Key Terms

asteroid

a stony or metallic object orbiting the Sun that is smaller than a planet but that shows no evidence of an atmosphere or of other types of activity associated with comets

asteroid belt

the region of the solar system between the orbits of Mars and Jupiter in which most asteroids are located; the main belt, where the orbits are generally the most stable, extends from 2.2 to 3.3 AU from the Sun

comet

a small body of icy and dusty matter that revolves about the Sun; when a comet comes near the Sun, some of its material vaporizes, forming a large head of tenuous gas and often a tail

Kuiper belt

a region of space beyond Neptune that is dynamically stable (like the asteroid belt); the source region for most short-period comets

near-Earth asteroid (NEA)

an Earth-approaching asteroid, one whose orbit could bring it on a collision course with our planet

near-Earth object (NEO)

a comet or asteroid whose path intersects the orbit of Earth

nucleus (of a comet)

the solid chunk of ice and dust in the head of a comet

Oort cloud

the large spherical region around the Sun from which most “new” comets come; a reservoir of objects with aphelia at about 50,000 AU

tail

(of a comet) a tail consisting of two parts: the dust tail is made of dust loosened by the sublimation of ice in a comet that is then pushed by photons from the Sun into a curved stream; the ion tail is a stream of ionized particles evaporated from a comet and then swept away from the Sun by the solar wind