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Asteroids
Asteroids are rocky and metallic
objects with an irregular shape
that orbit the Sun but are too
small and numerous to be
considered planets. They are
known as minor planets.
Asteroids range in size from
Ceres, which has a diameter of
about 1000 km, down to the size
of pebbles. Sixteen asteroids
have a diameter of 240 km or
greater. They have been found
inside Earth's orbit to beyond
Saturn's orbit. Most, however, are contained within the asteroid belt that
exists between the orbits of Mars and Jupiter. Some have orbits that cross
Earth's path and some have even hit the Earth in times past. One of the best
preserved examples is Barringer Meteor Crater near Winslow, Arizona.
Asteroids are material left over from the formation of the solar system. One
theory suggests that they are the remains of a planet that was destroyed in a
massive collision long ago. More likely, asteroids are material that never
formed into a planet. Asteroids are composed of several different materials
and differ in composition. This is how scientists classify asteroids. Much of
our understanding about asteroids comes from examining pieces of space
debris that fall to the surface of Earth. Asteroids that are on a collision course
with Earth are called meteoroids.
Meteors, Meteoroids, & Meteorites
The term meteor comes from the Greek meteoron, meaning phenomenon in
the sky. It is used to describe the streak of light produced as matter in the
solar system falls into Earth's atmosphere creating temporary light resulting
from atmospheric friction. This typically occurs at heights of 80 to 110
kilometers (50 to 68 miles) above Earth's surface. A meteoroid is a chunk of
metal, rock, or dust revolving around the sun or any object in interplanetary
space that is too small to be called an asteroid or a comet. In fact, meteors,
meteoroids, and meteorites can be smaller than a grain
of sand. Meteors are meteoroids that pass through and
burn up in the Earth’s atmosphere and are often called
shooting stars. A meteorite is a meteoroid that passes
through Earth’s atmosphere and hits the Earth’s
surface.
One of the primary goals of studying meteorites is to determine the history
and origin of the celestial object they broke off of. Meteorites are classified
based on their composition. Several meteorites sampled from Antarctica
since 1981 have been shown to have originated from the moon. Astronomers
compared the composition of these meteorites to moon rocks obtained by the
Apollo missions of 1969-1972 and found that they are made of the same
types of materials. Sources of other specific meteorites remain unproven,
although another set of eight meteorites are suspected to have come from
Mars. All other groups are presumed to have formed from asteroids or
comets; the majority of meteorites are believed to be fragments of asteroids.
Meteoroids may become detached from passing comets or form from
collisions between asteroids. Meteorites fall all over Earth. Most of them
look just like stones, so nobody notices them. A few meteorites are made
almost entirely of iron and nickel, and so are unusually heavy for their size.
This makes them more likely to be identified as meteorites than as Earth
rocks.
Comets
Comets are small, fragile, irregularly shaped
bodies composed of chunks of ice and dust. They
have highly elliptical orbits that bring them very
close to the Sun and swing them deeply into
space, often beyond the orbit of Pluto. Comets
can range in size from 0.7 to 20 km in diameter.
As comets approach the Sun they develop enormous tails of luminous
material that extend for millions of kilometers away from the Sun. When far
from the Sun, the center is very cold and its material is frozen solid. In this
state comets are sometimes referred to as a "dirty snowball," since over half
of their material is ice. When a comet approaches the Sun, the surface of the
center begins to warm, forming the comet's coma (outer layer) of gas and
dust. When the center is frozen, it can be seen only by reflected sunlight.
However, when a coma develops, dust reflects still more sunlight, and gas in
the coma absorbs ultraviolet radiation and begins to glow. Closer to the Sun,
it produces a glow that is more intense than reflected light off it’s surface.
This means that comets can produce their own light when a coma develops.
The sun’s corona produces a stream of particles called the solar wind. Solar
wind pushes the gas from a comet away from the sun. Gas and dust form the
comet’s tail. The tail looks like hair; in fact, the name comet means “long–
haired star” in Greek. A comet’s tail can be hundreds of millions of
kilometers long and stretch across most of the sky. The material is stretched
out very thinly, however, so there isn’t much mass in a comet tail.
In 1705, Edmond Halley, an English astronomer, calculated the orbits of 24
comets that people had observed over hundreds of years. Halley realized that
several of the comets seemed to have the same orbit and suggested that they
were actually the same comet. Halley calculated that this comet appeared
about every 76 years, and predicted that it would reappear in 1758. When
this prediction came true, the comet was named Halley’s Comet. In 1986,
the last time Halley’s Comet appeared in our sky, scientists all over the world
awaited the arrival of Halley’s comet.