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Appendix C
The Probability of Collisions with Earth

        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 sensibly 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, as noted in Section 2, 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, many times, and
suffers the insult of a major impact.

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 bodies of this type burning up high in the atmosphere.
In fact, up to a diameter of about 10 m (33 ft.) most stony
meteoroids are destroyed in the atmosphere in a terminal
explosion. 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
fireball was seen streaking across the sky all the way from
Kentucky to New York. A 27-lb. 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 other well documented
incursions in this century. A 10-m body typically has the kinetic
energy of about five Hiroshima fission bombs, 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 10-m 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 U.S. tons of fragments reached the
ground, the largest intact fragment weighing 3,839 lb. The
fragments covered an area of about 1-2 km^2 (0.6-1.2 mi.^2),
within which there were 102 craters greater than 1 m in diameter,
the largest of them 26.5 m (87 ft), and about 100 more smaller
craters. If this small iron meteorite 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 supersonic auto
suddenly drop in! 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 m that start to become really
worrisome. The 1908 Tunguska event described in Section 7 was a
stony meteorite in the 100-m class. The famous meteor crater in
northern Arizona, some 4,000 ft. in diameter and 600 ft. deep, was
created 50,000 years ago by a nickel-iron meteorite perhaps 60 m
in diameter. It probably survived nearly intact until impact, at
which time it was pulverized and largely vaporized as its
6-7x10^16 joules of kinetic energy were rapidly dissipated. An
explosion equivalent to some 15 million tons of TNT creates quite
a bang! Falls of this class occur once or twice every 1,000 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 km (15 mi.) in diameter with the city of Nurdlingen in the
middle. Fifteen million years ago a 1,500-m (5,000-ft.) asteroid
or comet hit there, excavating more than a trillion tons of
material and scattering it all over central Europe. This sort of
thing happens about once every million years or so. Another step
upward in size takes us to Chicxulub, described in detail in
Appendix B, 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
"suspicious."

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 km. A working group chaired
by D. Morrison estimates that there are some 2,100 such asteroids
larger than 1Jkm and perhaps 320,000 larger than 100 m, the size
that caused the Tunguska event and the Arizona meteor crater. An
impact by one of the latter in the wrong place would be a great
catastrophe, but it would not threaten civilization. An impact by
an 8-km object is in the mass extinction category. In addition
there are many comets in the 1-10-km 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 1,000 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 ridiculously small as compared to death
by lightning, 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. Once discovered, with a few years of warning, there is
every reason to believe that a space mission could be mounted "to
shove them aside."