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ON A CLEAR NIGHT you may see several shooting stars in the space
of an hour. A shooting star, or meteor, looks like a streak of light that
suddenly darts across the sky and disappears. A meteor occurs when
a piece of dust from space, called a meteoroid, burns up as it enters
Earth’s atmosphere. As the meteor plummets to Earth at a speed of
about 150,000 mph (240,000 km/h), friction with the air produces
intense heat, which leaves a bright glow in the sky. Meteors usually
burn up about 56 miles (90 km) from Earth’s surface.
Many meteoroids are fragments from comets that orbit the sun.
A comet appears as a faint, fuzzy point of light that moves across the
night sky for weeks or months. As it nears the sun, the comet grows
a “tail.” Then it swings past the
sun and travels away, becoming
smaller and fainter. Comets often
reappear at regular intervals
(every few years) as they travel
past Earth on their orbits.
Comets and meteors
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SunMETEORITES
Huge lumps of rock called meteorites pass through Earth’s
atmosphere without burning up completely. About 25,000 years ago,
a meteorite that weighed about 900,000 tons caused a crater in Arizona
(above), 4,000 ft (1,200 m) across. Some scientists believe that the
impact of a huge meteorite about 66 million years ago may have
destroyed many animal species.
METEORS
There are two ways in which meteors
occur: individually and in showers.
This spectacular meteor shower
(left) occurred in 1833. Similar
impressive displays occur every
33 years during November. At this
time Earth passes through a swarm
of meteors, called the Leonids,
that spread out along the orbit
of a comet.Gas tail can be
up to 62 million miles
(100 million km) long. The
gas tail has a bluish glow. This is
because the heat of the sun makes the
gas molecules emit blue-colored light.COMETS
A comet consists
of a central core, or
nucleus, of dust and
ice; a cloud of gas
and dust around the
nucleus, called the
coma; and one or more
tails. Astronomers have
observed hundreds of
comets and believe that
about one billion other
comets orbit the sun
unseen, far beyond the
most distant planet.The solar wind – a blast of charged particles that
stream from the sun – blows the comet’s gas tail
away from the sun. When the comet approaches
the sun, its tails follow. The tails lead when the
comet moves away from the sun.
The size of a comet’s nucleus can range
from a few hundred yards across to more
than about 6 miles (10 km) across.
COMET TAIL
As a comet approaches Earth,
the heat of the sun turns the ice
into gas. The gas escapes, along
with dust, and forms one
or more tails (the gas and
dust form separate
tails). The tails
always point away
from the sun. They
get shorter as the comet
moves away from the sun.HALLEY’S COMET
The English astronomer Edmund
Halley (1656-1742) was the first
to realize that some comets
appear regularly. In 1705, he
showed that the comet now called
Halley’s Comet returns past
In 2004, the Stardust Earth every 75 or 76 years.
spacecraft flew past
comet Wild 2, sending
back many pictures,
including this enhanced,
composite image.Chinese astronomers
probably observed Halley’s
Comet more than 2,200
years ago. The comet also
appears in the 11th-century
Bayeux tapestry, which
shows the Norman
Conquest of England.Dust tail can be up to about
600,000 miles (1 million km)
long. It shines white because
the particles of dust
reflect sunlight.