March 26, 2013
The Daily Revolution
The solar system contains not only planets and
moons but other small bodies, including
comets, asteroids, and meteoroids. Scientists
study these objects to learn about the
composition of the solar system.
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can be pulled toward the sun. Another recently
discovered region where comets exist is the
Kuiper (KIE puhr) belt, which is the region
outside the orbit of Neptune.
Comets
A small body of ice, rock, and cosmic dust
loosely packed together is called a comet.
Some scientists refer to comets as “dirty
snowballs” because of their composition.
Comets formed in the cold, outer solar system.
Nothing much has happened to comets since
the birth of the solar system 4.6 billion years
ago. Comets are probably left over from the
time when the planets formed. As a result, each
comet is a sample of the early solar system.
Scientists want to learn more about comets to
piece together the history of our solar system.
Comet Tails
When a comet passes close enough to the sun,
solar radiation heats the ice so that the comet
gives off gas and dust in the form of a long tail.
Sometimes, a comet has two tails—an ion tail
and a dust tail. The ion tail is made of
electrically charged particles called ions. The
solid center of a comet is called its nucleus.
Comet nuclei can range in size from less than
half a kilometer to more than 100 km in
diameter.
Comet Orbits
The orbits of all bodies that move around the
sun are ellipses. Ellipses are circles that are
somewhat stretched out of shape. The orbits of
most planets are close to perfect circles, but the
orbits of comets are very elongated.
Asteroids
A comet’s ion tail always points away from the
sun. The reason is that the ion tail is blown
away from the sun by solar wind, which is also
made of ions. The dust tail tends to follow the
comet’s orbit around the sun. Dust tails do not
always point away from the sun. When a comet
is close to the sun, its tail can extend millions
of kilometers through space!
Small, rocky bodies that revolve around the sun
are called asteroids. They range in size from a
few meters to more than 900 km in diameter.
Asteroids have irregular shapes, although some
of the larger ones are spherical. Most asteroids
orbit the sun in the asteroid belt. The asteroid
belt is a wide region between the orbits of Mars
and Jupiter. Like comets, asteroids are thought
to be material left over from the formation of
the solar system.
Comet Origins
Types of Asteroids
Where do comets come from? Many scientists
think that comets come from the Oort (AWRT)
cloud, a spherical region that surrounds the
solar system. When the gravity of a passing
planet or star disturbs part of this cloud, comets
The composition of asteroids varies depending
on where they are located within the asteroid
belt. In the outermost region of the asteroid
belt, asteroids have dark reddish brown to black
March 26, 2013
The Daily Revolution
surfaces. This coloring may indicate that the
asteroids are rich in organic material. Asteroids
that have dark gray surfaces are rich in carbon.
In the innermost part of the asteroid belt are
light gray asteroids that have either a stony or
metallic composition.
meteorites are some of the building blocks of
planets.
Meteoroids
Meteoroids are similar to but much smaller
than asteroids. A meteoroid is a small, rocky
body that revolves around the sun. Most
meteoroids are probably pieces of asteroids. A
meteoroid that enters Earth’s atmosphere and
strikes the ground is called a meteorite. As a
meteoroid falls into Earth’s atmosphere, the
meteoroid moves so fast that its surface melts.
As the meteoroid burns up, it gives off an
enormous amount of light and thermal energy.
From the ground, you see a spectacular streak
of light, or a shooting star. A meteor is the
bright streak of light caused by a meteoroid or
comet dust burning up in the atmosphere.
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Meteor Showers
Many of the meteors that we see come from
very small (dust-sized to pebble-sized) rocks.
Even so, meteors can be seen on almost any
night if you are far enough away from a city to
avoid the glare of its lights. At certain times of
the year, you can see large numbers of meteors.
These events are called meteor showers.
Meteor showers happen when Earth passes
through the dusty debris that comets leave
behind.
Types of Meteorites
Like their asteroid relatives, meteorites have
different compositions. There are three major
types of meteorites—stony, metallic, and stonyiron meteorites. Many of the stony meteorites
probably come from carbon-rich asteroids.
Stony meteorites may contain organic materials
and water. Scientists use meteorites to study the
early solar system. Like comets and asteroids,