near-Earth object

NASA's Center for Near Earth Object Studies defines a near-Earth object as an asteroid or comet whose closest approach to the Sun, its perihelion distance, is less than 1.3 astronomical units. One astronomical unit is the average distance from the Earth to the Sun, so the definition draws a boundary a little way outside our own orbit and counts everything that comes inside it. Almost all of them are asteroids, sorted into four orbital families named Atira, Aten, Apollo and Amor. Comets qualify too, but only short period ones, meaning those that go round the Sun in under 200 years.

A smaller subset carries the label potentially hazardous. To earn it an object must come within 0.05 astronomical units of Earth's orbit, roughly 7.5 million kilometres, and be bright enough to imply a diameter of about 140 metres or more. Neither condition says an impact is expected. They mark out the objects worth tracking closely, which is why surveys that catalogue near-Earth objects are treated as the first stage of planetary defence rather than an academic exercise.