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EARTH Phần 2 pot
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In addition to the big planets, the solar system contains many billions of
smaller orbiting objects. Many of these are lumps of rock, iron, and nickel left
over from the formation of the planets. These include the asteroids that
mainly orbit the Sun between Mars and Jupiter. There are also comets—big
chunks of ice and dust that loop around the Sun before vanishing
into the far reaches of the solar system. Smaller pieces
of rock and ice shoot through Earth’s sky as
meteors. Some of these pieces may even fall to
Earth as meteorites.
ASTEROIDS, METEORITES,
AND COMETS
COMETS
There are billions of
comets in the Oort Cloud, a
region of the solar system beyond the
orbit of Neptune. A few of these icy bodies
travel close to the Sun. As they approach, they are
blasted by solar radiation that makes them trail
long tails of glowing dust and gas. After several
weeks, the comets vanish, but some reappear
many years later. This is Halley’s Comet, which
orbits the Sun every 76 years.
IMPACT CRATERS
This crater in Arizona is one of about 170 that have
been found on Earth. Formed by an asteroid strike
about 50,000 years ago, it is ¾ miles (1.2 km) across.
The impact would have caused a colossal explosion,
killing everything in the region. Luckily, these large
impacts are very rare. The last occurred in 1908,
when an asteroid exploded high above
a remote region of Siberia
called Tunguska.
Length
Orbital period
Discovery date
IDA
1884
1,768 days
33 miles (53 km)
Orbital speed 11 miles (18 km) per sec
Length
Orbital period
Discovery date
GASPRA
1916
1,200 days
11 miles (18 km)
Orbital speed 12 miles (20 km) per sec
Length
Orbital period
Discovery date
EROS
1898
643 days
20 miles (33 km)
Orbital speed 15 miles (24 km) per sec
ASTEROIDS
The Asteroid Belt between the orbits of Mars
and Jupiter contains vast numbers of asteroids.
Most are too small to have names, but a few,
such as Gaspra and Ida, are big enough to have
been photographed by passing space probes.
Some asteroids orbit outside the main belt,
including Eros, which passes within
14 million miles (22 million km) of Earth.
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