History Channel
Season. 3 Episode. 6
http://www.history.com/shows/the-universe
http://en.wikipedia.org/wiki/The_Universe_%28TV_series%29
EXCERPTS:
Asteroids and Comets
hold secrets of universe
and building blocks of life
and
also enemy.
Asteroids
"Because they were formed at the same times as Earth
asteroids contain the same basic elements
we find on our planet to build things.
Fragments of asteroids
that actually survive an impact
with the Earth's surface
are called METEORITES.
Asteroid Belt - After Mars
Kuiper Belt - After Neptune - Ice and Dust bodies
Oort Cloud - couple of light years from Sun,
"In 1986, the Giotto spacecraft
became the first mission to fly by a comet.
It was Comet Halley.
the only one known to return within our human lifetime.
also visible to the naked eye.
"The so-called carbonaceous chondrite types of asteroids
which are typically found
in the outermost reaches of the main asteroid belt
have an almost exact match
between their deuterium-to-hydrogen ratios and Earth's oceans.
"Jupiter's gravity or light pressure from the Sun
can bump these cosmic relics out of the asteroid belt.
Similarly, the gravity from planet Neptune
can nudge comets out of the Kuiper Belt.
"65 million years ago, a cosmic object
roughly six miles across
impacted the Yucatan Peninsula
near the modern village of Chicxulub, Mexico.
It produced kinetic engergy equivalent to
eight billion Hiroshima atomic bombs.
Many scientists now believe the impact event
extinguished 75% of animal and plant species.
The next Chicxulub-sized impact
is estimated to occur 35 million years from now
but one can never by too certain when it comes to space rocks.
Somwhere on the order of 85 to 95 percent
of the objects that collide with the Earth
come from the main asteroid belt.
Although most potentially hazardous objects are asteroids
comets reamin a legitimate threat.
These icy bodies, which come from the outer solar syatem
can travel three times faster than asteroids.
so it takes only 9 months
for them to get from the orbit of Jupter
to the orbit of Earth.
Fortunately, Jupiter acts as a planetary shield.
It deflects or absorbs comets that drift towards Earth.
And scientists now have the pictures to prove it.
On July 22, 1994, telescopes around the world
caught a glimpse of one of the most violent comet impacts
in the history of humanity.
A String of about 20 comet fragments named Shoemaker-Levy 9
crashed into the upper cloud decks of Jupiter.
On June 30, 1908,
the biggest cosmic disaster in recorded human history
on Earth occurred in Russia.
A large object exploded in Siberia's remote Tunguska wilderness.
smallaer asteroids can create significant damage at the surface.
It's estimated to happen once every 500 years.
Believe it or not,
Earth takes a hit
from soccoer ball-sized space rocks every day.
Most go unseen.
But in 1992, amateur video caught a meteorite.
flash across the nighttime sky.
Congress mandated that NASA must detect
over 90% of near-Earth objects.
over a half-mile in diameter.
APOPHIS, size of city
This space rock will make a close pass
near Earth on April 13, 2029.
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