Kirkwood gaps
Regions in the main asteroid belt that
have been cleared of asteroids by the perturbing
effects of Jupiter, named for Daniel Kirkwood
who discovered them. The Kirkwood gaps are due to resonances
with Jupiter's orbital period. For example, an asteroid with a semimajor
axis of 3.3 AU makes two circuits around the Sun in the time it takes
Jupiter to make one and is thus said to be in a 2:1 resonance orbit with
Jupiter. Once every two orbits, Jupiter and such an asteroid would be in
the same relative positions, so that the asteroid would experience a force
in a fixed direction. Repeated applications of this force would eventually
change the semimajor axes of asteroids in such orbits, creating gaps at
that distance. Gaps occur at 4:1, 7:2, 3:1, 5:2, 7:3, and 2:1 resonances,
while concentrations occur at the 3:2 (Hilda
group), 4:3 (Thule), and 1:1 (Trojan
group) resonances. The presence of secular
resonances complicates the situation, particularly at the inner edges of
the belt. An adequate explanation of why some resonances produce gaps and
others produce concentrations has yet to be found.
Related
category
• ASTEROIDS
AND OTHER MINOR PLANETS: TYPES AND GROUPS
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