RISC DISC 2/RISC_DISC_2.iso/resources/nasa/shoemaker/info/collisions_with_earth

Text File | 1995-05-03 | 6.7 KB | 107 lines

The Probability of Collisions with Earth

Comet Impact '94
Fact Sheet

Most bodies in the solar system with a visible solid surface exhibit craters.
On Earth we see very few because geological processes such as weathering and
erosion soon destroy the obvious evidence.  On bodies with no atmosphere, such
as Mercury or the Moon, craters are everywhere. without going into detail,
there is strong evidence of a period of intense cratering in the solar system
that ended about 3.9 billion years ago.  Since that time cratering appears to
have continued at a much slower and fairly uniform rate.  The cause of the
craters is impacts by comets and asteroids.  Most asteroids follow simple
circular orbits between the planets Mars and Jupiter, but all of these
asteroids are perturbed, occasionally by each other and more regularly and
dramatically by Jupiter. As a result some find themselves in orbits that cross
that of Mars or even Earth. Comets on the other hand follow highly elongated
orbits that often come close to Earth or other major bodies to begin with.
These orbits are greatly affected if they come anywhere near Jupiter. Over the
eons every moon and planet finds itself in the wrong place in its orbit at the
wrong time and suffers the insult of a major impact.

The Earth's atmosphere protects us from the multitude of small debris, the size
of grains of sand or pebbles, thousands of which pelt our planet every day.
The meteors in our night sky are visible evidence of this small debris burning
up high in the atmosphere.  In fact, up to a diameter of about l0-meters (33
feet), most stony meteoroids are destroyed in the atmosphere in thermal
explosions.  Obviously some fragments do reach the ground, because we have
stony meteorites in our museums.  Such falls are known to cause property damage
from time to time.  On October 9, 1992, a fire ball was seen streaking across
the sky all the way from Kentucky to New York. A 27-pound stony meteorite
(chondrite) from the fireball fell in Peekskill, New York, punching a hole in
the rear end of an automobile parked in a driveway and coming to rest in a
shallow depression beneath it.  Falls into a Connecticut dining room and an
Alabama bedroom are well documented incursions in this century.  A l0-meter
body typically has the kinetic energy of about five nuclear warheads of the
size dropped on Hiroshima, however, and the shock wave it creates can do
considerable damage even if nothing but comparatively small fragments survive
to reach the ground.

Many fragments of a l0-meter iron meteoroid will reach the ground.  The only
well-studied example of such a fall in recent times took place in the
Sikhote-Alin Mountains of eastern Siberia on February 12, 1947.  About 150 US
tons of fragments reached the ground, the largest intact fragment weighing
3,839 pounds.  The fragments covered an area of about 1 x 2

kilometers (0.6 x 1.2 miles), within which there were 102 craters greater than
1 meter in diameter, the largest of them 26.5 meters (87 feet), and about 100
more smaller craters.  If this small iron meteoroid had landed in a city, it
obviously would have created quite a stir.  The effect of the larger pieces
would be comparable to having a car suddenly drop in at supersonic speeds!
Such an event occurs about once per decade somewhere on Earth, but most of them
are never recorded, occurring at sea or in some remote region such as
Antarctica It is a fact that there is no record in modern times of any person
being killed by a meteorite.

It is the falls larger than 10 meters that start to become really worrisome.
The 1908 Tunguska event was a stony meteorite in the 100-meter class.  The
famous meteor crater in northern Arizona, some 1219 meters (4,000 feet) in
diameter and 183 meters (600 feet) deep, was created 50,000 years ago by a
nickel-iron meteorite perhaps 60 meters in diameter.  It probably survived
nearly intact until impact, at which time it was pulverized and largely
vaporized as its 6-7 x 1016 joules* of kinetic energy were rapidly dissipated
in an explosion equivalent to some 15 million tons of TNT!  Falls of this class
occur once or twice every 1000 years.

There are now over 100 ring-like structures on Earth recognized as definite
impact craters.  Most of them are not obviously craters, their identity masked
by heavy erosion over the centuries, but the minerals and shocked rocks present
make it clear that impact was their cause.  The Ries Crater in Bavaria is a
lush green basin some 25 kilometers (15 miles) in diameter with the city of
Nordlingen in the middle.  Fifteen million years ago a 1500-meter (5000 feet)
asteroid or comet hit there, excavating more than a trillion tons of material
and scattering it all over Europe. This sort of thing happens about once every
million years or so.  Another step upward in size take us to Chicxulub, an
event that occurs once in 50-100 million years.  Chicxulub is the largest
crater known which seems definitely to have an impact origin, but there are a
few ring-like structures that are 2-3 times larger yet about which geologists
are uncertain.

There are now more than 150 asteroids known that come nearer to the Sun than
the outermost point of Earth's orbit.  These range in diameter from a few
meters to about 8 kilometers.  A working group chaired by Dr. David Morrison,
NASA Ames Research Center, estimates that there are some 2,100 such asteroids
larger than 1 kilometer and perhaps 320,000 larger than 100 meters, the size
that caused the Tunguska event and the Arizona meteor crater.  An impact by one
of these larger meteors in the wrong place would be a great catastrophe, but it
would not threaten civilization.  An impact by an 8-kilometer object is
damaging enough to cause mass extinctions.  In addition there are many comets
in the l-10-kilometer class, 15 of them in short period orbits that pass inside
Earth's orbit, and an unknown number of long-period comets. virtually any
short-period comet among the 100 or so not currently coming near to Earth could
become dangerous after a close passage by Jupiter.

This all sounds pretty scary.  However, as noted earlier, no human in the past
1000 years is known to have been killed by a meteorite or by the effects of one
impacting. (There are ancient Chinese records of such deaths.) An individual's
chance of being killed by a meteorite is small as compared to death by
lighting, volcanism, earthquake, or hurricane, to say nothing of the multitude
of human-aided events.  That small probability was unlikely to have been any
consolation to the dinosaurs, however.  For this reason astronomers today are
conducting ever-increasing searches for all of the larger asteroids that could
become dangerous.

*joule: a unit of measurement, the amount of energy corresponding to one watt
acting for one second.