Download Comets People were very superstitious in ancient times. They

Comets
People were very superstitious in ancient times. They
believed that fuzzy, white streaks that sometimes
appeared in the night sky meant an important event was
about to happen. They believed that these comets were
omens (signs) that bad things were about to happen.
Astronomers now know that a comet is a ball of ice, rock,
and dust. A comet is like a dirty snowball speeding though
space.
WHAT ARE COMETS?
Astronomers in the 1500s and 1600s began to study comets.
They found that comets are bodies in space, not part of the
atmosphere.
A British astronomer named Edmond Halley discovered that
comets actually orbit the Sun. They return again and again, but
they go so far from the Sun that their returns take a long time.
He studied a comet that showed up in 1682. He proposed that it
was the same comet that had appeared in 1607 and 1531.
Halley predicted that the comet would return again around 1758.
The comet returned as he had said it would. The comet came to
be known as Halley’s Comet. The orbit of Halley’s Comet takes it
close to the Sun about every 76 years. The last time Halley’s
Comet came near the Sun was in 1986.
Astronomers now study comets with telescopes and spacecraft.
They are learning more about what comets are made of, where
they come from, and how they move in space.
WHAT DOES A COMET LOOK LIKE?
A comet looks like a fuzzy, white ball with one or more fat,
white tails trailing behind it. A comet has three main parts. It has
a center made of ice and rock. A hazy cloud of gas called the
coma surrounds the center. A comet can also have one or more
tails that trail behind it.
Comets only have tails when they get near the Sun. When a
comet comes near the Sun, heat makes some of the ice turn to
gas. The comet gets very bright. This gas streams away and
carries dust with it, making one or more tails. The tails always
point away from the Sun.
HOW DO COMETS TRAVEL IN SPACE?
Comets orbit the Sun just as the planets do. That is why
astronomers see the same comet return over and over again.
But comets have huge oval-shaped orbits. They swing far out to
the edge of the solar system.
It takes Earth 365 days to go around the Sun. The orbits of
some comets are so big that it takes them hundreds of years to
go around the Sun once.
WHERE DO COMETS COME FROM?
Comets come from places in the outer solar system called the
Kuiper Belt and the Oort Cloud. The Kuiper Belt and the Oort
Cloud are made up of chunks of ice and rock. Comets that orbit
the Sun in less than 200 years come from the Kuiper Belt. The
Kuiper Belt is just beyond the planet Neptune. Comets that take
longer than 200 years to go around the Sun come from the Oort
Cloud. The Oort Cloud is far out at the edge of the solar system,
beyond the planet Pluto.
Astronomers think that comets formed soon after the solar
system began about 5 billion years ago. The solar system
formed from a big cloud of gas and dust. The Sun formed in the
center of the cloud. Gas and dust farther out clumped together
to make the planets. Comets may have formed from leftover gas
and dust at the edge of the solar system where it is very cold.
COULD A COMET HIT EARTH?
We know that comets can hit planets. In 1994, telescopes and
spacecraft watched Comet Shoemaker-Levy 9 crash into the
planet Jupiter. The crash sent up fireballs that were bigger than
Earth!
Some scientists think that comets hit Earth long ago. A comet or
asteroid (space rock) crashing into Earth 65 million years ago
may have killed off the dinosaurs. The crash would have sent up
a huge cloud of dust. The dust could have blocked out the Sun’s
light. Earth would have become cold and dark enough to kill off
the plants that dinosaurs ate.
On a dark, clear night you can sometimes see streaks of light
shooting across the sky. Some of these shooting stars are bits
left behind by comets. They give off light as they burn up while
falling through the atmosphere. So in a way, comets are
constantly falling on Earth!
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