps are:

4: 1 response
It is 2.07  AU and limits the main belt inward. On this side of this gap are the asteroids of the Hungaria group.
3: 1 resonance (Hestia gap)
It is 2.50 AU and forms the border between the inner and outer main belt. It is the origin of the Alinda asteroids, which are directed to increasingly eccentric orbits by orbital disturbances.
5: 2 resonance
It is located at 2.82 AU and limits the Koronis - asteroids group inside.
7: 3 resonance
It is 2.96 AU and delimits the Koronis asteroid group to the outside.
2: 1 resonance (Hecuba gap)
It is 3.28 AU and forms the outer limit of the main belt of the asteroids. Beyond are the Cybele asteriods .

Stabilizing resonances

However, certain resonances also hold the asteroids in their orbit. These are for example:

9: 2 resonance (1.91 AU)
Hungaria group
7: 4 resonance (3.58 AU)
Cybele group
3: 2 resonance (3.97 AU)
Hilda group
4: 3 resonance (4.29 AU)
Thule group
1: 1 resonance (5.20 AU)

These stabilizing resonances are outside the graphic above.

Web links

Commons : Kirkwood Gap  - Collection of images, videos and audio files


  1. a b IAU Minor Planet Center