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.
Griqua asteroids The
Griqua asteroids (also known as the "Griquas") 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 (at least) 500 million years. Known members of the Griqua group include
(articles in boldface): •
1362 Griqua •
3688 Navajo •
4177 Kohman •
11665 Dirichlet •
13963 Euphrates • • • • • •
Zhongguo asteroids While the
Griquas are
asteroids in a marginally unstable orbit, the
Zhongguo asteroids (or "Zhongguos"; named after
3789 Zhongguo) are in a rather stable
2:1 resonance with
Jupiter. The group of 26 identified members, with a lifetime of more than 530 million (or even 1 billion) years, can be further divided into two clusters in the pseudo-proper element space: •
3789 Zhongguo • •
11266 Macke •
11573 Helmholtz •
14871 Pyramus • '''''' • •
22740 Rayleigh • • •
31249 Renéefleming • • • • • '''''' • • •
45511 Anneblack • • • • • • •
Strongly unstable group Related but distinct from the "long-lived" Griquas and Zhongguos, is the group of "unstable" Hecuba-gap asteroids: •
1921 Pala •
1922 Zulu •
5201 Ferraz-Mello •
5370 Taranis () •
8373 Stephengould •
9767 Midsomer Norton • • • () • • •
65541 Kasbek • • • • 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 (with their current eccentricity taken from the
JPL SBDB in parentheses) are
5370 Taranis (0.64), (0.65), (0.70), (0.66), (0.72). == References ==