Alinda group (of asteroids)
The Alinda group is a group of asteroids with a mean orbital radius of about 2.5 AU and an orbital eccentricity between about 0.4 and 0.65. These objects are held in this region by the 1:3 orbital resonance with Jupiter, which results in a 4:1 resonance with Earth. An object in this resonance has its orbital eccentricity steadily increased by gravitational interactions with Jupiter until it eventually has a close encounter with an inner planet that breaks the resonance. Some Alindas have perihelia very close to Earth's orbit, resulting in a series of close encounters at almost exactly four-year intervals, due to the 4:1 resonance. One consequence of this is that if an Alinda asteroid happens to be in an unfavorable position for viewing at the time of its close approach to Earth (for instance, at a small elongation from the Sun), then this situation may persist for decades. As of 2004, for example, the Alinda asteroids 1981 VA and 1915 Quetzalcoatl had not been observed since 1985, and 2608 Seneca had not been observed since 1994. The namesake is 887 Alinda, discovered by Max Wolf in 1918.