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My photos of Meteor Crater in Arizona were taken on a visit
in the mid 1990s. For more information about the impacts of meteorites on earth,
visit;
Falling from the Sky: A Meteorite Reference web site.
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 at:
PHA Close Approaches to the Earth
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 shown
below;
Future Close Approach Table
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. |
