アテン群
is currently an Aten asteroid (Earth-crossing but with a period less than a year). Its semi-major axis (currently 0.9994 AU) is similar to that of Earth (0.9992 AU), but it has a relatively high eccentricity (0.4607) and moderate orbital inclination (10.1910°). Gravitational interaction with Earth causes its orbit to change so that its average period is one year (This means it alternates between being an Aten asteroid and being an Apollo asteroid.) Its mean longitude (around the Sun) is similar to that of Earth, which means that it is fairly close to Earth (less than about 1.5 AU). This makes it a quasi-satellite. It moves in a kidney-shaped path going from east to west relative to Earth ("retrograde", the opposite way from the Moon). The relative mean longitude compared to Earth (that is, its mean longitude minus that of Earth) librates around zero. This means that the center of the "kidney" moves back and forth over a period of years, from being in front of Earth to being behind Earth, but this movement is centered on Earth.