Definitions and Discoveries of Small Solar System Bodies

While there are now clear definitions for the terms asteroid, meteorite, and impact crater, these definitions have been hard won through centuries of scientific investigation. For millennia, meteorites have been recognized as unusual rocks, but asteroids were entirely unknown until the early 17th century, and impact craters have only been recognized as such for a century or less. Fundamental beliefs about the stability of the universe and the safety of Earth stood in the way of this science more, perhaps, than other disciplines. For centuries, the deeply held beliefs that all objects orbited in their own paths, never wavering nor departing, made the idea that meteorites might fall to Earth from space impossible. The idea that catastrophically large impacts might occur on the Earth, threatening life itself was completely unthinkable.

What Are Asteroids, Comets, Meteorites, and Meteors?

Asteroids and comets are bodies that orbit the Sun, just as the planets do. While there is a continuum of sizes of objects in the solar system, from the Sun to a grain of interplanetary dust (see figure on page 2), until the early 19th century and the development of telescopes, observers were unable to detect any but the largest planets. Planets, therefore, dominate the image of the solar system.The great abundance of smaller bodies is still being discovered, and now is categorized under the name "minor planets."

Comets are icy bodies in highly elliptical orbits around the Sun. Asteroids are, by definition, rocky bodies less than 621 miles (1,000 km)

in diameter that orbit the Sun. Although comets appear from Earth to be bright objects, the comet Halley, for example, absorbs 96 percent of the light that strikes it, making it one of the darkest bodies in the solar

Continuum of Sizes

Sun: 432,000 miles (695,000 km)

Jupiter: 44,424 miles (71,492 km) Earth: 3,986 miles (6,378 km)

Jupiter: 44,424 miles (71,492 km) Earth: 3,986 miles (6,378 km)

Rather than a family of planets interspersed by some asteroids, the solar system consists of bodies over a continuum of sizes—from the Sun, with a radius of 434,000 miles (695,000 km), to interplanetary dust, about 10 trillion times smaller.

system. In contrast to comets, asteroids were not thought to shed gas and therefore lack the beautiful streaming tails of comets. Recently, though, some asteroids have been found that give off a comet-like coma (the head of a comet consisting of a cloud of gas and dust) so the delineation between comets and asteroids is not as clear as had previously been presented. Some asteroids may have begun as comets and since lost all their ices and gases. Two asteroids that have gassy comas only at their orbital perihelia (their closest approach to the Sun) are 2060 Chiron and 4015 Wilson-Harrington.

2060 Chiron is a member of the Centaur family of asteroids, orbiting between Saturn and Neptune. Its unstable orbit suggests it was once a Kuiper belt object that received some sort of gravitational perturbation from a larger body that threw it into its current orbit. 2060 Chiron has a hazy, luminous cloud around it, indicating that it is degassing or shedding ices. Chiron is therefore one of the enigmatic objects that lies somewhere between the definition of an asteroid and that of a comet.

To add to the confusion between designations, until recently all asteroids were thought to orbit in a direct sense (see figure below), in the same direction around the Sun as the major planets (it has been

All planets and most asteroids orbit the Sun directly, although some comets and asteroids orbit in a retrograde sense.

Direct and Retrograde Orbit

P=perihelion or perigee, the closest approach of the satellite to the body it orbits A=aphelion or apogee, the farthest remove of the satellite from the body it orbits

Looking down on the north pole of a planet or moon, an orbit in this direction is called direct, or prograde. An orbit in the opposite direction is called indirect, or retrograde.

The same terms are used for the body's rotation around its axis: This planet is rotating in a prograde direction.

known for a long time that some comets orbit in a retrograde sense). In 1999 for the first time, two objects found to have retrograde orbits failed to show any icy content or gassy tails, even when examined closely and at length. These objects are named 1999 LD31 and 1999 LE and were discovered only four nights apart. 1999 LD and 1999 LE have orbital inclinations of 160° and 152°, respectively, so not only do they orbit in a retrograde sense, but their orbits lie far out of the ecliptic plane.

The line between comets and asteroids thus continues to blur. Some asteroids have gassy tails, some orbit in a retrograde sense, and some have orbits outside the ecliptic plane, all attributes previously reserved for comets. In general, though, bodies considered to be asteroid-like are thought to have formed in the denser, hotter parts of the inner solar system where the terrestrial planets formed.The bodies considered more comet-like acquired their volatiles (elements and molecules that evaporate or liquefy at relatively low temperatures, such as water and carbon dioxide) toward the outer solar system, in the region of the gas giant planets. It makes sense, then, that all these bodies would have a range of volatile contents. Comets and asteroids are all remnant planetesimals (material that failed to be incorporated into a planet early in solar system formation) and thus are probably formed from the same materials as the planets that formed near them.

Meteorites are rocky bodies that have fallen to Earth. Before they fell, they were considered meteoroids if they were between four thousandths of an inch (100 |Xm) and about four inches (10 cm) and considered asteroids if they were larger. Meteorites are related to asteroids:The only idea scientists have for the process that creates meteorites is the collision and breakup of asteroids. Recently it has been shown that large, irregularly shaped asteroids, such as 6 Hebe, 4 Vesta, and 8 Flora, are associated with swarms of small asteroids and meteoroids, totaling in the tens of millions. It is thought that these swarms are the source of the meteorites that collide with Earth.

The term meteor is only applicable to a visual phenomenon on Earth (see table on page 5). Meteors are grains of dust that make streaks of light as they enter the Earth's atmosphere. Bodies larger than dust that enter the atmosphere can also glow brightly, and these are usually referred to as fireballs. Some fireballs have glowed so brightly that they illuminated 400,000 square miles (150,000 km2) of land surface

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