DEFINITION OF ORIGIN OF METEORITES AND OTHER EARTH ORBIT-CROSSING OBJECTS

Most meteorites originate in the asteroid belt, situated

between the orbits of Mars and Jupiter. There are at least one

million asteroids in this belt with diameters greater than 0.6

miles (1 km), 1,000 with diameters greater than 18 miles (30

km), and 200 with diameters greater than 60 miles (100 km).

Asteroids and meteorites are distinguished only by their size,

asteroids being greater than 328 feet (100 m) in diameter.

Meteorites are referred to as meteors only after they enter the

Earth’s atmosphere. These are thought to be either remnants

of a small planet that was destroyed by a large impact event,

or perhaps fragments of rocky material that failed to coalesce

into a planet, probably due to the gravitational effects of the

nearby massive planet of Jupiter. Most scientists favor the

second hypothesis but recognize that collisions between asteroids

have fragmented a large body to expose a planet-like

core and mantle now preserved in the asteroid belt.

Collisions between asteroids can alter their orbits and

cause them to head into an Earth orbit-crossing path. At this

point, the asteroid becomes hazardous to life on Earth and is

known as an Apollo object. Presently, about 150 Apollo

objects with diameters of greater than 0.6 miles (1 km) are

known, but there are bound to be many more. In 1996 an

asteroid about one-quarter-mile across barely missed hitting

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

to the Moon. A similar near-miss event was recorded in

2001; Asteroid 2001 YB5 passed Earth at a distance of twice

that to the Moon. Had YB5 hit Earth, it would have released

energy equivalent to 350,000 times the energy released during

the nuclear bomb blast in Hiroshima.

The objects that are in an Earth orbit-crossing path

could not have been in this path for very long, because gravitational

influences of the Earth, Mars, and Venus would

cause them to hit one of the planets or be ejected from the

solar system within about 100 million years. The abundance

of asteroids in an Earth orbit-crossing path demonstrates that

ongoing collisions in the asteroid belt are replenishing the

source of potential impacts on Earth. A few rare meteorites

found on Earth have chemical signatures that suggest they

originated on Mars and on the Moon, probably being ejected

toward the Earth from giant impacts on those bodies.

Other objects from space (such as comets) may collide

with Earth. Comets are masses of ice and carbonaceous material

mixed with silicate minerals that are thought to originate

in the outer parts of the solar system, in a region called the

Oort Cloud. Other comets have a closer origin, in the Kuiper

Belt just beyond the orbit of Neptune. There is considerable

debate about whether small icy Pluto, long considered the

small outermost planet, should actually be classified as a

large Kuiper Belt object. Comets may be less common near

Earth than meteorites, but they still may hit the Earth with

severe consequences. There are estimated to be more than a

trillion comets in our solar system. Since they are lighter than

asteroids, and have water-rich and carbon-rich compositions,

many scientists have speculated that cometary impact may

have brought water, the atmosphere, and even life to Earth.

See also ASTEROID; COMET.

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