Why do asteroid orbits change near gravitational resonances with Jupiter?

Asteroid orbits change near gravitational resonances with Jupiter primarily due to the repeated application of gravitational forces when an asteroid and Jupiter reach the same relative positions at regular intervals.1

When an asteroid is in a mean-motion resonance—where its orbital period is a simple ratio of Jupiter's—it experiences a gravitational force in a fixed direction during each encounter. These repeated perturbations can lead to two distinct outcomes:1

• Orbital Evolution and Gaps: For many resonances, such as the 3:1, 5:2, and 2:1, these perturbations create chaotic orbital behavior. In these chaotic zones, an asteroid's orbital eccentricity can become large, eventually causing the orbit to cross that of Mars. A close encounter with Mars can then remove the asteroid from the resonance, resulting in the population depletions known as Kirkwood gaps.231
• Stabilization: Conversely, some resonances can result in the long-term stabilization of orbits, as seen in groups like the Trojan asteroids, which cluster near the 1:1 resonance.1

Additionally, secular resonances—involving the interaction of orbital nodes and perihelia—operate over millions of years to further alter an asteroid's eccentricity and inclination.1