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How do Aten asteroids differ from Atira asteroids?
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Small body
Asteroid Ida and its satellite, Dactyl, photographed by the Galileo spacecraft on August 28, 1993, from a distance of about 10,870 km (6,750 miles). Ida is about 56 km (35 miles) long and shows the irregular shape and impact craters characteristic of many asteroids. The Galileo image revealed that Ida is accompanied by a tiny companion about 1.5 km (1 mile) wide, the first proof that some asteroids have natural satellites.
Aten and Atira asteroids are both classes of near-Earth asteroids (NEAs), but they are distinguished primarily by their orbital relationship to Earth:
- Aten asteroids: These asteroids have a mean distance from the Sun of less than 1 astronomical unit (AU). They are characterized by aphelion distances greater than or equal to 0.983 AU (Earth's perihelion distance), which means they cross Earth’s orbit when they are near the farthest points of their own orbits.1
- Atira asteroids: This class consists of asteroids with orbits located entirely inside that of Earth. They have mean distances from the Sun of less than 1 AU and aphelion distances less than 0.983 AU. Consequently, Atira asteroids do not cross Earth’s orbit.1
In summary, while both groups have orbits with a semimajor axis of less than 1 AU, Aten asteroids are defined by their ability to cross Earth's orbit, whereas Atira asteroids remain strictly interior to it.21