DEFINITION OF COMET

Wanderers of the solar system, occasionally appearing

as bright objects that move across the night sky, growing

long brilliant tails that have mystified people for thousands of

years. They are thought to be made of stony inner cores covered

by icy outer layers, often described as dirty snowballs

because small rock fragments may be mixed with the icy

outer layers. Most are only a few kilometers to tens of kilometers

across. Comets represent primitive material formed

during the early accretion of the outer solar system, and as

such they are analogous to asteroids, which represent primitive

material that formed in the early inner solar system.

Most comets are located in one of two prominent belts in the

outer solar system. These are known as the Kuiper belt, outside

the orbit of Neptune, and the Oort cloud, forming the

outer reaches of the solar system.

The Kuiper belt contains many comets outside the orbit

of Neptune, and the objects Pluto and its large moon Charon

are considered by many to represent large comets. Most

comets reside in the Oort cloud, a spherical cloud located a

light-year (approximately 6 trillion miles) from the Sun.

There are thought to be a trillion comets in the Oort cloud,

with a total mass of about 40 Earths. Occasionally, gravitational

interactions between comets in the Oort cloud will

deflect one from this far region and cause it to have an orbit

that intersects the inner solar system. When these comets pass

though the inner solar system, heat from the Sun vaporizes

some of the ice that forms outer layers of the comet, and the

solar wind causes this vapor tail (consisting of both gas and

dust) to become elongated in a direction pointing directly

away from the Sun. These tails may be hundreds of millions

of kilometers long, and some are clearly visible from Earth.

The gravity of objects in the solar system (such as Jupiter and

the Sun) can further distort the orbits of these errant comets,

causing them to obtain regular elliptical orbits circling the

Sun with a predictable period. Such comets include the

famous Halley’s comet, Hale-Bopp, and others.

Comets have occasionally hit the Earth and other planets,

forming impact craters, and very likely brought many of

the volatile elements, water, and the atmosphere to our planet.

Some scientists believe that comets may also have brought

primitive organic molecules or even life to Earth. A cometary

impact with Earth is thought to have caused a huge explosion

and fireball over Tunguska, Siberia, in 1908. The explosion

from this event leveled 1,158 square miles (3,000 km2) of forest

and sent shock waves around the planet.

There are approximately 10 million comets that have

orbits that cross the paths that the planets take around the

Sun. Some of these cross the Earth’s orbit, leaving a trail of

dust particles behind. When the Earth crosses these cometary

dust trails, the dust enters the upper atmosphere and burns

up, forming spectacular meteorite showers. Halley’s comet

has left a large trail of dust particles in its wake during its

numerous orbits around the Sun. The Earth crosses this trail

two times a year, producing the Eta Aquarid meteor shower

in early May, and the Oronoid meteor shower in mid–late

May. Other meteor showers including the Perseids, Leonids,

and Geminids, probably also originating from dust trails

from comets, but these comets have probably lost their icy

outer layers and have yet to be identified.

It is estimated that 25,000 small comets enter the Earth’s

atmosphere every day, or about several million per year.

These objects are mostly less than 30 feet (10 m) wide but

carry 100 tons of water. Cumulatively over the age of the

Earth, these minicomets would have added a volume of water

to the atmosphere equivalent to the present-day oceans.

These comets also carry organic molecules and provide one

plausible mechanism to bring complex organic molecules to

Earth that could have helped the development of life.

See also ASTEROID; METEOR; LIFE’S ORIGINS AND EARLY

EVOLUTION.

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