A Hecuba-gap asteroid is a member of a dynamical group of resonant asteroids located in the Hecuba gap at 3.27 AU – one of the largest Kirkwood gaps in the asteroid belt, which is considered the borderline separating the outer main belt asteroids from the Cybeles. A Hecuba-gap asteroid stays in a 2:1 mean motion resonance with the gas giantJupiter, which may gradually perturbe its orbits over a long period until it either intersect with the orbit of Mars or Jupiter itself. Depending on the dynamical stability of an asteroid's orbit in the Hecuba gap, three subgroups have been proposed. These are the marginally unstable [|Griqua asteroids], with an estimated lifetime of more than 100 million years, the stable [|Zhongguo asteroids], and an unnamed, strongly unstable population of asteroids with a dynamical lifetime of less than 70 million years. Moving further out the Solar System, the asteroids in the Hecuba gap are followed by the resonant Cybeles, Hildas, Thules and Jupiter trojans.
Description
Both the Zhongguo and Griqua asteroids 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.
Griqua asteroids
The Griqua asteroids are a dynamical group of asteroids with marginally unstable orbits. The group derives its name from the asteroid 1362 Griqua. The Griquas are located in the Hecuba gap – one of the largest Kirkwood gaps in the asteroid belt at 3.27 AU – and stay in a 2:1 mean motion resonance with the gas giant Jupiter, which gradually perturbes their orbits over a long period until they either intersect with the orbit of Mars or Jupiter itself. The group has an estimated lifetime of 100 to 500 million years. Known members of the Griqua group include ':
While the Griquas are asteroids in a marginally unstable orbit, the Zhongguo asteroids are in a rather stable 1:2 resonance with Jupiter. The group of 26 identified members, with a lifetime of more than 530 million years, can be further divided into two clusters in the pseudo-proper element space:
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.
Among the unstable population, some asteroids have such a high eccentricity with a perihelion of less than 1.3 AU, that qualifies them as a near-Earth object. These five bodies are 5370 Taranis, , , , .
