However, a volatile-rich object will develop an atmosphere only if it is heated by the Sun, whereas a comet that is far from the Sun, or an old comet that has lost most of its volatile inventory and is insulated with a regolith deposit, also can look like an asteroid. The near-Earth asteroids are categorized as Amors, Apollos, and Atens, according to whether their orbits lie outside that of the Earth, cross that of the Earth with period greater than 1 year, or cross that of the Earth with a period less than 1 year, respectively. These are the asteroids that have the potential to impact our planet. For any particular NEO, it will not be clear whether it is in fact an ECA until an accurate orbit is calculated. In 1989 there were 90 known ECAs, while 128 ECAs were known at the time the Spaceguard Survey Report was written in 1992.
Active comets can also cross the Earth's orbit with the potential for collision.
Although about 700 longer-period comets are known to have passed through the inner solar system during recorded history, their total population is difficult to characterize since the majority remain unobservable in the outermost regions of the solar system. The well-observed ECAs exhibit a diversity in infrared mineralogy approaching that in the rest of the asteroid population. There are also a large number of S-class asteroids. The nucleus of Halley is irregular and dark, with an average diameter of about 9 km. The physical configuration of comets is even less well understood than that of the small asteroids, and many comets have been observed to split under rather modest tidal and thermal forces.