The asterisk (er, asteriod) on comets.

• Author: Miller, Judith-Braffman
• Position: Science & Technology - Comets and asteroids that cannot be easily categorized as either one or the other

LIKE MASKED entertainers at a 17th-century ball, the true identity of certain bewitching objects that perform a weird, mysterious pageant upon our solar system's festive stage cannot be determined easily--until their masks have fallen off.

Comets and asteroids are small remnant bodies left over from our primordial solar system, which was born about 4,456,000,000 years ago. Although they represent two distinct populations of relatively small objects that dance around the sun, certain members of the two groups have masked their identifies very well and cannot easily be categorized as either one or the other.

Comets are small, icy, dusty bodies that travel a long and treacherous path in long elliptical orbits. They are famous for their flashing, thrashing tails that stream out whenever their orbits take them sufficiently close to the sun. These alien visitors from the outer limits streak into Earth's inner, warm region from two remote, dark, and frozen realms. The first is the Kuiper Belt, which circles around the sun beyond the orbit of Neptune. The second is the Oort cloud, an enormous sphere of icy objects thought to encircle our entire solar system. Comets are fragile and ephemeral objects, sometimes dismissively referred to as "dirty snowballs" or "icy mudballs," depending on the observer's point of view.

Every time a wandering comet makes its hazardous, long journey into the warm inner solar system, it loses some of its mass by way of sublimation of its surface ices and gas--the frozen surface of the nucleus turns to gas and then forms a cloud called a coma. Radiation emanating from the sun pushes grains of dust away from the coma, and this creates the famous bright, dusty, flashing tails that comets are so famous for.

Asteroids, meanwhile, primarily are found in the Main Asteroid Belt between Mars and Jupiter, although there are other large populations of these rocky objects that dwell elsewhere. Rocky, metallic asteroids traveling in the Main Belt are not supposed to exhibit comet-like behavior. Asteroids should not develop comae or flashing, thrashing tails, but some do.

Comets and asteroids are planetesimals: the leftover remains of a vast population of primordial objects that were the building blocks of the major planets of the solar system, which came into being when a relatively small dense blob--embedded within a gigantic, frigid, cold molecular cloud---collapsed under its own weight. Most of the material that composed the...