Britannica
Sources used in this answer
How do Amor asteroids differ from Apollo asteroids?
Created from Britannica articles
using AI.
AI can make mistakes, so verify using Britannica articles.
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.
Amor and Apollo asteroids are both classified as near-Earth asteroids (NEAs), but they are distinguished primarily by their orbital paths relative to Earth's orbit.1
- Apollo asteroids are defined as Earth-crossing asteroids. They have mean distances from the Sun greater than or equal to 1 astronomical unit (AU) and perihelion distances less than or equal to Earth’s aphelion distance of 1.017 AU. Consequently, they cross Earth’s orbit when they are near the closest points to the Sun in their own orbits.21
- Amor asteroids have perihelion distances greater than 1.017 AU (Earth's aphelion) but no greater than 1.3 AU. Because their perihelion is outside Earth's orbit, they do not currently cross it. However, due to strong gravitational perturbations caused by close approaches to Earth, the orbital elements of these asteroids can change significantly over time, and about half of the known Amors are considered part-time Earth crossers.1