An asteroid is a large rocky body found in outer space. They range from boulder size all the way up to about 600 miles (965 kilometers) wide; they generally are classified as either carbonaceous, stony, or metal. The majority of asteroids are found along the plane, or ecliptic, of the solar system. The larger ones are sometimes called minor planets because of their propensity to orbit along the same plane as the major planets. Most of the asteroids revolve around the Sun in a tight band between the orbits of Mars and Jupiter, called the asteroid belt. Italian astronomer Giuseppe Piazzi (1746–1826) discovered the first asteroid, Ceres, in 1801.

Comets are a collection of dust, gases, and ice that orbit the Sun. Once described as “dirty snowballs,” many comets are now thought to be more like “mud-balls,” most carrying more dust than ice. In general, comets are composed of carbon dioxide, frozen water, methane, ammonia, and materials such as silicates and organic compounds.