Long-term changes in the orbit of Mars are believed to be a critical factor in the current removal of Hungaria asteroids. At the highest eccentricities, similar to the extreme values observed today or even slightly greater, Mars will perturb Hungaria asteroids and force them into ever more eccentric and unstable orbits when their ascending node is close in longitude to Mars's aphelion. This ultimately leads over millions of years to the formation of the short-lived Amor asteroids and Earth-crossers.
