NASA defines near-Earth objects (NEO) as asteroids or comets with orbits that bring them within 120 million miles (195 million kilometers) of the sun, meaning they are close to Earth's orbit. The vast majority of these NEOs are asteroids, or near-Earth asteroids (NEAs).
There are over 600,000 known asteroids in the solar system, and over 20,000 of these are NEAs, according to the European Space Agency (ESA). Most of the rest are located in the main asteroid belt between Mars and Jupiter. NEAs are thought to make their way out of the asteroid belt and toward Earth's orbit as they are nudged by the gravity of other solar system bodies. The majority of NEOs range from about 10 feet to 25 miles (3 meters to 40 km) wide, according to NASA.
