Definition of Asteroid

Asteroids are classified based on their composition and

include three main classes—P-type or primitive asteroids are

rich in carbon and water, and are thought to represent unaltered

material left over from the formation of the solar system.

They reside primarily in outer parts of the asteroid belt.

C-type asteroids are metamorphic and, like the P-type asteroids,

contain abundant carbon, but their water has been

removed during heating and metamorphism. They reside

mainly in the center part of the asteroid belt. S-type asteroids

are igneous and represent the most common type of meteorite

found on Earth. These asteroids were partially melted and

have a composition similar to that of the Earth’s mantle. Stype

asteroids reside in the inner part of the asteroid belt.

Some of these classes are thought by some scientists to represent

a planet that broke up during a massive collision early in

the history of the solar system. The metallic or iron meteorites

would represent the core of this hypothesized destroyed

planet, whereas the stony meteorite would represent the mantle

and crust. Other scientists believe that the asteroids never

formed a single planet but represent several different planetsimals

that never coalesced but have experienced many collisions,

forming the metamorphism and partial melting

observed in some meteorites.

More than 2,000 asteroids that are larger than a kilometer

wide are in Earth-crossing orbits. When asteroids enter

the Earth’s atmosphere, their outer surface burns up and creates

a fiery streak moving across the sky. Asteroids that enter

the Earth’s atmosphere are known as meteorites. Small meteorites

burn up completely before hitting the Earth, whereas

larger ones may reach the Earth before burning up.

Asteroids, comets, and meteorites have been objects of

fascination, speculation, and fear for most of recorded

human history. Early peoples thought that fiery streaks in the

sky were omens of ill fortune and sought refuge from their

evil powers. Impacts of comets and meteorites with Earth are

now recognized as the main cause for several periods of mass

extinction on the planet, including termination of the

dinosaurs 65 million years ago. Comets and meteorites may

also have brought much of the water, air, and perhaps even

life to Earth, and the planets themselves coalesced from

numerous smaller asteroids, comets, and interplanetary dust.

Asteroids, comets, and meteorites were therefore essential for

the formation of Earth and life and also responsible for the

extinction of many species.

New asteroids are constantly being discovered as our

powers of observation become better. The mass of currently

known asteroids in the main asteroid belt is about half of

that of the Earth’s moon, but it was probably much higher in

the earlier history of the solar system before many of the

asteroids collided with other planets. Some large asteroids

were discovered in the last century. For instance, the nearly

620-mile (1,000-km) wide asteroid Ceres was discovered in

1801 by the Italian astronomer Giuseppe Piazzi. The three

largest known asteroids are Ceres, Pallas, and Vesta, each

several hundred kilometers wide. More than 1,000 asteroids

are larger than 20 miles (30 km) wide, and it is estimated that

more than a million are more than 2,625 feet (800 m) across.

We only know the orbits of about 18,000 of these asteroids.

The Earth has experienced large impacts from meteorites.

Several have caused 50 to 90 percent of all species

alive on the planet at the time of the impact to go extinct,

paving the way for the evolution and diversification of new

organisms. The impact of a six-mile (10-km) wide meteorite

with the Yucatán Peninsula is now thought to have ended the

reign of the dinosaurs. The impact instantly formed a fireball

1,240 miles (2,000 km) across, followed by tsunami hundreds

of meters tall. The dust thrown out of the deep crater

excavated by the impact plunged the world into a fiery darkness,

then months or even years of freezing temperatures. As

soon as the dust settled, carbon dioxide released by the

impact caused Earth to soar into an intense greenhouse

warming. Few species handled these changing environmental

stresses well, and 65 percent of all species went extinct.

NASA and other agencies have recently begun to plan

possible defenses that the human race might mount against

any asteroid or comets on an impact course with Earth. The

future of the human race may well depend on increased

awareness of how to handle this potential threat. In 1996

an asteroid about 2,300 feet (700 m) across barely missed

hitting Earth, speeding past at a distance about equal to the

distance to the moon. The sobering reality of this near collision

is that the asteroid was not even spotted until a few

days before it sped past Earth. What if the object was bigger,

or slightly closer? Would it have been stoppable, and if

not, what would have been the consequences of its collision

with Earth?

See also COMET; METEOR.

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