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Movies like Deep
Impact and Space Cowboys presented
stories about major meteorite impacts on Earth.
These movies discussed what might happen if a meteor
or asteroid was discovered on a collision course
with Earth. Both movies involved space ships going
to the asteroid and trying to blow it up or deflect
it with nuclear weapons. In the end, mankind was
saved just in time.
Is an asteroid or
meteorite impact possible? Could this really happen,
or is it just “Hollywood” fantasy? If so, when will
it happen?
We know in the
geologic past that meteorites have impacted the
earth. One good example is the meteorite crater left
in the Arizona desert (Meteorite Crater). I took
several photos of Meteor Crater in Arizona on a
visit in the mid 1990s. Several of these photos are
located on my website.
Roy A. Gallant, in his
book Meteorite Hunter, ISBN 007137224-5,
writes;
“According to NASA, a
10-meter-diameter cosmic missile passes closer to us
than the moon’s distance each day. An object 100
meters in diameter crosses earth’s orbit at about
the moon’s distance on average of once a month. In
January 1991 a 10-meter-diameter object missed earth
by only half the moon’s distance just 12 hours after
astronomers spotted it. On December 9, 1994,
Asteroid 1994XM missed us by 100 kilometers. It was
13 meters in diameter. The size of a house, the
small asteroid would have completely wiped out the
greater New York area had it made a direct hit on
Manhattan. We can expect to get hit by such
house-size objects about once every 100 years. Every
1000 to 3000 years we can expect to be hit by Near
Earth Objects (NEOs) ranging in size from 100 to
several hundred meters. Fortunately we don’t have to
worry about those stoney objects smaller than about
50 meters in diameter because most burn up in the
atmosphere. But if an object is made of iron, then
some worry is justified. For instance, a metal
asteroid about 30 meters in diameter carved out a
crater 1.2 kilometers across in the Arizona desert
50,000 years ago.
Asteroids about 100
meters and larger deserve our greatest respect, and
we know of some 100,000 of them that inhabit the
Solar System this side of Mars. A direct hit by one
of these would wipe out a continent. They visit
earth once every 50,000 to 500,000 years. More
troublesome are the 1,000 to 2,000 NEOs roughly 1
kilometer and larger in diameter that collide with
Earth once every 300,000 years or so. There are real
Earth crunchers that cause mass extinctions.
Chicxulub was one, but that was 65 million years
ago. If our numbers game is a reliable one, then
where are all the more recent impact sites? In any
case, as one writer put it, “we live in a cosmic
shooting gallery.”
One thing that alerted
astronomers to just how often Earth is targeted by
bombs from space was a U.S. military report made
public early in 1994. According the report, from
1975 to 1992 military satellites detected 136
high-altitude explosions with a force of 500 to
15,000 tons of high explosives –in effect, small
atomic bombs. This report went on to reveal, to the
astonishment of the scientific community, that the
objects entered the atmosphere at 16 to 48
kilometers per second, that they exploded 27 to 32
kilometers above the ground, and that there probably
were 10 times more events than were detected. If
that were so, then there are about eighty such
explosive events a year.
Currently, the
University of Arizona’s Spacewatch program detects
about twenty new NEOs a month. According to
Spacewatch director Robert S. McMillan as many as
nine hundred 1-kilometer-diameter asteroids capable
of wreaking global havoc may pose a collision threat
to Earth.
Just how much energy
does a moderately sized asteroid – say, a
1-kilometer-wide object pack? Tom Gehrels says we
can use the mathematical expression 1/2mv2 to
calculate the kinetic energy. Let m represent the
object’s mass and v its velocity of entry into the
atmosphere. If the object has a density of 3grams
per cubic centimeter, which we get from meteorites,
and the entry velocity is 20 kilometers per second,
then an asteroid 1 kilometer in diameter packs a
striking force of millions of times the explosive
forces of the atomic bomb dropped on Hiroshima.
Gehrels is reassuring when he tells us that the
Spacewatch team knows of only about ten
Chicxulub-size objects with Earth-crossing orbits.”
A list of Potentially
Hazardous Asteroids (PHAs) is updated by the
Spacewatch project. This list is located on the
following educational website:
http://cfa-www.harvard.edu/iau/lists/PHACloseApp.html
According to the PHA
website:
“… the mean distance
of the moon is 0.0026 AU = 384400 km = 238900 miles.
(1 AU is approximately the mean distance of the
earth from the sun = 149597870 km = 92955810 miles.”
AU stand for Astronomical distance Unit: One AU is about 1.5x10^8 km,
(roughly the average distance between the Earth and the Sun).
NASA and JPL have put
together a Future Close Approach table. This table
predicts close approaches between 2001 and May 2040,
for projected approaches within 0.2 au. A link to
this table is at http://ssd.jpl.nasa.gov/cgi-bin/ca_future?date.
From the studies
that have been done, and the research that is
ongoing, it is impossible to think that a major
meteorite impact will not happen. Like earthquakes
on major fault zones, meteorite impacts are part of
our physical world. The only real variable is time.
H. Court Young
Geologist, author and publisher
Promoting awareness through the written word
http://www.hcourtyoung.com
http://www.tmcco.com
(303) 726 8320
©August, 2007 |
The
Orphan Boy mine was started in Colorado's glory days. It was reopened in
1948 by one man, Herbert T. Young, who had a passion for mining. He was
a member of the "greatest generation."