AHecuba-gap asteroid is a member of adynamical group of resonantasteroids located in theHecuba gap at 3.27 AU – one of the largestKirkwood gaps in theasteroid belt, which is considered the borderline separating theouter main belt asteroids from theCybeles. A Hecuba-gap asteroid stays in a 2:1mean motion resonance with the gas giantJupiter, which may graduallyperturbe its orbits over a long period until it either intersect with the orbit ofMars or Jupiter itself. Depending on the dynamical stability of an asteroid'sorbit in the Hecuba gap, three subgroups have been proposed. These are the marginally unstableGriqua asteroids, with an estimated lifetime of more than 100 million years, the stableZhongguo asteroids (more than 500 million or even 1 billion years), and an unnamed, strongly unstable population of asteroids with a dynamical lifetime of less than 70 million years.[1][2][3]
Moving further out theSolar System, the asteroids in the Hecuba gap are followed by the resonantCybeles (7:4),Hildas (3:2),Thules (4:3) andJupiter trojans (1:1).
Both the Zhongguo (stable) and Griqua asteroids (marginally stable) are also described as the "long-lived" group with lifetimes over 70 million years in order to distinct them from the "short-lived" population. The division between the Griquas and Zhongguo into clearly distinct groups in terms of lifetime has also been questioned and follow-up studies have moved the threshold lifetime between the two long-lived groups from 500 million to 1 billion years.[2]
TheGriqua asteroids (also known as the "Griquas") are adynamical group ofasteroids with marginally unstableorbits. The group derives its name from the asteroid1362 Griqua. The Griquas are located in theHecuba gap – one of the largestKirkwood gaps in theasteroid belt at 3.27 AU – and stay in a 2:1mean motion resonance with the gas giantJupiter, which graduallyperturbes their orbits over a long period until they either intersect with the orbit ofMars or Jupiter itself. The group has an estimated lifetime of 100 to (at least) 500 million years. Known members of the Griqua group include(articles in boldface):[1]: 422
While theGriquas areasteroids in a marginally unstable orbit, theZhongguo asteroids (or "Zhongguos"; named after3789 Zhongguo) are in a rather stable1:2 resonance withJupiter. The group of 26 identified members, with a lifetime of more than 530 million (or even 1 billion[2]) years, can be further divided into two clusters in the pseudo-proper element space:[1]: 423
Related but distinct from the "long-lived" Griquas and Zhongguos, is the group of "unstable" Hecuba-gap asteroids:[1]: 422, 423
In 2005, further research expanded this group of 16 bodies with an additional 31 multi-opposition asteroids, that show life-times of less than 70 million years. It is also thought that approximately 25% of the short-lived population are "extremely unstable" with lifetimes of less than 2 million years.[2]
Among the unstable population, some asteroids have such a higheccentricity with aperihelion of less than 1.3 AU, that qualifies them as anear-Earth object. These five bodies (with their current eccentricity taken from theJPL SBDB in parentheses) are5370 Taranis (0.64),1995 QN3 (0.65),2003 UL12 (0.70),1997 YM3 (0.66),2000 WL10 (0.72).[2]