Near Earth Object

A Near-Earth Object (NEO) is any small Solar System body whose orbit brings it into proximity with Earth. By convention, a Solar System body is a NEO if its closest approach to the Sun (perihelion) is less than 1.3 astronomical units (AU). If a NEO's orbit crosses the Earth's, and the object is larger than 140 meters (460 ft) across, it is considered a potentially hazardous object (PHO), and some of these can lead to an Extinction Level Event (ELE).

This is a major concern for the possible extinction of a Type 0 civilization, and Type I civilizations will have planetary defense systems against NEOs.