Page 54 - Ultimate Visual Dictionary (DK)

THE UNIVERSE

Asteroids, comets, OPTICAL IMAGE OF
HALLEY’S COMET
and meteoroids

ASTEROIDS, COMETS, AND METEOROIDS are all
debris remaining from the nebula from which the solar
system formed 4.6 billion years ago. Asteroids are
rocky bodies up to about 600 miles (1,000 km) in
diameter, although most are much smaller. Most of them
orbit the Sun in the asteroid belt, which lies between the
orbits of Mars and Jupiter. Cometary nuclei exist in
a huge cloud (called the Oort Cloud) that surrounds
ASTEROID 951 GASPRA the planetary part of the solar system. They are made
of frozen water and dust and are a few miles in FALSE-COLOR IMAGE OF
diameter. Occasionally, a comet is deflected from the Oort Cloud on to a long, HALLEY’S COMET
elliptical path that brings it much closer to the Sun. As the comet
approaches the Sun, the cometary nucleus starts to vaporize in the
High-intensity
heat, producing both a brightly shining coma (a huge sphere of
light emission
gas and dust around the nucleus), and a gas tail, and a dust tail.
Meteoroids are small chunks of stone or stone and iron, which
Nucleus
are fragments of asteroids or comets. Meteoroids range in size from
tiny dust particles to objects tens of meters across. If a meteoroid
enters the Earth’s atmosphere, it is heated by friction and
Medium-intensity
appears as a glowing streak of light called a meteor (also known
light emission
as a shooting star). Meteor showers occur when the Earth passes
through the trail of dust particles left by a comet. Most meteoroids
Low-intensity
burn up in the atmosphere. The remnants of the few that light emission
are large enough to reach the Earth’s surface are termed meteorites.
FALSE-COLOR IMAGE OF A METEORITES DEVELOPMENT OF COMET TAILS
LEONID METEOR SHOWER
Dust tail deflected by Gas tail pushed away
STONY METEORITE photons in sunlight from Sun by charged
and curved due to particles in solar wind
comet’s motion
Fusion crust
formed when
passing
through
Tails lengthen
atmosphere
as comet nears
Sun
Olivine
and pyroxene Sun
mineral interior Direction of comet’s
orbital motion
Coma surrounding
nucleus
STONY-IRON
Tails behind
METEORITE
nucleus Tails in front of
nucleus
Iron
Nucleus vaporized
by Sun’s heat, Dust Coma and tails
Stone (olivine) forming a coma Gas tail fade as comet moves
with two tails tail away from Sun
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