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Week 6 - Asteroids, Comets, and Meteors
Have you ever wondered why dinosaurs no longer exist? There are many theories about their
extinction, including competition with mammals, overpopulation, disease, and a series of
devastating volcanic eruptions. In 1980, a physicist named Luis Alvarez and his son Walter
Alvarez proposed that a large celestial body, such as an asteroid, comet, or meteor, crashed
into Earth millions of years ago. The impact caused a catastrophic climate change which caused
the extinction of dinosaur species. Although we may never know why dinosaurs died out, we
can study the celestial bodies which may have killed them. In this lesson, you will learn about
asteroids, comets, and meteors.
Do you have any cousins? Do you look similar? Even though you are related, you probably do
not look much like your cousins. You may share similar features or characteristics, but you are
still very different. Our solar system is filled with small, rocky fragments called asteroids.
Asteroids are like the cousins of planets. They have characteristics in common, such as an orbit
around the sun and a similar chemical composition, but they are still very different. For one,
most asteroids have irregular shapes which are not spheres. Asteroids are also much smaller
than planets. Of the 90,000 known asteroids in our solar system, only 16 have diameters
greater than 240 km (150 miles). Most asteroids are much smaller.
Asteroids are sometimes called "space rubble" because they are thought to be left over
material from the formation of our solar system billions of years ago. Scientists believe that
Jupiter's massive gravity may have prevented asteroids from forming planets. Imagine different
sized pieces of clay scattered on the floor. They pieces are too far apart to form one big ball of
clay, so they remain separate, smaller pieces. The asteroid belt found between Mars and
Jupiter is evidence which supports this theory.
Image courtesy of NASA
Scientists classify asteroids by both chemical composition and location in the solar system. Over
75 percent of known asteroids are very dark, carbon-containing rocks called C-types. About 17
percent are S-type asteroid made of bright iron, nickel, and magnesium. The remaining
asteroids are M-types made of pure nickel and iron. Main Belt asteroids are those found
between Mars and Jupiter, between two and four AU from the sun. Trojans are special
asteroids that have been captured by Jupiter's gravity and now orbit the gas giant instead of the
sun. Near-Earth Asteroids (NEAs) are the ones scientists watch most closely. These asteroids
come within 1.3 AU (121 million miles) of the sun, meaning they are less than 30 million miles
from the Earth.
What do black cats, mirrors, and comets have in common? All three have a superstitious
history. Crossing the path of a black cat, breaking a mirror, and seeing a comet were all
considered bad luck at one time or another. During the days of the Roman empire,
philosophers noted the passing of a comet shortly before the assassination of Julius Caesar.
Centuries later, a comet was blamed for the millions of Europeans killed during the Black
Plague. Today, we know that comets are not harbingers of doom, but small balls of ice, rock,
and dust left over from the creation of our solar system.
Like most asteroids, comets revolve around the sun, but their orbits are much more elliptical.
Some comets, called long-period comets, can take as long as 30 million years to complete a
single trip around the sun. These comets are difficult to predict and study because they are
seen so rarely. Short-period comets, which take less than 200 years to orbit the sun, are more
The characteristic "tail" of a comet is only visible when it is close to the sun. The center of a
comet is a solid mass, called the nucleus, made of ice, frozen gas, rock, and dust. The outer
layer of the nucleus is called the coma. It is made of water vapor, carbon dioxide, and other
gases. As a comet nears the sun, radiation changes some of its ice into gas. The gas mixes with
dust and streaks out behind the comet as a tail. Comet tails can be up to 10 million km (6
million miles) long. Since they are blown by solar winds, the tails always point away from the
sun. Eventually, the sun burns off most of the comet's ice and gas, and it becomes a dead
comet similar to an asteroid.
Have you ever seen a shooting star? Despite their name, shooting stars are not actually stars,
but pieces of rock called meteors. Small chunks of rock and metal moving through space are
called meteoroids. Meteoroids are similar to asteroids, but they are much smaller in size. When
a meteoroid falls through Earth's atmosphere, it becomes a meteor. Heat caused by intense air
friction vaporizes most of the meteor, leaving a bright trail of gas and dust. Most meteors burn
up in the atmosphere and fall to Earth's surface as tiny, dust-like grains called micrometeoroids.
Scientists estimate that anywhere between 1,000 tons and 10,000 of micrometeoroids fall on
Earth each day. Large chunks which strike the Earth are called meteorites.
Meteorites often look very similar to rocks on Earth, though they may have a burned
appearance. They may be only a few millimeters in diameter or larger than a boulder. The
largest meteorite, called Hoba, weighs almost 120,000 lbs. Scientists believe that meteors form
within our own solar system. Several meteorites found in Antarctica are believe to have
originated on Mars. Gases trapped inside the meteorites matched samples taken from Mars'
atmosphere during the Viking mission. Other meteorites have closely matched rocks from
Earth's moon.
Image courtesy of NASA
Are meteors dangerous? Yes and no. Since most meteors disintegrate in Earth's atmosphere,
the meteorites which make it to the surface are very small. The damage they cause is minor,
such as a broken window or dented car. However, scientists estimate at least large 1,000
meteors which pass through Earth's orbit. Every million years or so, one of them collides with
the Earth, creating a crater. Over 150 impact craters have been located on Earth. One of the
largest craters is the Barringer Crater, also called Meteor Crater, near Winslow, Arizona.
Scientists believe it was formed over 50,000 years ago by an iron meteor 30 to 50 m in
diameter. The crater caused by the meteor's impact is 1,200 m in diameter and 200 m deep.
Let's Review
Celestial Body
left over rock from formation of solar system
classified by chemical composition and location
most asteroids found between Mars and Jupiter
Trojans are asteroids captured by Jupiter's gravity
highly elliptical orbit
nucleus made of ice, rock, frozen gas, and dust
tail caused by heat from solar radiation
long-period comets can take millions of years to orbit the sun
meteors in space are called meteoroids
meteors which strike the Earth' surface are called meteorites
small, rocky bodies formed from our solar system
Orbiting around the sun in the space between Mars and Jupiter are more than 7,000
masses of rock called asteroids, or planetoids. Ceres, the largest asteroid, was discovered in
1801. Asteroids range in size from less than 1 kilometer to almost 1,000 km. Asteroids shine by
reflected sunlight and appear as stars when viewed through telescopes.
For many years astronomers believed that the space between Mars and Jupiter should be
occupied by a planet. Today, some scientists speculate that one or more planets existed
between Mars and Jupiter and that they disintegrated into the asteroids.
Asteroid Gaspra seen from the Galileo spacecraft. Long dimension is about 19 km. (Nasa)
Comets are members of the solar system. Most are captives
of the sun. Many travel around the sun in huge, elliptical
orbits. Some comets, however, appear to have parabolic
orbits, and will pass the sun only once. Sometimes, but not
very often, a comet can be seen in the night sky. The comet
appears as a bright ball of light with a long, glowing tail.
Comets move very fast, but they are so far away that they
appear to move slowly. A comet may be visible for several
Comets are made chiefly of frozen gases and dust.
Astronomers say that a distant comet is like a huge, dirty
snowball. The comet head may range from 1 to 30 km in
As a comet nears the sun, it is warmed. The frozen gases
change from ice to vapor. The sun's rays pass through the
gases and cause them to glow. The gases are very light. They
are pushed away from the sun to form the comet's tail. The
tail always points away from the sun. Even when a comet
moves away from the sun, the tail goes first.
By this time the comet is very large. The head may measure
a million kilometers across and the tail stretches through
millions of kilometers of space. As the comet moves away from the sun, the tail shrinks and
vanishes. The head shrinks and stops glowing.
The orbits of comets are of different sizes. Some take a few years to complete one orbit around
the sun, and some take many years. About a dozen comets are spotted each year, but only a
few can be seen without telescopes. The most famous comet is Halley's comet. It orbits the sun
once every 75 to 77 years. It was last seen in 1986. Several unmanned spacecraft collected
information about the comet.
Surpassing Halley's comet in size and brilliance was the Hale-Bopp comet, discovered in July of
1995. Almost four times larger than Halley's comet, Hale Bopp's 43,000-mile-per-hour path
made the comet visible to the naked eye during March and April of 1997.
You have learned about the planets and some of their characteristics. You know that
distances in space are measured in light-years. Now, you will learn about some of the other
celestial bodies in our solar system.
Asteroids. An asteroid is a very small celestial body with an irregular shape. Like the planets,
asteroids revolve around the sun. Most known asteroids orbit the sun between Mars and
Jupiter. This area is called the Asteroid Belt and contains millions of asteroids. Many asteroids
are influenced by the gravitational pull of Jupiter. Jupiter is so large that its pull of gravity on
the asteroids affects their pathways. The asteroids that follow Jupiter's path around the sun are
given the special name of Trojans. Mars can also affect asteroid orbits. Scientists believe that
Mars' moons Phobos and Deimos may actually be asteroids which were pulled into orbit around
the planet.
Asteroids were first observed during the early 1800s. Asteroids were given their name by the
astronomer William Herschel. The word means "star-like" in Greek. The first observed asteroid
was given the name Ceres. Asteroids range in size from Ceres, which is 980 miles in diameter, to
the size of small mountains.
Comets. A comet is a bright, celestial body made mainly of dust and small rock fragments. This
material is covered with frozen gases and water vapor. Like asteroids, comets revolve around
the sun in elliptical orbits. Comets orbits are very elongated. The orbits therefore often take
them to the outer edges of the solar system.
Halley's Comet
The word comet comes from a Latin word meaning "long-haired." This meaning refers to the
glow of gases that stream from a comet's tail. Scientists believe that comets have been around
since the formation of the solar system over 4 billion years ago. Each year a few "new" comets
are discovered. Many of these comets are short-period comets. This means they take less than
200 years to orbit the sun. Others are long-period comets which can take as long as 30 million
years to orbit the sun.
The center of a comet is called the nucleus. The nucleus is usually only a few miles wide. It is
made of ice, frozen gases, rock, and dust. As the comet nears the sun, it starts to warm up. The
sun's heat causes ice in the nucleus to vaporize into a gas. These gases stream out behind the
comet producing the characteristic tail. Comet tails can be thousands of miles long. They always
point away from the sun.
On rare occasions, celestial bodies collide with each other. Comets are no exception. One such
example is the Shoemaker-Levy 9 Comet. Pieces of this comet collided with Jupiter from July 16
to July 22, 1994. A photograph of this particular comet can be seen below. This picture was
taken by the Hubble Space Telescope in March of 1994.
Meteors. Millions of particles and small chunks of matter strike earth's atmosphere. These
particles glow very brightly in the night sky. They are called shooting stars, or meteors. A
meteor usually burns up in the atmosphere. Meteor fragments that do reach the ground are
called meteorites. Meteorites are composed of stone, or stone and iron combinations. Some
contain nickel.
According to scientists, anywhere between 1,000 and 10,000 tons of meteoritic material falls on
the earth every day. We do not notice this material because most particles are smaller than
specks of dust. Many meteors can be seen each night. When the number of meteors is very
large the event is called a meteor shower. Sometimes large meteorites strike the earth. Often
they look similar to rocks found on earth. Other times they have a burned appearance. The
largest meteorite is the 54,000 kg Hoba meteorite, which fell in southwest Africa.